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SSH session slow to start? It's the DNS stupid!

Ever tried logging into a machine with ssh and found you have to wait much longer than reasonable for the session to start? This happened to me a few times and was especially annoying with machines on my local network (or a VM attached to a virtual network) that should be letting me in immediately.

I eventually got mad enough to strace the SSH daemon and debug what was going on and it turns out it's a DNS thing. Basically the session is slow to start because the SSH server is trying to lookup the hostname of the SSH client and for whatever reason it's timing out (e.g., it can't reach a nameserver, because you happen to be offline)

There are a couple of very simple ways to fix that:

  • add "UseDNS no" to /etc/ssh/sshd_config
  • add the client's net address to the server's /etc/hosts

Was that helpful? Has this ever happened to you? Post a comment!


jgab's picture

Hi Liraz, thank you. I was having troubles to access through SFTP to my LAMP appliance installed on Virtualbox on my PC, and the problem was solved when I added "UseDNS no" to /etc/ssh/sshd_config.

(we recently configure a domain on our LAN with a Windows 2003 server)

Great! best regards,

Liraz Siri's picture

Occasionally I wonder whether anyone is finding these blog posts useful. Getting positive feedback is a big encouragement. Oh and sorry for the late reply, I've been monking out offline working on some new stuff!