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SSH Tips

Note: This article was originally published at Planet PHP on 5 December 2012.
Planet PHP

SSH is a way to access remote servers via the command line. It is a really fabulous secure communications channel that you can use for all kinds of things! Today, I thought I'd share some of my favorite SSH tips, from starting out, to using multiple SSH keys easily, to using SSH as a tunnel for something else entirely.

To SSH to a remote server, simply take a terminal program (Windows users should grab PuTTY) and type:

ssh username@server

You'll be prompted for your password (or passphrase for your key; more on those in a tick), and thena-apresto!a-ayou're in. If you don't supply the username, you'll be prompted for that first, and the server can take either a domain name like lornajane.net or an IP address.

SSH keys

While it's fine to use a username and password for SSH access, it's quicker and more secure to access using an SSH key. SSH keys are made of two parts: a public key and a private key. You generate the keys with the command ssh-keygen (Windows users, download PuTTYGen).

You will be prompted to answer some questions when creating the key, and, in general, the defaults are fine. One of the questions will ask you to create a passphrase. The passphrase is a password that must be used with the key when accessing a server. In general, it is recommended to have passphrases, but you might choose to omit them if, for example, this key will be used by an unsupervised automated user account.

There's an excellent GitHub resource on generating SSH keys that gives you more information, and it also provides a clue to some of the other things we can do with SSH!

Sharing SSH keys

Once you have your keys, you will need to send the public half (the filename will end with .pub) to anyone who needs to give you access to their server or other app. It is safe to email public keys and to share them, but keep your private keys safe, protected, and backed up. These are the real keys to the kingdom!

GitHub uses SSH keys to grant access to your GitHub account. You paste the contents of your public key into their site (look under aoSSH Keysa on your account page), and then you can access your repositories using the key.

Many keys

Some people only have one key, but I find it more helpful to have many keysa-aperhaps one for work use, and one for personal use, or one created for a specific project or system. This approach means that I can revoke an individual key if I need to, without needing to change all of my keys.

To SSH with a specific key is very simple. You can supply the information on which key to use with the -i switch to the ssh command. For example, I have a separate SSH key to use with GitHub:

ssh -i ~/.ssh./github_rsa git@github.com

Using multiple keys in this way is great for accessing servers, but many of the programs which rely on SSH, such as scp and git don't have an option to specify which key to use. For this situation (and many others), you can use the SSH config file.

SSH configuration file

The config file for SSH is the source of many very useful little tricks! (On Unix systems, it lives in ~/.ssh/config.) For example, you might set up an alias so that you can refer to a server by a shorter name:

Host lyra User=lorna Hostname=lyra.lornajane.net

Now, I can simply SSH to the alias, and it will pick up the hostname and username accordingly:

$ ssh lyra lorna@lyra.lornajane.net's password:

Using the config file, you can also tell SSH which key to use when accessing a different server. Here's the entry I use for accessing GitHub:

Host GitHub Hostname=github.com IdentityFile=~/.ssh/github_rsa

This does mean that I edit all the URLs I use to be GitHub, where normally there would be github.com. It reminds me that there's some magic somewhere!

SSH tunnels

Earlier, I mentioned that it's possible to use SSH connections as a transport for other protocols. I use this mostly to get around the issue that some clients' tools are locked down to my home static IP address. There's a server in the house th

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