SSH

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SSH (Secure SHell) is an encrypted terminal program that replaces the classic telnet tool on Unix-like operating systems.

In addition to remote terminal access provided by the main ssh binary, the SSH suite of programs has grown to include other tools such as scp (secure copy) and sftp (secure file transfer protocol).

Originally, SSH was not free. However, today the most popular and de-facto standard implementation of SSH is OpenBSD's OpenSSH, which comes pre-installed on Gentoo.

Installation

Most deployments will already have OpenSSH installed (through the net-misc/openssh package). This package uses the following USE flags:


→ Information about USE flags
USE flag Default Recommended Description
bindist No Disable EC/RC5 algorithms in OpenSSL for patent reasons.
hpn No Enable high performance ssh
pam No Adds support for PAM (Pluggable Authentication Modules) - DANGEROUS to arbitrarily flip
tcpd No Adds support for TCP wrappers
kerberos No Adds kerberos support
ldap No Add support for storing SSH public keys in LDAP
libedit No Use the libedit library (replacement for readline)
selinux No  !!internal use only!! Security Enhanced Linux support, this must be set by the selinux profile or breakage will occur
skey No Enable S/Key (Single use password) authentication support
static No  !!do not set this during bootstrap!! Causes binaries to be statically linked instead of dynamically
X No Adds support for X11
X509 No Adds support for X.509 certificate authentication

After changing USE flags, don't forget to rebuild OpenSSH:

root # emerge --ask net-misc/openssh

Running OpenSSH

Server

Add openssh to the default runlevel if this hasn't been done already.

root # rc-update add sshd default

Start sshd daemon with:

root # /etc/init.d/sshd start

The OpenSSH server can be controlled like any other OpenRC-managed service:

root # /etc/init.d/sshd start
root #
/etc/init.d/sshd stop
root #
/etc/init.d/sshd restart
Note
Active SSH connections to the server remain unaffected when issuing /etc/init.d/sshd restart.

Configuration

Create Keys

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.

Note
Substitute SecretPassphrase in the examples with your own passphrase.

To generate the key used for SSH protocol version 1 (which usually is not enabled anymore as it has been deprecated in favor of protocol version 2):

root # /usr/bin/ssh-keygen -t rsa1 -b 1024 -f /etc/ssh/ssh_host_key -N "SecretPassphrase"

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

root # /usr/bin/ssh-keygen -t dsa -f /etc/ssh/ssh_host_dsa_key -N "SecretPassphrase"
root #
/usr/bin/ssh-keygen -t rsa -f /etc/ssh/ssh_host_rsa_key -N "SecretPassphrase"

Server configuration

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.

You should also study this guide for a security focused configuration.

Client configuration

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

  • ~/.ssh/config
  • /etc/ssh/ssh_config

For more information read the ssh_config manual:

user $ man ssh_config

Passwordless Authentication

Handy for git server management.

Client

On the client run the following command:

user $ ssh-keygen -t rsa
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.. .. ..       |
| :. .  ! .       |
+-----------------+

Server

Make sure an account for the user exists on the server, and then place the clients' id_rsa.pub file into the ~/.ssh/authorized_keys file.

Single Machine Testing

The above procedure can be tested out locally:

user $ ssh-keygen -t rsa
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:
...
user $ mv ~/.ssh/id_rsa.pub ~/.ssh/authorized_keys
user $ ssh localhost

Troubleshooting

There are 3 different levels of debug modes that can help troubleshooting issues. With -v ssh prints debugging messages about its progress. This is helpful in debugging connection, authentication, and configuration problems. Multiple -v options increase the verbosity. The maximum is 3.

user $ ssh example.org -v
user $
ssh example.org -vv
user $
ssh example.org -vvv

Death of Long-lived Connections

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, 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!

Problem: After you have made the necessary changes to the configuration files for permitting X11 Forwarding, you find 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 as follows after logging in remotely:

user $ echo $DISPLAY
localhost:10.0

You should get something similar to "localhost:10.0" or "localhost2.local:10.0" using server side X11UseLocalhost no setting. If you're getting the usual ":0.0", check to make sure you are not unsetting or initializing the $DISPLAY variable within $HOME/.bash_profile. If so, remove or comment your custom initialization of $DISPLAY or prevent bash_profile from executing during SSH login:

user $ ssh -t me@localhost2 bash --noprofile

A trick would be to alias this within bashrc.

Intrusion Prevention

ssh is a commonly attacked service. sshguard & fail2ban monitor logs and black list remote users who have repeatedly failed to login.

See also