Fix slow connections to Ubuntu SSH servers

As of Ubuntu 8.10 and older, connections to SSH servers with default configs can be annoyingly slow. Thisterminal is because the OpenSSH daemon uses DNS to run a reverse lookup on the client to be sure that they aren’t faking their hostname. You can disable most look-ups by implementing the following setting:

As root edit “/etc/ssh/sshd_config” and add the line “UseDNS no“.

This can also be done by running the following command:

sudo echo "UseDNS no" >> /etc/ssh/sshd_config

Sources:

This issue is documented at OpenSSH.org

Fix and more details available at: http://tech.waltco.biz/2008/02/02/ssh-slow-to-connect-in-ubuntu-710-gutsy-gibbon/

11 thoughts on “Fix slow connections to Ubuntu SSH servers”

  1. On Ubuntu 10.04 (Lucid Lynx), I had a similar problem. I found out that the /etc/ssh/ssh_config (config file for the ssh client) was overriding the defaults with

    GSSAPIAuthentication yes
    GSSAPIDelegateCredentials no

    I commented out these two lines. Now ssh is not waiting for GSS anymore and the login speed is back to normal.

  2. Your sudo command doesn’t work, because the file redirect will be run as the regular user (outside of sudo).

    Here’s what I normally do:

    echo “UseDNS no” | sudo tee -a /etc/ssh/sshd_config

  3. The sudo used with redircet doesn’t work. Sudo does NOT traverse the redirection, so the only part of the commen you gave which has sudo powers is the echo command (not useful). The command that Dave gave is the correct way to do it. I suggest you ammend the guide. The ONLY way your command will work is if it is ran as root, but then you won’t need sudo at all.

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