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diff --git a/Documentation/networking/ip-sysctl.rst b/Documentation/networking/ip-sysctl.rst new file mode 100644 index 000000000..b47b3d0ce --- /dev/null +++ b/Documentation/networking/ip-sysctl.rst @@ -0,0 +1,3067 @@ +.. SPDX-License-Identifier: GPL-2.0 + +========= +IP Sysctl +========= + +/proc/sys/net/ipv4/* Variables +============================== + +ip_forward - BOOLEAN + - 0 - disabled (default) + - not 0 - enabled + + Forward Packets between interfaces. + + This variable is special, its change resets all configuration + parameters to their default state (RFC1122 for hosts, RFC1812 + for routers) + +ip_default_ttl - INTEGER + Default value of TTL field (Time To Live) for outgoing (but not + forwarded) IP packets. Should be between 1 and 255 inclusive. + Default: 64 (as recommended by RFC1700) + +ip_no_pmtu_disc - INTEGER + Disable Path MTU Discovery. If enabled in mode 1 and a + fragmentation-required ICMP is received, the PMTU to this + destination will be set to the smallest of the old MTU to + this destination and min_pmtu (see below). You will need + to raise min_pmtu to the smallest interface MTU on your system + manually if you want to avoid locally generated fragments. + + In mode 2 incoming Path MTU Discovery messages will be + discarded. Outgoing frames are handled the same as in mode 1, + implicitly setting IP_PMTUDISC_DONT on every created socket. + + Mode 3 is a hardened pmtu discover mode. The kernel will only + accept fragmentation-needed errors if the underlying protocol + can verify them besides a plain socket lookup. Current + protocols for which pmtu events will be honored are TCP, SCTP + and DCCP as they verify e.g. the sequence number or the + association. This mode should not be enabled globally but is + only intended to secure e.g. name servers in namespaces where + TCP path mtu must still work but path MTU information of other + protocols should be discarded. If enabled globally this mode + could break other protocols. + + Possible values: 0-3 + + Default: FALSE + +min_pmtu - INTEGER + default 552 - minimum Path MTU. Unless this is changed mannually, + each cached pmtu will never be lower than this setting. + +ip_forward_use_pmtu - BOOLEAN + By default we don't trust protocol path MTUs while forwarding + because they could be easily forged and can lead to unwanted + fragmentation by the router. + You only need to enable this if you have user-space software + which tries to discover path mtus by itself and depends on the + kernel honoring this information. This is normally not the + case. + + Default: 0 (disabled) + + Possible values: + + - 0 - disabled + - 1 - enabled + +fwmark_reflect - BOOLEAN + Controls the fwmark of kernel-generated IPv4 reply packets that are not + associated with a socket for example, TCP RSTs or ICMP echo replies). + If unset, these packets have a fwmark of zero. If set, they have the + fwmark of the packet they are replying to. + + Default: 0 + +fib_multipath_use_neigh - BOOLEAN + Use status of existing neighbor entry when determining nexthop for + multipath routes. If disabled, neighbor information is not used and + packets could be directed to a failed nexthop. Only valid for kernels + built with CONFIG_IP_ROUTE_MULTIPATH enabled. + + Default: 0 (disabled) + + Possible values: + + - 0 - disabled + - 1 - enabled + +fib_multipath_hash_policy - INTEGER + Controls which hash policy to use for multipath routes. Only valid + for kernels built with CONFIG_IP_ROUTE_MULTIPATH enabled. + + Default: 0 (Layer 3) + + Possible values: + + - 0 - Layer 3 + - 1 - Layer 4 + - 2 - Layer 3 or inner Layer 3 if present + - 3 - Custom multipath hash. Fields used for multipath hash calculation + are determined by fib_multipath_hash_fields sysctl + +fib_multipath_hash_fields - UNSIGNED INTEGER + When fib_multipath_hash_policy is set to 3 (custom multipath hash), the + fields used for multipath hash calculation are determined by this + sysctl. + + This value is a bitmask which enables various fields for multipath hash + calculation. + + Possible fields are: + + ====== ============================ + 0x0001 Source IP address + 0x0002 Destination IP address + 0x0004 IP protocol + 0x0008 Unused (Flow Label) + 0x0010 Source port + 0x0020 Destination port + 0x0040 Inner source IP address + 0x0080 Inner destination IP address + 0x0100 Inner IP protocol + 0x0200 Inner Flow Label + 0x0400 Inner source port + 0x0800 Inner destination port + ====== ============================ + + Default: 0x0007 (source IP, destination IP and IP protocol) + +fib_sync_mem - UNSIGNED INTEGER + Amount of dirty memory from fib entries that can be backlogged before + synchronize_rcu is forced. + + Default: 512kB Minimum: 64kB Maximum: 64MB + +ip_forward_update_priority - INTEGER + Whether to update SKB priority from "TOS" field in IPv4 header after it + is forwarded. The new SKB priority is mapped from TOS field value + according to an rt_tos2priority table (see e.g. man tc-prio). + + Default: 1 (Update priority.) + + Possible values: + + - 0 - Do not update priority. + - 1 - Update priority. + +route/max_size - INTEGER + Maximum number of routes allowed in the kernel. Increase + this when using large numbers of interfaces and/or routes. + + From linux kernel 3.6 onwards, this is deprecated for ipv4 + as route cache is no longer used. + +neigh/default/gc_thresh1 - INTEGER + Minimum number of entries to keep. Garbage collector will not + purge entries if there are fewer than this number. + + Default: 128 + +neigh/default/gc_thresh2 - INTEGER + Threshold when garbage collector becomes more aggressive about + purging entries. Entries older than 5 seconds will be cleared + when over this number. + + Default: 512 + +neigh/default/gc_thresh3 - INTEGER + Maximum number of non-PERMANENT neighbor entries allowed. Increase + this when using large numbers of interfaces and when communicating + with large numbers of directly-connected peers. + + Default: 1024 + +neigh/default/unres_qlen_bytes - INTEGER + The maximum number of bytes which may be used by packets + queued for each unresolved address by other network layers. + (added in linux 3.3) + + Setting negative value is meaningless and will return error. + + Default: SK_WMEM_MAX, (same as net.core.wmem_default). + + Exact value depends on architecture and kernel options, + but should be enough to allow queuing 256 packets + of medium size. + +neigh/default/unres_qlen - INTEGER + The maximum number of packets which may be queued for each + unresolved address by other network layers. + + (deprecated in linux 3.3) : use unres_qlen_bytes instead. + + Prior to linux 3.3, the default value is 3 which may cause + unexpected packet loss. The current default value is calculated + according to default value of unres_qlen_bytes and true size of + packet. + + Default: 101 + +neigh/default/interval_probe_time_ms - INTEGER + The probe interval for neighbor entries with NTF_MANAGED flag, + the min value is 1. + + Default: 5000 + +mtu_expires - INTEGER + Time, in seconds, that cached PMTU information is kept. + +min_adv_mss - INTEGER + The advertised MSS depends on the first hop route MTU, but will + never be lower than this setting. + +fib_notify_on_flag_change - INTEGER + Whether to emit RTM_NEWROUTE notifications whenever RTM_F_OFFLOAD/ + RTM_F_TRAP/RTM_F_OFFLOAD_FAILED flags are changed. + + After installing a route to the kernel, user space receives an + acknowledgment, which means the route was installed in the kernel, + but not necessarily in hardware. + It is also possible for a route already installed in hardware to change + its action and therefore its flags. For example, a host route that is + trapping packets can be "promoted" to perform decapsulation following + the installation of an IPinIP/VXLAN tunnel. + The notifications will indicate to user-space the state of the route. + + Default: 0 (Do not emit notifications.) + + Possible values: + + - 0 - Do not emit notifications. + - 1 - Emit notifications. + - 2 - Emit notifications only for RTM_F_OFFLOAD_FAILED flag change. + +IP Fragmentation: + +ipfrag_high_thresh - LONG INTEGER + Maximum memory used to reassemble IP fragments. + +ipfrag_low_thresh - LONG INTEGER + (Obsolete since linux-4.17) + Maximum memory used to reassemble IP fragments before the kernel + begins to remove incomplete fragment queues to free up resources. + The kernel still accepts new fragments for defragmentation. + +ipfrag_time - INTEGER + Time in seconds to keep an IP fragment in memory. + +ipfrag_max_dist - INTEGER + ipfrag_max_dist is a non-negative integer value which defines the + maximum "disorder" which is allowed among fragments which share a + common IP source address. Note that reordering of packets is + not unusual, but if a large number of fragments arrive from a source + IP address while a particular fragment queue remains incomplete, it + probably indicates that one or more fragments belonging to that queue + have been lost. When ipfrag_max_dist is positive, an additional check + is done on fragments before they are added to a reassembly queue - if + ipfrag_max_dist (or more) fragments have arrived from a particular IP + address between additions to any IP fragment queue using that source + address, it's presumed that one or more fragments in the queue are + lost. The existing fragment queue will be dropped, and a new one + started. An ipfrag_max_dist value of zero disables this check. + + Using a very small value, e.g. 1 or 2, for ipfrag_max_dist can + result in unnecessarily dropping fragment queues when normal + reordering of packets occurs, which could lead to poor application + performance. Using a very large value, e.g. 50000, increases the + likelihood of incorrectly reassembling IP fragments that originate + from different IP datagrams, which could result in data corruption. + Default: 64 + +bc_forwarding - INTEGER + bc_forwarding enables the feature described in rfc1812#section-5.3.5.2 + and rfc2644. It allows the router to forward directed broadcast. + To enable this feature, the 'all' entry and the input interface entry + should be set to 1. + Default: 0 + +INET peer storage +================= + +inet_peer_threshold - INTEGER + The approximate size of the storage. Starting from this threshold + entries will be thrown aggressively. This threshold also determines + entries' time-to-live and time intervals between garbage collection + passes. More entries, less time-to-live, less GC interval. + +inet_peer_minttl - INTEGER + Minimum time-to-live of entries. Should be enough to cover fragment + time-to-live on the reassembling side. This minimum time-to-live is + guaranteed if the pool size is less than inet_peer_threshold. + Measured in seconds. + +inet_peer_maxttl - INTEGER + Maximum time-to-live of entries. Unused entries will expire after + this period of time if there is no memory pressure on the pool (i.e. + when the number of entries in the pool is very small). + Measured in seconds. + +TCP variables +============= + +somaxconn - INTEGER + Limit of socket listen() backlog, known in userspace as SOMAXCONN. + Defaults to 4096. (Was 128 before linux-5.4) + See also tcp_max_syn_backlog for additional tuning for TCP sockets. + +tcp_abort_on_overflow - BOOLEAN + If listening service is too slow to accept new connections, + reset them. Default state is FALSE. It means that if overflow + occurred due to a burst, connection will recover. Enable this + option _only_ if you are really sure that listening daemon + cannot be tuned to accept connections faster. Enabling this + option can harm clients of your server. + +tcp_adv_win_scale - INTEGER + Count buffering overhead as bytes/2^tcp_adv_win_scale + (if tcp_adv_win_scale > 0) or bytes-bytes/2^(-tcp_adv_win_scale), + if it is <= 0. + + Possible values are [-31, 31], inclusive. + + Default: 1 + +tcp_allowed_congestion_control - STRING + Show/set the congestion control choices available to non-privileged + processes. The list is a subset of those listed in + tcp_available_congestion_control. + + Default is "reno" and the default setting (tcp_congestion_control). + +tcp_app_win - INTEGER + Reserve max(window/2^tcp_app_win, mss) of window for application + buffer. Value 0 is special, it means that nothing is reserved. + + Possible values are [0, 31], inclusive. + + Default: 31 + +tcp_autocorking - BOOLEAN + Enable TCP auto corking : + When applications do consecutive small write()/sendmsg() system calls, + we try to coalesce these small writes as much as possible, to lower + total amount of sent packets. This is done if at least one prior + packet for the flow is waiting in Qdisc queues or device transmit + queue. Applications can still use TCP_CORK for optimal behavior + when they know how/when to uncork their sockets. + + Default : 1 + +tcp_available_congestion_control - STRING + Shows the available congestion control choices that are registered. + More congestion control algorithms may be available as modules, + but not loaded. + +tcp_base_mss - INTEGER + The initial value of search_low to be used by the packetization layer + Path MTU discovery (MTU probing). If MTU probing is enabled, + this is the initial MSS used by the connection. + +tcp_mtu_probe_floor - INTEGER + If MTU probing is enabled this caps the minimum MSS used for search_low + for the connection. + + Default : 48 + +tcp_min_snd_mss - INTEGER + TCP SYN and SYNACK messages usually advertise an ADVMSS option, + as described in RFC 1122 and RFC 6691. + + If this ADVMSS option is smaller than tcp_min_snd_mss, + it is silently capped to tcp_min_snd_mss. + + Default : 48 (at least 8 bytes of payload per segment) + +tcp_congestion_control - STRING + Set the congestion control algorithm to be used for new + connections. The algorithm "reno" is always available, but + additional choices may be available based on kernel configuration. + Default is set as part of kernel configuration. + For passive connections, the listener congestion control choice + is inherited. + + [see setsockopt(listenfd, SOL_TCP, TCP_CONGESTION, "name" ...) ] + +tcp_dsack - BOOLEAN + Allows TCP to send "duplicate" SACKs. + +tcp_early_retrans - INTEGER + Tail loss probe (TLP) converts RTOs occurring due to tail + losses into fast recovery (draft-ietf-tcpm-rack). Note that + TLP requires RACK to function properly (see tcp_recovery below) + + Possible values: + + - 0 disables TLP + - 3 or 4 enables TLP + + Default: 3 + +tcp_ecn - INTEGER + Control use of Explicit Congestion Notification (ECN) by TCP. + ECN is used only when both ends of the TCP connection indicate + support for it. This feature is useful in avoiding losses due + to congestion by allowing supporting routers to signal + congestion before having to drop packets. + + Possible values are: + + = ===================================================== + 0 Disable ECN. Neither initiate nor accept ECN. + 1 Enable ECN when requested by incoming connections and + also request ECN on outgoing connection attempts. + 2 Enable ECN when requested by incoming connections + but do not request ECN on outgoing connections. + = ===================================================== + + Default: 2 + +tcp_ecn_fallback - BOOLEAN + If the kernel detects that ECN connection misbehaves, enable fall + back to non-ECN. Currently, this knob implements the fallback + from RFC3168, section 6.1.1.1., but we reserve that in future, + additional detection mechanisms could be implemented under this + knob. The value is not used, if tcp_ecn or per route (or congestion + control) ECN settings are disabled. + + Default: 1 (fallback enabled) + +tcp_fack - BOOLEAN + This is a legacy option, it has no effect anymore. + +tcp_fin_timeout - INTEGER + The length of time an orphaned (no longer referenced by any + application) connection will remain in the FIN_WAIT_2 state + before it is aborted at the local end. While a perfectly + valid "receive only" state for an un-orphaned connection, an + orphaned connection in FIN_WAIT_2 state could otherwise wait + forever for the remote to close its end of the connection. + + Cf. tcp_max_orphans + + Default: 60 seconds + +tcp_frto - INTEGER + Enables Forward RTO-Recovery (F-RTO) defined in RFC5682. + F-RTO is an enhanced recovery algorithm for TCP retransmission + timeouts. It is particularly beneficial in networks where the + RTT fluctuates (e.g., wireless). F-RTO is sender-side only + modification. It does not require any support from the peer. + + By default it's enabled with a non-zero value. 0 disables F-RTO. + +tcp_fwmark_accept - BOOLEAN + If set, incoming connections to listening sockets that do not have a + socket mark will set the mark of the accepting socket to the fwmark of + the incoming SYN packet. This will cause all packets on that connection + (starting from the first SYNACK) to be sent with that fwmark. The + listening socket's mark is unchanged. Listening sockets that already + have a fwmark set via setsockopt(SOL_SOCKET, SO_MARK, ...) are + unaffected. + + Default: 0 + +tcp_invalid_ratelimit - INTEGER + Limit the maximal rate for sending duplicate acknowledgments + in response to incoming TCP packets that are for an existing + connection but that are invalid due to any of these reasons: + + (a) out-of-window sequence number, + (b) out-of-window acknowledgment number, or + (c) PAWS (Protection Against Wrapped Sequence numbers) check failure + + This can help mitigate simple "ack loop" DoS attacks, wherein + a buggy or malicious middlebox or man-in-the-middle can + rewrite TCP header fields in manner that causes each endpoint + to think that the other is sending invalid TCP segments, thus + causing each side to send an unterminating stream of duplicate + acknowledgments for invalid segments. + + Using 0 disables rate-limiting of dupacks in response to + invalid segments; otherwise this value specifies the minimal + space between sending such dupacks, in milliseconds. + + Default: 500 (milliseconds). + +tcp_keepalive_time - INTEGER + How often TCP sends out keepalive messages when keepalive is enabled. + Default: 2hours. + +tcp_keepalive_probes - INTEGER + How many keepalive probes TCP sends out, until it decides that the + connection is broken. Default value: 9. + +tcp_keepalive_intvl - INTEGER + How frequently the probes are send out. Multiplied by + tcp_keepalive_probes it is time to kill not responding connection, + after probes started. Default value: 75sec i.e. connection + will be aborted after ~11 minutes of retries. + +tcp_l3mdev_accept - BOOLEAN + Enables child sockets to inherit the L3 master device index. + Enabling this option allows a "global" listen socket to work + across L3 master domains (e.g., VRFs) with connected sockets + derived from the listen socket to be bound to the L3 domain in + which the packets originated. Only valid when the kernel was + compiled with CONFIG_NET_L3_MASTER_DEV. + + Default: 0 (disabled) + +tcp_low_latency - BOOLEAN + This is a legacy option, it has no effect anymore. + +tcp_max_orphans - INTEGER + Maximal number of TCP sockets not attached to any user file handle, + held by system. If this number is exceeded orphaned connections are + reset immediately and warning is printed. This limit exists + only to prevent simple DoS attacks, you _must_ not rely on this + or lower the limit artificially, but rather increase it + (probably, after increasing installed memory), + if network conditions require more than default value, + and tune network services to linger and kill such states + more aggressively. Let me to remind again: each orphan eats + up to ~64K of unswappable memory. + +tcp_max_syn_backlog - INTEGER + Maximal number of remembered connection requests (SYN_RECV), + which have not received an acknowledgment from connecting client. + + This is a per-listener limit. + + The minimal value is 128 for low memory machines, and it will + increase in proportion to the memory of machine. + + If server suffers from overload, try increasing this number. + + Remember to also check /proc/sys/net/core/somaxconn + A SYN_RECV request socket consumes about 304 bytes of memory. + +tcp_max_tw_buckets - INTEGER + Maximal number of timewait sockets held by system simultaneously. + If this number is exceeded time-wait socket is immediately destroyed + and warning is printed. This limit exists only to prevent + simple DoS attacks, you _must_ not lower the limit artificially, + but rather increase it (probably, after increasing installed memory), + if network conditions require more than default value. + +tcp_mem - vector of 3 INTEGERs: min, pressure, max + min: below this number of pages TCP is not bothered about its + memory appetite. + + pressure: when amount of memory allocated by TCP exceeds this number + of pages, TCP moderates its memory consumption and enters memory + pressure mode, which is exited when memory consumption falls + under "min". + + max: number of pages allowed for queueing by all TCP sockets. + + Defaults are calculated at boot time from amount of available + memory. + +tcp_min_rtt_wlen - INTEGER + The window length of the windowed min filter to track the minimum RTT. + A shorter window lets a flow more quickly pick up new (higher) + minimum RTT when it is moved to a longer path (e.g., due to traffic + engineering). A longer window makes the filter more resistant to RTT + inflations such as transient congestion. The unit is seconds. + + Possible values: 0 - 86400 (1 day) + + Default: 300 + +tcp_moderate_rcvbuf - BOOLEAN + If set, TCP performs receive buffer auto-tuning, attempting to + automatically size the buffer (no greater than tcp_rmem[2]) to + match the size required by the path for full throughput. Enabled by + default. + +tcp_mtu_probing - INTEGER + Controls TCP Packetization-Layer Path MTU Discovery. Takes three + values: + + - 0 - Disabled + - 1 - Disabled by default, enabled when an ICMP black hole detected + - 2 - Always enabled, use initial MSS of tcp_base_mss. + +tcp_probe_interval - UNSIGNED INTEGER + Controls how often to start TCP Packetization-Layer Path MTU + Discovery reprobe. The default is reprobing every 10 minutes as + per RFC4821. + +tcp_probe_threshold - INTEGER + Controls when TCP Packetization-Layer Path MTU Discovery probing + will stop in respect to the width of search range in bytes. Default + is 8 bytes. + +tcp_no_metrics_save - BOOLEAN + By default, TCP saves various connection metrics in the route cache + when the connection closes, so that connections established in the + near future can use these to set initial conditions. Usually, this + increases overall performance, but may sometimes cause performance + degradation. If set, TCP will not cache metrics on closing + connections. + +tcp_no_ssthresh_metrics_save - BOOLEAN + Controls whether TCP saves ssthresh metrics in the route cache. + + Default is 1, which disables ssthresh metrics. + +tcp_orphan_retries - INTEGER + This value influences the timeout of a locally closed TCP connection, + when RTO retransmissions remain unacknowledged. + See tcp_retries2 for more details. + + The default value is 8. + + If your machine is a loaded WEB server, + you should think about lowering this value, such sockets + may consume significant resources. Cf. tcp_max_orphans. + +tcp_recovery - INTEGER + This value is a bitmap to enable various experimental loss recovery + features. + + ========= ============================================================= + RACK: 0x1 enables the RACK loss detection for fast detection of lost + retransmissions and tail drops. It also subsumes and disables + RFC6675 recovery for SACK connections. + + RACK: 0x2 makes RACK's reordering window static (min_rtt/4). + + RACK: 0x4 disables RACK's DUPACK threshold heuristic + ========= ============================================================= + + Default: 0x1 + +tcp_reflect_tos - BOOLEAN + For listening sockets, reuse the DSCP value of the initial SYN message + for outgoing packets. This allows to have both directions of a TCP + stream to use the same DSCP value, assuming DSCP remains unchanged for + the lifetime of the connection. + + This options affects both IPv4 and IPv6. + + Default: 0 (disabled) + +tcp_reordering - INTEGER + Initial reordering level of packets in a TCP stream. + TCP stack can then dynamically adjust flow reordering level + between this initial value and tcp_max_reordering + + Default: 3 + +tcp_max_reordering - INTEGER + Maximal reordering level of packets in a TCP stream. + 300 is a fairly conservative value, but you might increase it + if paths are using per packet load balancing (like bonding rr mode) + + Default: 300 + +tcp_retrans_collapse - BOOLEAN + Bug-to-bug compatibility with some broken printers. + On retransmit try to send bigger packets to work around bugs in + certain TCP stacks. + +tcp_retries1 - INTEGER + This value influences the time, after which TCP decides, that + something is wrong due to unacknowledged RTO retransmissions, + and reports this suspicion to the network layer. + See tcp_retries2 for more details. + + RFC 1122 recommends at least 3 retransmissions, which is the + default. + +tcp_retries2 - INTEGER + This value influences the timeout of an alive TCP connection, + when RTO retransmissions remain unacknowledged. + Given a value of N, a hypothetical TCP connection following + exponential backoff with an initial RTO of TCP_RTO_MIN would + retransmit N times before killing the connection at the (N+1)th RTO. + + The default value of 15 yields a hypothetical timeout of 924.6 + seconds and is a lower bound for the effective timeout. + TCP will effectively time out at the first RTO which exceeds the + hypothetical timeout. + + RFC 1122 recommends at least 100 seconds for the timeout, + which corresponds to a value of at least 8. + +tcp_rfc1337 - BOOLEAN + If set, the TCP stack behaves conforming to RFC1337. If unset, + we are not conforming to RFC, but prevent TCP TIME_WAIT + assassination. + + Default: 0 + +tcp_rmem - vector of 3 INTEGERs: min, default, max + min: Minimal size of receive buffer used by TCP sockets. + It is guaranteed to each TCP socket, even under moderate memory + pressure. + + Default: 4K + + default: initial size of receive buffer used by TCP sockets. + This value overrides net.core.rmem_default used by other protocols. + Default: 131072 bytes. + This value results in initial window of 65535. + + max: maximal size of receive buffer allowed for automatically + selected receiver buffers for TCP socket. This value does not override + net.core.rmem_max. Calling setsockopt() with SO_RCVBUF disables + automatic tuning of that socket's receive buffer size, in which + case this value is ignored. + Default: between 131072 and 6MB, depending on RAM size. + +tcp_sack - BOOLEAN + Enable select acknowledgments (SACKS). + +tcp_comp_sack_delay_ns - LONG INTEGER + TCP tries to reduce number of SACK sent, using a timer + based on 5% of SRTT, capped by this sysctl, in nano seconds. + The default is 1ms, based on TSO autosizing period. + + Default : 1,000,000 ns (1 ms) + +tcp_comp_sack_slack_ns - LONG INTEGER + This sysctl control the slack used when arming the + timer used by SACK compression. This gives extra time + for small RTT flows, and reduces system overhead by allowing + opportunistic reduction of timer interrupts. + + Default : 100,000 ns (100 us) + +tcp_comp_sack_nr - INTEGER + Max number of SACK that can be compressed. + Using 0 disables SACK compression. + + Default : 44 + +tcp_slow_start_after_idle - BOOLEAN + If set, provide RFC2861 behavior and time out the congestion + window after an idle period. An idle period is defined at + the current RTO. If unset, the congestion window will not + be timed out after an idle period. + + Default: 1 + +tcp_stdurg - BOOLEAN + Use the Host requirements interpretation of the TCP urgent pointer field. + Most hosts use the older BSD interpretation, so if you turn this on + Linux might not communicate correctly with them. + + Default: FALSE + +tcp_synack_retries - INTEGER + Number of times SYNACKs for a passive TCP connection attempt will + be retransmitted. Should not be higher than 255. Default value + is 5, which corresponds to 31seconds till the last retransmission + with the current initial RTO of 1second. With this the final timeout + for a passive TCP connection will happen after 63seconds. + +tcp_syncookies - INTEGER + Only valid when the kernel was compiled with CONFIG_SYN_COOKIES + Send out syncookies when the syn backlog queue of a socket + overflows. This is to prevent against the common 'SYN flood attack' + Default: 1 + + Note, that syncookies is fallback facility. + It MUST NOT be used to help highly loaded servers to stand + against legal connection rate. If you see SYN flood warnings + in your logs, but investigation shows that they occur + because of overload with legal connections, you should tune + another parameters until this warning disappear. + See: tcp_max_syn_backlog, tcp_synack_retries, tcp_abort_on_overflow. + + syncookies seriously violate TCP protocol, do not allow + to use TCP extensions, can result in serious degradation + of some services (f.e. SMTP relaying), visible not by you, + but your clients and relays, contacting you. While you see + SYN flood warnings in logs not being really flooded, your server + is seriously misconfigured. + + If you want to test which effects syncookies have to your + network connections you can set this knob to 2 to enable + unconditionally generation of syncookies. + +tcp_migrate_req - BOOLEAN + The incoming connection is tied to a specific listening socket when + the initial SYN packet is received during the three-way handshake. + When a listener is closed, in-flight request sockets during the + handshake and established sockets in the accept queue are aborted. + + If the listener has SO_REUSEPORT enabled, other listeners on the + same port should have been able to accept such connections. This + option makes it possible to migrate such child sockets to another + listener after close() or shutdown(). + + The BPF_SK_REUSEPORT_SELECT_OR_MIGRATE type of eBPF program should + usually be used to define the policy to pick an alive listener. + Otherwise, the kernel will randomly pick an alive listener only if + this option is enabled. + + Note that migration between listeners with different settings may + crash applications. Let's say migration happens from listener A to + B, and only B has TCP_SAVE_SYN enabled. B cannot read SYN data from + the requests migrated from A. To avoid such a situation, cancel + migration by returning SK_DROP in the type of eBPF program, or + disable this option. + + Default: 0 + +tcp_fastopen - INTEGER + Enable TCP Fast Open (RFC7413) to send and accept data in the opening + SYN packet. + + The client support is enabled by flag 0x1 (on by default). The client + then must use sendmsg() or sendto() with the MSG_FASTOPEN flag, + rather than connect() to send data in SYN. + + The server support is enabled by flag 0x2 (off by default). Then + either enable for all listeners with another flag (0x400) or + enable individual listeners via TCP_FASTOPEN socket option with + the option value being the length of the syn-data backlog. + + The values (bitmap) are + + ===== ======== ====================================================== + 0x1 (client) enables sending data in the opening SYN on the client. + 0x2 (server) enables the server support, i.e., allowing data in + a SYN packet to be accepted and passed to the + application before 3-way handshake finishes. + 0x4 (client) send data in the opening SYN regardless of cookie + availability and without a cookie option. + 0x200 (server) accept data-in-SYN w/o any cookie option present. + 0x400 (server) enable all listeners to support Fast Open by + default without explicit TCP_FASTOPEN socket option. + ===== ======== ====================================================== + + Default: 0x1 + + Note that additional client or server features are only + effective if the basic support (0x1 and 0x2) are enabled respectively. + +tcp_fastopen_blackhole_timeout_sec - INTEGER + Initial time period in second to disable Fastopen on active TCP sockets + when a TFO firewall blackhole issue happens. + This time period will grow exponentially when more blackhole issues + get detected right after Fastopen is re-enabled and will reset to + initial value when the blackhole issue goes away. + 0 to disable the blackhole detection. + + By default, it is set to 0 (feature is disabled). + +tcp_fastopen_key - list of comma separated 32-digit hexadecimal INTEGERs + The list consists of a primary key and an optional backup key. The + primary key is used for both creating and validating cookies, while the + optional backup key is only used for validating cookies. The purpose of + the backup key is to maximize TFO validation when keys are rotated. + + A randomly chosen primary key may be configured by the kernel if + the tcp_fastopen sysctl is set to 0x400 (see above), or if the + TCP_FASTOPEN setsockopt() optname is set and a key has not been + previously configured via sysctl. If keys are configured via + setsockopt() by using the TCP_FASTOPEN_KEY optname, then those + per-socket keys will be used instead of any keys that are specified via + sysctl. + + A key is specified as 4 8-digit hexadecimal integers which are separated + by a '-' as: xxxxxxxx-xxxxxxxx-xxxxxxxx-xxxxxxxx. Leading zeros may be + omitted. A primary and a backup key may be specified by separating them + by a comma. If only one key is specified, it becomes the primary key and + any previously configured backup keys are removed. + +tcp_syn_retries - INTEGER + Number of times initial SYNs for an active TCP connection attempt + will be retransmitted. Should not be higher than 127. Default value + is 6, which corresponds to 63seconds till the last retransmission + with the current initial RTO of 1second. With this the final timeout + for an active TCP connection attempt will happen after 127seconds. + +tcp_timestamps - INTEGER + Enable timestamps as defined in RFC1323. + + - 0: Disabled. + - 1: Enable timestamps as defined in RFC1323 and use random offset for + each connection rather than only using the current time. + - 2: Like 1, but without random offsets. + + Default: 1 + +tcp_min_tso_segs - INTEGER + Minimal number of segments per TSO frame. + + Since linux-3.12, TCP does an automatic sizing of TSO frames, + depending on flow rate, instead of filling 64Kbytes packets. + For specific usages, it's possible to force TCP to build big + TSO frames. Note that TCP stack might split too big TSO packets + if available window is too small. + + Default: 2 + +tcp_tso_rtt_log - INTEGER + Adjustment of TSO packet sizes based on min_rtt + + Starting from linux-5.18, TCP autosizing can be tweaked + for flows having small RTT. + + Old autosizing was splitting the pacing budget to send 1024 TSO + per second. + + tso_packet_size = sk->sk_pacing_rate / 1024; + + With the new mechanism, we increase this TSO sizing using: + + distance = min_rtt_usec / (2^tcp_tso_rtt_log) + tso_packet_size += gso_max_size >> distance; + + This means that flows between very close hosts can use bigger + TSO packets, reducing their cpu costs. + + If you want to use the old autosizing, set this sysctl to 0. + + Default: 9 (2^9 = 512 usec) + +tcp_pacing_ss_ratio - INTEGER + sk->sk_pacing_rate is set by TCP stack using a ratio applied + to current rate. (current_rate = cwnd * mss / srtt) + If TCP is in slow start, tcp_pacing_ss_ratio is applied + to let TCP probe for bigger speeds, assuming cwnd can be + doubled every other RTT. + + Default: 200 + +tcp_pacing_ca_ratio - INTEGER + sk->sk_pacing_rate is set by TCP stack using a ratio applied + to current rate. (current_rate = cwnd * mss / srtt) + If TCP is in congestion avoidance phase, tcp_pacing_ca_ratio + is applied to conservatively probe for bigger throughput. + + Default: 120 + +tcp_tso_win_divisor - INTEGER + This allows control over what percentage of the congestion window + can be consumed by a single TSO frame. + The setting of this parameter is a choice between burstiness and + building larger TSO frames. + + Default: 3 + +tcp_tw_reuse - INTEGER + Enable reuse of TIME-WAIT sockets for new connections when it is + safe from protocol viewpoint. + + - 0 - disable + - 1 - global enable + - 2 - enable for loopback traffic only + + It should not be changed without advice/request of technical + experts. + + Default: 2 + +tcp_window_scaling - BOOLEAN + Enable window scaling as defined in RFC1323. + +tcp_shrink_window - BOOLEAN + This changes how the TCP receive window is calculated. + + RFC 7323, section 2.4, says there are instances when a retracted + window can be offered, and that TCP implementations MUST ensure + that they handle a shrinking window, as specified in RFC 1122. + + - 0 - Disabled. The window is never shrunk. + - 1 - Enabled. The window is shrunk when necessary to remain within + the memory limit set by autotuning (sk_rcvbuf). + This only occurs if a non-zero receive window + scaling factor is also in effect. + + Default: 0 + +tcp_wmem - vector of 3 INTEGERs: min, default, max + min: Amount of memory reserved for send buffers for TCP sockets. + Each TCP socket has rights to use it due to fact of its birth. + + Default: 4K + + default: initial size of send buffer used by TCP sockets. This + value overrides net.core.wmem_default used by other protocols. + + It is usually lower than net.core.wmem_default. + + Default: 16K + + max: Maximal amount of memory allowed for automatically tuned + send buffers for TCP sockets. This value does not override + net.core.wmem_max. Calling setsockopt() with SO_SNDBUF disables + automatic tuning of that socket's send buffer size, in which case + this value is ignored. + + Default: between 64K and 4MB, depending on RAM size. + +tcp_notsent_lowat - UNSIGNED INTEGER + A TCP socket can control the amount of unsent bytes in its write queue, + thanks to TCP_NOTSENT_LOWAT socket option. poll()/select()/epoll() + reports POLLOUT events if the amount of unsent bytes is below a per + socket value, and if the write queue is not full. sendmsg() will + also not add new buffers if the limit is hit. + + This global variable controls the amount of unsent data for + sockets not using TCP_NOTSENT_LOWAT. For these sockets, a change + to the global variable has immediate effect. + + Default: UINT_MAX (0xFFFFFFFF) + +tcp_workaround_signed_windows - BOOLEAN + If set, assume no receipt of a window scaling option means the + remote TCP is broken and treats the window as a signed quantity. + If unset, assume the remote TCP is not broken even if we do + not receive a window scaling option from them. + + Default: 0 + +tcp_thin_linear_timeouts - BOOLEAN + Enable dynamic triggering of linear timeouts for thin streams. + If set, a check is performed upon retransmission by timeout to + determine if the stream is thin (less than 4 packets in flight). + As long as the stream is found to be thin, up to 6 linear + timeouts may be performed before exponential backoff mode is + initiated. This improves retransmission latency for + non-aggressive thin streams, often found to be time-dependent. + For more information on thin streams, see + Documentation/networking/tcp-thin.rst + + Default: 0 + +tcp_limit_output_bytes - INTEGER + Controls TCP Small Queue limit per tcp socket. + TCP bulk sender tends to increase packets in flight until it + gets losses notifications. With SNDBUF autotuning, this can + result in a large amount of packets queued on the local machine + (e.g.: qdiscs, CPU backlog, or device) hurting latency of other + flows, for typical pfifo_fast qdiscs. tcp_limit_output_bytes + limits the number of bytes on qdisc or device to reduce artificial + RTT/cwnd and reduce bufferbloat. + + Default: 1048576 (16 * 65536) + +tcp_challenge_ack_limit - INTEGER + Limits number of Challenge ACK sent per second, as recommended + in RFC 5961 (Improving TCP's Robustness to Blind In-Window Attacks) + Note that this per netns rate limit can allow some side channel + attacks and probably should not be enabled. + TCP stack implements per TCP socket limits anyway. + Default: INT_MAX (unlimited) + +tcp_ehash_entries - INTEGER + Show the number of hash buckets for TCP sockets in the current + networking namespace. + + A negative value means the networking namespace does not own its + hash buckets and shares the initial networking namespace's one. + +tcp_child_ehash_entries - INTEGER + Control the number of hash buckets for TCP sockets in the child + networking namespace, which must be set before clone() or unshare(). + + If the value is not 0, the kernel uses a value rounded up to 2^n + as the actual hash bucket size. 0 is a special value, meaning + the child networking namespace will share the initial networking + namespace's hash buckets. + + Note that the child will use the global one in case the kernel + fails to allocate enough memory. In addition, the global hash + buckets are spread over available NUMA nodes, but the allocation + of the child hash table depends on the current process's NUMA + policy, which could result in performance differences. + + Note also that the default value of tcp_max_tw_buckets and + tcp_max_syn_backlog depend on the hash bucket size. + + Possible values: 0, 2^n (n: 0 - 24 (16Mi)) + + Default: 0 + +UDP variables +============= + +udp_l3mdev_accept - BOOLEAN + Enabling this option allows a "global" bound socket to work + across L3 master domains (e.g., VRFs) with packets capable of + being received regardless of the L3 domain in which they + originated. Only valid when the kernel was compiled with + CONFIG_NET_L3_MASTER_DEV. + + Default: 0 (disabled) + +udp_mem - vector of 3 INTEGERs: min, pressure, max + Number of pages allowed for queueing by all UDP sockets. + + min: Number of pages allowed for queueing by all UDP sockets. + + pressure: This value was introduced to follow format of tcp_mem. + + max: This value was introduced to follow format of tcp_mem. + + Default is calculated at boot time from amount of available memory. + +udp_rmem_min - INTEGER + Minimal size of receive buffer used by UDP sockets in moderation. + Each UDP socket is able to use the size for receiving data, even if + total pages of UDP sockets exceed udp_mem pressure. The unit is byte. + + Default: 4K + +udp_wmem_min - INTEGER + UDP does not have tx memory accounting and this tunable has no effect. + +RAW variables +============= + +raw_l3mdev_accept - BOOLEAN + Enabling this option allows a "global" bound socket to work + across L3 master domains (e.g., VRFs) with packets capable of + being received regardless of the L3 domain in which they + originated. Only valid when the kernel was compiled with + CONFIG_NET_L3_MASTER_DEV. + + Default: 1 (enabled) + +CIPSOv4 Variables +================= + +cipso_cache_enable - BOOLEAN + If set, enable additions to and lookups from the CIPSO label mapping + cache. If unset, additions are ignored and lookups always result in a + miss. However, regardless of the setting the cache is still + invalidated when required when means you can safely toggle this on and + off and the cache will always be "safe". + + Default: 1 + +cipso_cache_bucket_size - INTEGER + The CIPSO label cache consists of a fixed size hash table with each + hash bucket containing a number of cache entries. This variable limits + the number of entries in each hash bucket; the larger the value is, the + more CIPSO label mappings that can be cached. When the number of + entries in a given hash bucket reaches this limit adding new entries + causes the oldest entry in the bucket to be removed to make room. + + Default: 10 + +cipso_rbm_optfmt - BOOLEAN + Enable the "Optimized Tag 1 Format" as defined in section 3.4.2.6 of + the CIPSO draft specification (see Documentation/netlabel for details). + This means that when set the CIPSO tag will be padded with empty + categories in order to make the packet data 32-bit aligned. + + Default: 0 + +cipso_rbm_structvalid - BOOLEAN + If set, do a very strict check of the CIPSO option when + ip_options_compile() is called. If unset, relax the checks done during + ip_options_compile(). Either way is "safe" as errors are caught else + where in the CIPSO processing code but setting this to 0 (False) should + result in less work (i.e. it should be faster) but could cause problems + with other implementations that require strict checking. + + Default: 0 + +IP Variables +============ + +ip_local_port_range - 2 INTEGERS + Defines the local port range that is used by TCP and UDP to + choose the local port. The first number is the first, the + second the last local port number. + If possible, it is better these numbers have different parity + (one even and one odd value). + Must be greater than or equal to ip_unprivileged_port_start. + The default values are 32768 and 60999 respectively. + +ip_local_reserved_ports - list of comma separated ranges + Specify the ports which are reserved for known third-party + applications. These ports will not be used by automatic port + assignments (e.g. when calling connect() or bind() with port + number 0). Explicit port allocation behavior is unchanged. + + The format used for both input and output is a comma separated + list of ranges (e.g. "1,2-4,10-10" for ports 1, 2, 3, 4 and + 10). Writing to the file will clear all previously reserved + ports and update the current list with the one given in the + input. + + Note that ip_local_port_range and ip_local_reserved_ports + settings are independent and both are considered by the kernel + when determining which ports are available for automatic port + assignments. + + You can reserve ports which are not in the current + ip_local_port_range, e.g.:: + + $ cat /proc/sys/net/ipv4/ip_local_port_range + 32000 60999 + $ cat /proc/sys/net/ipv4/ip_local_reserved_ports + 8080,9148 + + although this is redundant. However such a setting is useful + if later the port range is changed to a value that will + include the reserved ports. Also keep in mind, that overlapping + of these ranges may affect probability of selecting ephemeral + ports which are right after block of reserved ports. + + Default: Empty + +ip_unprivileged_port_start - INTEGER + This is a per-namespace sysctl. It defines the first + unprivileged port in the network namespace. Privileged ports + require root or CAP_NET_BIND_SERVICE in order to bind to them. + To disable all privileged ports, set this to 0. They must not + overlap with the ip_local_port_range. + + Default: 1024 + +ip_nonlocal_bind - BOOLEAN + If set, allows processes to bind() to non-local IP addresses, + which can be quite useful - but may break some applications. + + Default: 0 + +ip_autobind_reuse - BOOLEAN + By default, bind() does not select the ports automatically even if + the new socket and all sockets bound to the port have SO_REUSEADDR. + ip_autobind_reuse allows bind() to reuse the port and this is useful + when you use bind()+connect(), but may break some applications. + The preferred solution is to use IP_BIND_ADDRESS_NO_PORT and this + option should only be set by experts. + Default: 0 + +ip_dynaddr - INTEGER + If set non-zero, enables support for dynamic addresses. + If set to a non-zero value larger than 1, a kernel log + message will be printed when dynamic address rewriting + occurs. + + Default: 0 + +ip_early_demux - BOOLEAN + Optimize input packet processing down to one demux for + certain kinds of local sockets. Currently we only do this + for established TCP and connected UDP sockets. + + It may add an additional cost for pure routing workloads that + reduces overall throughput, in such case you should disable it. + + Default: 1 + +ping_group_range - 2 INTEGERS + Restrict ICMP_PROTO datagram sockets to users in the group range. + The default is "1 0", meaning, that nobody (not even root) may + create ping sockets. Setting it to "100 100" would grant permissions + to the single group. "0 4294967294" would enable it for the world, "100 + 4294967294" would enable it for the users, but not daemons. + +tcp_early_demux - BOOLEAN + Enable early demux for established TCP sockets. + + Default: 1 + +udp_early_demux - BOOLEAN + Enable early demux for connected UDP sockets. Disable this if + your system could experience more unconnected load. + + Default: 1 + +icmp_echo_ignore_all - BOOLEAN + If set non-zero, then the kernel will ignore all ICMP ECHO + requests sent to it. + + Default: 0 + +icmp_echo_enable_probe - BOOLEAN + If set to one, then the kernel will respond to RFC 8335 PROBE + requests sent to it. + + Default: 0 + +icmp_echo_ignore_broadcasts - BOOLEAN + If set non-zero, then the kernel will ignore all ICMP ECHO and + TIMESTAMP requests sent to it via broadcast/multicast. + + Default: 1 + +icmp_ratelimit - INTEGER + Limit the maximal rates for sending ICMP packets whose type matches + icmp_ratemask (see below) to specific targets. + 0 to disable any limiting, + otherwise the minimal space between responses in milliseconds. + Note that another sysctl, icmp_msgs_per_sec limits the number + of ICMP packets sent on all targets. + + Default: 1000 + +icmp_msgs_per_sec - INTEGER + Limit maximal number of ICMP packets sent per second from this host. + Only messages whose type matches icmp_ratemask (see below) are + controlled by this limit. For security reasons, the precise count + of messages per second is randomized. + + Default: 1000 + +icmp_msgs_burst - INTEGER + icmp_msgs_per_sec controls number of ICMP packets sent per second, + while icmp_msgs_burst controls the burst size of these packets. + For security reasons, the precise burst size is randomized. + + Default: 50 + +icmp_ratemask - INTEGER + Mask made of ICMP types for which rates are being limited. + + Significant bits: IHGFEDCBA9876543210 + + Default mask: 0000001100000011000 (6168) + + Bit definitions (see include/linux/icmp.h): + + = ========================= + 0 Echo Reply + 3 Destination Unreachable [1]_ + 4 Source Quench [1]_ + 5 Redirect + 8 Echo Request + B Time Exceeded [1]_ + C Parameter Problem [1]_ + D Timestamp Request + E Timestamp Reply + F Info Request + G Info Reply + H Address Mask Request + I Address Mask Reply + = ========================= + + .. [1] These are rate limited by default (see default mask above) + +icmp_ignore_bogus_error_responses - BOOLEAN + Some routers violate RFC1122 by sending bogus responses to broadcast + frames. Such violations are normally logged via a kernel warning. + If this is set to TRUE, the kernel will not give such warnings, which + will avoid log file clutter. + + Default: 1 + +icmp_errors_use_inbound_ifaddr - BOOLEAN + + If zero, icmp error messages are sent with the primary address of + the exiting interface. + + If non-zero, the message will be sent with the primary address of + the interface that received the packet that caused the icmp error. + This is the behaviour many network administrators will expect from + a router. And it can make debugging complicated network layouts + much easier. + + Note that if no primary address exists for the interface selected, + then the primary address of the first non-loopback interface that + has one will be used regardless of this setting. + + Default: 0 + +igmp_max_memberships - INTEGER + Change the maximum number of multicast groups we can subscribe to. + Default: 20 + + Theoretical maximum value is bounded by having to send a membership + report in a single datagram (i.e. the report can't span multiple + datagrams, or risk confusing the switch and leaving groups you don't + intend to). + + The number of supported groups 'M' is bounded by the number of group + report entries you can fit into a single datagram of 65535 bytes. + + M = 65536-sizeof (ip header)/(sizeof(Group record)) + + Group records are variable length, with a minimum of 12 bytes. + So net.ipv4.igmp_max_memberships should not be set higher than: + + (65536-24) / 12 = 5459 + + The value 5459 assumes no IP header options, so in practice + this number may be lower. + +igmp_max_msf - INTEGER + Maximum number of addresses allowed in the source filter list for a + multicast group. + + Default: 10 + +igmp_qrv - INTEGER + Controls the IGMP query robustness variable (see RFC2236 8.1). + + Default: 2 (as specified by RFC2236 8.1) + + Minimum: 1 (as specified by RFC6636 4.5) + +force_igmp_version - INTEGER + - 0 - (default) No enforcement of a IGMP version, IGMPv1/v2 fallback + allowed. Will back to IGMPv3 mode again if all IGMPv1/v2 Querier + Present timer expires. + - 1 - Enforce to use IGMP version 1. Will also reply IGMPv1 report if + receive IGMPv2/v3 query. + - 2 - Enforce to use IGMP version 2. Will fallback to IGMPv1 if receive + IGMPv1 query message. Will reply report if receive IGMPv3 query. + - 3 - Enforce to use IGMP version 3. The same react with default 0. + + .. note:: + + this is not the same with force_mld_version because IGMPv3 RFC3376 + Security Considerations does not have clear description that we could + ignore other version messages completely as MLDv2 RFC3810. So make + this value as default 0 is recommended. + +``conf/interface/*`` + changes special settings per interface (where + interface" is the name of your network interface) + +``conf/all/*`` + is special, changes the settings for all interfaces + +log_martians - BOOLEAN + Log packets with impossible addresses to kernel log. + log_martians for the interface will be enabled if at least one of + conf/{all,interface}/log_martians is set to TRUE, + it will be disabled otherwise + +accept_redirects - BOOLEAN + Accept ICMP redirect messages. + accept_redirects for the interface will be enabled if: + + - both conf/{all,interface}/accept_redirects are TRUE in the case + forwarding for the interface is enabled + + or + + - at least one of conf/{all,interface}/accept_redirects is TRUE in the + case forwarding for the interface is disabled + + accept_redirects for the interface will be disabled otherwise + + default: + + - TRUE (host) + - FALSE (router) + +forwarding - BOOLEAN + Enable IP forwarding on this interface. This controls whether packets + received _on_ this interface can be forwarded. + +mc_forwarding - BOOLEAN + Do multicast routing. The kernel needs to be compiled with CONFIG_MROUTE + and a multicast routing daemon is required. + conf/all/mc_forwarding must also be set to TRUE to enable multicast + routing for the interface + +medium_id - INTEGER + Integer value used to differentiate the devices by the medium they + are attached to. Two devices can have different id values when + the broadcast packets are received only on one of them. + The default value 0 means that the device is the only interface + to its medium, value of -1 means that medium is not known. + + Currently, it is used to change the proxy_arp behavior: + the proxy_arp feature is enabled for packets forwarded between + two devices attached to different media. + +proxy_arp - BOOLEAN + Do proxy arp. + + proxy_arp for the interface will be enabled if at least one of + conf/{all,interface}/proxy_arp is set to TRUE, + it will be disabled otherwise + +proxy_arp_pvlan - BOOLEAN + Private VLAN proxy arp. + + Basically allow proxy arp replies back to the same interface + (from which the ARP request/solicitation was received). + + This is done to support (ethernet) switch features, like RFC + 3069, where the individual ports are NOT allowed to + communicate with each other, but they are allowed to talk to + the upstream router. As described in RFC 3069, it is possible + to allow these hosts to communicate through the upstream + router by proxy_arp'ing. Don't need to be used together with + proxy_arp. + + This technology is known by different names: + + In RFC 3069 it is called VLAN Aggregation. + Cisco and Allied Telesyn call it Private VLAN. + Hewlett-Packard call it Source-Port filtering or port-isolation. + Ericsson call it MAC-Forced Forwarding (RFC Draft). + +shared_media - BOOLEAN + Send(router) or accept(host) RFC1620 shared media redirects. + Overrides secure_redirects. + + shared_media for the interface will be enabled if at least one of + conf/{all,interface}/shared_media is set to TRUE, + it will be disabled otherwise + + default TRUE + +secure_redirects - BOOLEAN + Accept ICMP redirect messages only to gateways listed in the + interface's current gateway list. Even if disabled, RFC1122 redirect + rules still apply. + + Overridden by shared_media. + + secure_redirects for the interface will be enabled if at least one of + conf/{all,interface}/secure_redirects is set to TRUE, + it will be disabled otherwise + + default TRUE + +send_redirects - BOOLEAN + Send redirects, if router. + + send_redirects for the interface will be enabled if at least one of + conf/{all,interface}/send_redirects is set to TRUE, + it will be disabled otherwise + + Default: TRUE + +bootp_relay - BOOLEAN + Accept packets with source address 0.b.c.d destined + not to this host as local ones. It is supposed, that + BOOTP relay daemon will catch and forward such packets. + conf/all/bootp_relay must also be set to TRUE to enable BOOTP relay + for the interface + + default FALSE + + Not Implemented Yet. + +accept_source_route - BOOLEAN + Accept packets with SRR option. + conf/all/accept_source_route must also be set to TRUE to accept packets + with SRR option on the interface + + default + + - TRUE (router) + - FALSE (host) + +accept_local - BOOLEAN + Accept packets with local source addresses. In combination with + suitable routing, this can be used to direct packets between two + local interfaces over the wire and have them accepted properly. + default FALSE + +route_localnet - BOOLEAN + Do not consider loopback addresses as martian source or destination + while routing. This enables the use of 127/8 for local routing purposes. + + default FALSE + +rp_filter - INTEGER + - 0 - No source validation. + - 1 - Strict mode as defined in RFC3704 Strict Reverse Path + Each incoming packet is tested against the FIB and if the interface + is not the best reverse path the packet check will fail. + By default failed packets are discarded. + - 2 - Loose mode as defined in RFC3704 Loose Reverse Path + Each incoming packet's source address is also tested against the FIB + and if the source address is not reachable via any interface + the packet check will fail. + + Current recommended practice in RFC3704 is to enable strict mode + to prevent IP spoofing from DDos attacks. If using asymmetric routing + or other complicated routing, then loose mode is recommended. + + The max value from conf/{all,interface}/rp_filter is used + when doing source validation on the {interface}. + + Default value is 0. Note that some distributions enable it + in startup scripts. + +src_valid_mark - BOOLEAN + - 0 - The fwmark of the packet is not included in reverse path + route lookup. This allows for asymmetric routing configurations + utilizing the fwmark in only one direction, e.g., transparent + proxying. + + - 1 - The fwmark of the packet is included in reverse path route + lookup. This permits rp_filter to function when the fwmark is + used for routing traffic in both directions. + + This setting also affects the utilization of fmwark when + performing source address selection for ICMP replies, or + determining addresses stored for the IPOPT_TS_TSANDADDR and + IPOPT_RR IP options. + + The max value from conf/{all,interface}/src_valid_mark is used. + + Default value is 0. + +arp_filter - BOOLEAN + - 1 - Allows you to have multiple network interfaces on the same + subnet, and have the ARPs for each interface be answered + based on whether or not the kernel would route a packet from + the ARP'd IP out that interface (therefore you must use source + based routing for this to work). In other words it allows control + of which cards (usually 1) will respond to an arp request. + + - 0 - (default) The kernel can respond to arp requests with addresses + from other interfaces. This may seem wrong but it usually makes + sense, because it increases the chance of successful communication. + IP addresses are owned by the complete host on Linux, not by + particular interfaces. Only for more complex setups like load- + balancing, does this behaviour cause problems. + + arp_filter for the interface will be enabled if at least one of + conf/{all,interface}/arp_filter is set to TRUE, + it will be disabled otherwise + +arp_announce - INTEGER + Define different restriction levels for announcing the local + source IP address from IP packets in ARP requests sent on + interface: + + - 0 - (default) Use any local address, configured on any interface + - 1 - Try to avoid local addresses that are not in the target's + subnet for this interface. This mode is useful when target + hosts reachable via this interface require the source IP + address in ARP requests to be part of their logical network + configured on the receiving interface. When we generate the + request we will check all our subnets that include the + target IP and will preserve the source address if it is from + such subnet. If there is no such subnet we select source + address according to the rules for level 2. + - 2 - Always use the best local address for this target. + In this mode we ignore the source address in the IP packet + and try to select local address that we prefer for talks with + the target host. Such local address is selected by looking + for primary IP addresses on all our subnets on the outgoing + interface that include the target IP address. If no suitable + local address is found we select the first local address + we have on the outgoing interface or on all other interfaces, + with the hope we will receive reply for our request and + even sometimes no matter the source IP address we announce. + + The max value from conf/{all,interface}/arp_announce is used. + + Increasing the restriction level gives more chance for + receiving answer from the resolved target while decreasing + the level announces more valid sender's information. + +arp_ignore - INTEGER + Define different modes for sending replies in response to + received ARP requests that resolve local target IP addresses: + + - 0 - (default): reply for any local target IP address, configured + on any interface + - 1 - reply only if the target IP address is local address + configured on the incoming interface + - 2 - reply only if the target IP address is local address + configured on the incoming interface and both with the + sender's IP address are part from same subnet on this interface + - 3 - do not reply for local addresses configured with scope host, + only resolutions for global and link addresses are replied + - 4-7 - reserved + - 8 - do not reply for all local addresses + + The max value from conf/{all,interface}/arp_ignore is used + when ARP request is received on the {interface} + +arp_notify - BOOLEAN + Define mode for notification of address and device changes. + + == ========================================================== + 0 (default): do nothing + 1 Generate gratuitous arp requests when device is brought up + or hardware address changes. + == ========================================================== + +arp_accept - INTEGER + Define behavior for accepting gratuitous ARP (garp) frames from devices + that are not already present in the ARP table: + + - 0 - don't create new entries in the ARP table + - 1 - create new entries in the ARP table + - 2 - create new entries only if the source IP address is in the same + subnet as an address configured on the interface that received the + garp message. + + Both replies and requests type gratuitous arp will trigger the + ARP table to be updated, if this setting is on. + + If the ARP table already contains the IP address of the + gratuitous arp frame, the arp table will be updated regardless + if this setting is on or off. + +arp_evict_nocarrier - BOOLEAN + Clears the ARP cache on NOCARRIER events. This option is important for + wireless devices where the ARP cache should not be cleared when roaming + between access points on the same network. In most cases this should + remain as the default (1). + + - 1 - (default): Clear the ARP cache on NOCARRIER events + - 0 - Do not clear ARP cache on NOCARRIER events + +mcast_solicit - INTEGER + The maximum number of multicast probes in INCOMPLETE state, + when the associated hardware address is unknown. Defaults + to 3. + +ucast_solicit - INTEGER + The maximum number of unicast probes in PROBE state, when + the hardware address is being reconfirmed. Defaults to 3. + +app_solicit - INTEGER + The maximum number of probes to send to the user space ARP daemon + via netlink before dropping back to multicast probes (see + mcast_resolicit). Defaults to 0. + +mcast_resolicit - INTEGER + The maximum number of multicast probes after unicast and + app probes in PROBE state. Defaults to 0. + +disable_policy - BOOLEAN + Disable IPSEC policy (SPD) for this interface + +disable_xfrm - BOOLEAN + Disable IPSEC encryption on this interface, whatever the policy + +igmpv2_unsolicited_report_interval - INTEGER + The interval in milliseconds in which the next unsolicited + IGMPv1 or IGMPv2 report retransmit will take place. + + Default: 10000 (10 seconds) + +igmpv3_unsolicited_report_interval - INTEGER + The interval in milliseconds in which the next unsolicited + IGMPv3 report retransmit will take place. + + Default: 1000 (1 seconds) + +ignore_routes_with_linkdown - BOOLEAN + Ignore routes whose link is down when performing a FIB lookup. + +promote_secondaries - BOOLEAN + When a primary IP address is removed from this interface + promote a corresponding secondary IP address instead of + removing all the corresponding secondary IP addresses. + +drop_unicast_in_l2_multicast - BOOLEAN + Drop any unicast IP packets that are received in link-layer + multicast (or broadcast) frames. + + This behavior (for multicast) is actually a SHOULD in RFC + 1122, but is disabled by default for compatibility reasons. + + Default: off (0) + +drop_gratuitous_arp - BOOLEAN + Drop all gratuitous ARP frames, for example if there's a known + good ARP proxy on the network and such frames need not be used + (or in the case of 802.11, must not be used to prevent attacks.) + + Default: off (0) + + +tag - INTEGER + Allows you to write a number, which can be used as required. + + Default value is 0. + +xfrm4_gc_thresh - INTEGER + (Obsolete since linux-4.14) + The threshold at which we will start garbage collecting for IPv4 + destination cache entries. At twice this value the system will + refuse new allocations. + +igmp_link_local_mcast_reports - BOOLEAN + Enable IGMP reports for link local multicast groups in the + 224.0.0.X range. + + Default TRUE + +Alexey Kuznetsov. +kuznet@ms2.inr.ac.ru + +Updated by: + +- Andi Kleen + ak@muc.de +- Nicolas Delon + delon.nicolas@wanadoo.fr + + + + +/proc/sys/net/ipv6/* Variables +============================== + +IPv6 has no global variables such as tcp_*. tcp_* settings under ipv4/ also +apply to IPv6 [XXX?]. + +bindv6only - BOOLEAN + Default value for IPV6_V6ONLY socket option, + which restricts use of the IPv6 socket to IPv6 communication + only. + + - TRUE: disable IPv4-mapped address feature + - FALSE: enable IPv4-mapped address feature + + Default: FALSE (as specified in RFC3493) + +flowlabel_consistency - BOOLEAN + Protect the consistency (and unicity) of flow label. + You have to disable it to use IPV6_FL_F_REFLECT flag on the + flow label manager. + + - TRUE: enabled + - FALSE: disabled + + Default: TRUE + +auto_flowlabels - INTEGER + Automatically generate flow labels based on a flow hash of the + packet. This allows intermediate devices, such as routers, to + identify packet flows for mechanisms like Equal Cost Multipath + Routing (see RFC 6438). + + = =========================================================== + 0 automatic flow labels are completely disabled + 1 automatic flow labels are enabled by default, they can be + disabled on a per socket basis using the IPV6_AUTOFLOWLABEL + socket option + 2 automatic flow labels are allowed, they may be enabled on a + per socket basis using the IPV6_AUTOFLOWLABEL socket option + 3 automatic flow labels are enabled and enforced, they cannot + be disabled by the socket option + = =========================================================== + + Default: 1 + +flowlabel_state_ranges - BOOLEAN + Split the flow label number space into two ranges. 0-0x7FFFF is + reserved for the IPv6 flow manager facility, 0x80000-0xFFFFF + is reserved for stateless flow labels as described in RFC6437. + + - TRUE: enabled + - FALSE: disabled + + Default: true + +flowlabel_reflect - INTEGER + Control flow label reflection. Needed for Path MTU + Discovery to work with Equal Cost Multipath Routing in anycast + environments. See RFC 7690 and: + https://tools.ietf.org/html/draft-wang-6man-flow-label-reflection-01 + + This is a bitmask. + + - 1: enabled for established flows + + Note that this prevents automatic flowlabel changes, as done + in "tcp: change IPv6 flow-label upon receiving spurious retransmission" + and "tcp: Change txhash on every SYN and RTO retransmit" + + - 2: enabled for TCP RESET packets (no active listener) + If set, a RST packet sent in response to a SYN packet on a closed + port will reflect the incoming flow label. + + - 4: enabled for ICMPv6 echo reply messages. + + Default: 0 + +fib_multipath_hash_policy - INTEGER + Controls which hash policy to use for multipath routes. + + Default: 0 (Layer 3) + + Possible values: + + - 0 - Layer 3 (source and destination addresses plus flow label) + - 1 - Layer 4 (standard 5-tuple) + - 2 - Layer 3 or inner Layer 3 if present + - 3 - Custom multipath hash. Fields used for multipath hash calculation + are determined by fib_multipath_hash_fields sysctl + +fib_multipath_hash_fields - UNSIGNED INTEGER + When fib_multipath_hash_policy is set to 3 (custom multipath hash), the + fields used for multipath hash calculation are determined by this + sysctl. + + This value is a bitmask which enables various fields for multipath hash + calculation. + + Possible fields are: + + ====== ============================ + 0x0001 Source IP address + 0x0002 Destination IP address + 0x0004 IP protocol + 0x0008 Flow Label + 0x0010 Source port + 0x0020 Destination port + 0x0040 Inner source IP address + 0x0080 Inner destination IP address + 0x0100 Inner IP protocol + 0x0200 Inner Flow Label + 0x0400 Inner source port + 0x0800 Inner destination port + ====== ============================ + + Default: 0x0007 (source IP, destination IP and IP protocol) + +anycast_src_echo_reply - BOOLEAN + Controls the use of anycast addresses as source addresses for ICMPv6 + echo reply + + - TRUE: enabled + - FALSE: disabled + + Default: FALSE + +idgen_delay - INTEGER + Controls the delay in seconds after which time to retry + privacy stable address generation if a DAD conflict is + detected. + + Default: 1 (as specified in RFC7217) + +idgen_retries - INTEGER + Controls the number of retries to generate a stable privacy + address if a DAD conflict is detected. + + Default: 3 (as specified in RFC7217) + +mld_qrv - INTEGER + Controls the MLD query robustness variable (see RFC3810 9.1). + + Default: 2 (as specified by RFC3810 9.1) + + Minimum: 1 (as specified by RFC6636 4.5) + +max_dst_opts_number - INTEGER + Maximum number of non-padding TLVs allowed in a Destination + options extension header. If this value is less than zero + then unknown options are disallowed and the number of known + TLVs allowed is the absolute value of this number. + + Default: 8 + +max_hbh_opts_number - INTEGER + Maximum number of non-padding TLVs allowed in a Hop-by-Hop + options extension header. If this value is less than zero + then unknown options are disallowed and the number of known + TLVs allowed is the absolute value of this number. + + Default: 8 + +max_dst_opts_length - INTEGER + Maximum length allowed for a Destination options extension + header. + + Default: INT_MAX (unlimited) + +max_hbh_length - INTEGER + Maximum length allowed for a Hop-by-Hop options extension + header. + + Default: INT_MAX (unlimited) + +skip_notify_on_dev_down - BOOLEAN + Controls whether an RTM_DELROUTE message is generated for routes + removed when a device is taken down or deleted. IPv4 does not + generate this message; IPv6 does by default. Setting this sysctl + to true skips the message, making IPv4 and IPv6 on par in relying + on userspace caches to track link events and evict routes. + + Default: false (generate message) + +nexthop_compat_mode - BOOLEAN + New nexthop API provides a means for managing nexthops independent of + prefixes. Backwards compatibilty with old route format is enabled by + default which means route dumps and notifications contain the new + nexthop attribute but also the full, expanded nexthop definition. + Further, updates or deletes of a nexthop configuration generate route + notifications for each fib entry using the nexthop. Once a system + understands the new API, this sysctl can be disabled to achieve full + performance benefits of the new API by disabling the nexthop expansion + and extraneous notifications. + Default: true (backward compat mode) + +fib_notify_on_flag_change - INTEGER + Whether to emit RTM_NEWROUTE notifications whenever RTM_F_OFFLOAD/ + RTM_F_TRAP/RTM_F_OFFLOAD_FAILED flags are changed. + + After installing a route to the kernel, user space receives an + acknowledgment, which means the route was installed in the kernel, + but not necessarily in hardware. + It is also possible for a route already installed in hardware to change + its action and therefore its flags. For example, a host route that is + trapping packets can be "promoted" to perform decapsulation following + the installation of an IPinIP/VXLAN tunnel. + The notifications will indicate to user-space the state of the route. + + Default: 0 (Do not emit notifications.) + + Possible values: + + - 0 - Do not emit notifications. + - 1 - Emit notifications. + - 2 - Emit notifications only for RTM_F_OFFLOAD_FAILED flag change. + +ioam6_id - INTEGER + Define the IOAM id of this node. Uses only 24 bits out of 32 in total. + + Min: 0 + Max: 0xFFFFFF + + Default: 0xFFFFFF + +ioam6_id_wide - LONG INTEGER + Define the wide IOAM id of this node. Uses only 56 bits out of 64 in + total. Can be different from ioam6_id. + + Min: 0 + Max: 0xFFFFFFFFFFFFFF + + Default: 0xFFFFFFFFFFFFFF + +IPv6 Fragmentation: + +ip6frag_high_thresh - INTEGER + Maximum memory used to reassemble IPv6 fragments. When + ip6frag_high_thresh bytes of memory is allocated for this purpose, + the fragment handler will toss packets until ip6frag_low_thresh + is reached. + +ip6frag_low_thresh - INTEGER + See ip6frag_high_thresh + +ip6frag_time - INTEGER + Time in seconds to keep an IPv6 fragment in memory. + +``conf/default/*``: + Change the interface-specific default settings. + + These settings would be used during creating new interfaces. + + +``conf/all/*``: + Change all the interface-specific settings. + + [XXX: Other special features than forwarding?] + +conf/all/disable_ipv6 - BOOLEAN + Changing this value is same as changing ``conf/default/disable_ipv6`` + setting and also all per-interface ``disable_ipv6`` settings to the same + value. + + Reading this value does not have any particular meaning. It does not say + whether IPv6 support is enabled or disabled. Returned value can be 1 + also in the case when some interface has ``disable_ipv6`` set to 0 and + has configured IPv6 addresses. + +conf/all/forwarding - BOOLEAN + Enable global IPv6 forwarding between all interfaces. + + IPv4 and IPv6 work differently here; e.g. netfilter must be used + to control which interfaces may forward packets and which not. + + This also sets all interfaces' Host/Router setting + 'forwarding' to the specified value. See below for details. + + This referred to as global forwarding. + +proxy_ndp - BOOLEAN + Do proxy ndp. + +fwmark_reflect - BOOLEAN + Controls the fwmark of kernel-generated IPv6 reply packets that are not + associated with a socket for example, TCP RSTs or ICMPv6 echo replies). + If unset, these packets have a fwmark of zero. If set, they have the + fwmark of the packet they are replying to. + + Default: 0 + +``conf/interface/*``: + Change special settings per interface. + + The functional behaviour for certain settings is different + depending on whether local forwarding is enabled or not. + +accept_ra - INTEGER + Accept Router Advertisements; autoconfigure using them. + + It also determines whether or not to transmit Router + Solicitations. If and only if the functional setting is to + accept Router Advertisements, Router Solicitations will be + transmitted. + + Possible values are: + + == =========================================================== + 0 Do not accept Router Advertisements. + 1 Accept Router Advertisements if forwarding is disabled. + 2 Overrule forwarding behaviour. Accept Router Advertisements + even if forwarding is enabled. + == =========================================================== + + Functional default: + + - enabled if local forwarding is disabled. + - disabled if local forwarding is enabled. + +accept_ra_defrtr - BOOLEAN + Learn default router in Router Advertisement. + + Functional default: + + - enabled if accept_ra is enabled. + - disabled if accept_ra is disabled. + +ra_defrtr_metric - UNSIGNED INTEGER + Route metric for default route learned in Router Advertisement. This value + will be assigned as metric for the default route learned via IPv6 Router + Advertisement. Takes affect only if accept_ra_defrtr is enabled. + + Possible values: + 1 to 0xFFFFFFFF + + Default: IP6_RT_PRIO_USER i.e. 1024. + +accept_ra_from_local - BOOLEAN + Accept RA with source-address that is found on local machine + if the RA is otherwise proper and able to be accepted. + + Default is to NOT accept these as it may be an un-intended + network loop. + + Functional default: + + - enabled if accept_ra_from_local is enabled + on a specific interface. + - disabled if accept_ra_from_local is disabled + on a specific interface. + +accept_ra_min_hop_limit - INTEGER + Minimum hop limit Information in Router Advertisement. + + Hop limit Information in Router Advertisement less than this + variable shall be ignored. + + Default: 1 + +accept_ra_min_lft - INTEGER + Minimum acceptable lifetime value in Router Advertisement. + + RA sections with a lifetime less than this value shall be + ignored. Zero lifetimes stay unaffected. + + Default: 0 + +accept_ra_pinfo - BOOLEAN + Learn Prefix Information in Router Advertisement. + + Functional default: + + - enabled if accept_ra is enabled. + - disabled if accept_ra is disabled. + +accept_ra_rt_info_min_plen - INTEGER + Minimum prefix length of Route Information in RA. + + Route Information w/ prefix smaller than this variable shall + be ignored. + + Functional default: + + * 0 if accept_ra_rtr_pref is enabled. + * -1 if accept_ra_rtr_pref is disabled. + +accept_ra_rt_info_max_plen - INTEGER + Maximum prefix length of Route Information in RA. + + Route Information w/ prefix larger than this variable shall + be ignored. + + Functional default: + + * 0 if accept_ra_rtr_pref is enabled. + * -1 if accept_ra_rtr_pref is disabled. + +accept_ra_rtr_pref - BOOLEAN + Accept Router Preference in RA. + + Functional default: + + - enabled if accept_ra is enabled. + - disabled if accept_ra is disabled. + +accept_ra_mtu - BOOLEAN + Apply the MTU value specified in RA option 5 (RFC4861). If + disabled, the MTU specified in the RA will be ignored. + + Functional default: + + - enabled if accept_ra is enabled. + - disabled if accept_ra is disabled. + +accept_redirects - BOOLEAN + Accept Redirects. + + Functional default: + + - enabled if local forwarding is disabled. + - disabled if local forwarding is enabled. + +accept_source_route - INTEGER + Accept source routing (routing extension header). + + - >= 0: Accept only routing header type 2. + - < 0: Do not accept routing header. + + Default: 0 + +autoconf - BOOLEAN + Autoconfigure addresses using Prefix Information in Router + Advertisements. + + Functional default: + + - enabled if accept_ra_pinfo is enabled. + - disabled if accept_ra_pinfo is disabled. + +dad_transmits - INTEGER + The amount of Duplicate Address Detection probes to send. + + Default: 1 + +forwarding - INTEGER + Configure interface-specific Host/Router behaviour. + + .. note:: + + It is recommended to have the same setting on all + interfaces; mixed router/host scenarios are rather uncommon. + + Possible values are: + + - 0 Forwarding disabled + - 1 Forwarding enabled + + **FALSE (0)**: + + By default, Host behaviour is assumed. This means: + + 1. IsRouter flag is not set in Neighbour Advertisements. + 2. If accept_ra is TRUE (default), transmit Router + Solicitations. + 3. If accept_ra is TRUE (default), accept Router + Advertisements (and do autoconfiguration). + 4. If accept_redirects is TRUE (default), accept Redirects. + + **TRUE (1)**: + + If local forwarding is enabled, Router behaviour is assumed. + This means exactly the reverse from the above: + + 1. IsRouter flag is set in Neighbour Advertisements. + 2. Router Solicitations are not sent unless accept_ra is 2. + 3. Router Advertisements are ignored unless accept_ra is 2. + 4. Redirects are ignored. + + Default: 0 (disabled) if global forwarding is disabled (default), + otherwise 1 (enabled). + +hop_limit - INTEGER + Default Hop Limit to set. + + Default: 64 + +mtu - INTEGER + Default Maximum Transfer Unit + + Default: 1280 (IPv6 required minimum) + +ip_nonlocal_bind - BOOLEAN + If set, allows processes to bind() to non-local IPv6 addresses, + which can be quite useful - but may break some applications. + + Default: 0 + +router_probe_interval - INTEGER + Minimum interval (in seconds) between Router Probing described + in RFC4191. + + Default: 60 + +router_solicitation_delay - INTEGER + Number of seconds to wait after interface is brought up + before sending Router Solicitations. + + Default: 1 + +router_solicitation_interval - INTEGER + Number of seconds to wait between Router Solicitations. + + Default: 4 + +router_solicitations - INTEGER + Number of Router Solicitations to send until assuming no + routers are present. + + Default: 3 + +use_oif_addrs_only - BOOLEAN + When enabled, the candidate source addresses for destinations + routed via this interface are restricted to the set of addresses + configured on this interface (vis. RFC 6724, section 4). + + Default: false + +use_tempaddr - INTEGER + Preference for Privacy Extensions (RFC3041). + + * <= 0 : disable Privacy Extensions + * == 1 : enable Privacy Extensions, but prefer public + addresses over temporary addresses. + * > 1 : enable Privacy Extensions and prefer temporary + addresses over public addresses. + + Default: + + * 0 (for most devices) + * -1 (for point-to-point devices and loopback devices) + +temp_valid_lft - INTEGER + valid lifetime (in seconds) for temporary addresses. + + Default: 172800 (2 days) + +temp_prefered_lft - INTEGER + Preferred lifetime (in seconds) for temporary addresses. + + Default: 86400 (1 day) + +keep_addr_on_down - INTEGER + Keep all IPv6 addresses on an interface down event. If set static + global addresses with no expiration time are not flushed. + + * >0 : enabled + * 0 : system default + * <0 : disabled + + Default: 0 (addresses are removed) + +max_desync_factor - INTEGER + Maximum value for DESYNC_FACTOR, which is a random value + that ensures that clients don't synchronize with each + other and generate new addresses at exactly the same time. + value is in seconds. + + Default: 600 + +regen_max_retry - INTEGER + Number of attempts before give up attempting to generate + valid temporary addresses. + + Default: 5 + +max_addresses - INTEGER + Maximum number of autoconfigured addresses per interface. Setting + to zero disables the limitation. It is not recommended to set this + value too large (or to zero) because it would be an easy way to + crash the kernel by allowing too many addresses to be created. + + Default: 16 + +disable_ipv6 - BOOLEAN + Disable IPv6 operation. If accept_dad is set to 2, this value + will be dynamically set to TRUE if DAD fails for the link-local + address. + + Default: FALSE (enable IPv6 operation) + + When this value is changed from 1 to 0 (IPv6 is being enabled), + it will dynamically create a link-local address on the given + interface and start Duplicate Address Detection, if necessary. + + When this value is changed from 0 to 1 (IPv6 is being disabled), + it will dynamically delete all addresses and routes on the given + interface. From now on it will not possible to add addresses/routes + to the selected interface. + +accept_dad - INTEGER + Whether to accept DAD (Duplicate Address Detection). + + == ============================================================== + 0 Disable DAD + 1 Enable DAD (default) + 2 Enable DAD, and disable IPv6 operation if MAC-based duplicate + link-local address has been found. + == ============================================================== + + DAD operation and mode on a given interface will be selected according + to the maximum value of conf/{all,interface}/accept_dad. + +force_tllao - BOOLEAN + Enable sending the target link-layer address option even when + responding to a unicast neighbor solicitation. + + Default: FALSE + + Quoting from RFC 2461, section 4.4, Target link-layer address: + + "The option MUST be included for multicast solicitations in order to + avoid infinite Neighbor Solicitation "recursion" when the peer node + does not have a cache entry to return a Neighbor Advertisements + message. When responding to unicast solicitations, the option can be + omitted since the sender of the solicitation has the correct link- + layer address; otherwise it would not have be able to send the unicast + solicitation in the first place. However, including the link-layer + address in this case adds little overhead and eliminates a potential + race condition where the sender deletes the cached link-layer address + prior to receiving a response to a previous solicitation." + +ndisc_notify - BOOLEAN + Define mode for notification of address and device changes. + + * 0 - (default): do nothing + * 1 - Generate unsolicited neighbour advertisements when device is brought + up or hardware address changes. + +ndisc_tclass - INTEGER + The IPv6 Traffic Class to use by default when sending IPv6 Neighbor + Discovery (Router Solicitation, Router Advertisement, Neighbor + Solicitation, Neighbor Advertisement, Redirect) messages. + These 8 bits can be interpreted as 6 high order bits holding the DSCP + value and 2 low order bits representing ECN (which you probably want + to leave cleared). + + * 0 - (default) + +ndisc_evict_nocarrier - BOOLEAN + Clears the neighbor discovery table on NOCARRIER events. This option is + important for wireless devices where the neighbor discovery cache should + not be cleared when roaming between access points on the same network. + In most cases this should remain as the default (1). + + - 1 - (default): Clear neighbor discover cache on NOCARRIER events. + - 0 - Do not clear neighbor discovery cache on NOCARRIER events. + +mldv1_unsolicited_report_interval - INTEGER + The interval in milliseconds in which the next unsolicited + MLDv1 report retransmit will take place. + + Default: 10000 (10 seconds) + +mldv2_unsolicited_report_interval - INTEGER + The interval in milliseconds in which the next unsolicited + MLDv2 report retransmit will take place. + + Default: 1000 (1 second) + +force_mld_version - INTEGER + * 0 - (default) No enforcement of a MLD version, MLDv1 fallback allowed + * 1 - Enforce to use MLD version 1 + * 2 - Enforce to use MLD version 2 + +suppress_frag_ndisc - INTEGER + Control RFC 6980 (Security Implications of IPv6 Fragmentation + with IPv6 Neighbor Discovery) behavior: + + * 1 - (default) discard fragmented neighbor discovery packets + * 0 - allow fragmented neighbor discovery packets + +optimistic_dad - BOOLEAN + Whether to perform Optimistic Duplicate Address Detection (RFC 4429). + + * 0: disabled (default) + * 1: enabled + + Optimistic Duplicate Address Detection for the interface will be enabled + if at least one of conf/{all,interface}/optimistic_dad is set to 1, + it will be disabled otherwise. + +use_optimistic - BOOLEAN + If enabled, do not classify optimistic addresses as deprecated during + source address selection. Preferred addresses will still be chosen + before optimistic addresses, subject to other ranking in the source + address selection algorithm. + + * 0: disabled (default) + * 1: enabled + + This will be enabled if at least one of + conf/{all,interface}/use_optimistic is set to 1, disabled otherwise. + +stable_secret - IPv6 address + This IPv6 address will be used as a secret to generate IPv6 + addresses for link-local addresses and autoconfigured + ones. All addresses generated after setting this secret will + be stable privacy ones by default. This can be changed via the + addrgenmode ip-link. conf/default/stable_secret is used as the + secret for the namespace, the interface specific ones can + overwrite that. Writes to conf/all/stable_secret are refused. + + It is recommended to generate this secret during installation + of a system and keep it stable after that. + + By default the stable secret is unset. + +addr_gen_mode - INTEGER + Defines how link-local and autoconf addresses are generated. + + = ================================================================= + 0 generate address based on EUI64 (default) + 1 do no generate a link-local address, use EUI64 for addresses + generated from autoconf + 2 generate stable privacy addresses, using the secret from + stable_secret (RFC7217) + 3 generate stable privacy addresses, using a random secret if unset + = ================================================================= + +drop_unicast_in_l2_multicast - BOOLEAN + Drop any unicast IPv6 packets that are received in link-layer + multicast (or broadcast) frames. + + By default this is turned off. + +drop_unsolicited_na - BOOLEAN + Drop all unsolicited neighbor advertisements, for example if there's + a known good NA proxy on the network and such frames need not be used + (or in the case of 802.11, must not be used to prevent attacks.) + + By default this is turned off. + +accept_untracked_na - INTEGER + Define behavior for accepting neighbor advertisements from devices that + are absent in the neighbor cache: + + - 0 - (default) Do not accept unsolicited and untracked neighbor + advertisements. + + - 1 - Add a new neighbor cache entry in STALE state for routers on + receiving a neighbor advertisement (either solicited or unsolicited) + with target link-layer address option specified if no neighbor entry + is already present for the advertised IPv6 address. Without this knob, + NAs received for untracked addresses (absent in neighbor cache) are + silently ignored. + + This is as per router-side behavior documented in RFC9131. + + This has lower precedence than drop_unsolicited_na. + + This will optimize the return path for the initial off-link + communication that is initiated by a directly connected host, by + ensuring that the first-hop router which turns on this setting doesn't + have to buffer the initial return packets to do neighbor-solicitation. + The prerequisite is that the host is configured to send unsolicited + neighbor advertisements on interface bringup. This setting should be + used in conjunction with the ndisc_notify setting on the host to + satisfy this prerequisite. + + - 2 - Extend option (1) to add a new neighbor cache entry only if the + source IP address is in the same subnet as an address configured on + the interface that received the neighbor advertisement. + +enhanced_dad - BOOLEAN + Include a nonce option in the IPv6 neighbor solicitation messages used for + duplicate address detection per RFC7527. A received DAD NS will only signal + a duplicate address if the nonce is different. This avoids any false + detection of duplicates due to loopback of the NS messages that we send. + The nonce option will be sent on an interface unless both of + conf/{all,interface}/enhanced_dad are set to FALSE. + + Default: TRUE + +``icmp/*``: +=========== + +ratelimit - INTEGER + Limit the maximal rates for sending ICMPv6 messages. + + 0 to disable any limiting, + otherwise the minimal space between responses in milliseconds. + + Default: 1000 + +ratemask - list of comma separated ranges + For ICMPv6 message types matching the ranges in the ratemask, limit + the sending of the message according to ratelimit parameter. + + The format used for both input and output is a comma separated + list of ranges (e.g. "0-127,129" for ICMPv6 message type 0 to 127 and + 129). Writing to the file will clear all previous ranges of ICMPv6 + message types and update the current list with the input. + + Refer to: https://www.iana.org/assignments/icmpv6-parameters/icmpv6-parameters.xhtml + for numerical values of ICMPv6 message types, e.g. echo request is 128 + and echo reply is 129. + + Default: 0-1,3-127 (rate limit ICMPv6 errors except Packet Too Big) + +echo_ignore_all - BOOLEAN + If set non-zero, then the kernel will ignore all ICMP ECHO + requests sent to it over the IPv6 protocol. + + Default: 0 + +echo_ignore_multicast - BOOLEAN + If set non-zero, then the kernel will ignore all ICMP ECHO + requests sent to it over the IPv6 protocol via multicast. + + Default: 0 + +echo_ignore_anycast - BOOLEAN + If set non-zero, then the kernel will ignore all ICMP ECHO + requests sent to it over the IPv6 protocol destined to anycast address. + + Default: 0 + +xfrm6_gc_thresh - INTEGER + (Obsolete since linux-4.14) + The threshold at which we will start garbage collecting for IPv6 + destination cache entries. At twice this value the system will + refuse new allocations. + + +IPv6 Update by: +Pekka Savola <pekkas@netcore.fi> +YOSHIFUJI Hideaki / USAGI Project <yoshfuji@linux-ipv6.org> + + +/proc/sys/net/bridge/* Variables: +================================= + +bridge-nf-call-arptables - BOOLEAN + - 1 : pass bridged ARP traffic to arptables' FORWARD chain. + - 0 : disable this. + + Default: 1 + +bridge-nf-call-iptables - BOOLEAN + - 1 : pass bridged IPv4 traffic to iptables' chains. + - 0 : disable this. + + Default: 1 + +bridge-nf-call-ip6tables - BOOLEAN + - 1 : pass bridged IPv6 traffic to ip6tables' chains. + - 0 : disable this. + + Default: 1 + +bridge-nf-filter-vlan-tagged - BOOLEAN + - 1 : pass bridged vlan-tagged ARP/IP/IPv6 traffic to {arp,ip,ip6}tables. + - 0 : disable this. + + Default: 0 + +bridge-nf-filter-pppoe-tagged - BOOLEAN + - 1 : pass bridged pppoe-tagged IP/IPv6 traffic to {ip,ip6}tables. + - 0 : disable this. + + Default: 0 + +bridge-nf-pass-vlan-input-dev - BOOLEAN + - 1: if bridge-nf-filter-vlan-tagged is enabled, try to find a vlan + interface on the bridge and set the netfilter input device to the + vlan. This allows use of e.g. "iptables -i br0.1" and makes the + REDIRECT target work with vlan-on-top-of-bridge interfaces. When no + matching vlan interface is found, or this switch is off, the input + device is set to the bridge interface. + + - 0: disable bridge netfilter vlan interface lookup. + + Default: 0 + +``proc/sys/net/sctp/*`` Variables: +================================== + +addip_enable - BOOLEAN + Enable or disable extension of Dynamic Address Reconfiguration + (ADD-IP) functionality specified in RFC5061. This extension provides + the ability to dynamically add and remove new addresses for the SCTP + associations. + + 1: Enable extension. + + 0: Disable extension. + + Default: 0 + +pf_enable - INTEGER + Enable or disable pf (pf is short for potentially failed) state. A value + of pf_retrans > path_max_retrans also disables pf state. That is, one of + both pf_enable and pf_retrans > path_max_retrans can disable pf state. + Since pf_retrans and path_max_retrans can be changed by userspace + application, sometimes user expects to disable pf state by the value of + pf_retrans > path_max_retrans, but occasionally the value of pf_retrans + or path_max_retrans is changed by the user application, this pf state is + enabled. As such, it is necessary to add this to dynamically enable + and disable pf state. See: + https://datatracker.ietf.org/doc/draft-ietf-tsvwg-sctp-failover for + details. + + 1: Enable pf. + + 0: Disable pf. + + Default: 1 + +pf_expose - INTEGER + Unset or enable/disable pf (pf is short for potentially failed) state + exposure. Applications can control the exposure of the PF path state + in the SCTP_PEER_ADDR_CHANGE event and the SCTP_GET_PEER_ADDR_INFO + sockopt. When it's unset, no SCTP_PEER_ADDR_CHANGE event with + SCTP_ADDR_PF state will be sent and a SCTP_PF-state transport info + can be got via SCTP_GET_PEER_ADDR_INFO sockopt; When it's enabled, + a SCTP_PEER_ADDR_CHANGE event will be sent for a transport becoming + SCTP_PF state and a SCTP_PF-state transport info can be got via + SCTP_GET_PEER_ADDR_INFO sockopt; When it's diabled, no + SCTP_PEER_ADDR_CHANGE event will be sent and it returns -EACCES when + trying to get a SCTP_PF-state transport info via SCTP_GET_PEER_ADDR_INFO + sockopt. + + 0: Unset pf state exposure, Compatible with old applications. + + 1: Disable pf state exposure. + + 2: Enable pf state exposure. + + Default: 0 + +addip_noauth_enable - BOOLEAN + Dynamic Address Reconfiguration (ADD-IP) requires the use of + authentication to protect the operations of adding or removing new + addresses. This requirement is mandated so that unauthorized hosts + would not be able to hijack associations. However, older + implementations may not have implemented this requirement while + allowing the ADD-IP extension. For reasons of interoperability, + we provide this variable to control the enforcement of the + authentication requirement. + + == =============================================================== + 1 Allow ADD-IP extension to be used without authentication. This + should only be set in a closed environment for interoperability + with older implementations. + + 0 Enforce the authentication requirement + == =============================================================== + + Default: 0 + +auth_enable - BOOLEAN + Enable or disable Authenticated Chunks extension. This extension + provides the ability to send and receive authenticated chunks and is + required for secure operation of Dynamic Address Reconfiguration + (ADD-IP) extension. + + - 1: Enable this extension. + - 0: Disable this extension. + + Default: 0 + +prsctp_enable - BOOLEAN + Enable or disable the Partial Reliability extension (RFC3758) which + is used to notify peers that a given DATA should no longer be expected. + + - 1: Enable extension + - 0: Disable + + Default: 1 + +max_burst - INTEGER + The limit of the number of new packets that can be initially sent. It + controls how bursty the generated traffic can be. + + Default: 4 + +association_max_retrans - INTEGER + Set the maximum number for retransmissions that an association can + attempt deciding that the remote end is unreachable. If this value + is exceeded, the association is terminated. + + Default: 10 + +max_init_retransmits - INTEGER + The maximum number of retransmissions of INIT and COOKIE-ECHO chunks + that an association will attempt before declaring the destination + unreachable and terminating. + + Default: 8 + +path_max_retrans - INTEGER + The maximum number of retransmissions that will be attempted on a given + path. Once this threshold is exceeded, the path is considered + unreachable, and new traffic will use a different path when the + association is multihomed. + + Default: 5 + +pf_retrans - INTEGER + The number of retransmissions that will be attempted on a given path + before traffic is redirected to an alternate transport (should one + exist). Note this is distinct from path_max_retrans, as a path that + passes the pf_retrans threshold can still be used. Its only + deprioritized when a transmission path is selected by the stack. This + setting is primarily used to enable fast failover mechanisms without + having to reduce path_max_retrans to a very low value. See: + http://www.ietf.org/id/draft-nishida-tsvwg-sctp-failover-05.txt + for details. Note also that a value of pf_retrans > path_max_retrans + disables this feature. Since both pf_retrans and path_max_retrans can + be changed by userspace application, a variable pf_enable is used to + disable pf state. + + Default: 0 + +ps_retrans - INTEGER + Primary.Switchover.Max.Retrans (PSMR), it's a tunable parameter coming + from section-5 "Primary Path Switchover" in rfc7829. The primary path + will be changed to another active path when the path error counter on + the old primary path exceeds PSMR, so that "the SCTP sender is allowed + to continue data transmission on a new working path even when the old + primary destination address becomes active again". Note this feature + is disabled by initializing 'ps_retrans' per netns as 0xffff by default, + and its value can't be less than 'pf_retrans' when changing by sysctl. + + Default: 0xffff + +rto_initial - INTEGER + The initial round trip timeout value in milliseconds that will be used + in calculating round trip times. This is the initial time interval + for retransmissions. + + Default: 3000 + +rto_max - INTEGER + The maximum value (in milliseconds) of the round trip timeout. This + is the largest time interval that can elapse between retransmissions. + + Default: 60000 + +rto_min - INTEGER + The minimum value (in milliseconds) of the round trip timeout. This + is the smallest time interval the can elapse between retransmissions. + + Default: 1000 + +hb_interval - INTEGER + The interval (in milliseconds) between HEARTBEAT chunks. These chunks + are sent at the specified interval on idle paths to probe the state of + a given path between 2 associations. + + Default: 30000 + +sack_timeout - INTEGER + The amount of time (in milliseconds) that the implementation will wait + to send a SACK. + + Default: 200 + +valid_cookie_life - INTEGER + The default lifetime of the SCTP cookie (in milliseconds). The cookie + is used during association establishment. + + Default: 60000 + +cookie_preserve_enable - BOOLEAN + Enable or disable the ability to extend the lifetime of the SCTP cookie + that is used during the establishment phase of SCTP association + + - 1: Enable cookie lifetime extension. + - 0: Disable + + Default: 1 + +cookie_hmac_alg - STRING + Select the hmac algorithm used when generating the cookie value sent by + a listening sctp socket to a connecting client in the INIT-ACK chunk. + Valid values are: + + * md5 + * sha1 + * none + + Ability to assign md5 or sha1 as the selected alg is predicated on the + configuration of those algorithms at build time (CONFIG_CRYPTO_MD5 and + CONFIG_CRYPTO_SHA1). + + Default: Dependent on configuration. MD5 if available, else SHA1 if + available, else none. + +rcvbuf_policy - INTEGER + Determines if the receive buffer is attributed to the socket or to + association. SCTP supports the capability to create multiple + associations on a single socket. When using this capability, it is + possible that a single stalled association that's buffering a lot + of data may block other associations from delivering their data by + consuming all of the receive buffer space. To work around this, + the rcvbuf_policy could be set to attribute the receiver buffer space + to each association instead of the socket. This prevents the described + blocking. + + - 1: rcvbuf space is per association + - 0: rcvbuf space is per socket + + Default: 0 + +sndbuf_policy - INTEGER + Similar to rcvbuf_policy above, this applies to send buffer space. + + - 1: Send buffer is tracked per association + - 0: Send buffer is tracked per socket. + + Default: 0 + +sctp_mem - vector of 3 INTEGERs: min, pressure, max + Number of pages allowed for queueing by all SCTP sockets. + + min: Below this number of pages SCTP is not bothered about its + memory appetite. When amount of memory allocated by SCTP exceeds + this number, SCTP starts to moderate memory usage. + + pressure: This value was introduced to follow format of tcp_mem. + + max: Number of pages allowed for queueing by all SCTP sockets. + + Default is calculated at boot time from amount of available memory. + +sctp_rmem - vector of 3 INTEGERs: min, default, max + Only the first value ("min") is used, "default" and "max" are + ignored. + + min: Minimal size of receive buffer used by SCTP socket. + It is guaranteed to each SCTP socket (but not association) even + under moderate memory pressure. + + Default: 4K + +sctp_wmem - vector of 3 INTEGERs: min, default, max + Only the first value ("min") is used, "default" and "max" are + ignored. + + min: Minimum size of send buffer that can be used by SCTP sockets. + It is guaranteed to each SCTP socket (but not association) even + under moderate memory pressure. + + Default: 4K + +addr_scope_policy - INTEGER + Control IPv4 address scoping - draft-stewart-tsvwg-sctp-ipv4-00 + + - 0 - Disable IPv4 address scoping + - 1 - Enable IPv4 address scoping + - 2 - Follow draft but allow IPv4 private addresses + - 3 - Follow draft but allow IPv4 link local addresses + + Default: 1 + +udp_port - INTEGER + The listening port for the local UDP tunneling sock. Normally it's + using the IANA-assigned UDP port number 9899 (sctp-tunneling). + + This UDP sock is used for processing the incoming UDP-encapsulated + SCTP packets (from RFC6951), and shared by all applications in the + same net namespace. This UDP sock will be closed when the value is + set to 0. + + The value will also be used to set the src port of the UDP header + for the outgoing UDP-encapsulated SCTP packets. For the dest port, + please refer to 'encap_port' below. + + Default: 0 + +encap_port - INTEGER + The default remote UDP encapsulation port. + + This value is used to set the dest port of the UDP header for the + outgoing UDP-encapsulated SCTP packets by default. Users can also + change the value for each sock/asoc/transport by using setsockopt. + For further information, please refer to RFC6951. + + Note that when connecting to a remote server, the client should set + this to the port that the UDP tunneling sock on the peer server is + listening to and the local UDP tunneling sock on the client also + must be started. On the server, it would get the encap_port from + the incoming packet's source port. + + Default: 0 + +plpmtud_probe_interval - INTEGER + The time interval (in milliseconds) for the PLPMTUD probe timer, + which is configured to expire after this period to receive an + acknowledgment to a probe packet. This is also the time interval + between the probes for the current pmtu when the probe search + is done. + + PLPMTUD will be disabled when 0 is set, and other values for it + must be >= 5000. + + Default: 0 + +reconf_enable - BOOLEAN + Enable or disable extension of Stream Reconfiguration functionality + specified in RFC6525. This extension provides the ability to "reset" + a stream, and it includes the Parameters of "Outgoing/Incoming SSN + Reset", "SSN/TSN Reset" and "Add Outgoing/Incoming Streams". + + - 1: Enable extension. + - 0: Disable extension. + + Default: 0 + +intl_enable - BOOLEAN + Enable or disable extension of User Message Interleaving functionality + specified in RFC8260. This extension allows the interleaving of user + messages sent on different streams. With this feature enabled, I-DATA + chunk will replace DATA chunk to carry user messages if also supported + by the peer. Note that to use this feature, one needs to set this option + to 1 and also needs to set socket options SCTP_FRAGMENT_INTERLEAVE to 2 + and SCTP_INTERLEAVING_SUPPORTED to 1. + + - 1: Enable extension. + - 0: Disable extension. + + Default: 0 + +ecn_enable - BOOLEAN + Control use of Explicit Congestion Notification (ECN) by SCTP. + Like in TCP, ECN is used only when both ends of the SCTP connection + indicate support for it. This feature is useful in avoiding losses + due to congestion by allowing supporting routers to signal congestion + before having to drop packets. + + 1: Enable ecn. + 0: Disable ecn. + + Default: 1 + + +``/proc/sys/net/core/*`` +======================== + + Please see: Documentation/admin-guide/sysctl/net.rst for descriptions of these entries. + + +``/proc/sys/net/unix/*`` +======================== + +max_dgram_qlen - INTEGER + The maximum length of dgram socket receive queue + + Default: 10 + |