Title: Bypassing ISP Blocked Ports Date: 2021-04-10 Category: Writing Summary: Bypass ISP blocked ports using VPN port forwarding for public access. Wide: true Short: 2 [TOC] 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 $5 per month virtual server with [Digital Ocean](https://digitalocean.com) that runs Debian GNU/Linux 10. 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. ## Server Setup Spin up a Debian 10 virtual server on your favourite hosting provider and set your user up as you would normally. You should probably harden this server. Assign a subdomain to it like `vpn.example.com`. Install the following requirements: ``` $ sudo apt update $ sudo apt install openvpn ufw ``` ### OpenVPN Server These steps roughly follow [this guide](https://wiki.debian.org/OpenVPN#TLS-enabled_VPN). Generate TLS certificates and keys: ``` $ cd /etc/openvpn $ sudo openvpn --genkey --secret static.key $ sudo make-cadir easy-rsa/ $ sudo chown -R tanner:tanner easy-rsa/ ``` Replace `tanner` with your Linux username, this is temporary. The `.rnd` file prevents a warning ``` $ cd easy-rsa/ $ ./easyrsa init-pki $ head /dev/urandom > pki/.rnd $ ./easyrsa build-ca ``` Enter a password 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 a client cert: ``` $ ./easyrsa build-client-full mediaserver nopass ``` 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. 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 + chmod: ``` $ cd /etc/openvpn $ sudo chown -R root:root easy-rsa/ $ sudo mkdir ccd $ sudo touch ccd/mediaserver ``` Replace `mediaserver` with whatever client name you used above. Edit it like so: Your home server will be `10.8.0.100` ``` ifconfig-push 10.8.0.100 255.255.255.0 ``` 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: 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 ``` ### 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 10.8.0.0/8 -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](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 [server /etc/openvpn/easy-rsa/pki/ca.crt] [server /etc/openvpn/easy-rsa/pki/issued/mediaserver.crt] [server /etc/openvpn/easy-rsa/pki/private/mediaserver.key] [server /etc/openvpn/static.key] ``` 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 [server /etc/openvpn/easy-rsa/pki/ca.crt] [server /etc/openvpn/easy-rsa/pki/issued/client.crt] [server /etc/openvpn/easy-rsa/pki/private/client.key] [server /etc/openvpn/static.key] ``` 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: 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.