Bugfix: Fix incorrect cksums

Presence of IPv6 Extension Headers was causing incorrect TCP/UDP/ICMP
cksums. Verified that with these changes, cksums are correct for all
combinations of L3 and L4 headers with/without options/extHdrs

Fixes #271

common/abstractprotocol.cpp

@@ -930,18 +930,27 @@ quint32 AbstractProtocol::protocolFrameHeaderCksum(int streamIndex,
     CksumType cksumType, CksumScope cksumScope) const
 {
     quint32 sum = 0;
-    quint16 cksum;
+    quint32 cksum;
     AbstractProtocol *p = prev;
 
     Q_ASSERT(cksumType == CksumIpPseudo);
 
+    // We may have extension headers between us and the IP header - skip 'em
     while (p)
     {
         cksum = p->protocolFrameCksum(streamIndex, cksumType);
-        sum += (quint16) ~cksum;
-        qDebug("%s: sum = %u, cksum = %u", __FUNCTION__, sum, cksum);
-        if (cksumScope == CksumScopeAdjacentProtocol)
-            goto out;
+        if (cksum <= 0xFFFF) // protocol has a valid pseudo cksum ie its IP
+        {
+            sum += (quint16) ~cksum;
+            // Ip4/6Protocol::protocolFrameCksum(CksumIpPseudo) only
+            // counts the src/dst IP (see Note in there)
+            // Count the payload length and protocolId here
+            sum += protocolFrameSize() + protocolFramePayloadSize();
+            sum += protocolId(ProtocolIdIp);
+            qDebug("%s: sum = %x, cksum = %x", __FUNCTION__, sum, cksum);
+            if (cksumScope == CksumScopeAdjacentProtocol)
+                goto out;
+        }
         p = p->prev;
     }
     if (parent)

common/ip4.cpp

@@ -859,14 +859,18 @@ quint32 Ip4Protocol::protocolFrameCksum(int streamIndex,
             sum += *((quint16*)(p + 14)); // src-ip lo
             sum += *((quint16*)(p + 16)); // dst-ip hi
             sum += *((quint16*)(p + 18)); // dst-ip lo
-            sum += qToBigEndian((quint16)
-                    protocolFramePayloadSize(streamIndex)); // len
-            sum += qToBigEndian((quint16) *(p + 9)); // proto
+
+            // XXX: payload length and protocol are also part of the
+            // pseudo cksum but for IPv6 we need to skip extension headers to
+            // get to them, so these two fields are counted in the
+            // pseudo cksum in AbstractProtocol::protocolFrameHeaderCksum()
+            // Although not needed for IPv4 case, we do the same for IPv4
+            // also, so that code there is common for IPv4 and IPv6
 
             while(sum>>16)
                 sum = (sum & 0xFFFF) + (sum >> 16);
 
-            return ~qFromBigEndian((quint16)sum);
+            return qFromBigEndian((quint16) ~sum);
         }
         default:
             break;

common/ip6.cpp

@@ -781,13 +781,18 @@ quint32 Ip6Protocol::protocolFrameCksum(int streamIndex,
         // src-ip, dst-ip
         for (int i = 8; i < fv.size(); i+=2)
             sum += *((quint16*)(p + i));
-        sum += *((quint16*)(p + 4)); // payload len
-        sum += qToBigEndian((quint16) *(p + 6)); // proto
+
+        // XXX: payload length and protocol are also part of the
+        // pseudo cksum but we need to skip extension headers to
+        // get to them as per RFC 8200 Section 8.1
+        // Since we can't traverse beyond our immediate neighboring
+        // protocol from here, these two fields are counted in the
+        // pseudo cksum in AbstractProtocol::protocolFrameHeaderCksum()
 
         while(sum>>16)
             sum = (sum & 0xFFFF) + (sum >> 16);
 
-        return ~qFromBigEndian((quint16)sum);
+        return qFromBigEndian((quint16) ~sum);
     }
     return AbstractProtocol::protocolFrameCksum(streamIndex, cksumType);
 }