How do I check if a port is open on Red Hat Linux/CentOS

Solution 1:

Hmm. Multicast, so telnet isn't going to work.

You can run tcpdump, I think, to see if traffic is getting to your server. Run something like this on your server:

tcpdump -i eth0 port 427 and see if anything shows up.

If nothing does, there's some sort of block elsewhere.

Check your local firewall with something like iptables -L -n to make sure the INPUT chain permits your traffic.

To further diagnose, you may need to start running traceroutes from your laptop, specifying the protocol and port (otherwise, it'll default to ICMP).

Solution 2:

Hmm, netstat -anp | grep 427 would do the trick. Maybe lsof -i | grep 427 as well.

To check from an outside linux host, you can use nmap. Perhaps nmap -sT -p 427 or nmap -sU -p 427 (for UDP).