Off late, a couple of friends at work are asking me how to check if we can connect to a port on a remote host. They have tried telnet-ing to the port but it does not work because they are UDP ports. After a bit of googling and reading the man page, this is how we can scan UDP ports using nc and nmap.
nc -uz remote-host udp-port
[root@server1 ~]# nc -z -u remote_host 514 Connection to remote_host 514 port [udp/syslog] succeeded! [root@server1 ~]#
If the remote host is not accepting the connection, it instantly returns the prompt without any message or takes a while sometimes.
nmap -sU -pPortNumer remote-host
[root@server1 ~]# nmap -sU -p514 remote_host Starting Nmap 4.11 ( http://www.insecure.org/nmap/) at 2015-05-11 09:00 Interesting ports on remote_host (IP_ADDRESS): PORT STATE SERVICE 514/udp open|filtered syslog Nmap finished: 1 IP address (1 host up) scanned in 0.398 seconds