SSH public keys
SSH Key Generation Overview with no password
This shows how to use unencrypted public keys for logging in to a remote SSH server without a password. The basic steps are:
- Create an RSA key-pair with an empty password (no encryption).
- Copy the public key to the remote server.
- Add the public key to the authorized_keys file on the remote server.
Here are those steps as you would actually type them (`ssh-copy-id` does steps 2 and 3):
ssh-keygen -q -t rsa -N '' -f ~/.ssh/id_rsa ssh-copy-id firstname.lastname@example.org
Here is what you would do without `ssh-copy-id`. This is what `ssh-copy-id` is doing (more or less):
ssh-keygen -q -t rsa -N '' -f ~/.ssh/id_rsa scp ~/.ssh/id_rsa.pub email@example.com:/tmp/id_rsa.pub ssh firstname.lastname@example.org "mkdir -p ~/.ssh;chmod 700 ~/.ssh;touch ~/.ssh/authorized_keys;cat /tmp/id_rsa.pub >> ~/.ssh/authorized_keys"
Usually it's bad to use unencrypted public keys for logging in to remote servers without a password. Only use unencrypted keys in limited circumstances where you want a privileged account to have automatic access to a remote server. Examples of this are accounts for backup processes. On the other hand, if you are just looking to make life easier because you don't like typing passwords, then you should learn about `ssh-agent`. That will give you most of the advantages of using unencrypted public keys without the risk.
The `ssh-copy-id` Script
OpenSSH comes with `ssh-copy-id` which makes it much easier to setup your local host to connect to a remote server via SSH. This will put your key on the remote server example.com. After doing this you will be able to connect using your key. If your key is unencrypted then you will be able to connect without a password. If the key is encrypted and ssh-agent is not running then you will have to enter your key password (not to be confused with the account password on the remote server).
SSH key fingerprint
Some tools will store public keys and then refer to them by their fingerprint. If you don't know the fingerprint to your own key then this can be confusing. This command will show the fingerprint of your default public key:
ssh-keygen -lf ~/.ssh/id_rsa.pub ssh-keygen -lf ~/.ssh/id_dsa.pub
Creating an encrypted key
Having an key pair with an unencrypted private key is dangerous. If anyone gets access to your private key this is as good as having your password. This creates an encrypted key in ~/.ssh/id_rsa:
If you want to change the default output to some other file besides ~/.ssh/id_rsa:
ssh-keygen -f /tmp/id_rsa.example
Unencrypt your existing key
You can remove the encryption from a key. If you do this then then you can connect from your local machine to the remote password without any password. You may also do this if you want to change the password. Just remove the old encryption then encrypt it again. To unencrypt:
openssl rsa -in ~/.ssh/id_rsa -out ~/.ssh/id_rsa
Encrypt your existing key
If you want to require a password then encrypt the key. This assumes that your key is not already encrypted. This will overwrite your key. To encrypt:
openssl rsa -des3 -in ~/.ssh/id_rsa -out ~/.ssh/id_rsa
Permission problems with SSH
Ssh is very picky about permissions on the ~/.ssh directory and files. Sometimes you may do something to mess up these permissions. Run the following to fix most permissions problems. You may have to do this on both the remote host and local host.
chmod 700 ~/.ssh chmod 600 ~/.ssh/id_rsa chmod 644 ~/.ssh/id_rsa.pub chmod 644 ~/.ssh/authorized_keys chmod 644 ~/.ssh/known_hosts
Also no directory above ~/.ssh can have 'group' or 'other' write permissions.
When you ssh to a remote server where you have previously added your public key to authorized_keys then the remote server will ask your ssh client to confirm your public key. If your private key is not encrypted then your ssh client will confirm the keys without your help. If your private key is encrypted then your ssh client will ask you for your password to decrypt your private key.
You can have ssh cache your private key password so that you only need to enter it once. To do this, ssh uses ssh-agent to cache passwords. The ssh-agent sits in the background and caches the password to your private keys. On most desktop distros such as Ubuntu an ssh-agent is started when you login. The ssh-agent doesn't do anything until you add a private key to the cache. This will add your private key to the ssh-agent cache.
The agent will ask you for your password. Now the agent running in the background has your password and private key. When the ssh client later needs to confirm a public key it will first ask the ssh-agent if it has the key and password cached. If it does then the ssh client can confirm your keys without your help.
Debugging Your Agent
Most distros that feature a display manager such as xdm, gdm, or kdm will start an ssh-agent when you login. This is started as part of the xinit process. For example if you use KDE, then kdm is your display manager. Kdm will start X, which starts startkde, which starts ssh-agent.
root 4100 0.0 0.0 2688 640 ? Ss 18:18 0:00 /usr/bin/kdm root 4116 2.6 1.5 35236 23788 tty7 Ss+ 18:18 0:16 \_ /usr/bin/X -br -nolisten tcp :0 vt7 -auth /var/run/xauth/A:0-HgufQc root 4130 0.0 0.0 3664 1388 ? S 18:18 0:00 \_ -:0 noah 4896 0.0 0.0 1660 504 ? Ss 18:19 0:00 \_ /bin/sh /usr/bin/startkde noah 5113 0.0 0.0 4480 1004 ? Ss 18:19 0:00 \_ /usr/bin/ssh-agent /usr/bin/dbus-launch --exit-with-session /usr/bin/startkde noah 5168 0.0 0.0 1580 352 ? S 18:19 0:00 \_ kwrapper ksmserver
You can check if an agent is running by using this command:
$ ps auxww | grep agent noah 5113 0.0 0.0 4480 1004 ? Ss 18:19 0:00 /usr/bin/ssh-agent /usr/bin/dbus-launch --exit-with-session /usr/bin/startkde
If you don't have an agent running you can put one in your .xsession. Or you can run it manually at any time from the command line:
$ exec ssh-agent bash
|local server||remote server|
ssh-keygen -t rsa
Press enter when it asks you for a passphrase. This will set no passphrase. Or use
ssh-keygen -t rsa -N
to set an empty new password.
This generates the following files under ~/.ssh/
|Copy id_rsa.pub to remote server:
scp ~/.ssh/id_rsa.pub email@example.com:/tmp/id_rsa.pub
You will still need your password at this point.
|Append /tmp/id_rsa.pub key to ~/.ssh/authorized_keys:
cat /tmp/id_rsa.pub >> ~/.ssh/authorized_keys
If you get an error saying "~/.ssh/authorized_keys: No such file or directory" it means that there is no .ssh directory for this user (this user has never used ssh before). Simply create an empty .ssh directory with 700 permissions:
mkdir ~/.ssh chmod 700 ~/.ssh
|You should now be able to ssh to the remote server without a password:
Things that often cause trouble
These are usually only problems when working with older SSH servers.
- The SSH2 protocol specifies a format for storing public keys. Some SSH servers (such as ssh.com's) require a public key in this format in order to accept authentication with the corresponding private key. Others, such as OpenSSH, use a different format. I don't know what to do about this.
- When cutting and pasting the public key BEWARE that it should be a single line. If you cut and paste from a terminal window then it is likely that you will get newline characters added where your terminal wrapped the line. If you use vi then the line may wrap and APPEAR to be multiple lines, but it is really one single line. When you paste it to a new window it may look the same, but the copy will likely contain newline characters. This will not work.
- Make sure you are using the correct version. Earlier versions of OpenSSH used two files, authorized_keys and authorized_keys2. Secure SSH uses something else with keys in an entirely different format.
Newbie SSH Notes
Create a key pair
This creates a Public Key and a Private Key. The Public Key is what you can give to other people. Key is a bit of a misnomer. It's really more of a Public Lock. You keep the key. You can give the lock to anyone and they can lock stuff with it, but they can't unlock it. They can give you the locked data and only you can unlock it with your Private Key.
You can further secure your Private Key by encrypting it so that you need a pass work to use it. This way even if someone gets a copy of your Private Key that won't be able to use it.