Beyond Linux® From Scratch (systemd Edition) - Version r13.1-469

Chapter 4. Security

OpenSSH-10.6p1

Introduction to OpenSSH

The OpenSSH package contains ssh clients and the sshd daemon. This is useful for encrypting authentication and subsequent traffic over a network. The ssh and scp commands are secure implementations of telnet and rcp.

[Note]

Note

Development versions of BLFS may not build or run some packages properly if LFS or dependencies have been updated since the most recent stable versions of the books.

Package Information

OpenSSH Dependencies

Optional

GDB-18.1 (for tests), Linux-PAM-1.7.3 (PAM configuration files from Shadow-4.20.3 are used to create openssh ones), Xorg Applications (or Xorg build environment, see Command Explanations), MIT Kerberos V5-1.22.2, libedit, LibreSSL Portable, OpenSC, and libsectok

Optional at runtime (used only to gather entropy)

Net-tools-2.10, and Sysstat-12.8.0

Installation of OpenSSH

OpenSSH runs as two processes when connecting to other computers. The first process is a privileged process and controls the issuance of privileges as necessary. The second process communicates with the network. Additional installation steps are necessary to set up the proper environment, which are performed by issuing the following commands as the root user: 

install -v -g sys -m700 -d /var/lib/sshd &&

groupadd -g 50 sshd        &&
useradd  -c 'sshd PrivSep' \
         -d /var/lib/sshd  \
         -g sshd           \
         -s /bin/false     \
         -u 50 sshd

Install OpenSSH by running the following commands: 

./configure --prefix=/usr                            \
            --sysconfdir=/etc/ssh                    \
            --with-privsep-path=/var/lib/sshd        \
            --with-default-path=/usr/bin             \
            --with-superuser-path=/usr/sbin:/usr/bin \
            --with-pid-dir=/run                      &&
make

To test the results, issue: make -j1 tests.

Now, as the root user: 

make install &&
install -v -m755    contrib/ssh-copy-id /usr/bin     &&

install -v -m644    contrib/ssh-copy-id.1 \
                    /usr/share/man/man1              &&
install -v -m755 -d /usr/share/doc/openssh-10.6p1    &&
install -v -m644    INSTALL LICENCE OVERVIEW README* \
                    /usr/share/doc/openssh-10.6p1

Command Explanations

--sysconfdir=/etc/ssh: This prevents the configuration files from being installed in /usr/etc.

--with-default-path=/usr/bin and --with-superuser-path=/usr/sbin:/usr/bin: These set the PATH setting to be consistent with the LFS and BLFS Shadow package.

--with-pid-dir=/run: This prevents OpenSSH from referring to the deprecated /var/run directory.

--with-pam: This option enables Linux-PAM support in the build. If you use this option, ensure that you create the PAM configuration in the Configuration Information section below.

--with-xauth=$XORG_PREFIX/bin/xauth: This sets the default location for the xauth binary for X authentication. The environment variable XORG_PREFIX should be set following Xorg build environment. This can also be controlled from sshd_config with the XAuthLocation keyword. You can omit this switch if xauth (one of Xorg Applications) is already installed.

--with-kerberos5=/usr: This option is used to include Kerberos 5 support in the build.

--with-libedit: This option enables line editing and history features for sftp.

Configuring OpenSSH

Config Files

~/.ssh/*, /etc/ssh/ssh_config, and /etc/ssh/sshd_config

There are no required changes to any of these files. However, you may wish to view the /etc/ssh/ files and make any changes appropriate for the security of your system. One recommended change is that you disable root login via ssh. Execute the following command as the root user to disable root login via ssh: 

echo "PermitRootLogin no" >> /etc/ssh/sshd_config

If you want to be able to log in without typing in your password, first create ~/.ssh/id_rsa and ~/.ssh/id_rsa.pub with ssh-keygen and then copy ~/.ssh/id_rsa.pub to ~/.ssh/authorized_keys on the remote computer that you want to log into. You'll need to change REMOTE_USERNAME and REMOTE_HOSTNAME for the username and hostname of the remote computer and you'll also need to enter your password for the ssh-copy-id command to succeed: 

ssh-keygen &&
ssh-copy-id -i ~/.ssh/id_ed25519.pub REMOTE_USERNAME@REMOTE_HOSTNAME

Once you've got passwordless logins working it's actually more secure than logging in with a password (as the private key is much longer than most people's passwords). If you would like to now disable password logins, as the root user: 

echo "PasswordAuthentication no" >> /etc/ssh/sshd_config &&
echo "KbdInteractiveAuthentication no" >> /etc/ssh/sshd_config

If you added Linux-PAM support and you want ssh to use it then you will need to add a configuration file for sshd and enable use of LinuxPAM. Note, ssh only uses PAM to check passwords, if you've disabled password logins these commands are not needed. If you want to use PAM, issue the following commands as the root user: 

sed 's@d/login@d/sshd@g' /etc/pam.d/login > /etc/pam.d/sshd &&
chmod 644 /etc/pam.d/sshd &&
echo "UsePAM yes" >> /etc/ssh/sshd_config

Additional configuration information can be found in the man pages for sshd, ssh and ssh-agent.

Systemd Unit

To start the SSH server at system boot, install the sshd.service unit included in the blfs-systemd-units-20261022 package.

[Note]

Note

Changing the setting of ListenAddress in /etc/sshd/sshd_config is unsupported with the BLFS sshd systemd unit. 

make install-sshd

You can then enable the unit at startup as the root user with: 

systemctl enable sshd
[Note]

Note

This package comes with two types of units: A service file and a socket file. The service file will start sshd daemon once at boot and it will keep running until the system shuts down. The socket file will make systemd listen on ssh port (Default 22, needs to be edited for anything else) and will start sshd daemon when something tries to connect to that port and stop the daemon when the connection is terminated. This is called socket activation and is analogous to using {,x}inetd on a SysVinit based system.

With the above command, the first method is used - sshd daemon is started at boot and stopped at shutdown. If the socket method is desired, you need to run instead (as the root user): 

systemctl enable sshd.socket

Contents

Installed Programs: scp, sftp, ssh, ssh-add, ssh-agent, ssh-copy-id, ssh-keygen, ssh-keyscan, and sshd
Installed Libraries: None
Installed Directories: /etc/ssh, /usr/share/doc/openssh-10.6p1, and /var/lib/sshd

Short Descriptions

scp

is a file copy program that acts like rcp except that it uses an encrypted protocol

sftp

is an FTP-like program that works over the SSH1 and SSH2 protocols

ssh

is an rlogin/rsh-like client program except that it uses an encrypted protocol

sshd

is a daemon that listens for ssh login requests

ssh-add

is a tool which adds keys to the ssh-agent

ssh-agent

is an authentication agent that can store private keys

ssh-copy-id

is a script that enables logins on remote machines using local keys

ssh-keygen

is a key generation tool

ssh-keyscan

is a utility for gathering public host keys from a number of hosts