eBPF + IPv6 + WireGuard

[Mike O'Connor]

Hi

Wireguard is a layer 3 system. As a result you must route traffic not bridge, this also means that ip forwarding must be enabled.

You will need to firewall the traffic or setup a separate routing table. 

Other routers in your network will need to know about the IP addresses allocated to Wireguard and have a route for this range.

Mike

> On 21 Dec 2021, at 3:21 am, Alex <wireguard at centromere.net> wrote:
> 
> Hi all,
> 
> I am championing WireGuard at work, and I have been granted permission
> to use it for establishing remote access to a private IPv6 VLAN for all
> employees. I have experimented with different approaches and ran in to
> an issue.
> 
> With the help of a machine dedicated fully to the job of remote access
> (Ubuntu 20.04 / Linux 5.4), I've managed to establish end-to-end
> connectivity between my work laptop and the servers. The VPN gateway
> has a wg0 interface for employees and a "private0" interface for
> private VLAN connectivity. The goal is to link the two.
> 
> I noticed an odd quirk: It only works when
> "net.ipv6.conf.all.forwarding" is 1. If this value is 0,
> "net.ipv6.conf.[wg0 | private0].forwarding" has no effect. In other
> words, I cannot seem to enable forwarding for *only* the interfaces
> that need it. It only works when forwarding is enabled for *all*
> interfaces. This is a problem because when the "all" value is set to 1,
> the machine will start to behave as a router on VLANs for which it is
> most definitely not a router.
> 
> For the servers, I am using the officially defined[0] subnet-router
> anycast address as the default IPv6 gateway, and it works well.
> However, when I flipped the switch on "net.ipv6.conf.all.forwarding",
> the VPN gateway started using NDP to announce that it was a router
> across *all* VLANs! Specifically, it was claiming ownership over our
> global subnet anycast address 2001:aaaa:bbbb:cccc::. This is neither
> true nor desired. The VPN gateway started to receive outbound non-VPN
> Internet traffic which broke Internet connectivity for all servers. 
> 
> This leads me to my first question: Why does
> "net.ipv6.conf.wg0.forwarding" have no effect?
> 
> In researching a solution, I decided to give eBPF a try. I attached an
> XDP program to "private0" (a layer 2 device, naturally) and
> successfully redirected packets to "wg0" (a layer 3 device) with
> bpf_redirect[1]. If the packets are unmodified, WireGuard will happily
> pass them to the remote hosts. This is an issue because the remote
> hosts are expecting to receive IP(v6) packets, not Ethernet frames. If
> I use bpf_xdp_adjust_head[1] to strip off the Ethernet frame, WireGuard
> will drop the packet[2] before it can be sent to the remote host.
> 
> My theory is that bpf_xdp_adjust_head is modifying the data pointer
> only and not any underlying structures that may be associated with the
> packet (sk_buff perhaps).
> 
> This leads me to my second question: Why can't I redirect traffic
> received on an L2 interface to an L3 interface, *even after stripping
> off the Ethernet frame?*
> 
> Finally, during this whole process I was using WireShark to inspect
> traffic received by the remote host (i.e. my work laptop). With the
> help of the extract-handshakes.sh script, I was able to decrypt traffic.
> I did discover a bug though. I believe "index_hashtable_insert" should
> actually be "wg_index_hashtable_insert"[3].
> 
> Does anyone have any insights as to what a proper solution would look
> like? Is there a way to achieve my goal without introducing eBPF? Is
> XDP completely unsuited for this particular purpose? Do I actually need
> to operate on the sk_buff level, as opposed to the xdp_buff level?
> 
> Thank you all for your time.
> 
> - Alex
> 
> [0] https://datatracker.ietf.org/doc/rfc1884/ (Section 2.5.1)
> [1] https://www.man7.org/linux/man-pages/man7/bpf-helpers.7.html
> [2] https://git.zx2c4.com/wireguard-linux/tree/drivers/net/wireguard/device.c#n131
> [3] https://git.zx2c4.com/wireguard-tools/tree/contrib/extract-handshakes/extract-handshakes.sh#n46