How to check if a port is open for remote system(ubuntu)

Question

Is it possible to check that if the ports are open for the remote system on ubuntu server?

I should able to check if a port(eg:ssh) on my machine is open for the remote machine.

Answer 1

use good old telnet:

[user@lappie ~]$ telnet host 22
Trying ip.adr.tld ...
Connected to host  (ip.addr.tld).
Escape character is '^]'.
SSH-2.0-OpenSSH_5.1p1 Debian-5

this is a successful attempt. An unsuccessful one should look like this;

[user@lappie ~]$ telnet host 23
Trying ip.adr.tld ...
telnet: connect to address ip.adr.tld: Connection refused
telnet: Unable to connect to remote host: Connection refused

or with nmap

[user@lappie ~]$ nmap host

Starting Nmap 5.21 ( http://nmap.org ) at 2010-10-07 11:25 CEST
Nmap scan report for host (ip.adr.tld)
Host is up (0.0052s latency).
rDNS record for ip.adr.tld : host.domain.tld
Not shown: 995 closed ports
PORT     STATE SERVICE
22/tcp   open  ssh
80/tcp   open  http
111/tcp  open  rpcbind
3000/tcp open  ppp
5666/tcp open  nrpe

Nmap done: 1 IP address (1 host up) scanned in 0.18 seconds

Answer 2

Use NMAP. Example:

nmap example.com

You can use IP address in place of domain name. Here is the full documentation: http://nmap.org/book/man.html

Answer 3

For a script, I use something like the following:

nmap example.com -p 22 -sV --version-all -oG - | grep -iq '22/open'

The return value tells you whether the port is open!

Answer 4

nmap example.com -p 22

Answer 5

Telnet will only work for TCP services, so if you're trying to see if your DHCP server (UDP/68) is running on a remote machine it won't work. Likewise nmap defaults to only scanning TCP ports.

For UDP ports use:

nmap -sU example.com -p 68