personal-site/content/bypassing-ports.md

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

[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 own username, this is temporary.

<span class="aside">The `.rnd` file prevents a warning</span>

```
$ 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, 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`:

<span class="aside">Can't use port 443 here since it'll be forwarded</span>

```
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:

<span class="aside">Your home server will be `10.8.0.100`</span>

```
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: <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
```

### 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
<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`:

<span class="aside">`redirect-gateway def1` forces traffic over the VPN</span>

```
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.

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.