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TCPHDR.C
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1993-08-09
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/* TCP header conversion routines
* Copyright 1991 Phil Karn, KA9Q
*/
#include "global.h"
#include "mbuf.h"
#include "tcp.h"
#include "ip.h"
#include "internet.h"
/* Convert TCP header in host format into mbuf ready for transmission,
* link in data (if any). If ph != NULL, compute checksum, otherwise
* take checksum from tcph->checksum
*/
struct mbuf *
htontcp(struct tcp *tcph,struct mbuf *data,struct pseudo_header *ph)
{
int16 hdrlen = TCPLEN;
struct mbuf *bp;
char *cp;
if(tcph->optlen > 0 && tcph->optlen <= TCP_MAXOPT){
hdrlen += tcph->optlen;
} else if(tcph->mss != 0){
hdrlen += MSS_LENGTH;
}
bp = pushdown(data,hdrlen);
cp = bp->data;
cp = put16(cp,tcph->source);
cp = put16(cp,tcph->dest);
cp = put32(cp,tcph->seq);
cp = put32(cp,tcph->ack);
*cp++ = hdrlen << 2; /* Offset field */
*cp = 0;
if(tcph->flags.congest)
*cp |= 64;
if(tcph->flags.urg)
*cp |= 32;
if(tcph->flags.ack)
*cp |= 16;
if(tcph->flags.psh)
*cp |= 8;
if(tcph->flags.rst)
*cp |= 4;
if(tcph->flags.syn)
*cp |= 2;
if(tcph->flags.fin)
*cp |= 1;
cp++;
cp = put16(cp,tcph->wnd);
if(ph == NULLHEADER){
/* Use user-supplied checksum */
cp = put16(cp,tcph->checksum);
} else {
/* Zero out checksum field for later recalculation */
*cp++ = 0;
*cp++ = 0;
}
cp = put16(cp,tcph->up);
/* Write options, if any */
if(hdrlen > TCPLEN){
if(tcph->mss != 0){
*cp++ = MSS_KIND;
*cp++ = MSS_LENGTH;
cp = put16(cp,tcph->mss);
} else {
memcpy(cp,tcph->options,tcph->optlen);
}
}
/* Recompute checksum, if requested */
if(ph != NULLHEADER)
put16(&bp->data[16],cksum(ph,bp,ph->length));
return bp;
}
/* Pull TCP header off mbuf */
int
ntohtcp(struct tcp *tcph,struct mbuf **bpp)
{
char hdrbuf[TCPLEN], *cp;
int flags, hdrlen, optlen, kind, i = pullup(bpp,hdrbuf,TCPLEN);
/* Note that the results will be garbage if the header is too short.
* We don't check for this because returned ICMP messages will be
* truncated, and we at least want to get the port numbers.
*/
tcph->source = get16(&hdrbuf[0]);
tcph->dest = get16(&hdrbuf[2]);
tcph->seq = get32(&hdrbuf[4]);
tcph->ack = get32(&hdrbuf[8]);
hdrlen = (hdrbuf[12] & 0xf0) >> 2;
flags = hdrbuf[13];
tcph->flags.congest = flags & 64;
tcph->flags.urg = flags & 32;
tcph->flags.ack = flags & 16;
tcph->flags.psh = flags & 8;
tcph->flags.rst = flags & 4;
tcph->flags.syn = flags & 2;
tcph->flags.fin = flags & 1;
tcph->wnd = get16(&hdrbuf[14]);
tcph->checksum = get16(&hdrbuf[16]);
tcph->up = get16(&hdrbuf[18]);
tcph->mss = 0;
tcph->optlen = hdrlen - TCPLEN;
/* Check for option field. Only space for one is allowed, but
* since there's only one TCP option (MSS) this isn't a problem
*/
if(i < TCPLEN || hdrlen < TCPLEN)
return -1; /* Header smaller than legal minimum */
if(tcph->optlen == 0)
return (int)hdrlen; /* No options, all done */
if(tcph->optlen > len_p(*bpp))
/* Remainder too short for options length specified */
return -1;
pullup(bpp,tcph->options,tcph->optlen); /* "Can't fail" */
/* Process options */
for(cp = tcph->options, i = tcph->optlen; i > 0; ) {
kind = *cp++;
/* Process single-byte options */
switch(kind){
case EOL_KIND:
case NOOP_KIND:
i--;
cp++;
if(kind == NOOP_KIND)
/* Go look for next option */
continue;
return hdrlen; /* End of options list */
}
/* All other options have a length field */
optlen = uchar(*cp++);
/* Process valid multi-byte options */
switch(kind){
case MSS_KIND:
if(optlen == MSS_LENGTH){
tcph->mss = get16(cp);
}
break;
}
optlen = max(2,optlen); /* Enforce legal minimum */
i -= optlen;
cp += optlen - 2;
}
return hdrlen;
}