netd: PPTP Helper

Enable the conntrack helper for PPTP connections when tethering
is started.

Bug: 112066349
Test: as follows
      - Boot device - Pass
      - Start tethering - Pass
      - Start PPTP on tethered host (USB) - Pass
      - make -j64 netd_unit_test
            adb shell /data/nativetest64/netd_unit_test/netd_unit_test
            - Pass

Change-Id: I547f8b877661c137646b663429d29737678652bf
diff --git a/server/TetherController.cpp b/server/TetherController.cpp
index 0ca1b77..c086465 100644
--- a/server/TetherController.cpp
+++ b/server/TetherController.cpp
@@ -646,17 +646,19 @@
     }
 
     std::vector<std::string> v4 = {
-        "*raw",
-        StringPrintf("%s %s -p tcp --dport 21 -i %s -j CT --helper ftp",
-                     op, LOCAL_RAW_PREROUTING, intIface),
-        "COMMIT",
-        "*filter",
-        StringPrintf("%s %s -i %s -o %s -m state --state ESTABLISHED,RELATED -g %s",
-                     op, LOCAL_FORWARD, extIface, intIface, LOCAL_TETHER_COUNTERS_CHAIN),
-        StringPrintf("%s %s -i %s -o %s -m state --state INVALID -j DROP",
-                     op, LOCAL_FORWARD, intIface, extIface),
-        StringPrintf("%s %s -i %s -o %s -g %s",
-                     op, LOCAL_FORWARD, intIface, extIface, LOCAL_TETHER_COUNTERS_CHAIN),
+            "*raw",
+            StringPrintf("%s %s -p tcp --dport 21 -i %s -j CT --helper ftp", op,
+                         LOCAL_RAW_PREROUTING, intIface),
+            StringPrintf("%s %s -p tcp --dport 1723 -i %s -j CT --helper pptp", op,
+                         LOCAL_RAW_PREROUTING, intIface),
+            "COMMIT",
+            "*filter",
+            StringPrintf("%s %s -i %s -o %s -m state --state ESTABLISHED,RELATED -g %s", op,
+                         LOCAL_FORWARD, extIface, intIface, LOCAL_TETHER_COUNTERS_CHAIN),
+            StringPrintf("%s %s -i %s -o %s -m state --state INVALID -j DROP", op, LOCAL_FORWARD,
+                         intIface, extIface),
+            StringPrintf("%s %s -i %s -o %s -g %s", op, LOCAL_FORWARD, intIface, extIface,
+                         LOCAL_TETHER_COUNTERS_CHAIN),
     };
 
     std::vector<std::string> v6 = {