4.27.12 ‘Replies to requests on closed channels’

The SSH protocol as published in RFC 4254 has an ambiguity which arises if one side of a connection tries to close a channel, while the other side simultaneously sends a request within the channel and asks for a reply. RFC 4254 leaves it unclear whether the closing side should reply to the channel request after having announced its intention to close the channel.

Discussion on the ietf-ssh mailing list in April 2014 formed a clear consensus that the right answer is no. However, because of the ambiguity in the specification, some SSH servers have implemented the other policy; for example, OpenSSH used to until it was fixed.

Because PuTTY sends channel requests with the ‘want reply’ flag throughout channels' lifetime (see section 4.27.5), it's possible that when connecting to such a server it might receive a reply to a request after it thinks the channel has entirely closed, and terminate with an error along the lines of ‘Received SSH2_MSG_CHANNEL_FAILURE for nonexistent channel 256’.