SSH(Secure Shell) Tips and Tricks

We use SSH to log into our servers, but it actually has a lot of neat tricks it can help us with as well! We'll cover some of them here.

Logging in

Of course, we use SSH to login:

$ ssh user@hostname

And, if needed, we can specify a different port:

$ ssh -p 2222 user@hostname

Sometimes, if we have a lot of SSH keys in our ~/.ssh directory, we'll often find that SSHing into servers with the intent of using a password results in a "too many authentication attempts" error. If we need to log into a server with a password, we can attempt to force password-based login. This will stop SSH from attempting to use your SSH keys first:

$ ssh -o "PubkeyAuthentication no" username@hostname

If you use AWS, and in other cases, you might get a PEM file to use as an identity. In this case, you might need to specify a specific identity file to use when logging in. We can do this with the -i flag:

$ ssh -i /path/to/identity.pem username@hostname

    You may need to set your permissions on the pem file so only the owner can read/write/execute it: chmod 0600 identity.pem or chmod u+rw identity.pem && chmod go-rwx identity.pem
2.
SSH Config

Here's something really powerful.

If you want to setup aliases for servers you access often, you can create an ~/.ssh/config file and specify each server you log into, along with the authentication method to use:

$ vim ~/.ssh/config

Host somealias
    HostName example.com
    Port 2222
    User someuser
    IdentityFile  ~/.ssh/id_example
    IdentitiesOnly yes

Host anotheralias
    HostName 192.168.33.10
    User anotheruser
    PubkeyAuthentication no

How aws
    HostName some.address.ec2.aws.com
    User awsuser
    IdentityFile  ~/.ssh/aws_identity.pem
    IdentitiesOnly yes

So, there's a few example entries you might find in the ~/.ssh/config file (you can have as many entries as you'd like).

Using a defined host ("alias") is as easy as this:

$ ssh somealias

Let's cover the options used above:

    HostName - The server host (domain or ipaddress)
    Port - The port to use when connecting
    User - The username to log in with
    IdentityFile - The SSH key identity to use to log in with, if using SSH key access
    IdentitiesOnly - "Yes" to specify only attempting to log in via SSH key
    PubkeyAuthentication - "No" to specify you wish to bypass attempting SSH key authentication

SSH Tunneling

SSH can be used for tunneling, which is essentially port forwarding. There's a few ways we can do this - Local (Outbound), Remote (Inboud), and some others (Dynamic and Agent Forwarding).
Local Port Forwarding

Local port forwarding is what you use when you need to tunnel through a server's firewall or other limitation.

A common example is attempting to connect to a remote database which is either behind a firewall or is only listening to local connection. For example, MySQL only listens to localhost connections by default. You can't remotely connect to it without editing the my.cnf file and have it listen on all networks. There's also a firewall preventing you connecting to MySQL's port 3306 anyway.

    Remote means any computer that isn't yours, which includes virtual machines (guests) inside of your host computer

Assuming we have SSH access to the remote server, we can get around this by creating a tunnel into the server. What might that look like?

ssh -L 3306:localhost:3306 username@hostname

Let's go over this:

    -L - Setup local port forwarding
    3306 - The local port to forward
    localhost:3306 - Within the remote server, what address and port to forward traffic to. Since the MySQL server is on the remote server, we're tunneling to the remote server's "localhost" on port 3306, which MySQL is listening to.
    username@localhost - The SSH username and host to connect to

I can then use my local client to connect to the remote server as if it's a local one:

Now, I used the same port locally and remotely, but I could have specified a different local port to use:

ssh -L 3307:localhost:3306 username@hostname

Then my local mysql client would have to connect to port 3307, which would still tunnel to the remote server's local 3306:

Remote Port Forwarding

Remote Port Forwarding is good if you need to share your local computer with others who are outside of your network.

To do this, we need a remote server all parties can reach. Something like a AWS or Digital Ocean server will do.

First, let's pretend our local computer has a web server running on port 8001:

# On our local machine:
$ curl localhost:8001
Hi!

We want our friends to see our website, which simply says "Hi!". Let's use a remote server to forward requests to our local computer:

# Still on our local machine:
ssh -R 9000:localhost:8001 username@hostname

Let's go over this command:

    -R - Using remote port forwarding
    9000 - The remote server's port to use (not our local server this time!)
    localhost:8001 - The local address to forward to. Since our webserver is on localhost port 8001, that's what we specify here. (Yep, the order of those arguments changed for -R over -L!)
    username@hostname - SSH access to the remote server

If our remote server's IP address was 123.123.123.123, then our friends can access our website at 123.123.123.123:9000, which will forward to our local site at localhost:8001!

Note: To accomplish this, your remote server's firewall must not block port 9000. You may also need to edit /etc/ssh/sshd_config and set the GatewayPorts directive to yes. (Don't forget to restart SSH after any changes to sshd_config).