15.11. OpenSSH

Contributed by Chern Lee.

OpenSSH is a set of network connectivity tools used to access remote machines securely. It can be used as a direct replacement for rlogin, rsh, rcp, and telnet. Additionally, TCP/IP connections can be tunneled/forwarded securely through SSH. OpenSSH encrypts all traffic to effectively eliminate eavesdropping, connection hijacking, and other network-level attacks.

OpenSSH is maintained by the OpenBSD project, and is based upon SSH v1.2.12 with all the recent bug fixes and updates. It is compatible with both SSH protocols 1 and 2.

15.11.1. Advantages of Using OpenSSH

Normally, when using telnet(1) or rlogin(1), data is sent over the network in an clear, un-encrypted form. Network sniffers anywhere in between the client and server can steal your user/password information or data transferred in your session. OpenSSH offers a variety of authentication and encryption methods to prevent this from happening.

15.11.2. Enabling sshd

The sshd is an option presented during a Standard install of FreeBSD. To see if sshd is enabled, check the rc.conf file for:

sshd_enable="YES"

This will load sshd(8), the daemon program for OpenSSH, the next time your system initializes. Alternatively, it is possible to use /etc/rc.d/sshd rc(8) script to start OpenSSH:

/etc/rc.d/sshd start

15.11.3. SSH Client

The ssh(1) utility works similarly to rlogin(1).

# ssh user@example.com
Host key not found from the list of known hosts.
Are you sure you want to continue connecting (yes/no)? yes
Host 'example.com' added to the list of known hosts.
user@example.com's password: *******

The login will continue just as it would have if a session was created using rlogin or telnet. SSH utilizes a key fingerprint system for verifying the authenticity of the server when the client connects. The user is prompted to enter yes only when connecting for the first time. Future attempts to login are all verified against the saved fingerprint key. The SSH client will alert you if the saved fingerprint differs from the received fingerprint on future login attempts. The fingerprints are saved in ~/.ssh/known_hosts, or ~/.ssh/known_hosts2 for SSH v2 fingerprints.

By default, recent versions of the OpenSSH servers only accept SSH v2 connections. The client will use version 2 if possible and will fall back to version 1. The client can also be forced to use one or the other by passing it the -1 or -2 for version 1 or version 2, respectively. The version 1 compatibility is maintained in the client for backwards compatibility with older versions.

15.11.4. Secure Copy

The scp(1) command works similarly to rcp(1); it copies a file to or from a remote machine, except in a secure fashion.

#  scp user@example.com:/COPYRIGHT COPYRIGHT
user@example.com's password: *******
COPYRIGHT            100% |*****************************|  4735
00:00
#

Since the fingerprint was already saved for this host in the previous example, it is verified when using scp(1) here.

The arguments passed to scp(1) are similar to cp(1), with the file or files in the first argument, and the destination in the second. Since the file is fetched over the network, through SSH, one or more of the file arguments takes on the form user@host:<path_to_remote_file>.

15.11.5. Configuration

The system-wide configuration files for both the OpenSSH daemon and client reside within the /etc/ssh directory.

ssh_config configures the client settings, while sshd_config configures the daemon.

Additionally, the sshd_program (/usr/sbin/sshd by default), and sshd_flags rc.conf options can provide more levels of configuration.

15.11.6. ssh-keygen

Instead of using passwords, ssh-keygen(1) can be used to generate DSA or RSA keys to authenticate a user:

% ssh-keygen -t dsa
Generating public/private dsa key pair.
Enter file in which to save the key (/home/user/.ssh/id_dsa):
Created directory '/home/user/.ssh'.
Enter passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved in /home/user/.ssh/id_dsa.
Your public key has been saved in /home/user/.ssh/id_dsa.pub.
The key fingerprint is:
bb:48:db:f2:93:57:80:b6:aa:bc:f5:d5:ba:8f:79:17 user@host.example.com

ssh-keygen(1) will create a public and private key pair for use in authentication. The private key is stored in ~/.ssh/id_dsa or ~/.ssh/id_rsa, whereas the public key is stored in ~/.ssh/id_dsa.pub or ~/.ssh/id_rsa.pub, respectively for DSA and RSA key types. The public key must be placed in ~/.ssh/authorized_keys of the remote machine in order for the setup to work. Similarly, RSA version 1 public keys should be placed in ~/.ssh/authorized_keys.

This will allow connection to the remote machine based upon SSH keys instead of passwords.

If a passphrase is used in ssh-keygen(1), the user will be prompted for a password each time in order to use the private key. ssh-agent(1) can alleviate the strain of repeatedly entering long passphrases, and is explored in the Τμήμα 15.11.7, «ssh-agent and ssh-add» section below.

Προειδοποίηση:

The various options and files can be different according to the OpenSSH version you have on your system; to avoid problems you should consult the ssh-keygen(1) manual page.

15.11.7. ssh-agent and ssh-add

The ssh-agent(1) and ssh-add(1) utilities provide methods for SSH keys to be loaded into memory for use, without needing to type the passphrase each time.

The ssh-agent(1) utility will handle the authentication using the private key(s) that are loaded into it. ssh-agent(1) should be used to launch another application. At the most basic level, it could spawn a shell or at a more advanced level, a window manager.

To use ssh-agent(1) in a shell, first it will need to be spawned with a shell as an argument. Secondly, the identity needs to be added by running ssh-add(1) and providing it the passphrase for the private key. Once these steps have been completed the user will be able to ssh(1) to any host that has the corresponding public key installed. For example:

% ssh-agent csh
% ssh-add
Enter passphrase for /home/user/.ssh/id_dsa:
Identity added: /home/user/.ssh/id_dsa (/home/user/.ssh/id_dsa)
%

To use ssh-agent(1) in X11, a call to ssh-agent(1) will need to be placed in ~/.xinitrc. This will provide the ssh-agent(1) services to all programs launched in X11. An example ~/.xinitrc file might look like this:

exec ssh-agent startxfce4

This would launch ssh-agent(1), which would in turn launch XFCE, every time X11 starts. Then once that is done and X11 has been restarted so that the changes can take effect, simply run ssh-add(1) to load all of your SSH keys.

15.11.8. SSH Tunneling

OpenSSH has the ability to create a tunnel to encapsulate another protocol in an encrypted session.

The following command tells ssh(1) to create a tunnel for telnet:

% ssh -2 -N -f -L 5023:localhost:23 user@foo.example.com
%

The ssh command is used with the following options:

-2

Forces ssh to use version 2 of the protocol. (Do not use if you are working with older SSH servers)

-N

Indicates no command, or tunnel only. If omitted, ssh would initiate a normal session.

-f

Forces ssh to run in the background.

-L

Indicates a local tunnel in localport:remotehost:remoteport fashion.

user@foo.example.com

The remote SSH server.

An SSH tunnel works by creating a listen socket on localhost on the specified port. It then forwards any connection received on the local host/port via the SSH connection to the specified remote host and port.

In the example, port 5023 on localhost is being forwarded to port 23 on localhost of the remote machine. Since 23 is telnet, this would create a secure telnet session through an SSH tunnel.

This can be used to wrap any number of insecure TCP protocols such as SMTP, POP3, FTP, etc.

Παράδειγμα 15.1. Using SSH to Create a Secure Tunnel for SMTP
% ssh -2 -N -f -L 5025:localhost:25 user@mailserver.example.com
user@mailserver.example.com's password: *****
% telnet localhost 5025
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
220 mailserver.example.com ESMTP

This can be used in conjunction with an ssh-keygen(1) and additional user accounts to create a more seamless/hassle-free SSH tunneling environment. Keys can be used in place of typing a password, and the tunnels can be run as a separate user.


15.11.8.1. Practical SSH Tunneling Examples

15.11.8.1.1. Secure Access of a POP3 Server

At work, there is an SSH server that accepts connections from the outside. On the same office network resides a mail server running a POP3 server. The network, or network path between your home and office may or may not be completely trustable. Because of this, you need to check your e-mail in a secure manner. The solution is to create an SSH connection to your office's SSH server, and tunnel through to the mail server.

% ssh -2 -N -f -L 2110:mail.example.com:110 user@ssh-server.example.com
user@ssh-server.example.com's password: ******

When the tunnel is up and running, you can point your mail client to send POP3 requests to localhost port 2110. A connection here will be forwarded securely across the tunnel to mail.example.com.

15.11.8.1.2. Bypassing a Draconian Firewall

Some network administrators impose extremely draconian firewall rules, filtering not only incoming connections, but outgoing connections. You may be only given access to contact remote machines on ports 22 and 80 for SSH and web surfing.

You may wish to access another (perhaps non-work related) service, such as an Ogg Vorbis server to stream music. If this Ogg Vorbis server is streaming on some other port than 22 or 80, you will not be able to access it.

The solution is to create an SSH connection to a machine outside of your network's firewall, and use it to tunnel to the Ogg Vorbis server.

% ssh -2 -N -f -L 8888:music.example.com:8000 user@unfirewalled-system.example.org
user@unfirewalled-system.example.org's password: *******

Your streaming client can now be pointed to localhost port 8888, which will be forwarded over to music.example.com port 8000, successfully evading the firewall.

15.11.9. The AllowUsers Users Option

It is often a good idea to limit which users can log in and from where. The AllowUsers option is a good way to accomplish this. For example, to only allow the root user to log in from 192.168.1.32, something like this would be appropriate in the /etc/ssh/sshd_config file:

AllowUsers root@192.168.1.32

To allow the user admin to log in from anywhere, just list the username by itself:

AllowUsers admin

Multiple users should be listed on the same line, like so:

AllowUsers root@192.168.1.32 admin

Σημείωση:

It is important that you list each user that needs to log in to this machine; otherwise they will be locked out.

After making changes to /etc/ssh/sshd_config you must tell sshd(8) to reload its config files, by running:

# /etc/rc.d/sshd reload

15.11.10. Further Reading

OpenSSH

ssh(1) scp(1) ssh-keygen(1) ssh-agent(1) ssh-add(1) ssh_config(5)

sshd(8) sftp-server(8) sshd_config(5)

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