5.8. Configuring your network servers and services.

Network servers and services are programs that allow a remote user to make use of your Linux machine. Server programs listen on network ports. Network ports are a means of addressing a particular service on any particular host. They are how a server knows the difference between an incoming telnet connection and an incoming ftp connection. The remote user establishes a network connection to your machine. The server program (the network daemon program) listening on that port accepts the connection and then executes. There are two ways that network daemons may operate. Both are commonly employed in practice. The two ways are:


The network daemon program listens on the designated network port. When an incoming connection is made, the daemon manages the network connection itself to provide the service.

slave to the inetd server

The inetd server is a special network daemon program that specializes in managing incoming network connections. It has a configuration file which tells it what program needs to be run upon receiving an incoming connection. Any service port may be configured for either of the tcp or udp protocols. The ports are described in another file that we will soon review..

There are two important files that need to be configured. They are the /etc/services file (which assigns names to port numbers), and the /etc/inetd.conf file (the configuration file for the inetd network daemon).

5.8.1. /etc/services

The /etc/services file is a simple database that associates a human friendly name to a machine friendly service port. Its format is quite simple. The file is a text file where each line represents and entry in the database. Each entry is comprised of three fields separated by any number of whitespace (tab or space) characters. The fields are:

name port/protocol aliases # comment


A single word name that represents the service being described.


This field is split into two subfields.


A number that specifies the port number where the named service will be available. Most of the common services have assigned service numbers. These are described in RFC-1340.


This subfield may be set to either tcp or udp.

It is important to note that an entry of 18/tcp is very different from an entry of 18/udp There is no technical reason why the same service needs to exist on both. Normally common sense prevails. It is only if a particular service is available via both tcp and udp that you will see an entry for both.


Other names that may be used to refer to this service entry.

Any text appearing in a line after a `#' character is ignored, and it is treated as a comment. An example /etc/services file.

All modern linux distributions provide a good /etc/services file. Just in case you happen to be building a machine from the ground up, here is a copy of the /etc/services file supplied with an old Debian distribution:

# /etc/services:
# $Id: Net-HOWTO.sgml,v 2001/01/17 19:55:16 lx Exp $
# Network services, Internet style
# Note that it is presently the policy of IANA to assign a single well-known
# port number for both TCP and UDP; hence, most entries here have two entries
# even if the protocol doesn't support UDP operations.
# Updated from RFC 1340, ``Assigned Numbers'' (July 1992).  Not all ports
# are included (only the more common ones):
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
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
time		37/tcp		timserver
time		37/udp		timserver
rlp		39/udp		resource	# resource location
nameserver	42/tcp		name		# IEN 116
whois		43/tcp		nicname
re-mail-ck	50/tcp				# Remote Mail Checking Protoconame server
re-mail-ck	50/udp				# Remote Mail Checking Protocol
domain		53/tcp		nameserver	# name-domain server
domain		53/udp		nameserver
mtp		57/tcp				# deprecated
bootps		67/tcname serverTP server
bootps		67/udp
bootpc		68/tcname serverTP client
bootpc		68/udp
tftp		69/udp
gopher		70/tcp				# Internet Gopher
gopher		70/udp
rje		77/tcp		netrjs
finger		79/tcp
www		80/tcp		http		# WorldWideWeb HTTP
www		80/udp				# HyperText Transfer Protocol
link		87/tcp		ttylink
kerberos	88/tcp		kerberos5 krb5	# Kerberos v5
kerberos	88/udp		kerberos5 krb5	# Kerberos v5
supdup		95/tcp
# 100 - reserved
hostnames	101/tcp		hostname	# usually from sri-nic
iso-tsap	102/tcp		tsap		# part of ISODE.
csnet-ns	105/tcp		cso-ns		# also used by CSO name server
csnet-ns	105/udp		cso-ns
rtelnet		107/tcp				# Remote Telnet
rtelnet		107/udp
pop-2		109/tcp		postoffice	# POP version 2
pop-2		109/udp
pop-3		110/tcp				# POP version 3
pop-3		110/udp
sunrpc		111/tcp		portmapper	# RPC 4.0 portmapper TCP
sunrpc		111/udp		portmapper	# RPC 4.0 portmapper UDP
auth		113/tcp		authentication tap ident
sftp		115/tcp
uucp-path	117/tcp
nntp		119/tcp		readnews untp	# USENET News Transfer Protocol
ntp		123/tcp
ntp		123/udp				# Network Time Protocol
netbios-ns	137/tcp				# NETBIOS Name Service
netbios-ns	137/udp
netbios-dgm	138/tcp				# NETBIOS Datagram Service
netbios-dgm	138/udp
netbios-ssn	139/tcp				# NETBIOS session service
netbios-ssn	139/udp
imap2		143/tcp				# Interim Mail Access Proto v2
imap2		143/udp
snmp		161/udp				# Simple Net Mgmt Proto
snmp-trap	162/udp		snmptrap	# Traps for SNMP
cmip-man	163/tcp				# ISO mgmt over IP (CMOT)
cmip-man	163/udp
cmip-agent	164/tcp
cmip-agent	164/udp
xdmcp		177/tcp				# X Display Mgr. Control Proto
xdmcp		177/udp
nextstep	178/tcp		NeXTStep NextStep	# NeXTStep window
nextstep	178/udp		NeXTStep NextStep	# server
bgp		179/tcp				# Border Gateway Proto.
bgp		179/udp
prospero	191/tcp				# Cliff Neuman's Prospero
prospero	191/udp
irc		194/tcp				# Internet Relay Chat
irc		194/udp
smux		199/tcp				# SNMP Unix Multiplexer
smux		199/udp
at-rtmp		201/tcp				# AppleTalk routing
at-rtmp		201/udp
at-nbp		202/tcp				# AppleTalk name binding
at-nbp		202/udp
at-echo		204/tcp				# AppleTalk echo
at-echo		204/udp
at-zis		206/tcp				# AppleTalk zone information
at-zis		206/udp
z3950		210/tcp		wais		# NISO Z39.50 database
z3950		210/udp		wais
ipx		213/tcp				# IPX
ipx		213/udp
imap3		220/tcp				# Interactive Mail Access
imap3		220/udp				# Protocol v3
ulistserv	372/tcp				# UNIX Listserv
ulistserv	372/udp
# UNIX specific services
exec		512/tcp
biff		512/udp		comsat
login		513/tcp
who		513/udp		whod
shell		514/tcp		cmd		# no passwords used
syslog		514/udp
printer		515/tcp		spooler		# line printer spooler
talk		517/udp
ntalk		518/udp
route		520/udp		router routed	# RIP
timed		525/udp		timeserver
tempo		526/tcp		newdate
courier		530/tcp		rpc
conference	531/tcp		chat
netnews		532/tcp		readnews
netwall		533/udp				# -for emergency broadcasts
uucp		540/tcp		uucpd		# uucp daemon
remotefs	556/tcp		rfs_server rfs	# Brunhoff remote filesystem
klogin		543/tcp				# Kerberized `rlogin' (v5)
kshell		544/tcp		krcmd		# Kerberized `rsh' (v5)
kerberos-adm	749/tcp				# Kerberos `kadmin' (v5)
webster		765/tcp				# Network dictionary
webster		765/udp
# From ``Assigned Numbers'':
#> The Registered Ports are not controlled by the IANA and on most systems
#> can be used by ordinary user processes or programs executed by ordinary
#> users.
#> Ports are used in the TCP [45,106] to name the ends of logical
#> connections which carry long term conversations.  For the purpose of
#> providing services to unknown callers, a service contact port is
#> defined.  This list specifies the port used by the server process as its
#> contact port.  While the IANA can not control uses of these ports it
#> does register or list uses of these ports as a convenience to the
#> community.
ingreslock	1524/tcp
ingreslock	1524/udp
prospero-np	1525/tcp		# Prospero non-privileged
prospero-np	1525/udp
rfe		5002/tcp		# Radio Free Ethernet
rfe		5002/udp		# Actually uses UDP only
bbs		7000/tcp		# BBS service
# Kerberos (Project Athena/MIT) services
# Note that these are for Kerberos v4 and are unofficial.  Sites running
# v4 should uncomment these and comment out the v5 entries above.
kerberos4	750/udp		kdc	# Kerberos (server) udp
kerberos4	750/tcp		kdc	# Kerberos (server) tcp
kerberos_master	751/udp			# Kerberos authentication
kerberos_master	751/tcp			# Kerberos authentication
passwd_server	752/udp			# Kerberos passwd server
krb_prop	754/tcp			# Kerberos slave propagation
krbupdate	760/tcp		kreg	# Kerberos registration
kpasswd		761/tcp		kpwd	# Kerberos "passwd"
kpop		1109/tcp		# Pop with Kerberos
knetd		2053/tcp		# Kerberos de-multiplexor
zephyr-srv	2102/udp		# Zephyr server
zephyr-clt	2103/udp		# Zephyr serv-hm connection
zephyr-hm	2104/udp		# Zephyr hostmanager
eklogin		2105/tcp		# Kerberos encrypted rlogin
# Unofficial but necessary (for NetBSD) services
supfilesrv	871/tcp			# SUP server
supfiledbg	1127/tcp		# SUP debugging
# Datagram Delivery Protocol services
rtmp		1/ddp			# Routing Table Maintenance Protocol
nbp		2/ddp			# Name Binding Protocol
echo		4/ddp			# AppleTalk Echo Protocol
zip		6/ddp			# Zone Information Protocol
# Debian GNU/Linux services
rmtcfg		1236/tcp		# Gracilis Packeten remote config server
xtel		1313/tcp		# french minitel
cfinger		2003/tcp		# GNU Finger
postgres	4321/tcp		# POSTGRES
mandelspawn	9359/udp	mandelbrot	# network mandelbrot
# Local services

In the real world, the actual file is always growing as new services are being created. If you fear your own copy is incomplete, I'd suggest to copy a new /etc/services from a recent distribution.

5.8.2. /etc/inetd.conf

The /etc/inetd.conf file is the configuration file for the inetd server daemon. Its function is to tell inetd what to do when it receives a connection request for a particular service. For each service that you wish to accept connections, you must tell inetd what network server daemon to run (and how to run it).

Its format is also fairly simple. It is a text file with each line describing a service that you wish to provide. Any text in a line following a `#' is both ignored, and it is considered a comment. Each line contains seven fields separated by any number of whitespace (tab or space) characters. The general format is as follows:

  service  socket_type  proto  flags  user  server_path  server_args


Is the service relevant to this configuration as taken from the /etc/services file.


This field describes the type of socket that this entry will consider relevant. Allowable values are: stream, dgram, raw, rdm, or seqpacket. This is a little technical in nature. As a rule of thumb nearly all tcp based services use stream, and nearly all udp based services use dgram. It is only very special types of server daemons that would use any of the other values.


The protocol to be considered valid for this entry. This should match the appropriate entry in the /etc/services file. It will typically be either tcp or udp. Sun RPC (Remote Procedure Call) based servers will use eitherrpc/tcp or rpc/udp.


There are really only two possible settings for this field. This field setting tells inetd whether the network server program frees the socket after it has been started (whether inetd can start another one on the next connection request), or, whether inetd should wait and assume that any server daemon already running will handle the new connection request. This is a little tricky to work out, but as a rule of thumb all tcp servers should have this entry set to nowait. Most udp servers should have this entry set to wait. Be warned there are some notable exceptions. You should let the example guide you if you are not sure.


This field describes which user account from /etc/passwd will be set as the owner of the network daemon when it is started. This is often useful if you want to safeguard against security risks. You can set the user of an entry to the nobody user. If the network server security is breached, the possible damage is minimized by using nobody. Typically this field is set to root, because many servers require root privileges in order to function correctly.


This field is pathname to the athoughctual server program to execute for this entry.


This field comprises the rest of the line and it is optional. This field is where you place any command line arguments that you wish to pass to the server daemon program when it is launched. An example /etc/inetd.conf

As for the /etc/services file all modern distributions will include a good /etc/inetd.conf file for you to work with. Here is the /etc/inetd.conf file from the Debian distribution.

# /etc/inetd.conf:  see inetd(8) for further informations.
# Internet server configuration database
# Modified for Debian by Peter Tobias <tobias@et-inf.fho-emden.de>
# <service_name> <sock_type> <proto> <flags> <user> <server_path> <args>
# Internal services
#echo		stream	tcp	nowait	root	internal
#echo		dgram	udp	wait	root	internal
discard		stream	tcp	nowait	root	internal
discard		dgram	udp	wait	root	internal
daytime		stream	tcp	nowait	root	internal
daytime		dgram	udp	wait	root	internal
#chargen	stream	tcp	nowait	root	internal
#chargen	dgram	udp	wait	root	internal
time		stream	tcp	nowait	root	internal
time		dgram	udp	wait	root	internal
# These are standard services.
telnet	stream	tcp	nowait	root	/usr/sbin/tcpd	/usr/sbin/in.telnetd
ftp	stream	tcp	nowait	root	/usr/sbin/tcpd	/usr/sbin/in.ftpd
#fsp	dgram	udp	wait	root	/usr/sbin/tcpd	/usr/sbin/in.fspd
# Shell, login, exec and talk are BSD protocols.
shell	stream	tcp	nowait	root	/usr/sbin/tcpd	/usr/sbin/in.rshd
login	stream	tcp	nowait	root	/usr/sbin/tcpd	/usr/sbin/in.rlogind
#exec	stream	tcp	nowait	root	/usr/sbin/tcpd	/usr/sbin/in.rexecd
talk	dgram	udp	wait	root	/usr/sbin/tcpd	/usr/sbin/in.talkd
ntalk	dgram	udp	wait	root	/usr/sbin/tcpd	/usr/sbin/in.ntalkd
# Mail, news and uucp services.
smtp	stream	tcp	nowait	root	/usr/sbin/tcpd	/usr/sbin/in.smtpd
#nntp	stream	tcp	nowait	news	/usr/sbin/tcpd	/usr/sbin/in.nntpd
#uucp	stream	tcp	nowait	uucp	/usr/sbin/tcpd	/usr/lib/uucp/uucico
#comsat	dgram	udp	wait	root	/usr/sbin/tcpd	/usr/sbin/in.comsat
# Pop et al
#pop-2	stream	tcp	nowait	root	/usr/sbin/tcpd	/usr/sbin/in.pop2d
#pop-3	stream	tcp	nowait	root	/usr/sbin/tcpd	/usr/sbin/in.pop3d
# `cfinger' is for the GNU finger server available for Debian.  (NOTE: The
# current implementation of the `finger' daemon allows it to be run as `root'.)
#cfinger stream	tcp	nowait	root	/usr/sbin/tcpd	/usr/sbin/in.cfingerd
#finger	stream	tcp	nowait	root	/usr/sbin/tcpd	/usr/sbin/in.fingerd
#netstat	stream	tcp	nowait	nobody	/usr/sbin/tcpd	/bin/netstat
#systat	stream	tcp	nowait	nobody	/usr/sbin/tcpd	/bin/ps -auwwx
# Tftp service is provided primarily for booting.  Most sites
# run this only on machines acting as "boot servers."
#tftp	dgram	udp	wait	nobody	/usr/sbin/tcpd	/usr/sbin/in.tftpd
#tftp	dgram	udp	wait	nobody	/usr/sbin/tcpd	/usr/sbin/in.tftpd /boot
#bootps	dgram	udp	wait	root	/usr/sbin/bootpd	bootpd -i -t 120
# Kerberos authenticated services (these probably need to be corrected)
#klogin		stream	tcp	nowait	root	/usr/sbin/tcpd	/usr/sbin/in.rlogind -k
#eklogin	stream	tcp	nowait	root	/usr/sbin/tcpd	/usr/sbin/in.rlogind -k -x
#kshell		stream	tcp	nowait	root	/usr/sbin/tcpd	/usr/sbin/in.rshd -k
# Services run ONLY on the Kerberos server (these probably need to be corrected)
#krbupdate	stream tcp	nowait	root	/usr/sbin/tcpd	/usr/sbin/registerd
#kpasswd	stream	tcp	nowait	root	/usr/sbin/tcpd	/usr/sbin/kpasswdd
# RPC based services
#mountd/1	dgram	rpc/udp	wait	root	/usr/sbin/tcpd	/usr/sbin/rpc.mountd
#rstatd/1-3	dgram	rpc/udp	wait	root	/usr/sbin/tcpd	/usr/sbin/rpc.rstatd
#rusersd/2-3	dgram	rpc/udp	wait	root	/usr/sbin/tcpd	/usr/sbin/rpc.rusersd
#walld/1	dgram	rpc/udp	wait	root	/usr/sbin/tcpd	/usr/sbin/rpc.rwalld
# End of inetd.conf.
ident		stream	tcp	nowait	nobody	/usr/sbin/identd	identd -i