SSH

Check install[edit | edit source]

Most deployments of Gentoo Linux will already have OpenSSH installed on the system. This can be checked by running the ssh command. If it is installed a usage statement should be printed:

usage: ssh [-1246AaCfgKkMNnqsTtVvXxYy] [-b bind_address] [-c cipher_spec]
           [-D [bind_address:]port] [-E log_file] [-e escape_char]
           [-F configfile] [-I pkcs11] [-i identity_file]
           [-L [bind_address:]port:host:hostport] [-l login_name] [-m mac_spec]
           [-O ctl_cmd] [-o option] [-p port]
           [-Q cipher | cipher-auth | mac | kex | key]
           [-R [bind_address:]port:host:hostport] [-S ctl_path] [-W host:port]
           [-w local_tun[:remote_tun]] [user@]hostname [command]

If no usage statement is printed ssh is either corrupted or not installed. It is also possible that a user is simply rebuilding OpenSSH to include a new USE configuration. Whatever the case, proceed on to view possible USE settings.

USE flags[edit | edit source]

Emerge[edit | edit source]

After changing the necessary USE flags, do not forget to install (or rebuild) OpenSSH:

Create keys[edit | edit source]

In order to provide a secure shell, cryptographic keys are used to manage the encryption, decryption, and hashing functionalities offered by SSH.

On the first start of the SSH service, system keys will be generated. Keys can be (re)generated using the ssh-keygen command.

To generate the keys for SSH protocol version 2 (DSA and RSA algorithms):

The article Secure Secure Shell suggests using Ed25519 and RSA public key algorithms with:

Server configuration[edit | edit source]

The SSH server is usually configured in the /etc/ssh/sshd_config file, though it is also possible to perform further configuration in OpenRC's /etc/conf.d/sshd, including changing the location of the configuration file. For detailed information on how to configure the server see the sshd_config man page.

Users should study Sven Vermeulen (SwifT)'s OpenSSH guide for a security focused configuration.

The server provides means to validate its configuration using test mode:

Client configuration[edit | edit source]

The ssh client and related programs (scp, sftp, etc.) can be configured using the following files:

• ~/.ssh/config
• /etc/ssh/ssh_config

For more information read the ssh_config manual:

Intrusion prevention[edit | edit source]

SSH is a commonly attacked service. Tools such as sshguard and fail2ban monitor logs and black list remote users who have repeatedly attempted, yet failed to login. Utilize them as needed to secure a frequently attacked system.

Running the SSH server[edit | edit source]

Created symlink from /etc/systemd/system/multi-user.target.wants/sshd.service to /usr/lib64/systemd/system/sshd.service.

Escape sequences[edit | edit source]

During an active SSH session, pressing the tilde (~) key starts an escape sequence. Enter the following for a list of options:

Connecting to a distant SSH server[edit | edit source]

Generating public/private rsa key pair.
Enter file in which to save the key (/home/larry/.ssh/id_rsa):
Enter passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved in /home/larry/.ssh/id_rsa.
Your public key has been saved in /home/larry/.ssh/id_rsa.pub.
The key fingerprint is:
de:ad:be:ef:15:g0:0d:13:37:15:ad:cc:dd:ee:ff:61 larry@client
The key's randomart image is:
+--[ RSA 2048]----+
|                 |
|     .           |
| . .. n   .      |
|   . (: . .      |
|  o   . . : .    |
| . ..: >.> .     |
|  * ?. .         |
| o.. .. ..       |
| :. .  ! .       |
+-----------------+

/usr/bin/ssh-copy-id: INFO: Source of key(s) to be installed: "/home/larry/.ssh/id_rsa.pub"
/usr/bin/ssh-copy-id: INFO: attempting to log in with the new key(s), to filter out any that are already installed
/usr/bin/ssh-copy-id: INFO: 1 key(s) remain to be installed -- if you are prompted now it is to install the new keys
larry@<server>'s password: 

Number of key(s) added: 1

Now try logging into the machine, with:   "ssh '<server>'"
and check to make sure that only the key(s) you wanted were added.

larry@<server>

Generating public/private rsa key pair.
Enter file in which to save the key (/home/larry/.ssh/id_rsa):
Enter passphrase (empty for no passphrase):
Enter same passphrase again:
...

Death of long-lived connections[edit | edit source]

Many internet access devices perform Network Address Translation (NAT), a process that enables devices on a private network such as that typically found in a home or business place to access foreign networks, such as the internet, despite only having a single IP address on that network. Unfortunately, not all NAT devices are created equal, and some of them incorrectly close long-lived, occasional-use TCP connections such as those used by SSH. This is generally observable as a sudden inability to interact with the remote server, even though the ssh client program has not exited.

In order to resolve the issue, OpenSSH clients and servers can be configured to send a 'keep alive', or invisible message aimed at maintaining and confirming the live status of the link:

• To enable keep alive for all clients connecting to your local server, set ClientAliveInterval 30 (or some other value, in seconds) within the /etc/ssh/sshd_config file.
• To enable keep alive for all servers connected to by your local client, set ServerAliveInterval 30 (or some other value, in seconds) within the /etc/ssh/ssh_config file.

X11 forwarding, not forwarding, or tunneling[edit | edit source]

Problem: After having made the necessary changes to the configuration files for permitting X11 forwarding, it is discovered X applications are executing on the server and are not being forwarded to the client.

Solution: What is likely occurring during SSH login into the remote server or host, the DISPLAY variable is either being unset or is being set after the SSH session sets it.

Test for this scenario perform the following after logging in remotely:

localhost:10.0

The output should be something similar to localhost:10.0 or localhost2.local:10.0 using server side X11UseLocalhost no setting. If the usual :0.0 is not displayed, check to make sure the DISPLAY variable within ~/.bash_profile is not being unset or re-initializing. If it is, remove or comment out any custom initialization of the DISPLAY variable to prevent the code in ~/.bash_profile from executing during a SSH login:

Be sure to substitute larry in the command above with the proper username.

A trick that works to complete this task would be to define an alias within the users' ~/.bashrc file.

ssh-agent[edit | edit source]

OpenSSH comes with ssh-agent, a daemon to cache and prevent from frequent ssh password entries. When run, the environment variable SSH_AUTH_SOCK is used to point to ssh-agent's communication socket. The normal way to setup ssh-agent is to run it as the top most process of the user's session. Otherwise the environment variables will not be visible inside the session.

Depending on the way the graphical user session is configured to launch, it can be tricky to find a suitable way to launch ssh-agent. As an example for the lightdm display manager, you may edit and change /etc/lightdm/Xsession from

into

To tell ssh-agent the password once per session, either run ssh-add manually or make use of the AddKeysToAgent option.

Recent Xfce[1] will start ssh-agent (and gpg-agent) automatically. If both are installed both will be started which makes identity management especially with SmartCards more complicated. Either stop XFCE from autostarting at least SSH's agent or disable both and use your shell, X-session or similar.