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Wile doing research I noticed that SSH both uses TCP and UDP. I completely understand the use of TCP, but UDP seems a bit strange. Why would I use an "unreliable" transport protocol with minimal handshaking for secure shell access?
The only use I can think of is for SCP, so (big) file transmission. But again, wouldn't TCP be more useful because of the handshaking stuff?
In fact, I am considering only opening TCP for SSH, but not knowing the implications of that would be quite detrimental.
EDIT
Turns out that there's not only a thesis about ssh over UDP, but also a fully fledged ssh-implementation, called mosh. However, the classic ssh still only uses TCP, as specified in its RFCs.
alex
Posted 2014-04-16T08:09:12.497
Reputation: 649
2Why do you think SSH uses UDP? I tried it with several SSH connections from Windows to Unix and Unix to Unix and I don't get a single UDP packet on port 22. Also the server only listens on TCP. – mtak – 2014-04-16T08:54:19.673
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Well, Wikipedia tells as much https://en.wikipedia.org/wiki/List_of_TCP_and_UDP_port_numbers Also, users on the web seem to tunnel some things through a ssh-udp combo, but it just wasn't clear why they'd use UDP and not TCP. Hence my question.
– alex – 2014-04-16T08:57:42.270In this thesis they tried to implement UDP for SSH transport, but they also mention that the default only uses TCP: ". OpenSSH is using TCP consistently for all its network connections and thus for its VPN feature.". The Wikipedia page might say it's UDP because the SSH developers made an initial request for UDP & TCP assignments, but the related RFC makes no mention of it. But I have to agree, an interesting question.
– mtak – 2014-04-16T09:10:45.090Huh. Didn't expect that. Well, write that up as an answer, you deserve some points. – alex – 2014-04-16T09:34:31.530