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Accepted
Mitch
Asked: 2010-05-13 09:37:18 +0800 CST

Restart SSH on a machine where SSH is the only mode of access

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I've made some changes to sshd_config file and therefore need to restart. I'm looking tips on safely restarting ssh when getting physical access to the server would be a huga PITA.

linux ssh

7 Answers

  1. Best Answer

    Restarting sshd while logged in via ssh will not disconnect your ssh connection.

    If you're worried about your configuration, log in a few times via ssh, and restart. If you can no longer ssh in, with new connections, you now have access to fix the problems.

    Mentioned below in a comment by @Milan Babuškov: sshd -t will test your configuration for syntax correctness, if you really want to be certain.

    Another suggestion, by @Ronald Pottol was to set up a cron task to restart the server with a known working configuration. Perhaps overkill, but if you're updating a mission critical server, etc... sometimes you can never be too careful.

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  2. If you have access to the hardware you may consider putting a terminal on the serial port /dev/ttyS0 . Then you can have a back door into your server.

    simply add 

    SO:2345:respawn:/sbin/mingetty ttySO

    to your /etc/inittab and a terminal will spawn over your serial port. You can use a serial port concentrator or use a null modem from the server next to it.

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  3. Don't worry, your current session won't be disconnected, even if there's a problem with the new configuration.

    After applying the new configuration and restarting sshd, just try to login a couple of times and take a look at the logs to see if everything is ok.

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  4. Or, use a cron or at job to start it back up, if you are feeling unlucky?

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  5. Could you not Just run a kill -HUP in the PID of the SSH service? Its not clean but it works

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  6. I have found that nowadays sshd does not disconnect your sessions when restarting, especially when it is a Redhat based distro. You could always write a small script that will automatically restore your sshd config from backup and restart sshd after 5 min as a cron or at job. This will ensure that even if you get disconnected, you can get back into your server at least.

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  7. I wouldn't recommend restarting/reloading SSHD on a sshd connection. I've seen many occasions where sshd just wouldn't start back up because of a syntax error in sshd_config.

    Even though everything is ok with the config file, it's risky.

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