18.1 Additional Topics

Most (but not all) of these topics are network related. Furthermore, none are specific to Linux; you can carry the knowledge you have of them to and from other Unix systems.

• Electronic mail Email is a big topic. Chapters 5 and 6 talked a little about the SMTP port (25) used for mail transfer, and they briefly mentioned Postfix and qmail, the two most viable mail transfer agents (MTAs) available now. There's more behind the MTA: for example, if you want to run an IMAP (Internet Message Access Protocol) server, you probably want the Cyrus package.

• Domain Name Service (DNS) Setting up a client is easy, as you saw in Chapter 5, but setting up a DNS server is trickier. The most common DNS server is BIND (Berkeley Internet Name Domain), but a newer DNS server named djbdns is also gaining popularity.

• Web servers In the Unix world, "Web server" usually means Apache, the most popular Web server in the world. However, many scripting languages and other tools have embedded Web server features that can aid debugging, status monitoring, and other tasks not directly related to serving Web pages to the general public.

• Virtual private networks (VPNs) A VPN allows you to connect several widely dispersed machines in a virtual network (for instance, a single subnet). Administrators typically create a VPN in conjunction with the IPSec protocol so that they can disperse sensitive data (that might otherwise be behind a network firewall) through an encrypted network channel across a possibly insecure link.

• The screen program Did you ever want to take your shell session wherever you go? You can do it with the screen program, a terminal multiplexer that supports multiple shells, disconnection, reconnection, scrollback, cut and paste, and more. This utility is a perennial favorite among seasoned Unix wizards. If you want some of the same functionality with a windowing system, check out VNC (Virtual Network Computing).

• DB files and DBM files When using many larger servers, you will encounter binary DB and DBM files. These files are very small databases that store blocks of data keyed by small strings, usually as some kind of hash table. For whatever reason, there are an absurd number of different DB/DBM file formats (DB, GDBM, NDBM, and SDBM, to name a few). Most packages that use DBM-style files come with their own manipulation utilities (for example, the postalias command that comes with Postfix), but scripting languages such as Perl include powerful interfaces that you can use if you need to do something beyond the ordinary with these database files.

• Relational databases You can run the powerful MySQL and PostgreSQL database servers on Linux. There are several commercial servers as well, but this book intentionally avoids talk of commercial software products.

• RCS and CVS The Revision Control System (RCS) is a way to track file revisions. Originally meant for software development, many systems administrators use RCS to control important system files so that they have a record of who made each change (and in theory, so that they can back out of harmful changes, though this rarely comes up in practice). The Concurrent Versions System (CVS) is a network-extensible revision control system that can handle several versions of the same package at once.

• Pluggable Authentication Modules (PAM) If you don't like the system of passwords in /etc/passwd, you can replace it with a PAM module that supports some other scheme.

• Network Information Service (NIS) NIS is a terrifying combination of RPC and DBM files, used by many systems to provide networked information such as passwd and group. It is an old system that isn't particularly robust, secure, or pleasant, but it is supported by most Unix platforms.

• Kerberos This is a powerful network authentication system.

• Lightweight Directory Access Protocol (LDAP) LDAP serves a bunch of data entries to network clients (as NIS does), except that where NIS allows only a single key per data entry, LDAP supports a hierarchy of keys for entries. LDAP is very extensible; OpenLDAP is a popular open source LDAP server.

• Network File System (NFS) This is the traditional method of sharing files across a network on a Unix cluster. NFS is a somewhat clunky system that is still in use because most Unix systems support it. Plus, the alternatives such as AFS (Andrew File System), DFS (Distributed File System), and 9P are too expensive, difficult to configure, and not terribly well understood.

• Secure Sockets Layer (SSL) Many network servers now offer SSL support to authenticate and encrypt individual TCP connections with the same technology used for secure Web sessions. If you want to set up a server that supports SSL, be prepared to learn all about public keys, private keys, certificates, signing, and more.