OpenSSH has a number of configuration options that can be employed to reduce risk. Most Linux distributions come with good middle-of-the-road settings. In some cases, these can be adjusted for better security. These include:
Logging: turn up the logging level to INFO or DEBUG. Examine your logs on a regular basis, or better yet, implement one of the many available log analysis tools to notify you of anomalies:
Privilege separation: make sure the UsePrivilegeSeparation option is enabled:
Protocol: limit OpenSSH to accept only protocol version 2:
Root logins: prevent root logins using passwords. This limits root access only to nonpassword methods like public key authentication. If you never directly log in as root, set this to no. You always can log in as a regular user and su to root or use sudo. Setting this to without-password allows only nonpassword methods, such as public key authentication:
File permissions: make sure StrictModes is enabled to prevent the use of insecure home directory and key file permissions:
Reverse name checking: require OpenSSH to check proper reverse hostname lookups. Note that you must have proper name lookups working (that is, DNS or /etc/hosts):
Prevent port forwarding: these two options prevent OpenSSH from setting up TCP port and X11 forwarding; if you do not need these features, disable them:
AllowTcpForwarding no X11Forwarding no
Disable all host-based authentication: always make sure these are disabled. These methods assume that the network can be trusted and allow .rhosts-style authentication based on hostname or IP. Never use these methods as primary authentication:
IgnoreRhosts yes HostbasedAuthentication no RhostsAuthentication no RhostsRSAAuthentication no
Using the above suggestions, you will be able to tighten access controls and eliminate sloppy trust relationships. As I mentioned earlier, staying current with updates or patches is critical to system security. In order to stay current, you need to be informed of when updates are released, so I suggest a multipronged approach. First, automate apt, yum or up2date to check nightly and report on missing updates. Second, subscribe to your Linux distribution's security mailing lists. Third, subscribe to one of the many security discussion groups, such as SecurityFocus. If you build OpenSSH from sources, join the OpenSSH mailing list to watch for updates.
System security requires a holistic approach. The methodologies provided in this article form the basis for securing OpenSSH, one small component of a modern complex Linux system.
After you believe you have secured your system, use scanning tools like the free Nessus Vulnerability Scanner (see Resources) and peer review from colleagues to check your work.
Resources for this article: /article/9023.
Matthew E. Hoskins is a Senior UNIX System Administrator for The New Jersey Institute of Technology where he maintains many of the corporate administrative systems. He enjoys trying to get wildly different systems and software working together, usually with a thin layer of Perl (locally known as “MattGlue”). When not hacking systems, he often can be found hacking in the kitchen. Matt is a member of the Society of Professional Journalists and can be reached at firstname.lastname@example.org.