4.2. Protocol Version Exchange

4.2. Protocol Version Exchange

When the connection has been established, both sides MUST send an identification string. This identification string MUST be:

SSH-protoversion-softwareversion SP comments CR LF

Since the protocol being defined in this set of documents is version 2.0, the protoversion MUST be "2.0". The comments string is OPTIONAL. If the comments string is included, a space character (denoted above as SP, ASCII 32) MUST separate the softwareversion and comments strings. The identification MUST be terminated by a single Carriage Return (CR) and a single Line Feed (LF) character (ASCII 13 and 10, respectively). Implementers who wish to maintain compatibility with older, undocumented versions of this protocol may want to process the identification string without expecting the presence of the carriage return character for reasons described in Section 5 of this document. The null character MUST NOT be sent. The maximum length of the string is 255 characters, including the Carriage Return and Line Feed.

The part of the identification string preceding the Carriage Return and Line Feed is used in the Diffie-Hellman key exchange (see Section 8).

The server MAY send other lines of data before sending the version string. Each line SHOULD be terminated by a Carriage Return and Line Feed. Such lines MUST NOT begin with "SSH-", and SHOULD be encoded in ISO-10646 UTF-8 [RFC3629] (language is not specified). Clients MUST be able to process such lines. Such lines MAY be silently ignored, or MAY be displayed to the client user. If they are displayed, control character filtering, as discussed in [SSH-ARCH], SHOULD be used. The primary use of this feature is to allow TCP-wrappers to display an error message before disconnecting.

Both the protoversion and softwareversion strings MUST consist of printable US-ASCII characters, with the exception of whitespace characters and the minus sign (-). The softwareversion string is primarily used to trigger compatibility extensions and to indicate the capabilities of an implementation. The comments string SHOULD contain additional information that might be useful in solving user problems. As such, an example of a valid identification string is:

SSH-2.0-billsSSH_3.6.3q3<CR><LF>

This identification string does not contain the optional comments string and is thus terminated by a CR and LF immediately after the softwareversion string.

Key exchange will begin immediately after sending this identifier. All packets following the identification string SHALL use the binary packet protocol, which is described in Section 6.