Difference between revisions of "Secure Shell"

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(Add warning about sshd.socket negating ListenAddress setting)
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The SSH client configuration file is {{ic|/etc/ssh/ssh_config}} or {{ic|~/.ssh/config}}.
 
The SSH client configuration file is {{ic|/etc/ssh/ssh_config}} or {{ic|~/.ssh/config}}.
  
An example configuration:
+
It is not longer needed to explicitly set {{ic|Protocol 2}}, it is commented out in the default configuration file. That means {{ic|Protocol 1}} will not be used as long as it is not explicitly enabled. (source: http://www.openssh.org/txt/release-5.4)
 
+
{{hc|/etc/ssh/ssh_config|
+
# $OpenBSD: ssh_config,v 1.26 2010/01/11 01:39:46 dtucker Exp $
+
 
+
# This is the ssh client system-wide configuration file. See
+
# ssh_config(5) for more information.  This file provides defaults for
+
# users, and the values can be changed in per-user configuration files
+
# or on the command line.
+
 
+
# Configuration data is parsed as follows:
+
#  1. command line options
+
#  2. user-specific file
+
#  3. system-wide file
+
# Any configuration value is only changed the first time it is set.
+
# Thus, host-specific definitions should be at the beginning of the
+
# configuration file, and defaults at the end.
+
 
+
# Site-wide defaults for some commonly used options.  For a comprehensive
+
# list of available options, their meanings and defaults, please see the
+
# ssh_config(5) man page.
+
 
+
# Host *
+
#  ForwardAgent no
+
#  ForwardX11 no
+
#  RhostsRSAAuthentication no
+
#  RSAAuthentication yes
+
#  PasswordAuthentication yes
+
#  HostbasedAuthentication no
+
#  GSSAPIAuthentication no
+
#  GSSAPIDelegateCredentials no
+
#  BatchMode no
+
#  CheckHostIP yes
+
#  AddressFamily any
+
#  ConnectTimeout 0
+
#  StrictHostKeyChecking ask
+
#  IdentityFile ~/.ssh/identity
+
#  IdentityFile ~/.ssh/id_rsa
+
#  IdentityFile ~/.ssh/id_dsa
+
#  Port 22
+
#  Protocol 2,1
+
#  Cipher 3des
+
#  Ciphers aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,aes128-cbc,3des-cbc
+
#  MACs hmac-md5,hmac-sha1,umac-64@openssh.com,hmac-ripemd160
+
#  EscapeChar ~
+
#  Tunnel no
+
#  TunnelDevice any:any
+
#  PermitLocalCommand no
+
#  VisualHostKey no
+
#  ProxyCommand ssh -q -W %h:%p gateway.example.com
+
}}
+
 
+
It is recommended to change the Protocol line into this:
+
Protocol 2
+
 
+
That means that only Protocol 2 will be used, since Protocol 1 is considered somewhat insecure.
+
  
 
====Daemon====
 
====Daemon====
Line 93: Line 38:
 
To allow access only for some users add this line:
 
To allow access only for some users add this line:
 
  AllowUsers    user1 user2
 
  AllowUsers    user1 user2
 +
 +
To allow access only for some groups:
 +
AllowGroups  group1 group2
  
 
To disable root login over SSH, change the PermitRootLogin line into this:
 
To disable root login over SSH, change the PermitRootLogin line into this:
Line 100: Line 48:
 
  Banner /etc/issue
 
  Banner /etc/issue
  
{{Tip| You may want to change the default port from 22 to any higher port (see [http://en.wikipedia.org/wiki/Security_through_obscurity security through obscurity]).}}
+
{{Tip|
 
+
* You may want to change the default port from 22 to any higher port (see [http://en.wikipedia.org/wiki/Security_through_obscurity security through obscurity]). Even though the port ssh is running on could be detected by using a port-scanner like {{Pkg|nmap}}, changing it will reduce the number of log entries caused by automated authentication attempts. To help select a port review the [[Wikipedia:List of TCP and UDP port numbers|list of TCP and UDP port numbers]]. You can also find port information locally in {{ic|/etc/services}}. Select an alternative port that is '''not''' already assigned to a common service to prevent conflicts.
Even though the port ssh is running on could be detected by using a port-scanner like nmap, changing it will reduce the number of log entries caused by automated authentication attempts. To help select a port review the [[Wikipedia:List of TCP and UDP port numbers|list of TCP and UDP port numbers]].
+
* Disabling password logins entirely will greatly increase security, see [[SSH Keys]] for more information.
 
+
}}
{{Tip|Disabling password logins entirely will greatly increase security, see [[SSH Keys]] for more information.}}
+
  
 
=== Managing the sshd daemon ===
 
=== Managing the sshd daemon ===
Line 113: Line 60:
 
  # systemctl enable sshd.service
 
  # systemctl enable sshd.service
  
{{Warning|Systemd is an asynchronous starting process. If you bind the SSH daemon to a specific IP address {{ic|ListenAddress 192.168.1.100}} it may fail to load during boot since the default sshd.service unit file has no dependency on network interfaces being enabled. When binding to an IP address, you will need to add {{ic|After=network.target}} to a custom sshd.service unit file. See [[Systemd#Replacing provided unit files]].}}
+
{{Warning|Systemd is an asynchronous starting process. If you bind the SSH daemon to a specific IP address {{ic|ListenAddress 192.168.1.100}} it may fail to load during boot since the default sshd.service unit file has no dependency on network interfaces being enabled. When binding to an IP address, you will need to add {{ic|After=network.target}} to a custom sshd.service unit file. See [[Systemd#Editing provided unit files]].}}
  
 
Or you can enable SSH Daemon socket so the daemon is started on the first incoming connection:
 
Or you can enable SSH Daemon socket so the daemon is started on the first incoming connection:
Line 283: Line 230:
  
 
=== Speeding up SSH ===
 
=== Speeding up SSH ===
 +
 
You can make all sessions to the same host use a single connection, which will greatly speed up subsequent logins, by adding these lines under the proper host in {{ic|/etc/ssh/ssh_config}}:
 
You can make all sessions to the same host use a single connection, which will greatly speed up subsequent logins, by adding these lines under the proper host in {{ic|/etc/ssh/ssh_config}}:
  ControlMaster auto
+
  Host examplehost.com
ControlPath ~/.ssh/socket-%r@%h:%p
+
  ControlMaster auto
 +
  ControlPersist yes
 +
  ControlPath ~/.ssh/socket-%r@%h:%p
  
Changing the ciphers used by SSH to less cpu-demanding ones can improve speed. In this aspect, the best choices are arcfour and blowfish-cbc. '''Please do not do this unless you know what you are doing; arcfour has a number of known weaknesses'''. To use them, run SSH with the {{Ic|"c"}} flag, like this:
+
See the {{ic|ssh_config(5)}} manual page for full description of these options.
 +
 
 +
Another option to improve speed is to enable compression with the {{ic|-C}} flag. A permanent solution is to add this line under the proper host in {{ic|/etc/ssh/ssh_config}}:
 +
Compression yes
 +
 
 +
Login time can be shortened by using the {{ic|-4}} flag, which bypasses IPv6 lookup. This can be made permanent by adding this line under the proper host in {{ic|/etc/ssh/ssh_config}}:
 +
AddressFamily inet
 +
 
 +
Changing the ciphers used by SSH to less cpu-demanding ones can improve speed. In this aspect, the best choices are arcfour and blowfish-cbc.
 +
 
 +
{{Warning|Please do not do this unless you know what you are doing; arcfour has a number of known weaknesses.}}
 +
 
 +
To use alternative ciphers, run SSH with the {{ic|-c}} flag:
 
  $ ssh -c arcfour,blowfish-cbc user@server-address
 
  $ ssh -c arcfour,blowfish-cbc user@server-address
 +
 
To use them permanently, add this line under the proper host in {{ic|/etc/ssh/ssh_config}}:
 
To use them permanently, add this line under the proper host in {{ic|/etc/ssh/ssh_config}}:
 
  Ciphers arcfour,blowfish-cbc
 
  Ciphers arcfour,blowfish-cbc
Another option to improve speed is to enable compression with the {{Ic|"C"}} flag. A permanent solution is to add this line under the proper host in {{ic|/etc/ssh/ssh_config}}:
 
Compression yes
 
Login time can be shorten by using the {{Ic|"4"}} flag, which bypasses IPv6 lookup. This can be made permanent by adding this line under the proper host in {{ic|/etc/ssh/ssh_config}}:
 
AddressFamily inet
 
Another way of making these changes permanent is to create an alias in {{ic|~/.bashrc}}:
 
alias ssh='ssh -C4c arcfour,blowfish-cbc'
 
  
 
=== Mounting a remote filesystem with SSHFS ===
 
=== Mounting a remote filesystem with SSHFS ===
Line 315: Line 272:
  
 
=== Saving connection data in ssh config ===
 
=== Saving connection data in ssh config ===
Whenever you want to connect to a ssh server, you usually have to type at least its address and the username. To save that typing work for servers you regularly connect to, you can use the personal {{ic|$HOME/.ssh/config}} or the global {{ic|/etc/ssh/ssh_config}} files as shown in the following example:
+
Whenever you want to connect to a ssh server, you usually have to type at least its address and the username. To save that typing work for servers you regularly connect to, you can use the personal {{ic|~/.ssh/config}} or the global {{ic|/etc/ssh/ssh_config}} files as shown in the following example:
  
{{hc|$HOME/.ssh/config|
+
{{hc|~/.ssh/config|
 
Host myserver
 
Host myserver
 
     HostName 123.123.123.123
 
     HostName 123.123.123.123
Line 343: Line 300:
 
  $ sshfs -o reconnect,compression=yes,transform_symlinks,ServerAliveInterval=45,ServerAliveCountMax=2,ssh_command='autossh -M 0' username@example.com: /mnt/example  
 
  $ sshfs -o reconnect,compression=yes,transform_symlinks,ServerAliveInterval=45,ServerAliveCountMax=2,ssh_command='autossh -M 0' username@example.com: /mnt/example  
 
Connecting through a SOCKS-proxy set by [[ Proxy_settings ]]:
 
Connecting through a SOCKS-proxy set by [[ Proxy_settings ]]:
  $ autossh -M 0 "ServerAliveInterval 45" -o "ServerAliveCountMax 2" -NCD 8080 username@example.com  
+
  $ autossh -M 0 -o "ServerAliveInterval 45" -o "ServerAliveCountMax 2" -NCD 8080 username@example.com  
 
With the {{ic|-f}} option autossh can be made to run as a background process. Running it this way however means the passprase cannot be entered interactively.
 
With the {{ic|-f}} option autossh can be made to run as a background process. Running it this way however means the passprase cannot be entered interactively.
  
Line 355: Line 312:
 
   
 
   
 
  [Service]
 
  [Service]
  ExecStart=/usr/bin/autossh -M 0 2222:localhost:2222 foo@bar.com
+
  ExecStart=/usr/bin/autossh -M 0 -NL 2222:localhost:2222 -o TCPKeepAlive=yes foo@bar.com
 
   
 
   
 
  [Install]
 
  [Install]
Line 376: Line 333:
  
 
== Troubleshooting ==
 
== Troubleshooting ==
 +
=== SSH connection left hanging after poweroff/reboot ===
 +
SSH connection hangs after poweroff or reboot if systemd stop network before sshd. To fix that problem, comment and change the {{ic|After}} statement:
 +
{{hc|/usr/lib/systemd/system/systemd-user-sessions.service|2=
 +
#After=remote-fs.target
 +
After=network.target}}
 +
 
=== Connection refused or timeout problem ===
 
=== Connection refused or timeout problem ===
  

Revision as of 07:23, 28 November 2013

Secure Shell (SSH) is a network protocol that allows data to be exchanged over a secure channel between two computers. Encryption provides confidentiality and integrity of data. SSH uses public-key cryptography to authenticate the remote computer and allow the remote computer to authenticate the user, if necessary.

SSH is typically used to log into a remote machine and execute commands, but it also supports tunneling, forwarding arbitrary TCP ports and X11 connections; file transfer can be accomplished using the associated SFTP or SCP protocols.

An SSH server, by default, listens on the standard TCP port 22. An SSH client program is typically used for establishing connections to an sshd daemon accepting remote connections. Both are commonly present on most modern operating systems, including Mac OS X, GNU/Linux, Solaris and OpenVMS. Proprietary, freeware and open source versions of various levels of complexity and completeness exist.

(Source: Wikipedia:Secure Shell)

OpenSSH

OpenSSH (OpenBSD Secure Shell) is a set of computer programs providing encrypted communication sessions over a computer network using the ssh protocol. It was created as an open source alternative to the proprietary Secure Shell software suite offered by SSH Communications Security. OpenSSH is developed as part of the OpenBSD project, which is led by Theo de Raadt.

OpenSSH is occasionally confused with the similarly-named OpenSSL; however, the projects have different purposes and are developed by different teams, the similar name is drawn only from similar goals.

Installing OpenSSH

Install openssh from the official repositories.

Configuring SSH

Client

The SSH client configuration file is /etc/ssh/ssh_config or ~/.ssh/config.

It is not longer needed to explicitly set Protocol 2, it is commented out in the default configuration file. That means Protocol 1 will not be used as long as it is not explicitly enabled. (source: http://www.openssh.org/txt/release-5.4)

Daemon

The SSH daemon configuration file can be found and edited in /etc/ssh/sshd_config.

To allow access only for some users add this line:

AllowUsers    user1 user2

To allow access only for some groups:

AllowGroups   group1 group2

To disable root login over SSH, change the PermitRootLogin line into this:

PermitRootLogin no

To add a nice welcome message edit the file /etc/issue and change the Banner line into this:

Banner /etc/issue
Tip:
  • You may want to change the default port from 22 to any higher port (see security through obscurity). Even though the port ssh is running on could be detected by using a port-scanner like nmap, changing it will reduce the number of log entries caused by automated authentication attempts. To help select a port review the list of TCP and UDP port numbers. You can also find port information locally in /etc/services. Select an alternative port that is not already assigned to a common service to prevent conflicts.
  • Disabling password logins entirely will greatly increase security, see SSH Keys for more information.

Managing the sshd daemon

You can start the sshd daemon with the following command:

# systemctl start sshd

You can enable the sshd daemon at startup with the following command:

# systemctl enable sshd.service
Warning: Systemd is an asynchronous starting process. If you bind the SSH daemon to a specific IP address ListenAddress 192.168.1.100 it may fail to load during boot since the default sshd.service unit file has no dependency on network interfaces being enabled. When binding to an IP address, you will need to add After=network.target to a custom sshd.service unit file. See Systemd#Editing provided unit files.

Or you can enable SSH Daemon socket so the daemon is started on the first incoming connection:

# systemctl enable sshd.socket

If you use a different port than the default 22, you have to set "ListenStream" in the unit file. Copy /lib/systemd/system/sshd.socket to /etc/systemd/system/sshd.socket to keep your unit file from being overwritten on upgrades. In /etc/systemd/system/sshd.socket change "ListenStream" the appropriate port.

Warning: Using sshd.socket effectively negates the ListenAddress setting, so using the default sshd.socket will allow connections over any address. To achieve the effect of setting ListenAddress, you must create a custom unit file and modify ListenStream (ie. ListenStream=192.168.1.100:22 is equivalent to ListenAddress 192.168.1.100). However, doing so has the same drawback as setting ListenAddress: the socket will fail to start if the network is not up in time.

Connecting to the server

To connect to a server, run:

$ ssh -p port user@server-address

Protecting SSH

Allowing remote log-on through SSH is good for administrative purposes, but can pose a threat to your server's security. Often the target of brute force attacks, SSH access needs to be limited properly to prevent third parties gaining access to your server.

  • Use non-standard account names and passwords
  • Only allow incoming SSH connections from trusted locations
  • Use fail2ban or sshguard to monitor for brute force attacks, and ban brute forcing IPs accordingly
Protecting against brute force attacks

Brute forcing is a simple concept: One continuously tries to log in to a webpage or server log-in prompt like SSH with a high number of random username and password combinations. You can protect yourself from brute force attacks by using an automated script that blocks anybody trying to brute force their way in, for example fail2ban or sshguard.

Deny root login

It is generally considered bad practice to allow the user root to log in over SSH: The root account will exist on nearly any Linux system and grants full access to the system, once login has been achieved. Sudo provides root rights for actions requiring these and is the more secure solution, third parties would have to find a username present on the system, the matching password and the matching password for sudo to get root rights on your system. More barriers to be breached before full access to the system is reached.

Configure SSH to deny remote logins with the root user by editing /etc/ssh/sshd_config and look for this section:

# Authentication:

#LoginGraceTime 2m
#PermitRootLogin yes
#StrictModes yes
#MaxAuthTries 6
#MaxSessions 10

Now simply change #PermitRootLogin yes to no, and uncomment the line:

PermitRootLogin no

Next, restart the SSH daemon:

# systemctl restart sshd

You will now be unable to log in through SSH under root, but will still be able to log in with your normal user and use su - or sudo to do system administration.

Other SSH clients and servers

Apart from OpenSSH, there are many SSH clients and servers avaliable.

Dropbear

Dropbear is a SSH-2 client and server. dropbearAUR is available in the AUR.

The commandline ssh client is named dbclient.

SSH alternative: Mobile Shell - responsive, survives disconnects

From the Mosh website:

Remote terminal application that allows roaming, supports intermittent connectivity, and provides intelligent local echo and line editing of user keystrokes. Mosh is a replacement for SSH. It's more robust and responsive, especially over Wi-Fi, cellular, and long-distance links.

Install mosh from the official repositories or the latest revision mosh-gitAUR in the AUR.

Tips and tricks

Encrypted SOCKS tunnel

This is highly useful for laptop users connected to various unsafe wireless connections. The only thing you need is an SSH server running at a somewhat secure location, like your home or at work. It might be useful to use a dynamic DNS service like DynDNS so you do not have to remember your IP-address.

Step 1: start the connection

You only have to execute this single command to start the connection:

$ ssh -TND 4711 user@host

where "user" is your username at the SSH server running at the "host". It will ask for your password, and then you're connected! The "N" flag disables the interactive prompt, and the "D" flag specifies the local port on which to listen on (you can choose any port number if you want). The "T" flag disables pseudo-tty allocation.

It's nice to add the verbose "-v" flag, because then you can verify that it's actually connected from that output.

Step 2: configure your browser (or other programs)

The above step is completely useless if you do not configure your web browser (or other programs) to use this newly created socks tunnel. Since the current version of SSH supports both SOCKS4 and SOCKS5, you can use either of them.

  • For Firefox: Edit → Preferences → Advanced → Network → Connection → Setting:
Check the "Manual proxy configuration" radio button, and enter "localhost" in the "SOCKS host" text field, and then enter your port number in the next text field (I used 4711 above).

Firefox does not automatically make DNS requests through the socks tunnel. This potential privacy concern can be mitigated by the following steps:

  1. Type about:config into the Firefox location bar.
  2. Search for network.proxy.socks_remote_dns
  3. Set the value to true.
  4. Restart the browser.
  • For Chromium: You can set the SOCKS settings as environment variables or as command line options. I recommend to add one of the following functions to your .bashrc:
function secure_chromium {
    port=4711
    export SOCKS_SERVER=localhost:$port
    export SOCKS_VERSION=5
    chromium &
    exit
}

OR

function secure_chromium {
    port=4711
    chromium --proxy-server="socks://localhost:$port" &
    exit
}

Now open a terminal and just do:

$ secure_chromium

Enjoy your secure tunnel!

X11 forwarding

To run graphical programs through a SSH connection you can enable X11 forwarding. An option needs to be set in the configuration files on the server and client (here "client" means your (desktop) machine your X11 Server runs on, and you will run X applications on the "server").

Install xorg-xauth from the official repositories onto the server.

  • Enable the AllowTcpForwarding option in sshd_config on the server.
  • Enable the X11Forwarding option in sshd_config on the server.
  • Set the X11DisplayOffset option in sshd_config on the server to 10.
  • Enable the X11UseLocalhost option in sshd_config on the server.

Also:

  • Enable the ForwardX11 option in ssh_config on the client.
  • Enable the ForwardX11Trusted if gui is drawing badly.

You need to restart the ssh daemon on the server for these changes to take effect, of course.

To use the forwarding, log on to your server through ssh:

$ ssh -X -p port user@server-address

If you receive errors trying to run graphical applications try trusted forwarding instead:

$ ssh -Y -p port user@server-address

You can now start any X program on the remote server, the output will be forwarded to your local session:

$ xclock


If you get "Cannot open display" errors try the following command as the non root user:

$ xhost +

the above command will allow anybody to forward X11 applications. To restrict forwarding to a particular host type:

$ xhost +hostname

where hostname is the name of the particular host you want to forward to. Type "man xhost" for more details.

Be careful with some applications as they check for a running instance on the local machine. Firefox is an example. Either close running Firefox or use the following start parameter to start a remote instance on the local machine

$ firefox -no-remote

If you get "X11 forwarding request failed on channel 0" when you connect (and the server /var/log/errors.log shows "Failed to allocate internet-domain X11 display socket"), try to either

  • Enable the AddressFamily any option in sshd_config on the server, or
  • Set the AddressFamily option in sshd_config on the server to inet.

Setting it to inet may fix problems with Ubuntu clients on IPv4.

For running X applications as other user on the SSH server you need to xauth add the authentication line taken from xauth list of the SSH logged in user.

Forwarding other ports

In addition to SSH's built-in support for X11, it can also be used to securely tunnel any TCP connection, by use of local forwarding or remote forwarding.

Local forwarding opens a port on the local machine, connections to which will be forwarded to the remote host and from there on to a given destination. Very often, the forwarding destination will be the same as the remote host, thus providing a secure shell and, e.g. a secure VNC connection, to the same machine. Local forwarding is accomplished by means of the -L switch and it's accompanying forwarding specification in the form of <tunnel port>:<destination address>:<destination port>.

Thus:

$ ssh -L 1000:mail.google.com:25 192.168.0.100

will use SSH to login to and open a shell on 192.168.0.100, and will also create a tunnel from the local machine's TCP port 1000 to mail.google.com on port 25. Once established, connections to localhost:1000 will connect to the Gmail SMTP port. To Google, it will appear that any such connection (though not necessarily the data conveyed over the connection) originated from 192.168.0.100, and such data will be secure as between the local machine and 192.168.0.100, but not between 192.168.0.100, unless other measures are taken.

Similarly:

$ ssh -L 2000:192.168.0.100:6001 192.168.0.100

will allow connections to localhost:2000 which will be transparently sent to the remote host on port 6001. The preceding example is useful for VNC connections using the vncserver utility--part of the tightvnc package--which, though very useful, is explicit about its lack of security.

Remote forwarding allows the remote host to connect to an arbitrary host via the SSH tunnel and the local machine, providing a functional reversal of local forwarding, and is useful for situations where, e.g., the remote host has limited connectivity due to firewalling. It is enabled with the -R switch and a forwarding specification in the form of <tunnel port>:<destination address>:<destination port>.

Thus:

$ ssh -R 3000:irc.freenode.net:6667 192.168.0.200

will bring up a shell on 192.168.0.200, and connections from 192.168.0.200 to itself on port 3000 (remotely speaking, localhost:3000) will be sent over the tunnel to the local machine and then on to irc.freenode.net on port 6667, thus, in this example, allowing the use of IRC programs on the remote host to be used, even if port 6667 would normally be blocked to it.

Both local and remote forwarding can be used to provide a secure "gateway," allowing other computers to take advantage of an SSH tunnel, without actually running SSH or the SSH daemon by providing a bind-address for the start of the tunnel as part of the forwarding specification, e.g. <tunnel address>:<tunnel port>:<destination address>:<destination port>. The <tunnel address> can be any address on the machine at the start of the tunnel, localhost, * (or blank), which, respectively, allow connections via the given address, via the loopback interface, or via any interface. By default, forwarding is limited to connections from the machine at the "beginning" of the tunnel, i.e. the <tunnel address> is set to localhost. Local forwarding requires no additional configuration, however remote forwarding is limited by the remote server's SSH daemon configuration. See the GatewayPorts option in sshd_config(5) for more information.

Speeding up SSH

You can make all sessions to the same host use a single connection, which will greatly speed up subsequent logins, by adding these lines under the proper host in /etc/ssh/ssh_config:

Host examplehost.com
  ControlMaster auto
  ControlPersist yes
  ControlPath ~/.ssh/socket-%r@%h:%p

See the ssh_config(5) manual page for full description of these options.

Another option to improve speed is to enable compression with the -C flag. A permanent solution is to add this line under the proper host in /etc/ssh/ssh_config:

Compression yes

Login time can be shortened by using the -4 flag, which bypasses IPv6 lookup. This can be made permanent by adding this line under the proper host in /etc/ssh/ssh_config:

AddressFamily inet

Changing the ciphers used by SSH to less cpu-demanding ones can improve speed. In this aspect, the best choices are arcfour and blowfish-cbc.

Warning: Please do not do this unless you know what you are doing; arcfour has a number of known weaknesses.

To use alternative ciphers, run SSH with the -c flag:

$ ssh -c arcfour,blowfish-cbc user@server-address

To use them permanently, add this line under the proper host in /etc/ssh/ssh_config:

Ciphers arcfour,blowfish-cbc

Mounting a remote filesystem with SSHFS

Please refer to the Sshfs article to use sshfs to mount a remote system - accessible via SSH - to a local folder, so you will be able to do any operation on the mounted files with any tool (copy, rename, edit with vim, etc.). Using sshfs instead of shfs is generally preferred as a new version of shfs hasn't been released since 2004.

Keep alive

Your ssh session will automatically log out if it is idle. To keep the connection active (alive) add this to ~/.ssh/config or to /etc/ssh/ssh_config on the client.

ServerAliveInterval 120

This will send a "keep alive" signal to the server every 120 seconds.

Conversely, to keep incoming connections alive, you can set

ClientAliveInterval 120

(or some other number greater than 0) in /etc/ssh/sshd_config on the server.

Saving connection data in ssh config

Whenever you want to connect to a ssh server, you usually have to type at least its address and the username. To save that typing work for servers you regularly connect to, you can use the personal ~/.ssh/config or the global /etc/ssh/ssh_config files as shown in the following example:

~/.ssh/config
Host myserver
    HostName 123.123.123.123
    Port 12345
    User bob
Host other_server
    HostName test.something.org
    User alice
    CheckHostIP no
    Cipher blowfish

Now you can simply connect to the server by using the name you specified:

$ ssh myserver

To see a complete list of the possible options, check out ssh_config's manpage on your system or the ssh_config documentation on the official website.

Autossh - automatically restarts SSH sessions and tunnels

When a ssh session or tunnel cannot be kept alive, because for example bad network conditions cause the sshd client to disconnect, you can use Autossh to automatically restart them. Autossh can be installed from the official repositories.

Usage examples:

$ autossh -M 0 -o "ServerAliveInterval 45" -o "ServerAliveCountMax 2" username@example.com

Combined with sshfs :

$ sshfs -o reconnect,compression=yes,transform_symlinks,ServerAliveInterval=45,ServerAliveCountMax=2,ssh_command='autossh -M 0' username@example.com: /mnt/example 

Connecting through a SOCKS-proxy set by Proxy_settings :

$ autossh -M 0 -o "ServerAliveInterval 45" -o "ServerAliveCountMax 2" -NCD 8080 username@example.com 

With the -f option autossh can be made to run as a background process. Running it this way however means the passprase cannot be entered interactively.

The session will end once you type exit in the session, or the autossh process receives a SIGTERM, SIGINT of SIGKILL signal.

If you want to automatically start autossh, it is now easy to get systemd to manage this for you. For example, you could create a systemd unit file like this:

[Unit]
Description=AutoSSH service for port 2222
After=network.target

[Service]
ExecStart=/usr/bin/autossh -M 0 -NL 2222:localhost:2222 -o TCPKeepAlive=yes foo@bar.com

[Install]
WantedBy=multi-user.target

Then place this in, for example, /etc/systemd/system/autossh.service. Of course, you can make this unit more complex if necessary (see the systemd documentation for details), and obviously you can use your own options for autossh.

You can then enable your autossh tunnels with, e.g.:

$ systemctl start autossh

(or whatever you called the service file)

If this works OK for you, you can make this permanent by running

$ systemctl enable autossh

That way autossh will start automatically at boot.

It is also easy to maintain several autossh processes, to keep several tunnels alive. Just create multiple .service files with different names.

Troubleshooting

SSH connection left hanging after poweroff/reboot

SSH connection hangs after poweroff or reboot if systemd stop network before sshd. To fix that problem, comment and change the After statement:

/usr/lib/systemd/system/systemd-user-sessions.service
#After=remote-fs.target
After=network.target

Connection refused or timeout problem

Is your router doing port forwarding?

SKIP THIS STEP IF YOU ARE NOT BEHIND A NAT MODEM/ROUTER (eg, a VPS or otherwise publicly addressed host). Most home and small businesses will have a NAT modem/router.

The first thing is to make sure that your router knows to forward any incoming ssh connection to your machine. Your external IP is given to you by your ISP, and it is associated with any requests coming out of your router. So your router needs to know that any incoming ssh connection to your external IP needs to be forwarded to your machine running sshd.

Find your internal network address.

ip a

Find your interface device and look for the inet field. Then access your router's configuration web interface, using your router's IP (find this on the web). Tell your router to forward it to your inet IP. Go to [1] for more instructions on how to do so for your particular router.

Is SSH running and listening?

$ ss -tnlp

If the above command do not show SSH port is open, SSH is NOT running. Check /var/log/messages for errors etc.

Are there firewall rules blocking the connection?

Iptables may be blocking connections on port 22. Check this with:

# iptables -nvL

and look for rules that might be dropping packets on the INPUT chain. Then, if necessary, unblock the port with a command like:

# iptables -I INPUT 1 -p tcp --dport 22 -j ACCEPT

For more help configuring firewalls, see firewalls.

Is the traffic even getting to your computer?

Start a traffic dump on the computer you're having problems with:

# tcpdump -lnn -i any port ssh and tcp-syn

This should show some basic information, then wait for any matching traffic to happen before displaying it. Try your connection now. If you do not see any output when you attempt to connect, then something outside of your computer is blocking the traffic (e. g., hardware firewall, NAT router etc.).

Your ISP or a third party blocking default port?

Note: Try this step if you KNOW you aren't running any firewalls and you know you have configured the router for DMZ or have forwarded the port to your computer and it still doesn't work. Here you will find diagnostic steps and a possible solution.

In some cases, your ISP might block the default port (SSH port 22) so whatever you try (opening ports, hardening the stack, defending against flood attacks, et al) ends up useless. To confirm this, create a server on all interfaces (0.0.0.0) and connect remotely.

If you get an error message comparable to this:

ssh: connect to host www.inet.hr port 22: Connection refused

That means the port ISN'T being blocked by the ISP, but the server doesn't run SSH on that port (See security through obscurity).

However, if you get an error message comparable to this:

ssh: connect to host 111.222.333.444 port 22: Operation timed out 

That means that something is rejecting your TCP traffic on port 22. Basically that port is stealth, either by your firewall or 3rd party intervetion (like an ISP blocking and/or rejecting incoming traffic on port 22). If you know you aren't running any firewall on your computer, and you know that Gremlins aren't growing in your routers and switches, then your ISP is blocking the traffic.

To double check, you can run Wireshark on your server and listen to traffic on port 22. Since Wireshark is a Layer 2 Packet Sniffing utility, and TCP/UDP are Layer 3 and above (See IP Network stack), if you don't receive anything while connecting remotely, a third party is most likely to be blocking the traffic on that port to your server.

Diagnosis via Wireshark

Install Wireshark with the wireshark-cli package, available in the official repositories.

And then run it using,

tshark -f "tcp port 22" -i NET_IF

where NET_IF is the network interface for a WAN connection (see ip a to check). If you aren't receiving any packets while trying to connect remotely, you can be very sure that your ISP is blocking the incoming traffic on port 22.

Possible solution

The solution is just to use some other port that the ISP isn't blocking. Open the /etc/ssh/sshd_config and configure the file to use different ports. For example, add:

Port 22
Port 1234

Also make sure that other "Port" configuration lines in the file are commented out. Just commenting "Port 22" and putting "Port 1234" won't solve the issue because then sshd will only listen on port 1234. Use both lines to run the SSH server on both ports.

Restart the server systemctl restart sshd.service and you're almost done. You still have to configure your client(s) to use the other port instead of the default port. There are numerous solutions to that problem, but let's cover two of them here.

Read from socket failed: connection reset by peer

Recent versions of openssh sometimes fail with the above error message, due to a bug involving elliptic curve cryptography. In that case add the following line to ~/.ssh/config:

HostKeyAlgorithms ssh-rsa-cert-v01@openssh.com,ssh-dss-cert-v01@openssh.com,ssh-rsa-cert-v00@openssh.com,ssh-dss-cert-v00@openssh.com,ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,ssh-rsa,ssh-dss

With openssh 5.9, the above fix doesn't work. Instead, put the following lines in ~/.ssh/config:

Ciphers aes128-ctr,aes192-ctr,aes256-ctr,aes128-cbc,3des-cbc 
MACs hmac-md5,hmac-sha1,hmac-ripemd160

See also the discussion on the openssh bug forum.

"[your shell]: No such file or directory" / ssh_exchange_identification problem

One possible cause for this is the need of certain SSH clients to find an absolute path (one returned by whereis -b [your shell], for instance) in $SHELL, even if the shell's binary is located in one of the $PATH entries. Another reason can be that the user is no member of the network group.

"Terminal unknown" or "Error opening terminal" error message

With ssh it is possible to receive errors like "Terminal unknown" upon logging in. Starting ncurses applications like nano fails with the message "Error opening terminal". There are two methods to this problem, a quick one using the $TERM variable and a profound one using the terminfo file.

Workaround by setting the $TERM variable

After connecting to the remote server set the $TERM variable to "xterm" with the following command.

TERM=xterm

This method is a workaround and should be used on ssh servers you do seldomly connect to, because it can have unwanted side effects. Also you have to repeat the command after every connection, or alternatively set it in ~.bashrc .

Solution using terminfo file

A profound solution is transferring the terminfo file of the terminal on your client computer to the ssh server. In this example we cover how to setup the terminfo file for the "rxvt-unicode-256color" terminal. Create the directory containing the terminfo files on the ssh server, while you are logged in to the server issue this command:

mkdir -p ~/.terminfo/r/

Now copy the terminfo file of your terminal to the new directory. Replace "rxvt-unicode-256color" with your client's terminal in the following command and ssh-server with the relevant user and server adress.

$ scp /usr/share/terminfo/r/rxvt-unicode-256color ssh-server:~/.terminfo/r/

After logging in and out from the ssh server the problem should be fixed.

See also

Links & references