Test UDP Connectivity

Using nmap you can easily test if a UDP port is open or closed on a server or workstation.

To test a listening UDP port, simply use nmap.

# nmap -p  -sU -P0 [host name | ip address]


# nmap -p 123 -sU -P0 example.com


# nmap -p 123 -sU -P0 123.123.123.123

Example of checking TFTP (UDP 69) on a remote server with the IP address of 10.4.212.6 with a closed port and 10.4.24.6 with an open port.

[root@usna0netweb02 ~]# nmap -p 69 -sU -P0 10.4.212.6

Starting Nmap 4.11 ( http://www.insecure.org/nmap/ ) at 2011-07-19 05:20 EDT

mass_dns: warning: Unable to determine any DNS servers. Reverse DNS is disabled. Try using --system-dns or specify valid servers with --dns_servers

Interesting ports on 10.4.212.6:

PORT STATE SERVICE

69/udp closed tftp

Nmap finished: 1 IP address (1 host up) scanned in 0.062 seconds

[root@usna0netweb02 ~]# nmap -p 69 -sU -P0 10.4.24.6

Starting Nmap 4.11 ( http://www.insecure.org/nmap/ ) at 2011-07-19 05:23 EDT

mass_dns: warning: Unable to determine any DNS servers. Reverse DNS is disabled. Try using --system-dns or specify valid servers with --dns_servers

Interesting ports on 10.4.24.6:

PORT STATE SERVICE

69/udp open|filtered tftp

Nmap finished: 1 IP address (1 host up) scanned in 2.019 seconds

[root@usna0netweb02 ~]#