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 a 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 port/protocol [aliases]Here, service specifies the service name, port defines the port the service is offered on, and protocol defines which transport protocol is used. Commonly, this is either udp or 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 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 system. Nevertheless, we give a small excerpt from that file below.
# 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 (Contr telnet 23/tcp # Virtual Terminal Protocol smtp 25/tcp # Simple Mail Transfer Protocol nntp 119/tcp readnews # Network News Transfer Protoco # # 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 use syslog 514/udp # remote system logging printer 515/tcp spooler # remote print spooling route 520/udp router routed # routing information protocolNote that, for example, the echo service is offered on port 7 for both TCP and UDP, and that port 512 is used for two different services, namely the COMSAT daemon (which notifies users of newly arrived mail, see xbiff(1x)), over UDP, and for remote execution (rexec(1)), using TCP.
Similar to 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 below: # # Internet (IP) protocols # ip 0 IP # internet protocol, pseudo protocol 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
Next: Remote Procedure Call Up: Various Network Applications Previous: The tcpd access control
Thu Mar 7 23:22:06 EST 1996