dsvpn: A C repository from rh314

DSVPN is a Dead Simple VPN, designed to address the most common use case for using a VPN:

[client device] ---- (untrusted/restricted network) ---- [vpn server] ---- [the Internet]

Features:

Runs on TCP. Works pretty much everywhere, including on public WiFi where only TCP/443 is open or reliable.
Secure. Doesn't perform any heap memory allocations. Uses modern cryptography.
Small (~25 KB), with an equally small and readable code base. No external dependencies
Works out of the box. No lousy documentation to read. No configuration file. No post-configuration. Run a single-line command on the server, a similar one on the client and you're done. No firewall and routing rules to manually mess with.
Works with Linux (client, server) and MacOS/OpenBSD (client). Adding support for other operating systems is trivial.
Blocks IPv6 on the client to prevent IPv6 leaks.

Optimized ARM (NEON) implementation
Make everything non-blocking, reduce the number of system calls

Maybe:

The ability to run custom commands after the link is up
Support for multiple clients
pledge(), chroot(), non-root.

Non-features:

Anything else. Including supporting operating systems I don't use.

Never ever:

Any feature request mentioning systemd.

Installation

make

Secret key

DSVPN uses a shared secret. Create it with the following command:

dd if=/dev/urandom of=vpn.key count=1 bs=32

And copy it on the server and the client.

Example usage on the server

sudo ./dsvpn server vpn.key auto 1959

Here, I use port 1959. Everything else is set to the default values. If you want to use the default port (443), it doesn't even have to be specified, so the parameters can just be server vpn.key

Example usage on the client

sudo ./dsvpn client vpn.key 34.216.127.34 1959

This is a MacOS client, connecting to the VPN server 34.216.127.34 on port 1959. The port number is optional here as well.

That's it

You are connected. Just hit Ctrl-C to disconnect.

Evaggelos Balaskas wrote a great blog post walking through the whole procedure: A Dead Simple VPN - Thank you Evaggelos!

A note on DNS

If you were previously using a DNS resolver only accessible from the local network, it won't be accessible through the VPN. That might be the only thing you may have to change. Use a public resolver, a local resolver, or DNSCrypt.

Or send a pull request implementing the required commands to change and revert the DNS settings, or redirect DNS queries to another resolver, for all supported operating systems.

Advanced configuration

dsvpn   "server"
        <key file>
        <vpn server ip>|"auto"
        <vpn server port>|"auto"
        <tun interface>|"auto"
        <local tun ip>|"auto"
        <remote tun ip>"auto"
        <external ip>|"auto"

dsvpn   "client"
        <key file>
        <vpn server ip>
        <vpn server port>|"auto"
        <tun interface>|"auto"
        <local tun ip>|"auto"
        <remote tun ip>|"auto"
        <gateway ip>|"auto"

server|client: use server on the server, and client on clients.
<key file>: path to the file with the secret key (e.g. vpn.key).
<vpn server ip>: on the client, it should be the IP address or the hostname of the server. On the server, it doesn't matter, so you can just use auto.
<vpn server port>: the TCP port to listen to/connect to for the VPN. Use 443 or anything else. auto will use 443.
<tun interface>: this is the name of the VPN interface. On Linux, you can set it to anything. Or MacOS, it has to follow a more boring pattern. If you feel lazy, just use auto here.
<local tun ip>: local IP address of the tunnel. Use any private IP address that you don't use here.
<remote tun ip>: remote IP address of the tunnel. See above. The local and remote tunnel IPs must the same on the client and on the server, just reversed. For some reason, I tend to pick 192.168.192.254 for the server, and 192.168.192.1 for the client. These values will be used if you put auto for the local and remote tunnel IPs.
<external ip> (server only): the external IP address of the server. Can be left to "auto".
<gateway ip> (client only): the internal router IP address. The first line printed by netstat -rn will tell you (gateway).

If all the remaining parameters of a command would be auto, they don't have to be specified.

Why

I needed a VPN that works in an environment where only TCP/80 and TCP/443 are open.

WireGuard doesn't work over TCP.

GloryTun is excellent, but requires post-configuration and the maintained branch uses UDP.

OpenVPN is horribly difficult to set up.

Sshuttle is very nice and I've been using it a lot in the past, but it's not a VPN. It doesn't tunnel non-TCP traffic. It also requires a full Python install, which I'd rather avoid on my router.

Everything else I looked at was either too difficult to use, slow, bloated, didn't work on MacOS, didn't work on small devices, was complicated to cross-compile due to dependencies, wasn't maintained, or didn't feel secure.

TCP-over-TCP is not as bad as some documents describe. It works surprisingly well in practice, especially with modern congestion control algorithms (BBR). For traditional algorithms that rely on packet loss, DSVPN has the ability to couple the inner and outer congestion controllers, by setting the BUFFERBLOAT_CONTROL macro to 1 (this requires more CPU cycles, though).

Cryptography

The cryptographic primitives used in DSVPN are available as a standalone project: Charm.

Guarantees, support, feature additions

None.

This is a weekend project, and this is what I use, because it solves a problem I had. Extending it to solve different problems is not planned, but feel free to fork it and tailor it to your needs!

rh314/dsvpn