solstag
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solstag

Why sshd won't talk on port 1723 but will on any other?

October 23, 2016 109 views
Networking Debian

Ni!

I've correctly configured sshd to listen on port 1723, that is:

  1. I have no other process listening on port 1723 (according to netstat)
  2. Changing the number to any other available port works fine

Apparently, see logs below, it gets to the point that a connection (a tcp connection) is established, but doesn't get through on the ssh level.

I need it to listen on port 1723 because, on a network I use often, it is the only allowed port besides 80 and the server already listens on 80 for web connections. I have never configured a VPN (port 1723 is usually a VPN port) on this server.

Any ideas? Any chance this is a problem from DigitalOcean's side? Something within sshd?

This is what I get from the outputs/logs after the connection times out:

Client side:

$ ssh -vvv
Connection established.
...
ssh_exchange_identification: read: Connection reset by peer

Server side:

Log:
sshd[xxx]: Did not receive identification string from x.x.x.x

Netstat:
tcp        0     39 x.x.x.x:1723    y.y.y.y:44983    ESTABLISHED 23401/sshd: [accepted]

Thanks!

1 comment
  • Did you tried it from any other connection if you have possibility? Maybe your ISP blocks port 1723 so you can't use to connect to SSH via that port. Check also if you use firewall on client machine that it doesn't block that port.

1 Answer

Hi @xMudrii , thanks, I had tried under two different and quite distinct networks - one wifi another cellphone - and got the same result, so I assumed it was not a network problem, but since you've mentioned it again I've now I've tried under a third network and it does work. So it is indeed the client networks blocking, not an issue with my server at all. Thanks.

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