How to allow public access to an SSH tunnel

Whenever you create an SSH tunnel or port forwarding, by default, it is only available for the local host. Unless specified, it listens to the loopback IP address or the localhost (127.0.0.1). This IP address is not accessible from other hosts, which causes the forwarded port to not be available from the outside.

You can allow public access to your SSH tunnel by creating a port forwarding or tunnel that listens on an accessible IP address and configuring necessary firewall rules to allow incoming connection to the tunnel.

1. Get IP addresses of your host.

   $ ip a
   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: ens33: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP group default qlen 1000
       link/ether 00:0c:29:08:63:73 brd ff:ff:ff:ff:ff:ff
       altname enp2s1
       inet 192.168.111.27/24 brd 192.168.111.255 scope global dynamic noprefixroute ens33
          valid_lft 66553sec preferred_lft 66553sec
       inet6 fe80::fc5d:1d5c:ae0e:68f1/64 scope link noprefixroute 
          valid_lft forever preferred_lft forever

2. Create a tunnel or port forwarding by specifying IP address to listen to.

   $ ssh -fN -D 192.168.111.27:8080 192.168.111.29
   user@192.168.111.29's password:

   Related: How to create an SSH SOCKS proxy

3. Check if tunnel currently running on specified IP address.

   $ ss -natp | grep 8080
   LISTEN 0      128    192.168.111.27:8080         0.0.0.0:*    users:(("ssh",pid=2966,fd=4))

4. Configure firewall to allow remote access to the tunnel port.

   $ sudo ufw allow 8080 # Ubuntu and Debian variance
   $ sudo firewall-cmd --zone=public --permanent --add-port=8080/tcp && sudo firewall-cmd --reload # CentOS and Red Hat variance

5. Access tunnel from other host to test.

   $ curl --proxy socks5://192.168.111.27:8080 https://ifconfig.me
   115.131.92.137