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11. Additional Messages

11. Additional Messages​

Either party may send any of the following messages at any time.

11.1. Disconnection Message​

      byte      SSH_MSG_DISCONNECT
      uint32    reason code
      string    description in ISO-10646 UTF-8 encoding [RFC3629]
      string    language tag [RFC3066]

This message causes immediate termination of the connection. All implementations MUST be able to process this message; they SHOULD be able to send this message.

The sender MUST NOT send or receive any data after this message, and the recipient MUST NOT accept any data after receiving this message. The Disconnection Message 'description' string gives a more specific explanation in a human-readable form. The Disconnection Message 'reason code' gives the reason in a more machine-readable format (suitable for localization), and can have the values as displayed in the table below. Note that the decimal representation is displayed in this table for readability, but the values are actually uint32 values.

           Symbolic name                                reason code
           -------------                                -----------
      SSH_DISCONNECT_HOST_NOT_ALLOWED_TO_CONNECT             1
      SSH_DISCONNECT_PROTOCOL_ERROR                          2
      SSH_DISCONNECT_KEY_EXCHANGE_FAILED                     3
      SSH_DISCONNECT_RESERVED                                4
      SSH_DISCONNECT_MAC_ERROR                               5
      SSH_DISCONNECT_COMPRESSION_ERROR                       6
      SSH_DISCONNECT_SERVICE_NOT_AVAILABLE                   7
      SSH_DISCONNECT_PROTOCOL_VERSION_NOT_SUPPORTED          8
      SSH_DISCONNECT_HOST_KEY_NOT_VERIFIABLE                 9
      SSH_DISCONNECT_CONNECTION_LOST                        10
      SSH_DISCONNECT_BY_APPLICATION                         11
      SSH_DISCONNECT_TOO_MANY_CONNECTIONS                   12
      SSH_DISCONNECT_AUTH_CANCELLED_BY_USER                 13
      SSH_DISCONNECT_NO_MORE_AUTH_METHODS_AVAILABLE         14
      SSH_DISCONNECT_ILLEGAL_USER_NAME                      15

If the 'description' string is displayed, the control character filtering discussed in [SSH-ARCH] should be used to avoid attacks by sending terminal control characters.

Requests for assignments of new Disconnection Message 'reason code' values (and associated 'description' text) in the range of 0x00000010 to 0xFDFFFFFF MUST be done through the IETF CONSENSUS method, as described in [RFC2434]. The Disconnection Message 'reason code' values in the range of 0xFE000000 through 0xFFFFFFFF are reserved for PRIVATE USE. As noted, the actual instructions to the IANA are in [SSH-NUMBERS].

11.2. Ignored Data Message​

      byte      SSH_MSG_IGNORE
      string    data

All implementations MUST understand (and ignore) this message at any time (after receiving the identification string). No implementation is required to send them. This message can be used as an additional protection measure against advanced traffic analysis techniques.

11.3. Debug Message​

      byte      SSH_MSG_DEBUG
      boolean   always_display
      string    message in ISO-10646 UTF-8 encoding [RFC3629]
      string    language tag [RFC3066]

All implementations MUST understand this message, but they are allowed to ignore it. This message is used to transmit information that may help debugging. If 'always_display' is TRUE, the message SHOULD be displayed. Otherwise, it SHOULD NOT be displayed unless debugging information has been explicitly requested by the user.

The 'message' doesn't need to contain a newline. It is, however, allowed to consist of multiple lines separated by CRLF (Carriage Return - Line Feed) pairs.

If the 'message' string is displayed, the terminal control character filtering discussed in [SSH-ARCH] should be used to avoid attacks by sending terminal control characters.

11.4. Reserved Messages​

An implementation MUST respond to all unrecognized messages with an SSH_MSG_UNIMPLEMENTED message in the order in which the messages were received. Such messages MUST be otherwise ignored. Later protocol versions may define other meanings for these message types.

      byte      SSH_MSG_UNIMPLEMENTED
      uint32    packet sequence number of rejected message
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