How to open port on Debian

Question

I try to open port 5431 so typed:

sudo iptables -A INPUT -p tcp --dport 5431 --jump ACCEPT
iptables-save

when I print the rules in a chain iptables -S then the output is:

-P INPUT ACCEPT
-P FORWARD ACCEPT
-P OUTPUT ACCEPT
-A INPUT -p tcp -m tcp --dport 5431 -j ACCEPT

so I tried to check open port by nmap from my machine:

mwalko@mwalko-X58A-UD3R:~$ nmap 10.1.2.30

Starting Nmap 7.01 ( https://nmap.org ) at 2018-04-20 16:46 CEST
Nmap scan report for static-30.vlan2.vlex.local (10.1.2.30)
Host is up (0.00027s latency).
Not shown: 997 closed ports
PORT     STATE SERVICE
22/tcp   open  ssh
111/tcp  open  rpcbind
5432/tcp open  postgresql

Nmap done: 1 IP address (1 host up) scanned in 0.05 seconds

and as can be seen above 5431 is still not open. How to open this port?

@Edit

root@dell1950:/sbin# netstat -ntulp
Active Internet connections (only servers)
Proto Recv-Q Send-Q Local Address           Foreign Address         State       PID/Program name    
tcp        0      0 0.0.0.0:22              0.0.0.0:*               LISTEN      923/sshd            
tcp        0      0 0.0.0.0:5432            0.0.0.0:*               LISTEN      927/postgres        
tcp        0      0 0.0.0.0:111             0.0.0.0:*               LISTEN      861/rpcbind         
tcp6       0      0 :::22                   :::*                    LISTEN      923/sshd            
tcp6       0      0 :::5432                 :::*                    LISTEN      927/postgres        
tcp6       0      0 :::111                  :::*                    LISTEN      861/rpcbind         
udp        0      0 0.0.0.0:111             0.0.0.0:*                           861/rpcbind         
udp        0      0 0.0.0.0:613             0.0.0.0:*                           861/rpcbind         
udp6       0      0 :::111                  :::*                                861/rpcbind         
udp6       0      0 :::613                  :::*                                861/rpcbind

Solution 1

I have used @Xenwar comment but had to specify port to make it work: nc -l -p 5431.

Now:

mwalko@mwalko-X58A-UD3R:~$ nmap 10.1.2.30

Starting Nmap 7.01 ( https://nmap.org ) at 2018-04-23 08:40 CEST
Nmap scan report for static-30.vlan2.vlex.local (10.1.2.30)
Host is up (0.00022s latency).
Not shown: 996 closed ports
PORT     STATE SERVICE
22/tcp   open  ssh
111/tcp  open  rpcbind
5431/tcp open  park-agent
5432/tcp open  postgresql

Nmap done: 1 IP address (1 host up) scanned in 0.05 seconds

Just noticed that nmap closes netcat's nc -l ;/

Solution 2

You are not opening the port, you are just letting the packet directed to such port not to be filtered by firewall.

A program should open the port (aka listening). Firewall just decides if packets go or not to a specific port (which could be open or close), but has nothing to do with how to handle packets.

Comments

[Michu93]

@GiacomoCatenzazzi, I got it wrong then, thanks +1! I used solution proposed by Xenwar but I still can't see this port by nmap.