A port is an application-specific or process-specific software construct serving as a communications endpoint and it is identified by its number such as TCP port number 80 . It is used by TCP and UDP of the Internet Protocol Suite. A port number is a 16-bit unsigned integer, thus ranging from 0 to 65535.
UNIX / Linux
+------------------+
| Networking stack |
| eth0 |
+------------------+
|
+------------------+
| Apache process |--> Binding port 80 @ 202.54.1.1 IP
+------------------+
$ less /etc/services
grep -w 80 /etc/services Sample outputs:
www 80/tcp http # WorldWideWeb HTTP www 80/udp # HyperText Transfer Protocol
Privileged Ports
Typically port number less than 1024 are used by well know network servers such as Apache. Under UNIX and Linux like oses root (super user) privileges are required to open privileged ports. Almost all clients uses a high port numbers for short term use. This is also known as an ephemeral port. For example Apache use TCP port 80Server Client w/ Firefox +----------+ +----------------+ | Apache | | connects using | | TCP Port | | an ephemeral | | 80 @ |<-----> eth0 <----> | port # | |202.54.1.2| | 46025 | +----------+ +----------------+The port numbers are divided into three ranges:
- Well Known Ports: those from 0 through 1023.
- Registered Ports: those from 1024 through 49151
- Dynamic and/or Private Ports: those from 49152 through 65535
# echo 1024 65535 > /proc/sys/net/ipv4/ip_local_port_rangeYou can also increase or decrease socket timeout (Linux specific example):
# echo 2000 > /proc/sys/net/ipv4/tcp_keepalive_timeCommon Well Known Port Numbers
The following are used by UNIX / Windows / Linux / BSD / OS X and all other server operating systems or network devices (see /etc/services file):- 21: FTP Server
- 22: SSH Server (remote login)
- 25: SMTP (mail server)
- 53: Domain Name System (Bind 9 server)
- 80: World Wide Web (HTTPD server)
- 110: POP3 mail server
- 143: IMAP mail server
- 443: HTTP over Transport Layer Security/Secure Sockets Layer (HTTPDS server)
- 445: microsoft-ds, Server Message Block over TCP
How Do I See Open Ports and Socket Information Under UNIX or Linux?
You can use the netstat command:# netstat -tulpnFreeBSD specific example:
# sockstat -lTo list open IPv4 connections use the lsof command:
# lsof -Pnl +M -i4The ss command is used to dump socket statistics. It allows showing information similar to netstat command. It can display more TCP and state information than other tools
# ss -s
# ss -l
# ss -pl
# ss -o state established '( dport = :smtp or sport = :smtp )'Examples
Each TCP or UDP port is opened using a UNIX service or daemon such as Apache web server. You can also write a program using C, C++, Perl, Shell or Bash to open any port. You can also use utilities such as nc command .Apache Server Example (open TCP port 80)
Start the Apache web server under FreeBSD as follows to open TCP port 80:# /usr/local/etc/rc.d/apache22 forcestartOR
# /usr/local/etc/rc.d/apache22 startTo displays listening sockets (open ports) under FreeBSD, enter:
# sockstat -l OR
# netstat -nat | grep LISTENYou should see port 80 opened under FreeBSD. Under CentOS or Redhat (RHEL) Linux, you can open port 80 using the following commands:
# service httpd start
# chkconfig httpd on
# netstat -tulpn | grep :80Firewall Configuration
All port numbers are encoded in the transport protocol packet header, and they can be read by other components of the network stack such as firewall. Firewall can be used for port forwarding or denying access to open port. For example, block an abusing IP address called 1.2.3.4 using UNIX firewall. In other words, Apache port is open but it may be blocked by UNIX (pf) or Linux (iptables) firewall. You also need to open port at firewall level. In this example, open tcp port 80 using Linux iptables firewall tool:# /sbin/iptables -A INPUT -m state --state NEW -m tcp -p tcp --dport 80 -j ACCEPT
# service iptables saveSee also:
- CentOS / Redhat Linux Iptables Firewall Configuration Tutorial
- Redhat / CentOS / Fedora Linux Open Port
- FreeBSD Setting up Firewall using IPFW
- OpenBSD PF Firewall Script – /etc/pf.conf File
nc Command Example
The nc (or netcat utility) is used for just about anything under the sun involving TCP or UDP. It can open TCP connections, send UDP packets, listen on arbitrary TCP and UDP ports, do port scanning, and deal with both IPv4 and IPv6. In this example, open port 5000 using nc command:$ nc -l 5000On a second console or from a second UNIX / Linux machine, connect to the machine and port being listened on:
$ nc localhost 5000OR
$ nc unix.system.ip.here 5000 In this example, send data from one computer to another:
$ nc -l 5555 > output.txtUsing a second machine, connect to the listening nc process (@ port 5555), feeding it the file which is to be transferred:
$ nc your.unix.systems.ip.here 5555 < input.txt You can run netstat command to view open ports:
$ netstat -a
$ netstat -nat | grep LISTENSample outputs:
tcp4 0 0 *.5555 *.* LISTEN
tcp4 0 0 10.1.3.29.53 *.* LISTEN
tcp4 0 0 192.168.56.1.53 *.* LISTEN
tcp4 0 0 115.242.47.238.53 *.* LISTEN
tcp4 0 0 127.0.0.1.953 *.* LISTEN
tcp4 0 0 127.0.0.1.53 *.* LISTEN
tcp4 0 0 127.0.0.1.631 *.* LISTEN
tcp6 0 0 ::1.631 *.* LISTEN
Python Example
Create a file called echo_server.py:#!/usr/bin/python # Demo server to open port 8888 # Modified from Python tutorial docs import socket HOST = '127.0.0.1' # Hostname to bind PORT = 8888 # Open non-privileged port 8888 s = socket.socket(socket.AF_INET, socket.SOCK_STREAM) s.bind((HOST, PORT)) s.listen(1) conn, addr = s.accept() print 'Connected by', addr while 1: data = conn.recv(1024) if not data: break conn.send(data) conn.close()Create a file called echo_client.py:
#!/usr/bin/python # Demo client program # Modified from Python tutorial docs import socket HOST = '127.0.0.1' # Set the remote host, for testing it is localhost PORT = 8000 # The same port as used by the server s = socket.socket(socket.AF_INET, socket.SOCK_STREAM) s.connect((HOST, PORT)) s.send('Where there is love there is life') data = s.recv(1024) s.close() print 'Received', repr(data)Save and close the file. Run it as follows:
$ chmod +x *.pyStart server, enter:
$ ./echo_server.py
$ netstat -nat | grep LISTENOn a second console connect to the localhost and port being listened on using echo_client.py:
$ ./echo_client.py
WARNING!
This beta release is not intended for production use! Be advised that
the current beta might replace your OpenOffice.org installation.
However, in my testing it did not replaced OpenOffice.org installation.
WARNING!
These examples may crash your computer if executed. Make a backup - it
cannot be stressed enough how important it is to make a backup of your
system before you do this.
