Testing UDP Availability

Occasionally I want to know if UDP packets are being dropped between 2 machines. When they are, lots of audio/video services (such as Asterisk) can be interrupted. How do I find out if UDP is enabled between 2 servers? Simple...I connect to one of them and run a command against the other.

nmap

Using nmap (network map) is one way. I specify the port(s) I want to scan with -p, the scan type you want (UDP is -sU), and the hostname or IP of the machine I want to scan. Specifying "don't ping in advance of connect" (-P0) is optional.

[me@myhost]# nmap -p 123 -sU -P0 myotherhost

Starting nmap 3.70 ( http://www.insecure.org/nmap/ ) at 2010-02-02 10:31 EST
Interesting ports on myotherhost (192.168.1.14):
PORT    STATE         SERVICE
123/udp open|filtered ntp

Nmap run completed -- 1 IP address (1 host up) scanned in 2.019 seconds

The result shows port 123 is open and be filtered, and that NTP is running there.

nc

Using nc (netcat) is another possibility. I specified verbose with -v, UDP with -u, and scan-only with -z.

[me@myhost]#  nc -v -u -z myotherhost 123
myotherhost [192.168.1.14] 123 (ntp) open

The result shows port 123 is open and the ntp daemon is listening.