Why does the udp port not open?

Question

I am investigating how to open a UDP port, specifically the port 1194. This is what I've done:

1. Check if the port is opened:

   % sudo nmap –sU –p 1194 <hostnameOfMyMachine>
   
   -> PORT     STATE  SERVICE
   -> 1194/udp closed unknown
   

2. Open the port:

   % sudo iptables -A INPUT -p udp --dport 1194 -d 0/0 -s 0/0 -j ACCEPT
   

Finally, I repeated the step 1) to check if the port has been opened but the same output is displayed:

-> PORT     STATE  SERVICE
-> 1194/udp closed unknown

Does anybody know why the port did not open? How can I open it?

% sudo netstat --inet --inet6 -lnp

Conexiones activas de Internet (solo servidores)
Proto  Recib Enviad Dirección local         Dirección remota       Estado       PID/Program name
udp        0      0 0.0.0.0:41503           0.0.0.0:*                           1372/avahi-daemon: 
udp        0      0 127.0.0.1:53            0.0.0.0:*                           2344/dnsmasq    
udp        0      0 0.0.0.0:68              0.0.0.0:*                           2303/dhclient   
udp        0      0 0.0.0.0:17500           0.0.0.0:*                           5935/dropbox    
udp        0      0 0.0.0.0:17500           0.0.0.0:*                           6025/dropbox    
udp        0      0 0.0.0.0:5353            0.0.0.0:*                           1372/avahi-daemon: 
udp6       0      0 :::44533                :::*                                1372/avahi-daemon: 
udp6       0      0 :::5353                 :::*                                1372/avahi-daemon: 


% sudo iptables -L INPUT -nv

Chain INPUT (policy ACCEPT 134K packets, 19M bytes)
 pkts bytes target     prot opt in     out     source               destination         
    3    84 ACCEPT     udp  --  *      *       0.0.0.0/0            0.0.0.0/0            udp dpt:1194
    0     0 ACCEPT     udp  --  *      *       0.0.0.0/0            0.0.0.0/0            udp dpt:1194
    0     0 ACCEPT     udp  --  *      *       0.0.0.0/0            0.0.0.0/0            udp dpt:1194

Answer 1

A UDP port is considered open by nmap if a packet sent to this port results in a reply from this port.

This, of course, means, that there needs to be some service running at the port. Port 1194 is typically OpenVPN.

Even if there is some service running at that port, it might be considered closed if a firewall is filtering packets to or from this port. That's why you've tried to add a rule to the firewall (iptables) to let these packets pass.

But this rule does not help if there is no service running on this port in the first place.

Answer 2

1. You need to put this port rule to your router and forward it to your machine.

   (If that is a VPN application, there is, of course, no need for port-forwarding.)

2. You need to configure whatever application for that port.

3. That application must be running.