fix(relay): XDP_PASS non-STUN UDP traffic (#10292)

To prevent userspace relaying, all traffic that seemingly looked like
STUN/TURN but we couldn't handle via the eBPF codepath we would
`XDP_DROP`.

This turned out to be too heavy-handed of an approach since it end up
matching DNS query responses as well due to them arriving within the
TURN ephemeral port range.

To fix this, we `XDP_PASS` the traffic up the stack so that the kernel
is able to match it to existing conntrack entries.

We've identified a minor race condition where the first few channel data
packets might be dropped when a channel is first being bound, but fixing
this will be saved for a later PR.

Related: https://github.com/firezone/infra/pull/132

rust/relay/ebpf-turn-router/src/main.rs

@@ -26,6 +26,7 @@ pub fn handle_turn(ctx: aya_ebpf::programs::XdpContext) -> u32 {
         | Error::InterfaceIpv6AddressAccessFailed
         | Error::PacketTooShort
         | Error::NotTurn
+        | Error::NoEntry(_)
         | Error::NotAChannelDataMessage
         | Error::UdpChecksumMissing
         | Error::Ipv4PacketWithOptions => {
@@ -34,16 +35,18 @@ pub fn handle_turn(ctx: aya_ebpf::programs::XdpContext) -> u32 {
             xdp_action::XDP_PASS
         }
 
-        Error::InterfaceIpv4AddressNotConfigured
-        | Error::PacketLoop
-        | Error::NoEntry(_)
-        | Error::InterfaceIpv6AddressNotConfigured => {
+        // TODO: Remove this when same-host relay-relay is supported.
+        Error::PacketLoop => {
             debug!(&ctx, "Dropping packet: {}", e);
 
             xdp_action::XDP_DROP
         }
 
-        Error::BadChannelDataLength | Error::XdpAdjustHeadFailed(_) => {
+        // These are exceptions and shouldn't happen in practice - WARN.
+        Error::BadChannelDataLength
+        | Error::InterfaceIpv4AddressNotConfigured
+        | Error::InterfaceIpv6AddressNotConfigured
+        | Error::XdpAdjustHeadFailed(_) => {
             warn!(&ctx, "Dropping packet: {}", e);
 
             xdp_action::XDP_DROP