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Q: My SSH connection is sometimes slow. It can take up to half a minute to connect to a remote host. What is the problem?
A: The SSH Daemon application uses DNS to do a reverse lookup of the host connecting to it. There can be several issues which make this reverse lookup query unsuccesfull. E.g. There might be a broken DNS on the internet which does not reply, the connection might be overloaded, causing dropped traffic etc.
You can do a couple of things to get rid of this: 1) You can try and add your source host to the /etc/hosts file. This will give sshd the reverse lookup it wants and speedup connection time. 2) You can try and run a local resolver (Which only does ‘recursive’ queries for your host and/or network). Using the “UseDNS no“ flag within OpenSSH will not actually speedup the process! If that all fails, there might be some more trouble then we currently expect. Try running some queries on the resolver defined in /etc/resolv.conf and see whether they get through, is the query answered quickly? etc. Feel free to post a message to a help-mailinglist in case this all does not help.