The port numbers on which certain “standard” services are
offered are defined in the Assigned Numbers RFC. To enable server and client
programs to convert service names to these numbers, at least part of
the list is kept on each host; it is stored in a file called
/etc/services. An entry is made up like this:
service specifies the service name,
port defines the port the service is offered on,
protocol defines which transport protocol
is used. Commonly, the latter field is either
tcp. It is possible for a service to be
offered for more than one protocol, as well as offering different services on
the same port as long as the protocols are different. The
aliases field allows you to specify alternative
names for the same service.
Usually, you don’t have to change the services file that comes along with the network software on your Linux system. Nevertheless, we give a small excerpt from that file in Example 12.2.
Example 12-2. A Sample /etc/services File
# The services file: # # well-known services echo 7/tcp # Echo echo 7/udp # discard 9/tcp sink null # Discard discard 9/udp sink null # daytime 13/tcp # Daytime daytime 13/udp # chargen 19/tcp ttytst source # Character Generator chargen 19/udp ttytst source # ftp-data 20/tcp # File Transfer Protocol (Data) ftp 21/tcp # File Transfer Protocol (Control) telnet 23/tcp # Virtual Terminal Protocol smtp 25/tcp # Simple Mail Transfer Protocol nntp 119/tcp readnews # Network News Transfer Protocol # # UNIX services exec 512/tcp # BSD rexecd biff 512/udp comsat # mail notification login 513/tcp # remote login who 513/udp whod # remote who and uptime shell 514/tcp cmd # remote command, no passwd used syslog 514/udp # remote system logging printer 515/tcp spooler # remote print spooling route 520/udp router routed # routing information protocol
Note that the echo service is offered on port 7 for both TCP and UDP, and that port 512 is used for two different services: remote execution (rexec) using TCP, and the COMSAT daemon, which notifies users of new mail, over UDP (see xbiff(1x)).
Like the services file, the networking library needs a way to translate
protocol names—for example, those used in the services file—to
protocol numbers understood by the IP layer on other hosts. This is done by
looking up the name in the
/etc/protocols file. It
contains one entry per line, each containing a protocol name, and the
associated number. Having to touch this file is even more unlikely than
having to meddle with
/etc/services. A sample file
is given in Example 12.3.
Example 12-3. A Sample /etc/protocols File
# # Internet (IP) protocols # ip 0 IP # internet protocol, pseudo protocol number icmp 1 ICMP # internet control message protocol igmp 2 IGMP # internet group multicast protocol tcp 6 TCP # transmission control protocol udp 17 UDP # user datagram protocol raw 255 RAW # RAW IP interface