If this looks like an interesting project to you, get in touch with the WireGuard development team.
WireGuard currently uses static addresses everywhere. This is because that is mostly a better way to design your network. But in some cases, insane people want dynamic IP addresses or other dynamic configuration.
This paper explores a couple approaches to WireGuard in-band configuration. Ultimately, however, custom in-band communication brings with it many difficulties, such as introducing new non-standard APIs and having to reinvent the wheel with reliable transport protocols.
Traditionally, configuration of IP addresses has been done at a layer in between 2 and 3 using DHCP. DHCP, however, has numerous drawbacks that make it unsuitable for use with WireGuard:
- Relies on broadcast.
- Unicast DHCP is not well supported nor mature.
- Two different protocols for v6 and v4, and v4-in-v6 dhcp is a bit insane.
- Cumbersome and old.
Fortunately IPv6 link-local addresses give us exactly the semantics we need for an in-band configuration protocol, without needing to introduce new layer 3 types. Since a server must know each of its clients public keys beforehand, it is not ridiculous to, at the same time, assign a unique link-local IP address to that client. In this setup, the server knows a priori the client’s public key and the client’s link local IPv6 address. The client needs only to know the server’s public key.
When wg-dynamic is run on a certain WireGuard interface, say wg0, it
determines whether that interface has a link-local IP address that is
actually a /128 and also has a peer whose allowedips include
fe80::/128. If not, it exits. If so, it initiates the protocol with
fe80::%wg0 -- notice the use of the link-local scope identifier.
In the protocol, the server -- communicated with via
assigns the client dynamically determined global IPv4 and IPv6
addresses and masks from an address pool, with a lease, and usual
dhcp-like renewal semantics commence. All leased IP addresses are
added to that peer’s set of allowed IPs, so that the client can
actually use them. The protocol could also push routes to be added to
allowedips and the routing table. The protocol would likely use TCP
rather than UDP, since we have the luxury of unicast and being inside
of a secure tunnel.
In the future, the protocol could also push information about other peers, transitioning wg-dynamic from a basic boring dhcp-substitute into a full fledged WireGuard mesh networking utility.
Thus, the above consists of implementing a lightweight client daemon and server daemon in C with no dependencies, to become part of the standard set of WireGuard tools. The UI of the client daemon is simplest in its usage:
$ wg-dynamic-client wg0
After this, it validates wg0 has the necessary configuration for it to function, and then it forks into the background, listens to up/down events, and lives until wg0 is removed, at which time it exits.
This will then hook nicely into wg-quick(8) configurations. The following configuration will initiate a wg-dynamic IP address assignment on the client side:
[Interface] PrivateKey = … Address = fe80::abcd:abcd:abcd:abcd/128, auto [Peer] PublicKey = … AllowedIPs = 0.0.0.0/0, ::/0
The “auto” in the list of addresses means “spawn wg-dynamic.” And
fe80::/128 is a subset of the included
this would be equally as valid:
[Interface] PrivateKey = … Address = fe80::abcd:abcd:abcd:abcd/128, auto [Peer] PublicKey = … AllowedIPs = fe80::/128
Though part of the standard set of WireGuard utilities, which is GPLv2, the code from wg-dynamic would likely be recycled in various client apps; thus MIT license is preferred for this.
The app will require various APIs and competencies:
- Standard linux socket programming.
- Rtnetlink for configuring device, listening to events, and so forth, preferably using libmnl.
- Genetlink for talking to wireguard, preferably using the mini wireguard library.
- Design of a new protocol wire format that is sane and appropriate for 2019.
- Implementation of code that parses untrusted packets, makes decisions with this data, and does not get your box owned in the process. This is harder than it may seem.