alias/alias_util.c

   1 /*
   2     Alias_util.h contains general utilities used by other functions
   3     in the packet aliasing module.  At the moment, there are functions
   4     for computing IP header and TCP packet checksums.
   5
   6     The checksum routines are based upon example code in a Unix networking
   7     text written by Stevens (sorry, I can't remember the title -- but
   8     at least this is a good author).
   9
  10     Initial Version:  August, 1996  (cjm)
  11
  12     Version 1.7:  January 9, 1997
  13          Added differential checksum update function.
  14 */
  15
  16 /*
  17 Note: the checksum routines assume that the actual checksum word has
  18 been zeroed out.  If the checksum workd is filled with the proper value,
  19 then these routines will give a result of zero (useful for testing
  20 purposes);
  21 */
  22
  23 #include <sys/types.h>
  24 #include <netinet/in_systm.h>
  25 #include <netinet/in.h>
  26 #include <netinet/ip.h>
  27 #include <netinet/tcp.h>
  28
  29 #include "alias.h"
  30 #include "alias_local.h"
  31
  32 u_short
  33 PacketAliasInternetChecksum(u_short *ptr, int nbytes)
  34 {
  35     int sum, oddbyte;
  36
  37     sum = 0;
  38     while (nbytes > 1)
  39     {
  40         sum += *ptr++;
  41         nbytes -= 2;
  42     }
  43     if (nbytes == 1)
  44     {
  45         oddbyte = 0;
  46         *((u_char *) &oddbyte) = *(u_char *) ptr;
  47         sum += oddbyte;
  48     }
  49     sum = (sum >> 16) + (sum & 0xffff);
  50     sum += (sum >> 16);
  51     return(~sum);
  52 }
  53
  54 u_short
  55 IpChecksum(struct ip *pip)
  56 {
  57     return( PacketAliasInternetChecksum((u_short *) pip,
  58             (pip->ip_hl << 2)) );
  59
  60 }
  61
  62 u_short
  63 TcpChecksum(struct ip *pip)
  64 {
  65     u_short *ptr;
  66     struct tcphdr *tc;
  67     int nhdr, ntcp, nbytes;
  68     int sum, oddbyte;
  69
  70     nhdr = pip->ip_hl << 2;
  71     ntcp = ntohs(pip->ip_len) - nhdr;
  72
  73     tc = (struct tcphdr *) ((char *) pip + nhdr);
  74     ptr = (u_short *) tc;
  75
  76 /* Add up TCP header and data */
  77     nbytes = ntcp;
  78     sum = 0;
  79     while (nbytes > 1)
  80     {
  81         sum += *ptr++;
  82         nbytes -= 2;
  83     }
  84     if (nbytes == 1)
  85     {
  86         oddbyte = 0;
  87         *((u_char *) &oddbyte) = *(u_char *) ptr;
  88         sum += oddbyte;
  89     }
  90
  91 /* "Pseudo-header" data */
  92     ptr = (u_short *) &(pip->ip_dst);
  93     sum += *ptr++;
  94     sum += *ptr;
  95     ptr = (u_short *) &(pip->ip_src);
  96     sum += *ptr++;
  97     sum += *ptr;
  98     sum += htons((u_short) ntcp);
  99     sum += htons((u_short) pip->ip_p);
 100
 101 /* Roll over carry bits */
 102     sum = (sum >> 16) + (sum & 0xffff);
 103     sum += (sum >> 16);
 104
 105 /* Return checksum */
 106     return((u_short) ~sum);
 107 }
 108
 109
 110 void
 111 DifferentialChecksum(u_short *cksum, u_short *new, u_short *old, int n)
 112 {
 113     int i;
 114     int accumulate;
 115
 116     accumulate = *cksum;
 117     for (i=0; i<n; i++)
 118     {
 119         accumulate -= *new++;
 120         accumulate += *old++;
 121     }
 122
 123     if (accumulate < 0)
 124     {
 125         accumulate = -accumulate;
 126         accumulate = (accumulate >> 16) + (accumulate & 0xffff);
 127         accumulate += accumulate >> 16;
 128         *cksum = (u_short) ~accumulate;
 129     }
 130     else
 131     {
 132         accumulate = (accumulate >> 16) + (accumulate & 0xffff);
 133         accumulate += accumulate >> 16;
 134         *cksum = (u_short) accumulate;
 135     }
 136 }
 137