[feature] respond to server check alive requests

[abdeldjalil-fellah]

when the ssh server is configured to check clients alive status with:
ClientAliveCountMax 2
ClientAliveInterval 5

the client should reply to the server requests to prevent the server from terminating the session but it does not as you can see in this server log:

 Jul 23 11:24:09 localhost sshd[5270]: debug3: send packet: type 81 [SSH_MSG_REQUEST_SUCCESS]
 Jul 23 11:24:09 localhost sshd[5270]: debug3: send packet: type 80 [SSH_MSG_GLOBAL_REQUEST]
 Jul 23 11:24:14 localhost sshd[5270]: debug3: send packet: type 80 [SSH_MSG_GLOBAL_REQUEST]
 Jul 23 11:24:19 localhost sshd[5270]: Timeout, client not responding from user user1 192.168.43.253 port 51236

with putty the server logs:

 Jul 23 11:44:20 localhost sshd[5364]: debug3: send packet: type 99 [SSH_MSG_CHANNEL_SUCCESS]
 Jul 23 11:44:20 localhost sshd[5360]: debug3: mm_request_receive entering
 Jul 23 11:44:20 localhost sshd[5360]: debug3: monitor_read: checking request 124
 Jul 23 11:44:25 localhost sshd[5364]: debug2: channel 2: request keepalive@openssh.com confirm 1
 Jul 23 11:44:25 localhost sshd[5364]: debug3: send packet: type 98 [SSH_MSG_CHANNEL_REQUEST]
 Jul 23 11:44:25 localhost sshd[5364]: debug3: receive packet: type 100 [SSH_MSG_CHANNEL_FAILURE]
 Jul 23 11:44:25 localhost sshd[5364]: debug1: Got 100/12 for keepalive
 Jul 23 11:44:30 localhost sshd[5364]: debug2: channel 2: request keepalive@openssh.com confirm 1
 Jul 23 11:44:30 localhost sshd[5364]: debug3: send packet: type 98 [SSH_MSG_CHANNEL_REQUEST]
 Jul 23 11:44:30 localhost sshd[5364]: debug3: receive packet: type 100 [SSH_MSG_CHANNEL_FAILURE]
 Jul 23 11:44:30 localhost sshd[5364]: debug1: Got 100/13 for keepalive

PS: does the client respond to tcp_keepalive messages (TCPKeepAlive yes)?

[JustinGrote]

I had the same issue trying to use with SISH which requests server keepalives. Both Putty and Openssh respond to the keepalives appropriately.

[JustinGrote]

@jalil1408 I am a C# noob but I managed to cobble together something that works. Attached is a patched zip of netstandard2.0 binaries.
Renci.SshNet-netstandard20-ServerKeepAliveResponse.zip

[kikaragyozov]

@daviburg Could we finally get official support for this via a simple SSH_MSG_REQUEST_FAILURE response?

We are not supporting an official definition within the SSH Protocol (rfc4254) as specified in section 4, whilst we support and do SSH_MSG_IGNORE to attempt to keep the connection alive as specified in RFC-4253. I believe both specifications go hand-in-hand. You can't implement one without the other, as both explain different parts of the entire picture.

[kikaragyozov]

I'm not sure if I tagged the correct person, @Rob-Hague perhaps?

[Rob-Hague]

@kikaragyozov you seem to understand the problem well. Do you want to raise a PR?

[JustinGrote]

FYI my PR is a working implementation, just needs tests and whatnot, so you could use that as a base.
#711