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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: - sstand-alone
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). 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 - name
A single word name that represents the service
being described. - port/protocol
This field is split into two subfields. - port
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. - protocol
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. - aliases
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. 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 1.1.1.1 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. 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 |
- service
Is the service relevant to this
configuration as taken from the /etc/services
file. - socket_type
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. - proto
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. - flags
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. - user
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. - server_path
This field is pathname to the athoughctual
server program to execute for this entry. - server_args
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.
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 |
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