Services files are found in the /etc directory on Unix-like OS, and in the %WINDOWS%\System32\Drivers\etc directories on Windows systems.
All services files use the same simple three-column structure.
Anything following a hash (#) character, including an entire line, is treated as a comment.
# servicename portnumber/porttype # comment xns-ch 54/udp # XNS Clearinghouse isi-gl 55/tcp # ISI Graphics Language isi-gl 55/udp # ISI Graphics Language xns-auth 56/tcp # XNS Authentication xns-auth 56/udp # XNS Authentication mtp 57/tcp # deprecated xns-mail 58/tcp # XNS Mail
The first column holds the service names. The second column associates a port number and type (tcp or udp) with the service name. The third and final column is an optional human-readable comment which typically details the nature or purpose of the service.
Note that services listed in this file may not actually be running or even installed on the given machine. Many services files include a long list of "well known" and otherwise "standard" port assignments, which may not be relevant in a given deployment.
The services file may be relevant in several scenarios --
# dbservername nettype hostname servicename options # comments infserver ontlitcp openlinux inf9srv s=2,b=5120 # comments