[client] Replace the eBPF WireGuard proxy with loopback endpoint addressing (#7316)

client/iface/wgproxy/loopback/addr.go

@@ -0,0 +1,70 @@
+//go:build linux && !android
+
+package loopback
+
+import (
+	"fmt"
+	"net/netip"
+)
+
+// Peer endpoints live in the upper half of 127.0.0.0/8. Everything in that
+// range is delivered to the loopback device without any address or route being
+// configured, and staying out of 127.0.0.0/9 keeps well-known squatters such as
+// 127.0.0.53 (systemd-resolved) and 127.0.1.1 out of the way.
+const (
+	addrRangeBase   uint32 = 0x7f800000 // 127.128.0.0
+	addrRangeSize   uint32 = 1 << 23    // /9
+	addrRangePrefix        = "127.128.0.0/9"
+)
+
+// allocator hands out one loopback address per relayed connection. The address
+// is the peer's identity: WireGuard sends to it, and the proxy recovers which
+// peer a packet belongs to from the destination address.
+type allocator struct {
+	cursor uint32
+}
+
+// next returns the first free address at or after the cursor, wrapping once.
+// inUse reports whether an address is already handed out.
+func (a *allocator) next(inUse func(netip.Addr) bool) (netip.Addr, error) {
+	for i := uint32(0); i < addrRangeSize; i++ {
+		a.cursor = (a.cursor + 1) % addrRangeSize
+		addr := addrFromOffset(a.cursor)
+		if !addr.IsValid() {
+			continue
+		}
+		if inUse(addr) {
+			continue
+		}
+		return addr, nil
+	}
+	return netip.Addr{}, fmt.Errorf("no free endpoint address in %s", addrRangePrefix)
+}
+
+// addrFromOffset maps an offset in the range to an address, skipping the .0 and
+// .255 hosts. They are unremarkable on loopback, but tools and firewall rules
+// tend to treat them as network and broadcast addresses.
+func addrFromOffset(offset uint32) netip.Addr {
+	last := offset & 0xff
+	if last == 0 || last == 0xff {
+		return netip.Addr{}
+	}
+
+	v := addrRangeBase + offset
+	return netip.AddrFrom4([4]byte{
+		byte(v >> 24),
+		byte(v >> 16),
+		byte(v >> 8),
+		byte(v),
+	})
+}
+
+// inRange reports whether addr is one this proxy could have handed out.
+func inRange(addr netip.Addr) bool {
+	if !addr.Is4() {
+		return false
+	}
+	b := addr.As4()
+	v := uint32(b[0])<<24 | uint32(b[1])<<16 | uint32(b[2])<<8 | uint32(b[3])
+	return v >= addrRangeBase && v < addrRangeBase+addrRangeSize && b[3] != 0 && b[3] != 0xff
+}