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exeral
Asked: 2018-08-02 23:53:54 +0800 CST

ssh config per dns domains

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I have two platforms with many servers I want to connect to via ssh.

I have in resolv.conf

search domain1.net domain2.org

So I only have the machine name to type for ssh.

I want to have an ssh config per domain:

  • user admin for every machine of domain1.net
  • user root for every machine of domain2.org

How can I achieve that ?

Host, Hostname directives seems to only works with what you type in the ssh command, not what is expanded with DNS suffixes

linux

4 Answers

  1. Best Answer

    I believe OpenSSH CAN do canonicalization. Please have a look at the following options in ssh_config man page:

    • CanonicalizeHostname
    • CanonicalizeFallbackLocal
    • CanonicalizeMaxDots
    • CanonicalizePermittedCNAMEs
    • Host
    • Match with canonical option

    Please note, this may require newer OpenSSH version than the one you have (I have 7.4p1).

    • 1

  2. As kasperd said, this is not really possible.

    Some workarounds:

    • I just use some small shell variables to expand $e to example.org for things I don't access so often. Others are listed in ~/.ssh/config anyway with a short name. You use it like so: 

      ssh www.$e

      With this, you can use the standard ~/.ssh/config method.

    • Another option: Write a small wrapper script or shell function for ssh that catches the hostname and expands it, something like this: 

      #!/bin/bash
hn=`host $1 | cut -d ' ' -f 1`
ssh $hn

      If you call it with sshwrapper.sh www, it will expand www to www.example.org and use this for ssh.

    • 0

  3. backing what @sven said, a tiny bash function can do the same:

    ssh() {
    declare h=$(host $1 | head -n 1 | cut -d ' ' -f 1)
    command ssh $h
}

    Put this somewhere in one of your sourced files, e.g. .profile, and it's always fixing up the name from the search order in /etc/resolv.conf

    • 0

  4. another approach that could come handy to others utilizes (relatively) new ssh_config features that allow for include files and even the include can be conditional.

    I'm just going to share an obfuscated example from my config that tests that if

    • my phone is connected over USB and
    • I don't have another network connection (I don't have a default route)

    Then it will include another file that routes some ssh things thru my phone

    Match exec "[ -e /dev/serial/by-id/usb-SAMSUNG_SAMSUNG_Android_XXXXXXXX-if01 ] && ! ip route| grep -q ^default"
    Include blahblah_termux.ssh

    You should be able to have a ~/.ssh/domain1.ssh & ~/.ssh/domain2.ssh and include those based on a Match test (your test would obviously be customized)

    • 0