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Bypassing ISP Blocked Ports

Bypass ISP blocked ports using VPN port forwarding for public access.

Sun 10 December 2023

My residential ISP blocks inbound traffic to common ports like 22, 80, and 443. I use an OpenVPN tunnel to forward these ports so that I can self-host a public media server. It does not require users to be on the VPN.

This article explains how I set it up and is targeted towards Linux sysadmins.

Overview

I have a cheap $6 per month virtual server with Digital Ocean that runs Debian GNU/Linux 12. An OpenVPN server is running on this virtual server.

My media server at home has an OpenVPN client connected to the server and is assigned a static IP on the VPN network.

The virtual server has routing enabled and forwards inbound traffic from the internet to my media server at home. This allows me to have external HTTP and SSH access.

a diagram of my setup. the client computer connecting to my home server through the cloud using a VPN tunnel.

Server Setup

Spin up a Debian 12 server on your favourite hosting provider. If you're using an older version of Debian, you can follow the old version of this article. You should harden this server. Assign a subdomain to it like vpn.example.com.

Install the following requirements:

$ sudo apt update
$ sudo apt install openvpn easy-rsa ufw
$ sudo ufw allow ssh
$ sudo ufw allow 1194  # openvpn's port

OpenVPN Server

These steps roughly follow this guide.

Generate TLS certificates and keys:

$ cd /etc/openvpn
$ sudo openvpn --genkey secret static.key
$ sudo make-cadir easy-rsa/
$ sudo chown -R tanner:tanner /etc/openvpn

Replace tanner with your Linux username, this is temporary.

(The certs will expire in 100 years)

$ cd easy-rsa/
$ export EASYRSA_CERT_EXPIRE=36500
$ export EASYRSA_CA_EXPIRE=36500
$ ./easyrsa init-pki
$ ./easyrsa build-ca

Enter passwords you won't forget in case you want to add another client later. The Common Name you choose is not important.

Generate Diffie–Hellman params:

$ ./easyrsa gen-dh

Generate a server cert:

$ ./easyrsa build-server-full server nopass

Generate client certs:

$ ./easyrsa build-client-full mediaserver nopass
$ ./easyrsa build-client-full anotherserver nopass
... etc

We make a mediaserver client because we want to assign a static IP to it. You need to make a different one for each client you want with a static IP.

Also, if you want generic clients that all get dynamic IPs for use on your laptop, phone, etc. to protect you from public WiFi (like a normal VPN), create only a single extra one:

$ ./easyrsa build-client-full client nopass  # optional

Leave off nopass if you want to password protect the config file keys when you set up a new client (PEM pass phrase).

Create the server config file /etc/openvpn/server.conf:

(Can't use port 443 here since it'll be forwarded)

port 1194
proto udp
dev tun
topology subnet
ca /etc/openvpn/easy-rsa/pki/ca.crt
cert /etc/openvpn/easy-rsa/pki/issued/server.crt
key /etc/openvpn/easy-rsa/pki/private/server.key
dh /etc/openvpn/easy-rsa/pki/dh.pem
tls-auth /etc/openvpn/static.key 0
client-config-dir /etc/openvpn/ccd
server 10.8.0.0 255.255.255.0
client-to-client
duplicate-cn
keepalive 10 120
cipher AES-256-GCM
auth SHA256
comp-lzo
max-clients 10
user nobody
group nogroup
persist-key
persist-tun

Assign a static IP:

(Your home server will be 10.8.0.100)

$ cd /etc/openvpn
$ mkdir ccd
$ echo "ifconfig-push 10.8.0.100 255.255.255.0" > mediaserver
$ echo "ifconfig-push 10.8.0.101 255.255.255.0" > anotherserver

Test your config by running:

$ sudo openvpn --config /etc/openvpn/server.conf

If you run ip addr in another terminal, you should see an entry like this:

5: tun0: <POINTOPOINT,MULTICAST,NOARP,UP,LOWER_UP> stuff
    link/none 
    inet 10.8.0.1/24 brd 10.8.0.255 scope global tun0
       valid_lft forever preferred_lft forever
    inet6 fe80::d9fc:b2f9:34e6:5ed2/64 scope link stable-privacy 
       valid_lft forever preferred_lft forever

Change back ownership:

$ sudo chown -R root:root /etc/openvpn

systemd

If it works fine, persist OpenVPN with systemd:

$ sudo systemctl enable openvpn@server
$ sudo systemctl start openvpn@server
$ sudo systemctl daemon-reload
$ sudo service openvpn restart

Test it works by rebooting:

$ sudo reboot
$ ssh vpn.example.com
$ ip addr

Port Forwarding

I use ufw to handle the iptables rules because I use it anyway as a firewall when I harden my servers.

Enable routing:

$ sudo sysctl net.ipv4.ip_forward=1

Edit /etc/sysctl.conf to set:

net.ipv4.ip_forward=1

Edit /etc/default/ufw to set:

DEFAULT_FORWARD_POLICY="ACCEPT"

Add this to the top of /etc/ufw/before.rules:

*nat
:POSTROUTING ACCEPT [0:0]

# ssh port forwarding
-A PREROUTING -d 123.123.123.123 -p tcp --dport 2222 -j DNAT --to-dest 10.8.0.100:2222
-A POSTROUTING -d 10.8.0.100 -p tcp --dport 2222 -j SNAT --to-source 10.8.0.1

# Allow traffic from OpenVPN client to eth0
-A POSTROUTING -s 10.8.0.0/24 -o eth0 -j MASQUERADE
COMMIT

Replace 123.123.123.123 with your VPN server's external IP address and eth0 with the external interface.

This will forward TCP traffic on port 2222 to your home server. If you want to use port 22, then you need to set the VPN SSH server to something else.

A full example of /etc/ufw/before.rules with other ports included can be found here:

https://txt.t0.vc/URUG

Apply the changes to ufw:

$ sudo ufw disable && sudo ufw enable

Client Setup

Switch to your home server or client machine.

Install OpenVPN:

$ sudo apt update
$ sudo apt install openvpn

Client Configs

For static IP clients (like your home server), create the config file /etc/openvpn/client.conf:

client
dev tun
proto udp
remote vpn.example.com 1194
resolv-retry infinite
nobind
persist-key
persist-tun
remote-cert-tls server
cipher AES-256-GCM
auth SHA256
comp-lzo
key-direction 1
<ca>
[server /etc/openvpn/easy-rsa/pki/ca.crt]
</ca>
<cert>
[server /etc/openvpn/easy-rsa/pki/issued/mediaserver.crt]
</cert>
<key>
[server /etc/openvpn/easy-rsa/pki/private/mediaserver.key]
</key>
<tls-auth>
[server /etc/openvpn/static.key]
</tls-auth>

Replace the [server ...] lines with the contents of that file on the VPN server, for example:

$ sudo cat /etc/openvpn/easy-rsa/pki/ca.crt
---> copy & paste result

Also replace vpn.example.com with the subdomain you assigned earlier.

For device clients (like your laptop and phone), create the config file client.ovpn:

(redirect-gateway def1 forces traffic over the VPN)

client
dev tun
proto udp
remote vpn.example.com 1194
redirect-gateway def1
resolv-retry infinite
nobind
persist-key
persist-tun
remote-cert-tls server
cipher AES-256-GCM
auth SHA256
comp-lzo
key-direction 1
<ca>
[server /etc/openvpn/easy-rsa/pki/ca.crt]
</ca>
<cert>
[server /etc/openvpn/easy-rsa/pki/issued/client.crt]
</cert>
<key>
[server /etc/openvpn/easy-rsa/pki/private/client.key]
</key>
<tls-auth>
[server /etc/openvpn/static.key]
</tls-auth>

The client.ovpn file is ready to be imported into your VPN clients.

Test your config by running:

$ sudo openvpn --config /etc/openvpn/client.conf

If you run ip addr in another terminal, you should see an entry like this:

7: tun0: <POINTOPOINT,MULTICAST,NOARP,UP,LOWER_UP> stuff
    link/none 
    inet 10.8.0.100/24 brd 10.8.0.255 scope global tun0
       valid_lft forever preferred_lft forever
    inet6 fe80::b2:ed71:6c98:4bc9/64 scope link stable-privacy 
       valid_lft forever preferred_lft forever

Try pinging the server:

$ ping 10.8.0.1
PING 10.8.0.1 (10.8.0.1) 56(84) bytes of data.
64 bytes from 10.8.0.1: icmp_seq=1 ttl=64 time=71.5 ms
64 bytes from 10.8.0.1: icmp_seq=2 ttl=64 time=73.0 ms
... etc

systemd

If it works fine, persist OpenVPN with systemd:

$ sudo chown root:root /etc/openvpn/client.conf
$ sudo chmod 600 /etc/openvpn/client.conf
$ sudo systemctl enable openvpn@client
$ sudo systemctl start openvpn@client
$ sudo systemctl daemon-reload
$ sudo service openvpn restart

Client Apps

On Android I use "OpenVPN for Android" and on Linux I use the network-manager-openvpn-gnome Debian package.

To add your VPN on Gnome, open VPN settings, import file, and select client.ovpn. If the private key is missing, select it from ~/.cert/nm-openvpn/.

Closing Thoughts

You should now be fine to access your home server from over the internet.

To forward additional ports, just edit the /etc/ufw/before.rules file like above and apply the changes to ufw.

You can now point a domain to your virtual server's IP and use that to connect to your home server. Use a CNAME to make it easy to change later:

NAME                    TYPE   VALUE
--------------------------------------------------
vpn.example.com.        A      123.123.123.123
myserver.example.com.   CNAME  vpn.example.com.

Finally, make sure any server programs are listening / bound to 10.8.0.100 or 0.0.0.0 so that they can get traffic from that interface.