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