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Short SSH host aliases

Typing ssh jnovak@phoebe.fzu.cz gets tedious, and it is easy to forget the username part. Without it, SSH logs in with your local username, the one you have on your laptop. If that differs from your account on the cluster (for example jan on the laptop and jnovak on Phoebe), the login fails with Permission denied even though your key is fine.

An entry in ~/.ssh/config on your laptop or workstation fixes both problems: it gives the host a short name (an alias) and remembers which username to use there.

Set up the aliases

Create or edit ~/.ssh/config and add one block per host. Replace <fzu-username> with your FZU username and <meta-login> with your MetaCentrum login, which may be different:

# Phoebe
Host phoebe
    HostName phoebe.fzu.cz
    User <fzu-username>

# Koios
Host koios
    HostName koios1.fzu.cz
    User <fzu-username>

# MetaCentrum frontend at FZU
Host metafzu
    HostName metafzu.fzu.cz
    User <meta-login>

What the keywords mean:

Keyword Meaning
Host the alias, the name you type after ssh; it can be anything you like
HostName the real name (or IP address) of the server
User the username on that server, used instead of your local one

SSH refuses a config file that other users can write to, so keep it private:

chmod 600 ~/.ssh/config

Now these commands are enough:

ssh phoebe
ssh koios
ssh metafzu

Windows

The OpenSSH client built into Windows 10 and 11 reads the same file, at C:\Users\<you>\.ssh\config. MobaXterm keeps its own session profiles instead; see SSH.

The alias works everywhere SSH does

Every tool built on SSH reads ~/.ssh/config, so the alias also works for copying files and for other programs:

scp results.tar.gz phoebe:                 # copy to your Phoebe home
scp koios:job/output.log .                 # copy from Koios
rsync -av data/ metafzu:project/data/      # sync a directory to MetaCentrum
sshfs phoebe: ~/phoebe_mount               # mount your Phoebe home, see "sshfs"

VS Code's Remote-SSH extension also lists the aliases from this file. Mounting with sshfs is described in Mount storage over SSH.

Check what SSH will do

ssh -G prints the settings SSH would use for a name, without connecting. Use it to confirm that the alias picks up the right host and user:

ssh -G koios | grep -E '^(hostname|user) '

hostname koios1.fzu.cz
user jnovak

Good to know

  • The alias matches only what you type. ssh phoebe uses the block above, but ssh phoebe.fzu.cz does not, and logs in with your local username again. If you want both names to work, list them both: Host phoebe phoebe.fzu.cz.
  • The first match wins. For each keyword, SSH uses the first value it finds. Put specific Host blocks at the top and general defaults in a Host * block at the end of the file.
  • A non-default key. If your cluster key is not one of the default files (such as ~/.ssh/id_ed25519), add IdentityFile ~/.ssh/<key-file> to the block. See SSH key.
  • MetaCentrum without passwords. To log in to metafzu.fzu.cz with a Kerberos ticket and have qsub/qstat work there, the alias needs a few more lines; see Passwordless SSH to MetaCentrum with Kerberos.
  • Keep idle connections open. If your connection drops after a while of inactivity, add ServerAliveInterval 60 to the block (or to Host *), so SSH sends a keep-alive message every minute.

The full list of keywords is in man ssh_config, also available online.