SSH

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SSH (Secure SHell) is the ubiquitous tool for logging into and working on remote machines securely. All sensitive information is strongly encrypted, and in addition to the remote shell, SSH supports file transfer, and port forwarding for arbitrary protocols, allowing secure access to remote services. It replaces the classic telnet, rlogin, and similar non-secure tools - but SSH is not just a remote shell, it is a complete environment for working with remote systems.

In addition to the main ssh command, the SSH suite of programs includes tools such as scp (Secure Copy Program), sftp (Secure File Transfer Protocol), or ssh-agent to help with key management. The standard SSH port is port 22.

Several versions of SSH have existed. Today the most popular implementation of SSH, and de-facto standard, is OpenBSD's OpenSSH. This comes pre-installed on Gentoo, and is published under a BSD ("and freer") license.

SSH is multi-platform, and is very widely used: OpenSSH is installed by default on most Unix-like OSs, on Windows10, on MacOS, and can be installed on Android or "jailbroken" iOS (SSH clients are available). This makes SSH a great tool for working with heterogeneous systems.

Installation

Check install

Deployments of Gentoo Linux should already have OpenSSH installed, as the net-misc/openssh package is part of the system set. The presence and proper functioning of OpenSSH can be checked by running the ssh command, which should output a usage statement:

user $ssh
usage: ssh [-46AaCfGgKkMNnqsTtVvXxYy] [-B bind_interface]
           [-b bind_address] [-c cipher_spec] [-D [bind_address:]port]
           [-E log_file] [-e escape_char] [-F configfile] [-I pkcs11]
           [-i identity_file] [-J [user@]host[:port]] [-L address]
           [-l login_name] [-m mac_spec] [-O ctl_cmd] [-o option] [-p port]
           [-Q query_option] [-R address] [-S ctl_path] [-W host:port]
           [-w local_tun[:remote_tun]] destination [command]

If no usage statement is printed, OpenSSH may be corrupt, or not installed. Try re-installation by following the emerge section, just as if rebuilding after a USE flag change. If OpenSSH were uninstalled, this should reinstall it. It should then remain installed, as part of the system set.

If this does not try to install OpenSSH, the package may have been masked, or even listed in package.provided, though this would be unusual.

USE flags

USE flags for net-misc/openssh Port of OpenBSD's free SSH release

+pie Build programs as Position Independent Executables (a security hardening technique)
+ssl Enable additional crypto algorithms via OpenSSL
audit Enable support for Linux audit subsystem using sys-process/audit
debug Enable extra debug codepaths, like asserts and extra output. If you want to get meaningful backtraces see https://wiki.gentoo.org/wiki/Project:Quality_Assurance/Backtraces
kerberos Add kerberos support
ldns Use LDNS for DNSSEC/SSHFP validation.
legacy-ciphers Enable support for deprecated, soon-to-be-dropped DSA keys. See https://marc.info/?l=openssh-unix-dev>m=170494903207436>w=2.
libedit Use the libedit library (replacement for readline)
livecd Enable root password logins for live-cd environment.
pam Add support for PAM (Pluggable Authentication Modules) - DANGEROUS to arbitrarily flip
security-key Include builtin U2F/FIDO support
selinux !!internal use only!! Security Enhanced Linux support, this must be set by the selinux profile or breakage will occur
static !!do not set this during bootstrap!! Causes binaries to be statically linked instead of dynamically
test Enable dependencies and/or preparations necessary to run tests (usually controlled by FEATURES=test but can be toggled independently)
verify-sig Verify upstream signatures on distfiles
xmss Enable XMSS post-quantum authentication algorithm
Data provided by the Gentoo Package Database · Last update: 2024-10-30 18:38 More information about USE flags

Emerge

After changing USE flags just for the OpenSSH package, rebuild OpenSSH for the new flags to be applied. As OpenSSH is in the system set, --oneshot should be used to avoid adding it to the world file:

root #emerge --ask --changed-use --oneshot net-misc/openssh

After changing any global USE flags in make.conf that affect the OpenSSH package, emerge world to update to the new USE flags:

root #emerge --ask --verbose --update --deep --newuse @world

Usage

Commands

OpenSSH provides several commands, see each command's man page for usage information:

Escape sequences

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

ssh>~?

Note that escapes are only recognized immediately after a newline. They may not always work with some shells, such as fish.

Passwordless authentication to a distant SSH server

Handy for git server management.

See also
For more details see the Security Handbook.
Warning
Leaving the passphrase empty implies the private key file will not be encrypted. An attacker having access to the local filesystem will be able to read the private key.
Note
The default file names of the keys must not be changed, or the server may persist in asking for a password even after running ssh-copy-id. The file name will be one of:
  • id_rsa
  • id_ecdsa
  • id_ed25519
depending on the key algorithm used.

Make sure an account for the user exists on the server. The clients' id_ed25519.pub will be copied to the server's ~/.ssh/authorized_keys file in the user's home directory.

Client

There are two alternatives for creating and storing SSH keys: the first is to use GnuPG, the second is to use SSH itself.

GnuPG

See Usage of GPG keys instead of SSH keys.

ssh-keygen

Clients need public and private keys. A pair may be created with (of course, not entering a passphrase):

user $ssh-keygen -t ed25519
Generating public/private ed25519 key pair.
Enter file in which to save the key (/home/larry/.ssh/id_ed25519): 
Enter passphrase (empty for no passphrase): 
Enter same passphrase again: 
Your identification has been saved in /home/larry/.ssh/id_ed25519
Your public key has been saved in /home/larry/.ssh/id_ed25519.pub
The key fingerprint is:
SHA256:riDdFuPhN7alEsAvm717gM1IZBP3DYXGo5apQG7OUM1 larry@client
The key's randomart image is:
+--[ED25519 256]--+
| .+   ..+o.      |
| o +   Ao+.o     |
|. o A Oo  . .    |
| =   + .o +      |
|  o   . S= + .   |
|     . *. B + .  |
|      o.=  * o   |
|     ...+ = .    |
|      .. .+=     |
+----[SHA256]-----+

Then authorize the public key with the server:

user $ssh-copy-id -i ~/.ssh/id_ed25519.pub <username>@<server>
/usr/bin/ssh-copy-id: INFO: Source of key(s) to be installed: "/home/larry/.ssh/id_ed25519.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.

Afterwards a passwordless login should be possible doing:

user $ssh <server>
larry@<server>

Server

The file /etc/ssh/sshd_config should be set to PasswordAuthentication no after the client adds their public key.

Then restart the sshd service to authenticate without passwords.

Single machine testing

The above procedure can be tested out locally:

user $ssh-keygen -t ed25519
Generating public/private ed25519 key pair.
Enter file in which to save the key (/home/larry/.ssh/id_ed25519): 
Enter passphrase (empty for no passphrase): 
Enter same passphrase again:
...
user $mv ~/.ssh/id_ed25519.pub ~/.ssh/authorized_keys
user $ssh localhost

Remote services over ssh

SSH may be used to access remote services, such as HTTP, HTTPS, fileshares, etc., through an encrypted "tunnel". Remote service access is detailed in the SSH tunneling and SSH jump host articles.

Copying files to a remote host

The SFTP command, a part of SSH, uses the SSH File Transfer Protocol to copy files to a remote host. rsync is also an alternative for this.

Note
The OpenSSH 8.0 release notes, from 2019, state "The scp protocol is outdated, inflexible and not readily fixed. We recommend the use of more modern protocols like sftp and rsync for file transfer instead.". The OpenSSH 8.8 release notes, from 2021, state "A near-future release of OpenSSH will switch scp(1) from using the legacy scp/rcp protocol to using SFTP by default.".

ssh-agent

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, edit and change /etc/lightdm/Xsession from:

user $exec $command

into:

user $exec ssh-agent $command

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

Recent Xfce 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 the shell, X-session or similar.

user $xfconf-query -c xfce4-session -p /startup/ssh-agent/enabled -n -t bool -s false
user $xfconf-query -c xfce4-session -p /startup/gpg-agent/enabled -n -t bool -s false

Configuration

Create keys

Important
This section pertains to the keys used by the system, not the user keys that reside in ~/.ssh.

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

On first run, the sshd init-script will generate all system keys if they don't already exist. Usually there is no need to recreate them (unless the system has been compromised), but if such a need arises, this can be done manually through the ssh-keygen command (RSA, ECDSA and Ed25519 algorithms):

root #ssh-keygen -t rsa -b 4096 -f /etc/ssh/ssh_host_rsa_key -N ""
root #ssh-keygen -t ecdsa -f /etc/ssh/ssh_host_ecdsa_key -N ""
root #ssh-keygen -t ed25519 -f /etc/ssh/ssh_host_ed25519_key -N ""
Important
To reduce the attack vector, enable only one trusted algorithm, as shown below:
FILE /etc/ssh/sshd_config
# Only Ed25519 host key
HostKey /etc/ssh/ssh_host_ed25519_key

And remove other HostKey keys, if present.

Check that only the key with the specified algorithm is used:

user $ssh-keyscan <SERVER IP>

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(5) man page.

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

root #/usr/sbin/sshd -t
Important
Always validate the configuration changes prior restarting the service in order to keep the remote login available.

Client configuration

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(5) man page.

Intrusion prevention

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.

Service

Commands to run the SSH server will depend on active init system.

OpenRC

Add the OpenSSH daemon to the default runlevel:

root #rc-update add sshd default

Start the sshd daemon with:

root #rc-service sshd start

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

root #rc-service sshd start
root #rc-service sshd stop
root #rc-service sshd restart
Note
Active SSH connections to the server remain unaffected when issuing rc-service sshd restart.

systemd

To have the OpenSSH daemon start when the system starts:

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

To start the OpenSSH daemon now:

root #systemctl start sshd.service

To check if the service has started:

root #systemctl status sshd.service

Tips

Terminal multiplexers to preserve sessions

It is possible to use a terminal multiplexer to resume a session after a dropped connection. Tmux and Screen are two popular multiplexers that can be used to be able to reconnect to a session, even if a command was running when the connection dropped out.

SSH over intermittent connections

When on unstable Internet connections, or when roaming between networks (such as when moving wifi networks), mosh can help avoid dropping SSH sessions.

Open new tabs for session with Kitty terminal

By using the SSH kitten for the Kitty terminal emulator, it is possible to open new "tabs", or windows, on the current SSH session without having log in again.

Kitty also provides other practical SSH functionality.

Benchmark the optimal rounds for an ed25519 key

Tip
It is highly advisable to conduct benchmarking for the ed25519 key generation process, particularly considering the default value of 16 rounds and the potential performance improvements achievable with higher round values.
FILE ssh-benchmark.shBenchmark SSH Ciphers
#!/bin/sh

rounds="16 32 64 100 150"
num_runs=20

for r in $rounds; do
    printf "Benchmarking 'ssh-keygen -t ed25519 -a %s' on average:\n" "$r"
    total_time=0
    i=1
    while [ $i -le $num_runs ]; do
        start_time=$(date +%s.%N)
        ssh-keygen -t ed25519 -a "$r" -f test -N test >/dev/null 2>&1
        end_time=$(date +%s.%N)
        runtime=$(echo "$end_time - $start_time" | bc)
        total_time=$(echo "$total_time + $runtime" | bc)
        rm test{,.pub} >/dev/null 2>&1
        printf "Run %s: %s seconds\n" "$i" "$runtime"
        i=$((i + 1))
    done
    average_time=$(echo "scale=3; $total_time / $num_runs"| bc)
    printf "Average execution time: %s seconds\n\n" "$average_time"
done
Note
It is highly recommended to perform benchmarking for ed25519 key generation, considering both security and performance aspects. While the highest recommended round may offer superior security, it's essential to find the optimal balance based on individual requirements and time constraints. By conducting benchmark tests, one can evaluate the trade-off between security and performance, ensuring the selection of an appropriate round value for ed25519 keys.

Benchmarking is a crucial process to measure the performance and efficiency of a system or a specific component, such as cryptographic algorithms. In the context of SSH (Secure Shell) ciphers, it is important to determine the optimal number of rounds for generating ed25519 keys.

The provided script, ssh-benchmark.sh, conducts benchmarking on the ssh-keygen command with different round values for ed25519 keys. The script executes the ssh-keygen command multiple times with varying round values and measures the execution time for each run. It then calculates the average execution time for each round value.

By benchmarking different round values, system administrators and security professionals can identify the optimal round value that strikes a balance between security and performance. Higher round values generally provide stronger security but can result in increased computational overhead. Finding the right balance ensures that ed25519 keys are generated efficiently without compromising security.

Benchmarking helps identify potential bottlenecks, vulnerabilities, or areas that require improvement in security systems. It assists in selecting the most suitable algorithms and configurations for a particular use case, ensuring that security measures are robust and effective.

Troubleshooting

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

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

Permissions are too open

An ssh connection will only work if the file permissions of the ~/.ssh directory and contents are correct.

  • The ~/.ssh directory permissions should be 700 (drwx------), i.e. the owner has full access and no one else has any access.
  • Under ~/.ssh:
    • public key files' permissions should be 644 (-rw-r--r--), i.e. anyone may read the file, only the owner can write.
    • all other files' permissions should be 600 (-rw-------), i.e. only the owner may read or write the file.

These permissions need to be correct on the client and server.

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, 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 the 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 the local client, set ServerAliveInterval 30 (or some other value, in seconds) within the /etc/ssh/ssh_config or ~/.ssh/config file.
  • Set TCPKeepAlive no to help eliminate disconnections.

For example, to modify the server's configuration, add following file:

FILE /etc/ssh/sshd_config.d/01_ClientAlive.confHelp disconnects occur less often (server)
# The following ClientAlive values will keep an inactive session open for 30 minutes
ClientAliveCountMax 60
ClientAliveInterval 30
#
# Deactivate TCPKeepAlive
TCPKeepAlive no

To modify the client's configuration, add following file:

FILE /etc/ssh/ssh_config.d/01_ServerAlive.confHelp disconnects occur less often (client)
# The following ServerAlive values will keep an inactive session open for 2 hours
ServerAliveInterval 60
ServerAliveCountMax 120

New key does not get used

This scenario covers the case when a key to access a remote system has been created, the public key installed on the remote system, but the remote system is (for some reason) not accessible via ssh. This can happen if the name of the keyfile is not known to ssh.

Confirm which key files ssh is trying by running it with one of the verbose options, as described at the start of the Troubleshooting section. The verbose output will include the names of the keyfiles it is trying, and the one (if any) that actually gets used.

The default key files for the system are listed in the /etc/ssh/ssh_config, see the commented-out lines containing IdentityFile directives.

There are several ways to use a key with a non-default name.

The key name can be specified on the command line every time:

user $ssh -i ~/.ssh/my_keyfile user@remotesys

Alternatively, add following ssh configuration file to add a special case for ssh to the remote system:

FILE /etc/ssh/ssh_config/02_remotesys.confDefine keyfiles to use for host remotesys
Host remotesys
    IdentityFile ~/.ssh/id_rsa
    IdentityFile ~/.ssh/my_keyfile

If any are specified, it appears to be necessary to specify all the desired keys on a remote host. Read up on the ssh IdentityFile.

X11 forwarding, not forwarding, or tunneling

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:

user $echo $DISPLAY
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:

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

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.

Wayland forwarding over ssh

Wayland-based systems require additional software for forwarding GUI applications.

Install Waypipe on both client and server:

root #emerge --ask gui-apps/waypipe

To run a remote command with waypipe, prefix the ssh command with waypipe. For example to open Kate on a remote server:

user $waypipe ssh <user>@<host> kate

Further it functions like normal ssh. So to open a secure shell that can launch GUI applications:

user $waypipe ssh <user>@<host>

See also

  • Autossh — a command that detects when SSH connections drop and automatically reconnects them.
  • dropbear — a lightweight SSH server. It runs on a variety of POSIX-based platforms.
  • Keychain — This document describes how to use SSH shared keys along with the keychain program.
  • Mosh — a SSH client server that is aware of connectivity problems of the original SSH implementation.
  • SCP — an interactive file transfer program, similar to the copy command, that copies files over an encrypted SSH transport.
  • SFTP — an interactive file transfer program, similar to FTP, which performs all operations over an encrypted SSH transport.
  • SSHFS — a secure shell client used to mount remote filesystems to local machines.
  • SSH tunneling — a method of connecting to machines on the other side of a gateway machine.
  • SSH jump host — employed as an alternative to SSH tunneling to access internal machines through a gateway.
  • rsync — a powerful file sync program capable of efficient file transfers and directory synchronization.
  • Securing the SSH service (Security Handbook)
  • Starting the SSH daemon — Gentoo Handbook — Installation

External resources

Retrieved from "https://wiki.gentoo.org/index.php?title=SSH&oldid=1317793"