Pi is listening but nothing except ssh can connect

Question

Ok, so I'm stumped. I have a new Raspbian install on a Model B Rev 2 and I can ssh using the IP Address just fine, but I can't connect on any other port outside the pi. Some details:

pi@dogwood:~ $ lsb_release -a
No LSB modules are available.

Distributor ID: Raspbian
Description:    Raspbian GNU/Linux 9.9 (stretch)
Release:    9.9
Codename:   stretch
pi@dogwood:~ $ 

I am trying to write a python REST API, but to troubleshoot I am using the netcat command like so:

pi@dogwood:~ $ nc -l localhost 5001

While that is waiting for a connection, I run netstat in another terminal:

pi@dogwood:~ $ sudo netstat -lpnt
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      340/sshd            
tcp6       0      0 :::22                   :::*                    LISTEN      340/sshd            
tcp6       0      0 ::1:5001                :::*                    LISTEN      11887/nc            

pi@dogwood:~ $ 

Which I think means it has bound to all IPs and is listening on port 5001. That ::1:5001 looks suspicious, but I don't know enough about IPv6.

So from here, I can then do a

pi@dogwood:~ $ curl http://localhost:5001

^C
pi@dogwood:~ $ 

And it makes a connection. The IP address of the pi is:

pi@dogwood:~ $ ip address
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
       valid_lft forever preferred_lft forever
    inet6 ::1/128 scope host 
       valid_lft forever preferred_lft forever
2: enxb827ebe4d83d: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP group default qlen 1000
    link/ether SO:ME:MAC:HE:RE brd ff:ff:ff:ff:ff:ff
    inet 192.168.2.11/24 brd 192.168.2.255 scope global enxb827ebe4d83d
       valid_lft forever preferred_lft forever
    inet6 fe80::f1bd:6ee:1851:66ba/64 scope link 
       valid_lft forever preferred_lft forever
pi@dogwood:~ $ 
pi@dogwood:~ $ 

However, if I use the ip address of the pi either locally on the pi:

pi@dogwood:~ $ curl http://192.168.2.11:5001
curl: (7) Failed to connect to 192.168.2.11 port 5001: Connection refused
pi@dogwood:~ $ 

or remotely, I get:

myuser@appletree:~$ curl http://192.168.2.11:5001
curl: (7) Failed to connect to 192.168.2.11 port 5001: Connection refused
myuser@appletree:~$ 

So obviously it's a firewall... right? Except that iptables has no rules:

pi@dogwood:~ $ sudo iptables -S
-P INPUT ACCEPT
-P FORWARD ACCEPT
-P OUTPUT ACCEPT
pi@dogwood:~ $ 

And ufw is not installed:

pi@dogwood:~ $ sudo ufw
sudo: ufw: command not found
pi@dogwood:~ $ 

Any suggestions?

Answer 1

Your process is listening on IPv6 address ::1 (that's the IPv6 equivalent of IPv4 127.0.0.1 (aka localhost)).

You need it to listen on :: (which is IP6ADDR_ANY as that's the IPv6 equivalent of IPv4 INADDR_ANY (0.0.0.0)).

Don't use nc -l localhost 5001

If you want to connect from any other machine on your LAN. Use nc -l -p 5001 or nc -l -6 -p 5001 or nc -l -4 -p 5001