diff options
Diffstat (limited to 'health/guides/tcp/10s_ipv4_tcp_resets_received.md')
| -rw-r--r-- | health/guides/tcp/10s_ipv4_tcp_resets_received.md | 67 |
1 files changed, 67 insertions, 0 deletions
diff --git a/health/guides/tcp/10s_ipv4_tcp_resets_received.md b/health/guides/tcp/10s_ipv4_tcp_resets_received.md new file mode 100644 index 00000000..c17954f2 --- /dev/null +++ b/health/guides/tcp/10s_ipv4_tcp_resets_received.md @@ -0,0 +1,67 @@ +### Understand the alert + +TCP reset is an abrupt closure of the session. It causes the resources allocated to the connection to be immediately released and all other information about the connection is erased. + +The Netdata Agent monitors the average number of sent TCP RESETS over the last 10 seconds. This can indicate a port scan or that a service running on the system has crashed. Additionally, it's a result of a high number of sent TCP RESETS. Furthermore, it can also indicate a SYN reset attack. + +### More about TCP Resets + +TCP uses a three-way handshake to establish a reliable connection. The connection is full duplex, and both sides synchronize (SYN) and acknowledge (ACK) each other. The exchange of these four flags +is performed in three steps: SYN, SYN-ACK, and ACK. + +When an unexpected TCP packet arrives at a host, that host usually responds by sending a reset packet back on the same connection. A reset packet is one with no payload and with the RST bit set in the TCP header flags. There are a few circumstances in which a TCP packet might not be expected. The most common cases are: + +1. A TCP packet received on a port that is not open. +2. An aborting connection +3. Half opened connections +4. Time wait assassination +5. Listening endpoint Queue is Full +6. A TCP Buffer Overflow + +Basically, A TCP Reset usually occurs when a system receives data which doesn't agree with its view of the connection. + +### Troubleshoot the alert + +- Use tcpdump to capture the traffic and use Wireshark to inspect the network packets. You must stop the capture after a certain observation period (60s up to 5 minutes). This command will create a dump file which can be interpreted by Wireshark that contains all the TCP packets with RST flag set. + ``` + tcpdump -i any 'tcp[tcpflags] & (tcp-rst) == (tcp-rst)' -s 65535 -w output.pcap + ``` + +- Counter measure on malicious TCP resets + +SYN cookie is a technique used to resist IP address spoofing attacks. In particular, the use of SYN cookies allows a server to avoid dropping connections when the SYN queue fills up. + +Enable SYN cookies in Linux: + + 1. Check if your system has the SYN cookies service enabled. If the value is 1, then the service is enabled, if not proceed to step 2. + ``` + cat /proc/sys/net/ipv4/tcp_syncookies + ``` + + 2. Bump this `net.ipv4.tcp_syncookies=1` value under `/etc/sysctl.conf` + + 3. Apply the configuration + ``` + sysctl -p + ``` + +Enable SYN cookies in FreeBSD: + + 1. Check if your system has the SYN cookies service enabled. If the value is 1, then the service is enabled, if not proceed to step 2. + ``` + sysctl net.inet.tcp.syncookies_only + ``` + + 2. Bump this `net.inet.tcp.syncookies_only=1` value under `/etc/sysctl.conf` + + 3. Apply the configuration + ``` + /etc/rc.d/sysctl reload + ``` + +The use of SYN cookies does not break any protocol specifications, and therefore should be compatible with all TCP implementations. There are, however, a few caveats that take effect when SYN cookies are in use. + +### Useful resources + +1. [TCP reset explanation](https://www.pico.net/kb/what-is-a-tcp-reset-rst/) +2. [TCP 3-way handshake on wikipedia](https://en.wikipedia.org/wiki/Handshaking) |