determine if udp socket can be accessed via external client

Question

I don't have access to company firewall server. but supposedly the port 1720 is open on my one ubuntu server. So I want to test it with netcat:

sudo nc -ul 1720

The port is listening on the machine ITSELF:

sudo netstat -tulpn | grep nc
udp        0      0 0.0.0.0:1720            0.0.0.0:*                           29477/nc

The port is open and in use on the machine ITSELF:

lsof -i -n -P | grep 1720
gateway 980 myuser    8u  IPv4 187284576      0t0  UDP *:1720

Checked the firewall on current server:

sudo ufw allow 1720/udp
Skipping adding existing rule
Skipping adding existing rule (v6)
sudo ufw status verbose | grep 1720
1720/udp                   ALLOW IN    Anywhere
1720/udp                   ALLOW IN    Anywhere (v6)

But I try echoing data to it from another computer (I replaced the x's with the real integers):

echo "Some data to send" | nc xx.xxx.xx.xxx 1720

But it didn't write anything. So then I try with telnet from the other computer as well:

telnet xx.xxx.xx.xxx 1720
Trying xx.xxx.xx.xxx...
telnet: connect to address xx.xxx.xx.xxx: Operation timed out
telnet: Unable to connect to remote host

Although I don't think telnet works with udp sockets.

I ran nmap from another computer within the same local network and this is what I got:

sudo nmap -v -A -sU -p 1720 xx.xxx.xx.xx

Starting Nmap 5.21 ( http://nmap.org ) at 2013-10-31 15:41 EDT
NSE: Loaded 36 scripts for scanning.
Initiating Ping Scan at 15:41
Scanning xx.xxx.xx.xx [4 ports]
Completed Ping Scan at 15:41, 0.10s elapsed (1 total hosts)
Initiating Parallel DNS resolution of 1 host. at 15:41
Completed Parallel DNS resolution of 1 host. at 15:41, 0.00s elapsed
Initiating UDP Scan at 15:41
Scanning xtremek.com (xx.xxx.xx.xx) [1 port]
Completed UDP Scan at 15:41, 0.07s elapsed (1 total ports)
Initiating Service scan at 15:41
Initiating OS detection (try #1) against xtremek.com (xx.xxx.xx.xx)
Retrying OS detection (try #2) against xtremek.com (xx.xxx.xx.xx)
Initiating Traceroute at 15:41
Completed Traceroute at 15:41, 0.01s elapsed
NSE: Script scanning xx.xxx.xx.xx.
NSE: Script Scanning completed.
Nmap scan report for xtremek.com (xx.xxx.xx.xx)
Host is up (0.00013s latency).
PORT     STATE  SERVICE VERSION
1720/udp closed unknown
Too many fingerprints match this host to give specific OS details
Network Distance: 1 hop

TRACEROUTE (using port 1720/udp)
HOP RTT     ADDRESS
1   0.13 ms xtremek.com (xx.xxx.xx.xx)

Read data files from: /usr/share/nmap
OS and Service detection performed. Please report any incorrect results at http://nmap.org/submit/ .
Nmap done: 1 IP address (1 host up) scanned in 2.04 seconds
           Raw packets sent: 27 (2128B) | Rcvd: 24 (2248B).

The only thing I can think of is a firewall or vpn issue. Is there anything else I can check for before requesting that they look at the firewall server again?

Answer 1

Several tools can be used to check for open ports or scan your computer. In general, because connections from localhost (your computer) will automatically be accepted, you should scan from a second computer on your LAN. nmap is one of several options.

nmap -v -A ip_address

Or if the server is pesky about pings ...

nmap -v -A -PN ip_address

nmap has additional capabilities as well =).

Additional tools you can use on your computer (rather then scanning from a remote location):

netstat -an | grep LISTEN | grep -v ^unix

netstat -ntulp

lsof -i -n -P 

Answer 2

How are you?

Do you try with netstat?

There is a lot of way to see if your port is open, may be this can help you:

sudo netstat -lp --inet

Its important sudo mode. This will list all your open port, and must appear. Another way to do same is:

sudo netstat -plut

Try it, and tell me.

Take Care.