Special commands

PuTTY

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3.1.3.2 Special commands

Depending on the protocol used for the current session, there may be a submenu of ‘special commands’. These are protocol-specific tokens, such as a ‘break’ signal, that can be sent down a connection in addition to normal data. Their precise effect is usually up to the server. Currently only Telnet, SSH, and serial connections have special commands.

The ‘break’ signal can also be invoked from the keyboard with Ctrl-Break.

The following special commands are available in Telnet:

  • Are You There
  • Break
  • Synch
  • Erase Character

    PuTTY can also be configured to send this when the Backspace key is pressed; see section 4.16.3.

  • Erase Line
  • Go Ahead
  • No Operation

    Should have no effect.

  • Abort Process
  • Abort Output
  • Interrupt Process

    PuTTY can also be configured to send this when Ctrl-C is typed; see section 4.16.3.

  • Suspend Process

    PuTTY can also be configured to send this when Ctrl-Z is typed; see section 4.16.3.

  • End Of Record
  • End Of File

In an SSH connection, the following special commands are available:

  • IGNORE message

    Should have no effect.

  • Repeat key exchange

    Only available in SSH-2. Forces a repeat key exchange immediately (and resets associated timers and counters). For more information about repeat key exchanges, see section 4.19.2.

  • Cache new host key type

    Only available in SSH-2. This submenu appears only if the server has host keys of a type that PuTTY doesn't already have cached, and so won't consider. Selecting a key here will allow PuTTY to use that key now and in future: PuTTY will do a fresh key-exchange with the selected key, and immediately add that key to its permanent cache (relying on the host key used at the start of the connection to cross-certify the new key). That key will be used for the rest of the current session; it may not actually be used for future sessions, depending on your preferences (see section 4.20.1).

    Normally, PuTTY will carry on using a host key it already knows, even if the server offers key formats that PuTTY would otherwise prefer, to avoid host key prompts. As a result, if you've been using a server for some years, you may still be using an older key than a new user would use, due to server upgrades in the meantime. The SSH protocol unfortunately does not have organised facilities for host key migration and rollover, but this allows you to manually upgrade.

  • Break

    Only available in SSH-2, and only during a session. Optional extension; may not be supported by server. PuTTY requests the server's default break length.

  • Signals (SIGINT, SIGTERM etc)

    Only available in SSH-2, and only during a session. Sends various POSIX signals. Not honoured by all servers.

With a serial connection, the only available special command is ‘Break’.

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