tomaswolf commented on issue #709:
URL: https://github.com/apache/mina-sshd/issues/709#issuecomment-2753610187

   Yes, OpenSSH 4.5p1 only handles SSH_MSG_CHANNEL_ERROR for keep-alive 
replies. See 
https://github.com/openssh/openssh-portable/blob/360357bee571a41dd4635332d486751d8120d3e2/serverloop.c#L1181C15-L1181C39.
   
   More modern OpenSSH also handles SSH_MSG_CHANNEL_SUCCESS; see 
https://github.com/openssh/openssh-portable/blob/6c49e5f7dcaf886b4a702a6c003cae9dca04d3ea/serverloop.c#L891-L894.
 Looks like it does so since 5.1p1 (commit 
https://github.com/openssh/openssh-portable/commit/b84886b).
   
   I remember having seen that it handles both success and error when I did 
commit fa1dd24d in #492, but I missed that older versions (from some 17 years 
ago!) would only handle the error reply.
   
   So for compatibility with such old OpenSSH we should sent back a 
SSH_MSG_CHANNEL_ERROR, at least if it is "keepalive@<!-- -->openssh.com". Don't 
know what other servers expect on their keepalive requests.
   
   As a work-around, you could register a `RequestHandler` via 
`AbstractChannel.addRequestHandler()` that handles this channel request and 
returns `RequestHandler.Result.ReplyFailure` for this channel message.


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