.: Advanced Linux Networking :.

You can configure inetd through the /etc/inetd.conf file. Aside from comments (lines that begin with a pound sign, #, that inetd ignores), inetd.conf consists of a series of lines, each of which defines a single server. A sample line resembles the following:

telnet stream tcp nowait root /usr/sbin/tcpd in.telnetd

Each line consists of a series of fields, which are separated by spaces or tabs. The meaning of each field is as follows:

• Server name— The first field is the name of the server protocol, as recorded in /etc/services. In the case of the preceding example, the server protocol name is telnet, and if you check /etc/services, you'll see that this name is associated with 23/tcp—in other words, TCP port 23. There must be an entry in /etc/services in order for inetd to handle a server. For this reason, you may need to edit /etc/services if you want inetd to handle some unusual server. In most cases, though, /etc/services has the appropriate entries already.
• Socket type— The second field relates the network socket type used by the protocol. Possible values are stream, dgram, raw, rdm, and seqpacket.
• Protocol type— The third field is the name of the protocol type, which in this context means a low-level network stack protocol such as TCP or UDP. Possible protocol values appear in /etc/protocols, but the most common values are tcp and udp.
• Wait/Nowait— The fourth field takes just one of two values, wait or nowait, and is meaningful only for datagram (dgram) socket types (other socket types conventionally use a nowait value). Most datagram servers connect to a socket and free up inetd to handle subsequent connection attempts. These servers are called multi-threaded, and they require a nowait entry. Servers that connect to a socket, process all input, and then time out are said to be single-threaded, and they require wait entries. You may optionally add a number to these values, separated by a period, as in wait.60. This specifies the maximum number of servers of the given type that inetd may launch in one minute. The default value is 40.
• Username— You can tell inetd to launch a server with a specific user's privileges. This can be an important security feature, because restricting privileges for servers that don't require extensive access to the system can prevent a bug from causing a security breach. For instance, the Apache Web server usually doesn't need unusual privileges, so you could launch it as nobody or as some special account intended only for Apache. The preceding example shows the username as root because root privileges are needed to launch the login processes required by a Telnet server. If you add a period and a group name, the specified group name is used for the server's group privileges. For instance, nobody.nogroup launches a server with the username nobody and the group nogroup.
• Server program— The sixth field specifies the name of the server program that inetd launches when it detects an incoming connection. The preceding example gives /usr/sbin/tcpd as this program name. In reality, tcpd is not a server; it's the program file for the TCP Wrappers program, which is described shortly. Most Linux distributions that use inetd also use TCP Wrappers, and so launch most inetd-mediated servers through tcpd. You can bypass TCP Wrappers for any server you like, although it's generally best to use TCP Wrappers for reasons that are described shortly.
• Server program arguments— The final field is optional. When present, it contains any arguments that are to be passed to the server program. These arguments might modify the server's behavior, tell it where its configuration files are, and so on. In the case of servers launched through TCP Wrappers, the argument is the name of the ultimate target server, such as in.telnetd in the preceding example. (You may add the ultimate server's arguments to this list, if it needs any.)

You can edit /etc/inetd.conf using any text editor you like. Be sure that any new entries you create, or existing ones you modify, span a single line. (Long filenames, server arguments, and the like sometimes produce lines long enough that some editors will try to wrap the line onto two lines, which will cause problems.) If you need to add an entry for a server you've installed, consult the server's documentation to learn what its inetd.conf entry should look like. In some cases, you can model an entry after an existing one, but without knowing what values to enter for the server name, socket type, and so on, such an entry might not work.

Most distributions that use inetd ship with an /etc/inetd.conf file that contains entries for many different servers. Many of these entries are commented out, so they're inactive. You can activate a server by uncommenting such a line, if the matching server is installed. Some inetd.conf files include multiple entries for any given service; for instance, a file might have two or three different entries for different FTP servers. If yours is like this, be sure you uncomment only the line corresponding to the specific server you've installed, such as ProFTPd or WU-FTPD.