Linux Networking No.6 : TCP/IP and Ports

The list of known services and their relative ports is generally found in /etc/services. The official and associated is managed by the IANA(Internet Assigned Numbers Authority).

We will look at the output of portscans. Beware that unauthorised portscanning is illegal although many use them.

Here is the output of a portscan:

Port state Services
21/tcp open ftp
22/tcp open ssh
23/tcp open telnet
25/tcp open smtp
70/tcp open gopher
79/tcp open finger
80/tcp open http

This shows open ports, these are ports being used by an application.

the /etc/services main ports (examples):
ftp-data 20/tcp
ftp 21/tcp
fsp 21/udp fspd
ssh 22/tcp # SSH Remote Login Protocol
ssh 22/udp
telnet 23/tcp
smtp 25/tcp mail
www 80/tcp http # WorldWideWeb HTTP
www 80/udp # HyperText Transfer Protocol
pop3 110/tcp pop-3 # POP version 3
pop3 110/udp pop-3
sunrpc 111/tcp portmapper # RPC 4.0 portmapper
sunrpc 111/udp portmapper
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
pwdgen 129/tcp # PWDGEN service
pwdgen 129/udp # PWDGEN service
loc-srv 135/tcp epmap # Location Service
loc-srv 135/udp epmap
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 imap # Interim Mail Access P 2 and 4
imap2 143/udp imap
snmp 161/tcp # Simple Net Mgmt Protocol
snmp 161/udp # Simple Net Mgmt Protocol
snmp-trap 162/tcp snmptrap # Traps for SNMP