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diff --git a/PROTOCOL b/PROTOCOL new file mode 100644 index 0000000..f75c1c0 --- /dev/null +++ b/PROTOCOL @@ -0,0 +1,499 @@ +This documents OpenSSH's deviations and extensions to the published SSH +protocol. + +Note that OpenSSH's sftp and sftp-server implement revision 3 of the SSH +filexfer protocol described in: + +https://www.openssh.com/txt/draft-ietf-secsh-filexfer-02.txt + +Newer versions of the draft will not be supported, though some features +are individually implemented as extensions described below. + +The protocol used by OpenSSH's ssh-agent is described in the file +PROTOCOL.agent + +1. Transport protocol changes + +1.1. transport: Protocol 2 MAC algorithm "umac-64@openssh.com" + +This is a new transport-layer MAC method using the UMAC algorithm +(rfc4418). This method is identical to the "umac-64" method documented +in: + +https://www.openssh.com/txt/draft-miller-secsh-umac-01.txt + +1.2. transport: Protocol 2 compression algorithm "zlib@openssh.com" + +This transport-layer compression method uses the zlib compression +algorithm (identical to the "zlib" method in rfc4253), but delays the +start of compression until after authentication has completed. This +avoids exposing compression code to attacks from unauthenticated users. + +The method is documented in: + +https://www.openssh.com/txt/draft-miller-secsh-compression-delayed-00.txt + +1.3. transport: New public key algorithms "ssh-rsa-cert-v01@openssh.com", + "ssh-dsa-cert-v01@openssh.com", + "ecdsa-sha2-nistp256-cert-v01@openssh.com", + "ecdsa-sha2-nistp384-cert-v01@openssh.com" and + "ecdsa-sha2-nistp521-cert-v01@openssh.com" + +OpenSSH introduces new public key algorithms to support certificate +authentication for users and host keys. These methods are documented +in the file PROTOCOL.certkeys + +1.4. transport: Elliptic Curve cryptography + +OpenSSH supports ECC key exchange and public key authentication as +specified in RFC5656. Only the ecdsa-sha2-nistp256, ecdsa-sha2-nistp384 +and ecdsa-sha2-nistp521 curves over GF(p) are supported. Elliptic +curve points encoded using point compression are NOT accepted or +generated. + +1.5 transport: Protocol 2 Encrypt-then-MAC MAC algorithms + +OpenSSH supports MAC algorithms, whose names contain "-etm", that +perform the calculations in a different order to that defined in RFC +4253. These variants use the so-called "encrypt then MAC" ordering, +calculating the MAC over the packet ciphertext rather than the +plaintext. This ordering closes a security flaw in the SSH transport +protocol, where decryption of unauthenticated ciphertext provided a +"decryption oracle" that could, in conjunction with cipher flaws, reveal +session plaintext. + +Specifically, the "-etm" MAC algorithms modify the transport protocol +to calculate the MAC over the packet ciphertext and to send the packet +length unencrypted. This is necessary for the transport to obtain the +length of the packet and location of the MAC tag so that it may be +verified without decrypting unauthenticated data. + +As such, the MAC covers: + + mac = MAC(key, sequence_number || packet_length || encrypted_packet) + +where "packet_length" is encoded as a uint32 and "encrypted_packet" +contains: + + byte padding_length + byte[n1] payload; n1 = packet_length - padding_length - 1 + byte[n2] random padding; n2 = padding_length + +1.6 transport: AES-GCM + +OpenSSH supports the AES-GCM algorithm as specified in RFC 5647. +Because of problems with the specification of the key exchange +the behaviour of OpenSSH differs from the RFC as follows: + +AES-GCM is only negotiated as the cipher algorithms +"aes128-gcm@openssh.com" or "aes256-gcm@openssh.com" and never as +an MAC algorithm. Additionally, if AES-GCM is selected as the cipher +the exchanged MAC algorithms are ignored and there doesn't have to be +a matching MAC. + +1.7 transport: chacha20-poly1305@openssh.com authenticated encryption + +OpenSSH supports authenticated encryption using ChaCha20 and Poly1305 +as described in PROTOCOL.chacha20poly1305. + +1.8 transport: curve25519-sha256@libssh.org key exchange algorithm + +OpenSSH supports the use of ECDH in Curve25519 for key exchange as +described at: +http://git.libssh.org/users/aris/libssh.git/plain/doc/curve25519-sha256@libssh.org.txt?h=curve25519 + +2. Connection protocol changes + +2.1. connection: Channel write close extension "eow@openssh.com" + +The SSH connection protocol (rfc4254) provides the SSH_MSG_CHANNEL_EOF +message to allow an endpoint to signal its peer that it will send no +more data over a channel. Unfortunately, there is no symmetric way for +an endpoint to request that its peer should cease sending data to it +while still keeping the channel open for the endpoint to send data to +the peer. + +This is desirable, since it saves the transmission of data that would +otherwise need to be discarded and it allows an endpoint to signal local +processes of the condition, e.g. by closing the corresponding file +descriptor. + +OpenSSH implements a channel extension message to perform this +signalling: "eow@openssh.com" (End Of Write). This message is sent by +an endpoint when the local output of a session channel is closed or +experiences a write error. The message is formatted as follows: + + byte SSH_MSG_CHANNEL_REQUEST + uint32 recipient channel + string "eow@openssh.com" + boolean FALSE + +On receiving this message, the peer SHOULD cease sending data of +the channel and MAY signal the process from which the channel data +originates (e.g. by closing its read file descriptor). + +As with the symmetric SSH_MSG_CHANNEL_EOF message, the channel does +remain open after a "eow@openssh.com" has been sent and more data may +still be sent in the other direction. This message does not consume +window space and may be sent even if no window space is available. + +NB. due to certain broken SSH implementations aborting upon receipt +of this message (in contravention of RFC4254 section 5.4), this +message is only sent to OpenSSH peers (identified by banner). +Other SSH implementations may be whitelisted to receive this message +upon request. + +2.2. connection: disallow additional sessions extension + "no-more-sessions@openssh.com" + +Most SSH connections will only ever request a single session, but a +attacker may abuse a running ssh client to surreptitiously open +additional sessions under their control. OpenSSH provides a global +request "no-more-sessions@openssh.com" to mitigate this attack. + +When an OpenSSH client expects that it will never open another session +(i.e. it has been started with connection multiplexing disabled), it +will send the following global request: + + byte SSH_MSG_GLOBAL_REQUEST + string "no-more-sessions@openssh.com" + char want-reply + +On receipt of such a message, an OpenSSH server will refuse to open +future channels of type "session" and instead immediately abort the +connection. + +Note that this is not a general defence against compromised clients +(that is impossible), but it thwarts a simple attack. + +NB. due to certain broken SSH implementations aborting upon receipt +of this message, the no-more-sessions request is only sent to OpenSSH +servers (identified by banner). Other SSH implementations may be +whitelisted to receive this message upon request. + +2.3. connection: Tunnel forward extension "tun@openssh.com" + +OpenSSH supports layer 2 and layer 3 tunnelling via the "tun@openssh.com" +channel type. This channel type supports forwarding of network packets +with datagram boundaries intact between endpoints equipped with +interfaces like the BSD tun(4) device. Tunnel forwarding channels are +requested by the client with the following packet: + + byte SSH_MSG_CHANNEL_OPEN + string "tun@openssh.com" + uint32 sender channel + uint32 initial window size + uint32 maximum packet size + uint32 tunnel mode + uint32 remote unit number + +The "tunnel mode" parameter specifies whether the tunnel should forward +layer 2 frames or layer 3 packets. It may take one of the following values: + + SSH_TUNMODE_POINTOPOINT 1 /* layer 3 packets */ + SSH_TUNMODE_ETHERNET 2 /* layer 2 frames */ + +The "tunnel unit number" specifies the remote interface number, or may +be 0x7fffffff to allow the server to automatically chose an interface. A +server that is not willing to open a client-specified unit should refuse +the request with a SSH_MSG_CHANNEL_OPEN_FAILURE error. On successful +open, the server should reply with SSH_MSG_CHANNEL_OPEN_SUCCESS. + +Once established the client and server may exchange packet or frames +over the tunnel channel by encapsulating them in SSH protocol strings +and sending them as channel data. This ensures that packet boundaries +are kept intact. Specifically, packets are transmitted using normal +SSH_MSG_CHANNEL_DATA packets: + + byte SSH_MSG_CHANNEL_DATA + uint32 recipient channel + string data + +The contents of the "data" field for layer 3 packets is: + + uint32 packet length + uint32 address family + byte[packet length - 4] packet data + +The "address family" field identifies the type of packet in the message. +It may be one of: + + SSH_TUN_AF_INET 2 /* IPv4 */ + SSH_TUN_AF_INET6 24 /* IPv6 */ + +The "packet data" field consists of the IPv4/IPv6 datagram itself +without any link layer header. + +The contents of the "data" field for layer 2 packets is: + + uint32 packet length + byte[packet length] frame + +The "frame" field contains an IEEE 802.3 Ethernet frame, including +header. + +2.4. connection: Unix domain socket forwarding + +OpenSSH supports local and remote Unix domain socket forwarding +using the "streamlocal" extension. Forwarding is initiated as per +TCP sockets but with a single path instead of a host and port. + +Similar to direct-tcpip, direct-streamlocal is sent by the client +to request that the server make a connection to a Unix domain socket. + + byte SSH_MSG_CHANNEL_OPEN + string "direct-streamlocal@openssh.com" + uint32 sender channel + uint32 initial window size + uint32 maximum packet size + string socket path + string reserved + uint32 reserved + +Similar to forwarded-tcpip, forwarded-streamlocal is sent by the +server when the client has previously send the server a streamlocal-forward +GLOBAL_REQUEST. + + byte SSH_MSG_CHANNEL_OPEN + string "forwarded-streamlocal@openssh.com" + uint32 sender channel + uint32 initial window size + uint32 maximum packet size + string socket path + string reserved for future use + +The reserved field is not currently defined and is ignored on the +remote end. It is intended to be used in the future to pass +information about the socket file, such as ownership and mode. +The client currently sends the empty string for this field. + +Similar to tcpip-forward, streamlocal-forward is sent by the client +to request remote forwarding of a Unix domain socket. + + byte SSH2_MSG_GLOBAL_REQUEST + string "streamlocal-forward@openssh.com" + boolean TRUE + string socket path + +Similar to cancel-tcpip-forward, cancel-streamlocal-forward is sent +by the client cancel the forwarding of a Unix domain socket. + + byte SSH2_MSG_GLOBAL_REQUEST + string "cancel-streamlocal-forward@openssh.com" + boolean FALSE + string socket path + +2.5. connection: hostkey update and rotation "hostkeys-00@openssh.com" +and "hostkeys-prove-00@openssh.com" + +OpenSSH supports a protocol extension allowing a server to inform +a client of all its protocol v.2 host keys after user-authentication +has completed. + + byte SSH_MSG_GLOBAL_REQUEST + string "hostkeys-00@openssh.com" + string[] hostkeys + +Upon receiving this message, a client should check which of the +supplied host keys are present in known_hosts. + +Note that the server may send key types that the client does not +support. The client should disgregard such keys if they are received. + +If the client identifies any keys that are not present for the host, +it should send a "hostkeys-prove@openssh.com" message to request the +server prove ownership of the private half of the key. + + byte SSH_MSG_GLOBAL_REQUEST + string "hostkeys-prove-00@openssh.com" + char 1 /* want-reply */ + string[] hostkeys + +When a server receives this message, it should generate a signature +using each requested key over the following: + + string "hostkeys-prove-00@openssh.com" + string session identifier + string hostkey + +These signatures should be included in the reply, in the order matching +the hostkeys in the request: + + byte SSH_MSG_REQUEST_SUCCESS + string[] signatures + +When the client receives this reply (and not a failure), it should +validate the signatures and may update its known_hosts file, adding keys +that it has not seen before and deleting keys for the server host that +are no longer offered. + +These extensions let a client learn key types that it had not previously +encountered, thereby allowing it to potentially upgrade from weaker +key algorithms to better ones. It also supports graceful key rotation: +a server may offer multiple keys of the same type for a period (to +give clients an opportunity to learn them using this extension) before +removing the deprecated key from those offered. + +2.6. connection: SIGINFO support for "signal" channel request + +The SSH channels protocol (RFC4254 section 6.9) supports sending a +signal to a session attached to a channel. OpenSSH supports one +extension signal "INFO@openssh.com" that allows sending SIGINFO on +BSD-derived systems. + +3. SFTP protocol changes + +3.1. sftp: Reversal of arguments to SSH_FXP_SYMLINK + +When OpenSSH's sftp-server was implemented, the order of the arguments +to the SSH_FXP_SYMLINK method was inadvertently reversed. Unfortunately, +the reversal was not noticed until the server was widely deployed. Since +fixing this to follow the specification would cause incompatibility, the +current order was retained. For correct operation, clients should send +SSH_FXP_SYMLINK as follows: + + uint32 id + string targetpath + string linkpath + +3.2. sftp: Server extension announcement in SSH_FXP_VERSION + +OpenSSH's sftp-server lists the extensions it supports using the +standard extension announcement mechanism in the SSH_FXP_VERSION server +hello packet: + + uint32 3 /* protocol version */ + string ext1-name + string ext1-version + string ext2-name + string ext2-version + ... + string extN-name + string extN-version + +Each extension reports its integer version number as an ASCII encoded +string, e.g. "1". The version will be incremented if the extension is +ever changed in an incompatible way. The server MAY advertise the same +extension with multiple versions (though this is unlikely). Clients MUST +check the version number before attempting to use the extension. + +3.3. sftp: Extension request "posix-rename@openssh.com" + +This operation provides a rename operation with POSIX semantics, which +are different to those provided by the standard SSH_FXP_RENAME in +draft-ietf-secsh-filexfer-02.txt. This request is implemented as a +SSH_FXP_EXTENDED request with the following format: + + uint32 id + string "posix-rename@openssh.com" + string oldpath + string newpath + +On receiving this request the server will perform the POSIX operation +rename(oldpath, newpath) and will respond with a SSH_FXP_STATUS message. +This extension is advertised in the SSH_FXP_VERSION hello with version +"1". + +3.4. sftp: Extension requests "statvfs@openssh.com" and + "fstatvfs@openssh.com" + +These requests correspond to the statvfs and fstatvfs POSIX system +interfaces. The "statvfs@openssh.com" request operates on an explicit +pathname, and is formatted as follows: + + uint32 id + string "statvfs@openssh.com" + string path + +The "fstatvfs@openssh.com" operates on an open file handle: + + uint32 id + string "fstatvfs@openssh.com" + string handle + +These requests return a SSH_FXP_STATUS reply on failure. On success they +return the following SSH_FXP_EXTENDED_REPLY reply: + + uint32 id + uint64 f_bsize /* file system block size */ + uint64 f_frsize /* fundamental fs block size */ + uint64 f_blocks /* number of blocks (unit f_frsize) */ + uint64 f_bfree /* free blocks in file system */ + uint64 f_bavail /* free blocks for non-root */ + uint64 f_files /* total file inodes */ + uint64 f_ffree /* free file inodes */ + uint64 f_favail /* free file inodes for to non-root */ + uint64 f_fsid /* file system id */ + uint64 f_flag /* bit mask of f_flag values */ + uint64 f_namemax /* maximum filename length */ + +The values of the f_flag bitmask are as follows: + + #define SSH_FXE_STATVFS_ST_RDONLY 0x1 /* read-only */ + #define SSH_FXE_STATVFS_ST_NOSUID 0x2 /* no setuid */ + +Both the "statvfs@openssh.com" and "fstatvfs@openssh.com" extensions are +advertised in the SSH_FXP_VERSION hello with version "2". + +3.5. sftp: Extension request "hardlink@openssh.com" + +This request is for creating a hard link to a regular file. This +request is implemented as a SSH_FXP_EXTENDED request with the +following format: + + uint32 id + string "hardlink@openssh.com" + string oldpath + string newpath + +On receiving this request the server will perform the operation +link(oldpath, newpath) and will respond with a SSH_FXP_STATUS message. +This extension is advertised in the SSH_FXP_VERSION hello with version +"1". + +3.6. sftp: Extension request "fsync@openssh.com" + +This request asks the server to call fsync(2) on an open file handle. + + uint32 id + string "fsync@openssh.com" + string handle + +One receiving this request, a server will call fsync(handle_fd) and will +respond with a SSH_FXP_STATUS message. + +This extension is advertised in the SSH_FXP_VERSION hello with version +"1". + +4. Miscellaneous changes + +4.1 Public key format + +OpenSSH public keys, as generated by ssh-keygen(1) and appearing in +authorized_keys files, are formatted as a single line of text consisting +of the public key algorithm name followed by a base64-encoded key blob. +The public key blob (before base64 encoding) is the same format used for +the encoding of public keys sent on the wire: as described in RFC4253 +section 6.6 for RSA and DSA keys, RFC5656 section 3.1 for ECDSA keys +and the "New public key formats" section of PROTOCOL.certkeys for the +OpenSSH certificate formats. + +4.2 Private key format + +OpenSSH private keys, as generated by ssh-keygen(1) use the format +described in PROTOCOL.key by default. As a legacy option, PEM format +(RFC7468) private keys are also supported for RSA, DSA and ECDSA keys +and were the default format before OpenSSH 7.8. + +4.3 KRL format + +OpenSSH supports a compact format for Key Revocation Lists (KRLs). This +format is described in the PROTOCOL.krl file. + +4.4 Connection multiplexing + +OpenSSH's connection multiplexing uses messages as described in +PROTOCOL.mux over a Unix domain socket for communications between a +master instance and later clients. + +$OpenBSD: PROTOCOL,v 1.36 2018/10/02 12:51:58 djm Exp $ |