Linux Network Administratoramp;#039;s Guide (3rd Edition) [Electronic resources] نسخه متنی

10.4. The Services and Protocols Files

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 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, the latter field 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 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 10-2.

Example 10-2. A sample /etc/services file

# /etc/services
tcpmux          1/tcp                           # TCP port service multiplexer
echo            7/tcp
echo            7/udp
discard         9/tcp           sink null
discard         9/udp           sink null
systat          11/tcp          users
daytime         13/tcp
daytime         13/udp
netstat         15/tcp
qotd            17/tcp          quote
msp             18/tcp                          # message send protocol
msp             18/udp                          # message send protocol
chargen         19/tcp          ttytst source
chargen         19/udp          ttytst source
ftp-data        20/tcp
ftp             21/tcp
fsp             21/udp          fspd
ssh             22/tcp                          # SSH Remote Login Protocol
ssh             22/udp                          # SSH Remote Login Protocol
telnet          23/tcp
# 24 - private
smtp            25/tcp          mail
# 26 - unassigned

Like the services file, the networking library needs a way to
translate protocol namesfor example, those used in the
services fileto 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 10-3.

Example 10-3. 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
esp     50      ESP             # Encap Security Payload for IPv6
ah      51      AH              # Authentication Header for IPv6
skip    57      SKIP            # SKIP
ipv6-icmp 58    IPv6-ICMP       # ICMP for IPv6
ipv6-nonxt 59   IPv6-NoNxt      # No Next Header for IPv6
ipv6-opts 60    IPv6-Opts       # Destination Options for IPv6
rspf    73      RSPF            # Radio Shortest Path First.