Beyond Linux From Scratch - Version 6.1

Chapter 21. Major Servers

OpenSSH-4.1p1

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.

Package Information

OpenSSH Dependencies

Required

OpenSSL-0.9.7g

Optional

Linux-PAM-0.80, tcpwrappers-7.6, X (XFree86-4.5.0 or X.org-6.8.2), MIT krb5-1.4.1 or Heimdal-0.7, JDK-1.5.0, Net-tools-1.60, Sysstat-6.0.0, OpenSC and libedit

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 the following commands: 

install -v -d -m700 /var/lib/sshd &&
chown 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

OpenSSH is very sensitive to changes in the linked OpenSSL libraries. If you recompile OpenSSL, OpenSSH may fail to startup. An alternative is to link against the static OpenSSL library. To link against the static library, execute the following command: 

sed -i "s:-lcrypto:/usr/lib/libcrypto.a:g" configure

Install OpenSSH by running the following commands: 

./configure --prefix=/usr --sysconfdir=/etc/ssh \
    --libexecdir=/usr/sbin --with-md5-passwords \
    --with-privsep-path=/var/lib/sshd

If you use Heimdal as your Kerberos5 implementation and you linked the Heimdal libraries into the build using the --with-kerberos5 parameter, you'll need to modify the Makefile or the build will fail. Use the following command: 

sed -i -e "s/lkrb5 -ldes/lkrb5/" Makefile

Continue the build: 

make

If you linked tcp_wrappers into the build using the --with-tcp-wrappers parameter, ensure you add 127.0.0.1 to the sshd line in /etc/hosts.allow if you have a restrictive /etc/hosts.deny file, or the testsuite will fail. To run the testsuite, issue: make -k tests.

Now, as the root user: 

make install

Command Explanations

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

--with-md5-passwords: This is required if you made the changes recommended by the shadowpasswd_plus LFS hint on your SSH server when you installed the Shadow Password Suite or if you access a SSH server that authenticates by user passwords encrypted with md5.

--libexecdir=/usr/sbin: This parameter changes the installation path of some programs to /usr/sbin instead of /usr/libexec.

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

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-6.1 package. 

make install-sshd

Contents

Installed Programs: scp, sftp, sftp-server, slogin, ssh, sshd, ssh-add, ssh-agent, ssh-keygen, ssh-keyscan, and ssh-keysign
Installed Libraries: None
Installed Directories: /etc/ssh 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 SSH1 and SSH2 protocols.

sftp-server

is an SFTP server subsystem.

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-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.

Last updated on 2005-08-01 13:29:19 -0600