If you are a devops engineer or a developer, at some point you will need to jump onto a server to configure some software, access logs and generally just check out what is happening. If you are like me, you’ll be doing it a lot.

You will likely be using ssh to login to the servers or at the very least you will have enabled ssh key access for updating your source code and pushing it to Github or Bitbucket. If you haven’t, then you should to improve security and make life easier.

It can be difficult to remember all those ssh options. Well, if you aren’t making the ssh config file do the hard work for you, then you are doing it wrong. Here are some helpful tips. (I’m going to assume you have basic knowledge about ssh keys, OK?)

What is the role of the ssh config file?

In short, it is for supplying configuration to ssh. The ssh command sources configuration parameters from the following sources in this order:

  1. command-line options
  2. the user’s configuration file in ~/.ssh/config
  3. the system-wide configuration file in /etc/ssh/ssh_config
  4. default values (these are listed in the system-wide config commented out)

Desired behaviour can be configured to always occur or specific behaviour for a particular host. So for example rather than always providing an IdentityFile (-i) option for a particular host, you can instead add this to the config file and save yourself a ton of typing.

What does one look like?

Here’s an extract from one that I use to make my life easier:

# specific host settings

Host github.com
  PreferredAuthentications publickey
  IdentityFile ~/.ssh/github_rsa

Host bitbucket.org
  PreferredAuthentications publickey
  IdentityFile ~/.ssh/bitbucket_rsa

# private CoreOS stuff

Host core-01
Host core-02
Host core-03
Host core-??
  User core
  IdentityFile ~/.vagrant.d/insecure_private_key
  StrictHostKeyChecking no
  UserKnownHostsFile /dev/null

# general settings

IdentitiesOnly yes
ServerAliveInterval 60

Let’s look at how this works in practice.

Host name matching is the key

For each configuration parameter, the first obtained value is the one that will be used. That’s how command line options take precedence over configuration file data. The same principle operates in the config file.

Each Host section defines options that will take effect for any host that matches the name referred to in the ssh command i.e.

ssh user@host <options>

Patterns can include *, ? and can be negated using !. This matching starts at the top of the file and therefore config should be entered with host specific values at the start and general values towards the end.

For example, when I enter ssh core-01 to log onto a vm running CoreOS, it is equivalent to me typing:

ssh core@ -i ~/.vagrant.d/insecure_private_key \
    -o StrictHostKeyChecking=no \
    -o UserKnownHostsFile=/dev/null \
    -o IdentitiesOnly=yes \
    -o ServerAliveInterval=60

Use a different ssh key-pair per host

There are a lot of discussions like this one on StackEchange regarding whether you should have separate private-public key pairs for each server or one pair to rule them all.

One argument against using multiple keys is that the more additional security you add, the more convenience you give up. With the ssh config approach, it is no less convenient to use a per-host keypair than one-pair for all hosts, apart from the act of generating the keys and adding them to the config.

I use a different pair per host. Should there be some form of compromise or some other reason that a given key-pair must be changed, there is minimal effort to effect that for one host without any need to alter my security with any other host or site.

Configuring each key in the ssh config makes this dead simple.

Prevent ‘known_hosts’ problems when using virtual machines

If you are in the habit of spinning up and tearing down vms on a regular basis, you will be aware of the problem whereby the ssh host key check fails and prevents you from accessing the vm.

ssh maintains a list of all hosts that have been accessed in ~/.ssh/known_hosts. Whenever a host is accessed, it’s previous entry is checked and if different, access will be prevented to avoid man-in-the-middle attacks. When vms are constantly being rebuilt, this behaviour is very frustrating and requires you to remove the offending key manually from known_hosts.

To avoid this, I use two settings:

  • UserKnownHostsFile=/dev/null

    Throw away the host key instead of adding it to the known_hosts file. It isn’t possible to suppress ssh trying to save the key but this has the same effect by disposing of it in /dev/null.

  • StrictHostKeyChecking=no

    Prevent ssh from pausing to ask if the key should be saved the first time a new host is accessed.

In combination, these settings prevent interruptions when using ssh to access vms that change frequently. It goes without saying that these settings are not recommended for public or long-lived servers as they protect you from potentially harmful attempts to compromise your security.

Other convenience settings

I’ll leave it up to the reader to explore the other settings in detail, but in short, here are the ones that I use to make life easier:

  • PreferredAuthentications=publickey

    Don’t fall back to other methods to access a server, just fail if public key access fails. If there is a networking problem or the public key has been deleted from authorized_keys on the host, then default behaviour is to fall back to asking for a userid / password. Since we know it is using an ssh key, don’t bother doing this, just fail.

  • IdentitiesOnly=yes

    Only use the identity files configured in the ssh config. This can avoid conflicts when there are other identities loaded via ssh-add. And there is no need to ssh-add any keys configured in the config file.

  • ServerAliveInterval=60

    This sets the frequency in seconds for ssh to send KeepAlive messages to the host. If no response is received to these messages, the connection will be closed. In effect, this prevents ssh sessions with little activity from being timed out and closed.

I hope this is as helpful for you as it has been for me. If you have any favourite tricks or settings, leave a comment below.