Add bypassing ports article
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content/bypassing-ports.md
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content/bypassing-ports.md
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Title: Bypassing ISP Blocked Ports
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Date: 2021-04-10
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Category: Writing
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Summary: Bypass ISP blocked ports using VPN port forwarding for public access.
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Wide: true
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[TOC]
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My residential ISP blocks inbound traffic to common ports like 22, 80, and 443.
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I use an OpenVPN tunnel to forward these ports so that I can self-host a
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public media server. It does __not__ require users to be on the VPN.
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This article explains how I set it up and is targeted towards Linux sysadmins.
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## Overview
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I have a cheap $5 per month virtual server with [Digital
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Ocean](https://digitalocean.com) that runs Debian GNU/Linux 10. An OpenVPN
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server is running on this virtual server.
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My media server at home has an OpenVPN client connected to the server and is
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assigned a static IP on the VPN network.
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The virtual server has routing enabled and forwards inbound traffic __from the
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internet__ to my media server at home. This allows me to have external HTTP and SSH
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access.
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## Server Setup
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Spin up a Debian 10 virtual server on your favourite hosting provider and set
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your user up as you would normally. You should probably harden this server.
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Assign a subdomain to it like `vpn.example.com`.
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Install the following requirements:
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```
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$ sudo apt update
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$ sudo apt install openvpn ufw
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```
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### OpenVPN Server
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These steps roughly follow [this
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guide](https://wiki.debian.org/OpenVPN#TLS-enabled_VPN).
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Generate TLS certificates and keys:
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```
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$ cd /etc/openvpn
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$ sudo openvpn --genkey --secret static.key
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$ sudo make-cadir easy-rsa/
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$ sudo chown -R tanner:tanner easy-rsa/
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```
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Replace `tanner` with your own username, this is temporary.
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<span class="aside">The `.rnd` file prevents a warning</span>
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```
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$ cd easy-rsa/
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$ ./easyrsa init-pki
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$ head /dev/urandom > pki/.rnd
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$ ./easyrsa build-ca
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```
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Enter a password you won't forget in case you want to add another client later.
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The Common Name you choose is not important.
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Generate Diffie–Hellman params:
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```
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$ ./easyrsa gen-dh
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```
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Generate a server cert:
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```
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$ ./easyrsa build-server-full server nopass
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```
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Generate a client cert:
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```
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$ ./easyrsa build-client-full mediaserver nopass
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```
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We make a `mediaserver` client because we want to assign a static IP to it. You
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need to make a different one for each client you want with a static IP.
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Also, if you want generic clients that all get dynamic IPs for use on your
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laptop, phone, etc. to protect you from public WiFi, create only a single extra one:
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```
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$ ./easyrsa build-client-full client nopass # optional
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```
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Leave off `nopass` if you want to password protect the config file when you set
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up a new client.
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Create the server config file `/etc/openvpn/server.conf`:
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```
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port 1194
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proto udp
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dev tun
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topology subnet
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ca /etc/openvpn/easy-rsa/pki/ca.crt
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cert /etc/openvpn/easy-rsa/pki/issued/server.crt
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key /etc/openvpn/easy-rsa/pki/private/server.key
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dh /etc/openvpn/easy-rsa/pki/dh.pem
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tls-auth /etc/openvpn/static.key 0
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client-config-dir /etc/openvpn/ccd
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server 10.8.0.0 255.255.255.0
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client-to-client
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duplicate-cn
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keepalive 10 120
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cipher AES-256-GCM
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auth SHA256
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comp-lzo
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max-clients 10
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user nobody
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group nogroup
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persist-key
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persist-tun
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```
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Assign a static IP + chmod:
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```
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$ cd /etc/openvpn
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$ sudo chown -R root:root easy-rsa/
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$ sudo mkdir ccd
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$ sudo touch ccd/mediaserver
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```
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Replace `mediaserver` with whatever client name you used above. Edit it like so:
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<span class="aside">Your home server will be `10.8.0.100`</span>
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```
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ifconfig-push 10.8.0.100 255.255.255.0
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```
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Test your config by running:
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```
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sudo openvpn --config /etc/openvpn/server.conf
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```
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If you run `ip addr` in another terminal, you should see an entry like this:
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```
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5: tun0: <POINTOPOINT,MULTICAST,NOARP,UP,LOWER_UP> stuff
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link/none
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inet 10.8.0.1/24 brd 10.8.0.255 scope global tun0
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valid_lft forever preferred_lft forever
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inet6 fe80::d9fc:b2f9:34e6:5ed2/64 scope link stable-privacy
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valid_lft forever preferred_lft forever
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```
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### systemd
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If it works fine, persist OpenVPN with systemd:
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```
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$ sudo systemctl enable openvpn@server
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$ sudo systemctl start openvpn@server
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$ sudo systemctl daemon-reload
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$ sudo service openvpn restart
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```
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Test it works by rebooting:
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```
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$ sudo reboot
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$ ssh vpn.example.com
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$ ip addr
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```
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### Port Forwarding
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I use `ufw` to handle the iptables rules because I use it anyway as a firewall
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when I harden my servers.
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Enable routing:
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```
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$ sudo sysctl net.ipv4.ip_forward=1
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```
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Edit `/etc/sysctl.conf` to set:
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```
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net.ipv4.ip_forward=1
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```
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Edit `/etc/default/ufw` to set:
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```
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DEFAULT_FORWARD_POLICY="ACCEPT"
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```
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Add this to the top of `/etc/ufw/before.rules`:
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```
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*nat
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:POSTROUTING ACCEPT [0:0]
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# ssh port forwarding
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-A PREROUTING -d 123.123.123.123 -p tcp --dport 2222 -j DNAT --to-dest 10.8.0.100:2222
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-A POSTROUTING -d 10.8.0.100 -p tcp --dport 2222 -j SNAT --to-source 10.8.0.1
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# Allow traffic from OpenVPN client to eth0
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-A POSTROUTING -s 10.8.0.0/8 -o eth0 -j MASQUERADE
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COMMIT
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```
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Replace `123.123.123.123` with your VPN server's external IP address and `eth0`
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with the external interface.
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This will forward TCP traffic on port 2222 to your home server. If you want to use
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port 22, then you need to set the VPN SSH server to something else.
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A full example of `/etc/ufw/before.rules` with other ports included can be found
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here:
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[https://txt.t0.vc/URUG](https://txt.t0.vc/URUG)
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Apply the changes to `ufw`:
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```
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$ sudo ufw disable && sudo ufw enable
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```
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## Client Setup
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Switch to your home server or client machine.
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Install openvpn:
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```
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$ sudo apt update
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$ sudo apt install openvpn
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```
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### Client Configs
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For static IP clients (like your home server), create the config file `/etc/openvpn/client.conf`:
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```
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client
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dev tun
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proto udp
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remote vpn.example.com 1194
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resolv-retry infinite
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nobind
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persist-key
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persist-tun
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remote-cert-tls server
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cipher AES-256-GCM
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auth SHA256
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comp-lzo
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key-direction 1
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<ca>
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[server /etc/openvpn/easy-rsa/pki/ca.crt]
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</ca>
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<cert>
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[server /etc/openvpn/easy-rsa/pki/issued/mediaserver.crt]
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</cert>
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<key>
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[server /etc/openvpn/easy-rsa/pki/private/mediaserver.key]
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</key>
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<tls-auth>
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[server /etc/openvpn/static.key]
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</tls-auth>
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```
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Replace the `[server ...]` lines with the contents of that file on the VPN
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server, for example:
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```
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$ sudo cat /etc/openvpn/easy-rsa/pki/ca.crt
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---> copy & paste result
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```
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Also replace `vpn.example.com` with the subdomain you assigned earlier.
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For device clients (like your laptop and phone), create the config file `client.ovpn`:
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<span class="aside">`redirect-gateway def1` forces traffic over the VPN</span>
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```
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client
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dev tun
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proto udp
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remote vpn.example.com 1194
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resolv-retry infinite
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nobind
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persist-key
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persist-tun
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remote-cert-tls server
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cipher AES-256-GCM
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auth SHA256
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comp-lzo
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key-direction 1
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redirect-gateway def1
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<ca>
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[server /etc/openvpn/easy-rsa/pki/ca.crt]
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</ca>
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<cert>
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[server /etc/openvpn/easy-rsa/pki/issued/client.crt]
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</cert>
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<key>
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[server /etc/openvpn/easy-rsa/pki/private/client.key]
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</key>
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<tls-auth>
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[server /etc/openvpn/static.key]
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</tls-auth>
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```
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The `client.ovpn` file is ready to be imported into your VPN clients.
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Test your config by running:
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```
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sudo openvpn --config /etc/openvpn/client.conf
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```
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If you run `ip addr` in another terminal, you should see an entry like this:
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```
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7: tun0: <POINTOPOINT,MULTICAST,NOARP,UP,LOWER_UP> stuff
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link/none
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inet 10.8.0.100/24 brd 10.8.0.255 scope global tun0
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valid_lft forever preferred_lft forever
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inet6 fe80::b2:ed71:6c98:4bc9/64 scope link stable-privacy
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valid_lft forever preferred_lft forever
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```
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Try pinging the server:
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```
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$ ping 10.8.0.1
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PING 10.8.0.1 (10.8.0.1) 56(84) bytes of data.
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64 bytes from 10.8.0.1: icmp_seq=1 ttl=64 time=71.5 ms
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64 bytes from 10.8.0.1: icmp_seq=2 ttl=64 time=73.0 ms
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... etc
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```
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### systemd
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If it works fine, persist OpenVPN with systemd:
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```
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$ sudo chown root:root /etc/openvpn/client.conf
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$ sudo chmod 600 /etc/openvpn/client.conf
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$ sudo systemctl enable openvpn@client
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$ sudo systemctl start openvpn@client
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$ sudo systemctl daemon-reload
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$ sudo service openvpn restart
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```
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### Client Apps
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On Android I use "OpenVPN for Android" and on Linux I use the
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`network-manager-openvpn-gnome` Debian package.
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To add your VPN on Gnome, open VPN settings, import file, and select
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`client.ovpn`. If the private key is missing, select it from
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`~/.cert/nm-openvpn/`.
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## Closing Thoughts
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You should now be fine to access your home server from over the internet.
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To forward additional ports, just edit the `/etc/ufw/before.rules` file like
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above.
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Finally, make sure any server programs are listening / bound to `10.8.0.100` or
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`0.0.0.0` so that they can get traffic from that interface.
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