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140 lines
3.0 KiB
C
140 lines
3.0 KiB
C
/*
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Alias_util.h contains general utilities used by other functions
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in the packet aliasing module. At the moment, there are functions
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for computing IP header and TCP packet checksums.
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The checksum routines are based upon example code in a Unix networking
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text written by Stevens (sorry, I can't remember the title -- but
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at least this is a good author).
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Initial Version: August, 1996 (cjm)
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Version 1.7: January 9, 1997
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Added differential checksum update function.
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*/
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/*
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Note: the checksum routines assume that the actual checksum word has
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been zeroed out. If the checksum workd is filled with the proper value,
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then these routines will give a result of zero (useful for testing
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purposes);
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*/
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#include <sys/types.h>
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#include <netinet/in_systm.h>
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#include <netinet/in.h>
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#include <netinet/ip.h>
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#include <netinet/tcp.h>
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#include "alias.h"
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#include "alias_local.h"
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u_short
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PacketAliasInternetChecksum(u_short *ptr, int nbytes)
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{
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int sum, oddbyte;
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sum = 0;
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while (nbytes > 1)
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{
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sum += *ptr++;
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nbytes -= 2;
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}
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if (nbytes == 1)
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{
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oddbyte = 0;
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((u_char *) &oddbyte)[0] = *(u_char *) ptr;
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((u_char *) &oddbyte)[1] = 0;
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sum += oddbyte;
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}
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sum = (sum >> 16) + (sum & 0xffff);
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sum += (sum >> 16);
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return(~sum);
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}
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u_short
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IpChecksum(struct ip *pip)
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{
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return( PacketAliasInternetChecksum((u_short *) pip,
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(pip->ip_hl << 2)) );
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}
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u_short
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TcpChecksum(struct ip *pip)
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{
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u_short *ptr;
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struct tcphdr *tc;
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int nhdr, ntcp, nbytes;
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int sum, oddbyte;
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nhdr = pip->ip_hl << 2;
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ntcp = ntohs(pip->ip_len) - nhdr;
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tc = (struct tcphdr *) ((char *) pip + nhdr);
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ptr = (u_short *) tc;
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/* Add up TCP header and data */
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nbytes = ntcp;
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sum = 0;
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while (nbytes > 1)
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{
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sum += *ptr++;
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nbytes -= 2;
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}
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if (nbytes == 1)
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{
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oddbyte = 0;
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((u_char *) &oddbyte)[0] = *(u_char *) ptr;
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((u_char *) &oddbyte)[1] = 0;
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sum += oddbyte;
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}
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/* "Pseudo-header" data */
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ptr = (u_short *) &(pip->ip_dst);
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sum += *ptr++;
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sum += *ptr;
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ptr = (u_short *) &(pip->ip_src);
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sum += *ptr++;
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sum += *ptr;
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sum += htons((u_short) ntcp);
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sum += htons((u_short) pip->ip_p);
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/* Roll over carry bits */
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sum = (sum >> 16) + (sum & 0xffff);
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sum += (sum >> 16);
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/* Return checksum */
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return((u_short) ~sum);
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}
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void
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DifferentialChecksum(u_short *cksum, u_short *new, u_short *old, int n)
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{
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int i;
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int accumulate;
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accumulate = *cksum;
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for (i=0; i<n; i++)
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{
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accumulate -= *new++;
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accumulate += *old++;
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}
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if (accumulate < 0)
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{
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accumulate = -accumulate;
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accumulate = (accumulate >> 16) + (accumulate & 0xffff);
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accumulate += accumulate >> 16;
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*cksum = (u_short) ~accumulate;
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}
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else
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{
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accumulate = (accumulate >> 16) + (accumulate & 0xffff);
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accumulate += accumulate >> 16;
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*cksum = (u_short) accumulate;
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}
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}
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