Beyond Linux® From Scratch - Version 7.5

Chapter 4. Security

OpenSSH-6.5p1

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 implementions of telnet and rcp respectively.

This package is known to build and work properly using an LFS-7.5 platform.

Package Information

OpenSSH Dependencies

Required

OpenSSL-1.0.1f

Optional

Linux-PAM-1.1.8, X Window System, MIT Kerberos V5-1.12.1, libedit, OpenSC, and libsectok

Optional Runtime (Used only to gather entropy)

OpenJDK-1.7.0.51/IcedTea-2.4.5, Net-tools-CVS_20101030, and Sysstat-10.2.1.

User Notes: http://wiki.linuxfromscratch.org/blfs/wiki/OpenSSH

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 -m700 -d /var/lib/sshd &&
chown   -v root:sys /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-md5-passwords              \
            --with-privsep-path=/var/lib/sshd &&
make

The testsuite requires an installed copy of scp to complete the multiplexing tests. To run the test suite, first copy the scp program to /usr/bin, making sure that you back up any existing copy first.

To test the results, issue: make 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-6.5p1           &&
install -v -m644 INSTALL LICENCE OVERVIEW README* /usr/share/doc/openssh-6.5p1

Command Explanations

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

--with-md5-passwords: This enables the use of MD5 passwords.

--with-pam: This parameter enables Linux-PAM support in the build.

--with-xauth=/usr/bin/xauth: Set the default location for the xauth binary for X authentication. Change the location if xauth will be installed to a different path. This can also be controlled from sshd_config with the XAuthLocation keyword. You can omit this switch if Xorg 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_HOSTNAME for the hostname of the remote computer and you'll also need to enter you password for the ssh command to succeed: 

ssh-keygen &&
public_key="$(cat ~/.ssh/id_rsa.pub)" &&
ssh REMOTE_HOSTNAME "echo ${public_key} >> ~/.ssh/authorized_keys" &&
unset public_key

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 "ChallengeResponseAuthentication no" >> /etc/ssh/sshd_config

If you added LinuxPAM 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.

Boot Script

To start the SSH server at system boot, install the /etc/rc.d/init.d/sshd init script included in the blfs-bootscripts-20140301 package. 

make install-sshd

Contents

Installed Programs: scp, sftp, sftp-server, slogin (symlink to ssh), ssh, sshd, ssh-add, ssh-agent, ssh-copy-id, ssh-keygen, ssh-keyscan, ssh-keysign, and ssh-pkcs11-helper
Installed Libraries: None
Installed Directories: /etc/ssh, /usr/lib/openssh, /usr/share/doc/openssh-6.5p1, and /var/lib/sshd

Short Descriptions

scp

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

sftp

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

sftp-server

is an SFTP server subsystem. This program is not normally called directly by the user.

slogin

is a symlink to ssh.

ssh

is an rlogin/rsh-like client program except 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 machine 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.

ssh-keysign

is used by ssh to access the local host keys and generate the digital signature required during hostbased authentication with SSH protocol version 2. This program is not normally called directly by the user.

ssh-pkcs11-helper

is a ssh-agent helper program for PKCS#11 support.

Last updated on 2014-02-19 12:16:20 -0800