The 4 most important files for SSH connections

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You’re higher armed to utilize the SSH software with an understanding of 4 key SSH recordsdata.

Picture: Jack Wallen

If you’re a Linux administrator, the worth of Safe Shell. With out this software, remoting right into a Linux server (or sending recordsdata by way of a safe channel) would possibly pose fairly the problem. You then get SSH, and you should utilize it with out thought. However for these new to Linux administration, you may not fairly perceive how SSH works. Positive, you’ll be able to in all probability sit down at a machine and challenge the command:

ssh [email protected]

You will log in, and you are able to do your work. What goes on behind that connection? What are the items that make it work?

I believed it will be a good suggestion to interrupt SSH all the way down to the 4 most vital recordsdata for SSH connections, so you can begin utilizing the software with a greater understanding of the way it works.

Let’s check out these key SSH recordsdata.

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known_hosts

This can be a crucial file for SSH. The ~/.ssh/known_hosts file incorporates the SSH fingerprints of machines you’ve got logged into. These fingerprints are generated from the distant server’s SSH key. Whenever you safe shell right into a distant machine for the primary time, you might be requested if you wish to proceed connecting (Determine A).

Figure AFigure A

Determine A: To attach, or to not join. That’s the query.

Whenever you reply sure to this query, the distant host fingerprint is then saved to the known_hosts file. That key will seem as a random string of characters. Every entry will start with |1| (Determine B).

Figure BFigure B

Determine B: A identified hosts file.

What occurs with the connection is that this:

  1. You try to make the connection to the distant server, by way of the consumer.
  2. The distant server sends it is public key to the consumer.
  3. The SSH consumer searches for the important thing fingerprint in ~/.ssh/known_hosts.
  4. The SSH consumer hundreds and verifies the important thing.
  5. Consumer authentication takes place
  6. If authentication is profitable, you might be logged into the distant machine.

Once more, throughout that connection course of, if the distant fingerprint will not be discovered, the SSH consumer will ask if you wish to proceed, and (if you say sure) save the fingerprint to ~/.ssh/known_hosts.

authorized_keys

Inside the ~/.ssh listing, there’s one other file referred to as authorized_keys. That is very completely different than the known_hosts file. What authorized_keys homes are all SSH authentication keys that had been copied to the server, from distant purchasers. That is used for SSH Key Authentication (See: The right way to arrange ssh key authentication).

To make key authentication work, the general public key of the consumer is copied into the ~/.ssh/authorized_keys file on the distant server. The simplest approach to do this is by utilizing the ssh-copy-id command on the consumer like so:

ssh-copy-id [email protected]

You can be prompted for the distant consumer’s password. Upon profitable authentication, the general public key from the consumer is copied into the ~/.ssh/authorized_keys file on the distant server. In the event you open that file, you may see every entry begins with ssh-rsa and ends with the [email protected] of the consumer machine (Determine C).

Figure CFigure C

Determine C: A pattern authorized_keys file.

As soon as that key’s saved in authorized_keys (on the distant server), you’ll be able to then log into that server (from the consumer whose public key has been saved) with SSH key authentication.

ssh_config

The /and so forth/ssh/ssh_config file is the file used for system-wide consumer configuration for SSH. The configurations discovered right here solely come into play when the ssh command is used to connect with one other host. Most frequently, you’ll not must edit this file.

sshd_config

The /and so forth/ssh/sshd_config file, however, is the configuration file for the SSH daemon. That is the place you configure the likes of:

  • Default SSH port.
  • Public Key Authentication.
  • Root login permission.
  • Password authentication.
  • X11Forwarding.

Say, for instance, you wish to solely enable key authentication and disable password authentication. On the distant server you’d edit the sshd_config file like so:

  • Change #PubkeyAuthentication sure to Pubkeyauthentication sure.
  • Change #PasswordAuthentication sure to PasswordAuthentication no.

As soon as you’ve got made these modifications, restart the ssh service, and the distant server will solely enable connections from consumer machines with an entry in ~/.ssh/authorized_keys. No entry within the distant server’s authorized_keys file? No entry.

Know these recordsdata

You’re higher armed to utilize the SSH software with an understanding of the above 4 SSH recordsdata. Do it is advisable to know the way these recordsdata perform? Not likely. However as an admin, you might be finest served when the instruments you utilize with greater than a cursory data.

For extra info, learn the next man pages:

man ssh
man ssh_config
man sshd_config

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