Kit: netcfg: add explicit IP mask routes

macOS will use the wrong source address unless we add explicit routes
that mention the self-pointing gateway. Actually, it won't add any
implicit routes on its own, so in order to route the masks of the
addresses, we have to add our own routes explicitly.

However, this still doesn't fix the problem while inside of the network
extension, even though it works outside it.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

2 files changed, 67 insertions, 18 deletions

diff --git a/Sources/WireGuardKit/IPAddressRange.swift b/Sources/WireGuardKit/IPAddressRange.swift
index 67c3955..ecb888e 100644
--- a/Sources/WireGuardKit/IPAddressRange.swift
+++ b/Sources/WireGuardKit/IPAddressRange.swift
@@ -64,4 +64,52 @@ extension IPAddressRange {
 
         return (address, networkPrefixLength)
     }
+
+    public func subnetMask() -> IPAddress {
+        if address is IPv4Address {
+            let mask = networkPrefixLength > 0 ? ~UInt32(0) << (32 - networkPrefixLength) : UInt32(0)
+            let bytes = Data([
+                UInt8(truncatingIfNeeded: mask >> 24),
+                UInt8(truncatingIfNeeded: mask >> 16),
+                UInt8(truncatingIfNeeded: mask >> 8),
+                UInt8(truncatingIfNeeded: mask >> 0)
+            ])
+            return IPv4Address(bytes)!
+        }
+        if address is IPv6Address {
+            var bytes = Data(repeating: 0, count: 16)
+            for i in 0..<Int(networkPrefixLength/8) {
+                bytes[i] = 0xff
+            }
+            let nibble = networkPrefixLength % 32
+            if nibble != 0 {
+                let mask = ~UInt32(0) << (32 - nibble)
+                let i = Int(networkPrefixLength / 32 * 4)
+                bytes[i + 0] = UInt8(truncatingIfNeeded: mask >> 24)
+                bytes[i + 1] = UInt8(truncatingIfNeeded: mask >> 16)
+                bytes[i + 2] = UInt8(truncatingIfNeeded: mask >> 8)
+                bytes[i + 3] = UInt8(truncatingIfNeeded: mask >> 0)
+            }
+            return IPv6Address(bytes)!
+        }
+        fatalError()
+    }
+
+    public func maskedAddress() -> IPAddress {
+        let subnet = subnetMask().rawValue
+        var masked = Data(address.rawValue)
+        if subnet.count != masked.count {
+            fatalError()
+        }
+        for i in 0..<subnet.count {
+            masked[i] &= subnet[i]
+        }
+        if subnet.count == 4 {
+            return IPv4Address(masked)!
+        }
+        if subnet.count == 16 {
+            return IPv6Address(masked)!
+        }
+        fatalError()
+    }
 }
diff --git a/Sources/WireGuardKit/PacketTunnelSettingsGenerator.swift b/Sources/WireGuardKit/PacketTunnelSettingsGenerator.swift
index 2d8cda1..7baa7cc 100644
--- a/Sources/WireGuardKit/PacketTunnelSettingsGenerator.swift
+++ b/Sources/WireGuardKit/PacketTunnelSettingsGenerator.swift
@@ -113,38 +113,26 @@ class PacketTunnelSettingsGenerator {
             networkSettings.mtu = NSNumber(value: mtu)
         }
 
-        let (ipv4Routes, ipv6Routes) = routes()
+        let (ipv4Addresses, ipv6Addresses) = addresses()
         let (ipv4IncludedRoutes, ipv6IncludedRoutes) = includedRoutes()
 
-        let ipv4Settings = NEIPv4Settings(addresses: ipv4Routes.map { $0.destinationAddress }, subnetMasks: ipv4Routes.map { $0.destinationSubnetMask })
+        let ipv4Settings = NEIPv4Settings(addresses: ipv4Addresses.map { $0.destinationAddress }, subnetMasks: ipv4Addresses.map { $0.destinationSubnetMask })
         ipv4Settings.includedRoutes = ipv4IncludedRoutes
         networkSettings.ipv4Settings = ipv4Settings
 
-        let ipv6Settings = NEIPv6Settings(addresses: ipv6Routes.map { $0.destinationAddress }, networkPrefixLengths: ipv6Routes.map { $0.destinationNetworkPrefixLength })
+        let ipv6Settings = NEIPv6Settings(addresses: ipv6Addresses.map { $0.destinationAddress }, networkPrefixLengths: ipv6Addresses.map { $0.destinationNetworkPrefixLength })
         ipv6Settings.includedRoutes = ipv6IncludedRoutes
         networkSettings.ipv6Settings = ipv6Settings
 
         return networkSettings
     }
 
-    private func ipv4SubnetMaskString(of addressRange: IPAddressRange) -> String {
-        let length: UInt8 = addressRange.networkPrefixLength
-        assert(length <= 32)
-        var octets: [UInt8] = [0, 0, 0, 0]
-        let subnetMask: UInt32 = length > 0 ? ~UInt32(0) << (32 - length) : UInt32(0)
-        octets[0] = UInt8(truncatingIfNeeded: subnetMask >> 24)
-        octets[1] = UInt8(truncatingIfNeeded: subnetMask >> 16)
-        octets[2] = UInt8(truncatingIfNeeded: subnetMask >> 8)
-        octets[3] = UInt8(truncatingIfNeeded: subnetMask)
-        return octets.map { String($0) }.joined(separator: ".")
-    }
-
-    private func routes() -> ([NEIPv4Route], [NEIPv6Route]) {
+    private func addresses() -> ([NEIPv4Route], [NEIPv6Route]) {
         var ipv4Routes = [NEIPv4Route]()
         var ipv6Routes = [NEIPv6Route]()
         for addressRange in tunnelConfiguration.interface.addresses {
             if addressRange.address is IPv4Address {
-                ipv4Routes.append(NEIPv4Route(destinationAddress: "\(addressRange.address)", subnetMask: ipv4SubnetMaskString(of: addressRange)))
+                ipv4Routes.append(NEIPv4Route(destinationAddress: "\(addressRange.address)", subnetMask: "\(addressRange.subnetMask())"))
             } else if addressRange.address is IPv6Address {
                 /* Big fat ugly hack for broken iOS networking stack: the smallest prefix that will have
                  * any effect on iOS is a /120, so we clamp everything above to /120. This is potentially
@@ -160,10 +148,23 @@ class PacketTunnelSettingsGenerator {
     private func includedRoutes() -> ([NEIPv4Route], [NEIPv6Route]) {
         var ipv4IncludedRoutes = [NEIPv4Route]()
         var ipv6IncludedRoutes = [NEIPv6Route]()
+
+        for addressRange in tunnelConfiguration.interface.addresses {
+            if addressRange.address is IPv4Address {
+                let route = NEIPv4Route(destinationAddress: "\(addressRange.maskedAddress())", subnetMask: "\(addressRange.subnetMask())")
+                route.gatewayAddress = "\(addressRange.address)"
+                ipv4IncludedRoutes.append(route)
+            } else if addressRange.address is IPv6Address {
+                let route = NEIPv6Route(destinationAddress: "\(addressRange.maskedAddress())", networkPrefixLength: NSNumber(value: addressRange.networkPrefixLength))
+                route.gatewayAddress = "\(addressRange.address)"
+                ipv6IncludedRoutes.append(route)
+            }
+        }
+
         for peer in tunnelConfiguration.peers {
             for addressRange in peer.allowedIPs {
                 if addressRange.address is IPv4Address {
-                    ipv4IncludedRoutes.append(NEIPv4Route(destinationAddress: "\(addressRange.address)", subnetMask: ipv4SubnetMaskString(of: addressRange)))
+                    ipv4IncludedRoutes.append(NEIPv4Route(destinationAddress: "\(addressRange.address)", subnetMask: "\(addressRange.subnetMask())"))
                 } else if addressRange.address is IPv6Address {
                     ipv6IncludedRoutes.append(NEIPv6Route(destinationAddress: "\(addressRange.address)", networkPrefixLength: NSNumber(value: addressRange.networkPrefixLength)))
                 }