[client] Assign the Android TUN address as a host prefix (#7414)

Android 16+ local network protection derives the blocked prefixes from the interface address prefix. A /16 address turns the whole overlay into a local network, so apps without ACCESS_LOCAL_NETWORK cannot reach any peer. Pass the address as /32 and /128 and add the overlay networks to the route list that the Android side turns into VPN routes, on both the initial create and the renew path.

client/iface/device/device_android.go

@@ -63,7 +63,12 @@ func (t *WGTunDevice) Create(routes []string, dns string, searchDomains []string
 		searchDomainsToString = ""
 	}
 
-	fd, err := t.tunAdapter.ConfigureInterface(t.address.String(), t.address.IPv6String(), int(t.mtu), dns, searchDomainsToString, routesString)
+	ipv6Host := ""
+	if t.address.HasIPv6() {
+		ipv6Host = t.address.IPv6HostPrefix().String()
+	}
+
+	fd, err := t.tunAdapter.ConfigureInterface(t.address.HostPrefix().String(), ipv6Host, int(t.mtu), dns, searchDomainsToString, routesString)
 	if err != nil {
 		log.Errorf("failed to create Android interface: %s", err)
 		return nil, err

client/iface/wgaddr/address.go

@@ -59,6 +59,19 @@ func (addr Address) IPv6Prefix() netip.Prefix {
 	return netip.PrefixFrom(addr.IPv6, addr.IPv6Net.Bits())
 }
 
+// HostPrefix returns the v4 address as a single-host prefix.
+func (addr Address) HostPrefix() netip.Prefix {
+	return netip.PrefixFrom(addr.IP, addr.IP.BitLen())
+}
+
+// IPv6HostPrefix returns the v6 address as a single-host prefix, or an invalid prefix when no v6 overlay address is assigned.
+func (addr Address) IPv6HostPrefix() netip.Prefix {
+	if !addr.HasIPv6() {
+		return netip.Prefix{}
+	}
+	return netip.PrefixFrom(addr.IPv6, addr.IPv6.BitLen())
+}
+
 // SetIPv6FromCompact decodes a compact prefix (5 or 17 bytes) and sets the IPv6 fields.
 // Returns an error if the bytes are invalid. A nil or empty input is a no-op.
 //

client/iface/wgaddr/address_test.go

@@ -0,0 +1,24 @@
+package wgaddr
+
+import (
+	"net/netip"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestAddress_HostPrefix(t *testing.T) {
+	addr := MustParseWGAddress("100.91.96.107/16")
+
+	assert.Equal(t, netip.MustParsePrefix("100.91.96.107/32"), addr.HostPrefix(), "v4 host prefix must be a single host")
+	assert.Equal(t, netip.MustParsePrefix("100.91.0.0/16"), addr.Network, "network must keep the overlay prefix length")
+	assert.False(t, addr.IPv6HostPrefix().IsValid(), "no v6 overlay means no v6 host prefix")
+}
+
+func TestAddress_IPv6HostPrefix(t *testing.T) {
+	addr := MustParseWGAddress("100.91.96.107/16")
+	addr.IPv6 = netip.MustParseAddr("fd00:1234::1")
+	addr.IPv6Net = netip.MustParsePrefix("fd00:1234::/64")
+
+	assert.Equal(t, netip.MustParsePrefix("fd00:1234::1/128"), addr.IPv6HostPrefix(), "v6 host prefix must be a single host")
+}