linux port numbers
This is an important linux port numbers mostly asked during Linux Interview.
I have collected some ports info to understanding the ports, their details and numbers.
Well known ports
22 – SSH(For secure remote administration which uses SSL to encrypt the
23 – Telnet (For insecure remote administration
25 – SMTP(Mail Transfer Agent for e-mail server such as SEND mail)
53 – DNS(Special service which uses both TCP and UDP)
67 – Bootp
68 – DHCP
69 – TFTP(Trivial file transfer protocol uses udp protocol for connection less
transmission of data)
80 – HTTP/WWW(apache)
88 – Kerberos
110 – POP3(Mail delivery Agent)
123 – NTP(Network time protocol used for time syncing uses UDP protocol)
137 – NetBIOS(nmbd)
139 – SMB-Samba(smbd)
143 – IMAP
161 – SNMP(For network monitoring)
389 – LDAP(For centralized administration)
443 – HTTPS(HTTP+SSL for secure web access)
514 – Syslogd(udp port)
636 – ldaps(both tcp and udp)
873 – rsync
989 – FTPS-data
990 – FTPS
993 – IMAPS
1194 – openVPN
1812 – RADIUS
995 – POP3s
2049 – NFS(nfsd, rpc.nfsd, rpc, portmap)
2401 – CVS server
3306 – MySql
3690 – SVN
where remote application can communicate. This is analogy to the physical ports
for entering in to a country from different sea ports.
Q. What is hardware port?
This is a physical peripheral connecting point to a machine from a physical
Q. What is a socket?
Socket is combination of software Port and IP address.
Q. What is the range of ports or how many ports are there?
Port numbers can vary from 0 to 65535, so total we can get 65536 ports
Q. Why port numbers are just 65536?
This is because limitation in TCP/IP stack where the port number field is just
16bit size. So we get only 2^16(2 to the power of 16) ports which are equal to
65536 available ports
Q.What are the well-known ports or assigned ports or default ports?
Well known ports are from 0 to 1023(total 2^10=1024 ports)
Q.What do you mean by default port?
Default port is a designated port for particular well-known service such as web
server, mail server, ftp server etc. By default FTP uses 21 port, DNS uses 53
and Apache uses 80 port.
Q.Can we change default port for a service(example Apache, squid)?
Yes, we can change. In Apache and DNS we can change this using listen
configuration entry in httpd.conf and named.conf. Squid have port entry in its
squid.conf file to mention port number.
Q.What are the protocol numbers for TCP and UDP?
Do not confuse this one with port numbers. TCP and UDP have their own numbers
in TCP/IP stack.
TCP protocol number:6
UDP protocol number:17
Q. Is there any way I can see all the port information in Linux?
Yes, you can get that from /etc/services files.
Q. How can I see open ports in Linux?
Use nmap command.