13.8. OpenSSH

Contributed by Chern Lee.

OpenSSH is a set of network connectivity tools used to provide secure access to remote machines. Additionally, TCP/IP connections can be tunneled or forwarded securely through SSH connections. OpenSSH encrypts all traffic to effectively eliminate eavesdropping, connection hijacking, and other network-level attacks.

OpenSSH is maintained by the OpenBSD project and is installed by default in FreeBSD. It is compatible with both SSH version 1 and 2 protocols.

When data is sent over the network in an unencrypted form, network sniffers anywhere in between the client and server can steal user/password information or data transferred during the session. OpenSSH offers a variety of authentication and encryption methods to prevent this from happening. More information about OpenSSH is available from http://www.openssh.com/.

This section provides an overview of the built-in client utilities to securely access other systems and securely transfer files from a FreeBSD system. It then describes how to configure a SSH server on a FreeBSD system. More information is available in the man pages mentioned in this chapter.

13.8.1. 使用 SSH 客戶端工具

To log into a SSH server, use ssh and specify a username that exists on that server and the IP address or hostname of the server. If this is the first time a connection has been made to the specified server, the user will be prompted to first verify the server's fingerprint:

# ssh user@example.com
The authenticity of host 'example.com (' can't be established.
ECDSA key fingerprint is 25:cc:73:b5:b3:96:75:3d:56:19:49:d2:5c:1f:91:3b.
Are you sure you want to continue connecting (yes/no)? yes
Permanently added 'example.com' (ECDSA) to the list of known hosts.
Password for user@example.com: user_password

SSH utilizes a key fingerprint system to verify the authenticity of the server when the client connects. When the user accepts the key's fingerprint by typing yes when connecting for the first time, a copy of the key is saved to .ssh/known_hosts in the user's home directory. Future attempts to login are verified against the saved key and ssh will display an alert if the server's key does not match the saved key. If this occurs, the user should first verify why the key has changed before continuing with the connection.

By default, recent versions of OpenSSH only accept SSHv2 connections. By default, the client will use version 2 if possible and will fall back to version 1 if the server does not support version 2. To force ssh to only use the specified protocol, include -1 or -2. Additional options are described in ssh(1).

Use scp(1) to securely copy a file to or from a remote machine. This example copies COPYRIGHT on the remote system to a file of the same name in the current directory of the local system:

# scp user@example.com:/COPYRIGHT COPYRIGHT
Password for user@example.com: *******
COPYRIGHT            100% |*****************************|  4735

Since the fingerprint was already verified for this host, the server's key is automatically checked before prompting for the user's password.

The arguments passed to scp are similar to cp. The file or files to copy is the first argument and the destination to copy to is the second. Since the file is fetched over the network, one or more of the file arguments takes the form user@host:<path_to_remote_file>. Be aware when copying directories recursively that scp uses -r, whereas cp uses -R.

To open an interactive session for copying files, use sftp. Refer to sftp(1) for a list of available commands while in an sftp session. 以金鑰為基礎的認證

Instead of using passwords, a client can be configured to connect to the remote machine using keys. To generate DSA or RSA authentication keys, use ssh-keygen. To generate a public and private key pair, specify the type of key and follow the prompts. It is recommended to protect the keys with a memorable, but hard to guess passphrase.

% 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): type some passphrase here which can contain spaces
Enter same passphrase again: type some passphrase here which can contain spaces
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

Depending upon the specified protocol, the private key is stored in ~/.ssh/id_dsa (or ~/.ssh/id_rsa), and the public key is stored in ~/.ssh/id_dsa.pub (or ~/.ssh/id_rsa.pub). The public key must be first copied to ~/.ssh/authorized_keys on the remote machine in order for key-based authentication to work.


Many users believe that keys are secure by design and will use a key without a passphrase. This is dangerous behavior. An administrator can verify that a key pair is protected by a passphrase by viewing the private key manually. If the private key file contains the word ENCRYPTED, the key owner is using a passphrase. In addition, to better secure end users, from may be placed in the public key file. For example, adding from="" in the front of ssh-rsa or rsa-dsa prefix will only allow that specific user to login from that IP address.

The various options and files can be different according to the OpenSSH version. To avoid problems, consult ssh-keygen(1).

If a passphrase is used, the user will be prompted for the passphrase each time a connection is made to the server. To load SSH keys into memory, without needing to type the passphrase each time, use ssh-agent(1) and ssh-add(1).

Authentication is handled by ssh-agent, using the private key(s) that are loaded into it. Then, ssh-agent should be used to launch another application such as a shell or a window manager.

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

% ssh-agent csh
% ssh-add
Enter passphrase for key '/usr/home/user/.ssh/id_dsa': type passphrase here
Identity added: /usr/home/user/.ssh/id_dsa (/usr/home/user/.ssh/id_dsa)

To use ssh-agent in Xorg, add an entry for it in ~/.xinitrc. This provides the ssh-agent services to all programs launched in Xorg. An example ~/.xinitrc might look like this:

exec ssh-agent startxfce4

This launches ssh-agent, which in turn launches XFCE, every time Xorg starts. Once Xorg has been restarted so that the changes can take effect, run ssh-add to load all of the SSH keys. SSH 通道

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

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

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

This example uses the following options:


Forces ssh to use version 2 to connect to the server.


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


Forces ssh to run in the background.


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


The login name to use on the specified remote SSH server.

An SSH tunnel works by creating a listen socket on localhost on the specified localport. It then forwards any connections received on localport via the SSH connection to the specified remotehost:remoteport. In the example, port 5023 on the client is forwarded to port 23 on the remote machine. Since port 23 is used by telnet, this creates an encrypted telnet session through an SSH tunnel.

This method can be used to wrap any number of insecure TCP protocols such as SMTP, POP3, and FTP, as seen in the following examples.

範例 13.1. 建立供 SMTP 使用的安全通道
% ssh -2 -N -f -L 5025:localhost:25 user@mailserver.example.com
user@mailserver.example.com's password: *****
% telnet localhost 5025
Connected to localhost.
Escape character is '^]'.
220 mailserver.example.com ESMTP

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

範例 13.2. 安全存取 POP3 伺服器

In this example, there is an SSH server that accepts connections from the outside. On the same network resides a mail server running a POP3 server. To check email in a secure manner, create an SSH connection to the 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: ******

Once the tunnel is up and running, point the email client to send POP3 requests to localhost on port 2110. This connection will be forwarded securely across the tunnel to mail.example.com.

範例 13.3. 跳過防火牆

Some firewalls filter both incoming and outgoing connections. For example, a firewall might limit access from remote machines to ports 22 and 80 to only allow SSH and web surfing. This prevents access to any other service which uses a port other than 22 or 80.

The solution is to create an SSH connection to a machine outside of the network's firewall and use it to tunnel to the desired service:

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

In this example, a streaming Ogg Vorbis client can now be pointed to localhost port 8888, which will be forwarded over to music.example.com on port 8000, successfully bypassing the firewall.

13.8.2. 開啟 SSH 伺服器

In addition to providing built-in SSH client utilities, a FreeBSD system can be configured as an SSH server, accepting connections from other SSH clients.

To see if sshd is operating, use the service(8) command:

# service sshd status

If the service is not running, add the following line to /etc/rc.conf.


This will start sshd, the daemon program for OpenSSH, the next time the system boots. To start it now:

# service sshd start

The first time sshd starts on a FreeBSD system, the system's host keys will be automatically created and the fingerprint will be displayed on the console. Provide users with the fingerprint so that they can verify it the first time they connect to the server.

Refer to sshd(8) for the list of available options when starting sshd and a more complete discussion about authentication, the login process, and the various configuration files.

At this point, the sshd should be available to all users with a username and password on the system.

13.8.3. SSH 伺服器安全性

While sshd is the most widely used remote administration facility for FreeBSD, brute force and drive by attacks are common to any system exposed to public networks. Several additional parameters are available to prevent the success of these attacks and will be described in this section.

It is a good idea to limit which users can log into the SSH server and from where using the AllowUsers keyword in the OpenSSH server configuration file. For example, to only allow root to log in from, add this line to /etc/ssh/sshd_config:

AllowUsers root@

To allow admin to log in from anywhere, list that user without specifying an IP address:

AllowUsers admin

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

AllowUsers root@ admin

After making changes to /etc/ssh/sshd_config, tell sshd to reload its configuration file by running:

# service sshd reload


When this keyword is used, it is important to list each user that needs to log into this machine. Any user that is not specified in that line will be locked out. Also, the keywords used in the OpenSSH server configuration file are case-sensitive. If the keyword is not spelled correctly, including its case, it will be ignored. Always test changes to this file to make sure that the edits are working as expected. Refer to sshd_config(5) to verify the spelling and use of the available keywords.

In addition, users may be forced to use two factor authentication via the use of a public and private key. When required, the user may generate a key pair through the use of ssh-keygen(1) and send the administrator the public key. This key file will be placed in the authorized_keys as described above in the client section. To force the users to use keys only, the following option may be configured:

AuthenticationMethods publickey


Do not confuse /etc/ssh/sshd_config with /etc/ssh/ssh_config (note the extra d in the first filename). The first file configures the server and the second file configures the client. Refer to ssh_config(5) for a listing of the available client settings.

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