Diff: main.cpp

Revision:

61:b3c1a04efd0a

Parent:

58:8f39f28f9b14

Child:

62:f192926e42f1

--- a/main.cpp	Mon Apr 24 05:39:02 2017 +0000
+++ b/main.cpp	Sun Jun 04 20:14:36 2017 +0000
@@ -80,7 +80,7 @@
 
 pppType ppp; // our global - definitely not thread safe
 
-// intitialize our globals
+// Initialize our globals
 void pppInitStruct()
 {
     ppp.online=0;
@@ -516,6 +516,7 @@
 
     int tcpSize = packetLength - headerSizeIP;
     int headerSizeTCP = ((offset[0]>>4)&0x0f)*4; // size of tcp header only
+    int protocolIP = protocol[0];
 
     unsigned int seq = (seqtcp[0]<<24)|(seqtcp[1]<<16)|(seqtcp[2]<<8)|(seqtcp[3]);
     unsigned int ack = (acktcp[0]<<24)|(acktcp[1]<<16)|(acktcp[2]<<8)|(acktcp[3]);
@@ -539,7 +540,7 @@
     int flagsOut = TCP_FLAG_ACK; // the default case is an ACK packet
     int fastResponse = 0; // normally you wait 200ms before sending a packet but this can make it faster
 
-    ppp.seq = ack;    // adopt their sequence number calculation in place of doing our own calculation
+    ppp.seq = ack;    // always adopt their sequence number calculation in place of doing our own calculation
 
     if ( ((flagsTCP & ~TCP_FLAG_ACK) == 0) && ((flagsTCP & TCP_FLAG_ACK) != 0) ) {
         if (incomingLen == 0) { // ignore - just an empty ack packet
@@ -620,19 +621,16 @@
 
     // Now we have to recalculate all the header sizes, swap IP address/port source and destination, and do the IP and TCP checksums
 
-    char src[4]; // temp hold
-    char dst[4]; // temp hold
-    memcpy(src, srcAdr,4);
-    memcpy(dst, dstAdr,4);
-    memcpy(srcAdr, dst,4);
-    memcpy(dstAdr, src,4); // swap ip address source/dest
+    char tempHold[12];
+    memcpy(tempHold, srcAdr,4);
+    memcpy(tempHold+4, dstAdr,4);
+    memcpy(srcAdr, tempHold+4,4);
+    memcpy(dstAdr, tempHold,4); // swap ip address source/dest
 
-    char psrc[2]; // temp hold
-    char pdst[2]; // temp hold
-    memcpy(psrc, srctcp,2);
-    memcpy(pdst, dsttcp,2);
-    memcpy(srctcp, pdst,2);
-    memcpy(dsttcp, psrc,2); // swap ip port source/dest
+    memcpy(tempHold, srctcp,2);
+    memcpy(tempHold+2, dsttcp,2);
+    memcpy(srctcp, tempHold+2,2);
+    memcpy(dsttcp, tempHold,2); // swap ip port source/dest
 
     ack = seq + incomingLen; // acknowledge the number of bytes that they sent by adding it to "our" sequence number
     seq = ppp.seq; // set up the sequence number we have to respond with
@@ -657,26 +655,28 @@
 
     // the header is all set up, now do the IP and TCP checksums
 
-    headercheck[0]=0;
-    headercheck[1]=0;
-    headerCheckSum(); // redo the ip header checksum
-
-    char pseudoHeader[12]; // we are building a fake TCP header
-    int sum;
-    memcpy( pseudoHeader+0, srcAdr, 8); // source and destination addresses.
-    pseudoHeader[8]=0;
-    pseudoHeader[9]=protocol[0];
-    pseudoHeader[10]=tcpSize>>8;
-    pseudoHeader[11]=tcpSize;
-    char temp[12]; // temporary storage for the 12 bytes that are in the way
-    memcpy(temp, s-12, 12); // save the 12 bytes that are in the way
-    memcpy( s-12, pseudoHeader, 12); // copy in the fake header
+    headercheck[0]=0; // IP header checksum
+    headercheck[1]=0; // IP header checksum
+    headerCheckSum(); // calculate the IP header checksum
+    
+    // now we have to build a 12-byte TCP "pseudo-header" in front of the TCP header (containing some IP header values) in order to correctly calculate the TCP checksum
+    
+    char * pseudoHeader = s-12;
+    memcpy(tempHold, pseudoHeader, 12); // save the 12 bytes in the IP header where the TCP pseudo-Header must be built
+    memcpy( pseudoHeader+0, srcAdr, 8); // IP source and destination addresses from IP header
+    memset( pseudoHeader+8, 0, 1); // reserved, set to zero
+    memset( pseudoHeader+9, protocolIP, 1); // protocol from IP header
+    memset( pseudoHeader+10, tcpSize>>8, 1); // size of TCP header
+    memset( pseudoHeader+11, tcpSize, 1); // size of TCP header
+    
+    // pseudo-header built, now we can calculate TCP checksum
+    
     checksumtcp[0]=0;
     checksumtcp[1]=0;
-    sum=dataCheckSum(s-12,tcpSize+12); // calculate the TCP checksum with the fake header
-    checksumtcp[0]=sum>>8;
-    checksumtcp[1]=sum;
-    memcpy( s-12, temp, 12); // restore the 12 bytes that the fake header overwrote
+    int pseudoHeaderSum=dataCheckSum(s-12,tcpSize+12); // calculate the TCP checksum starting at the pseudo-header
+    checksumtcp[0]=pseudoHeaderSum>>8;
+    checksumtcp[1]=pseudoHeaderSum;
+    memcpy( s-12, tempHold, 12); // restore the 12 bytes that the pseudo-header overwrote
     if (fastResponse==1) {
         fastResponse=0; // reset and skip 200 ms wait
     } else {