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_local.h"
30 
31 u_short
32 InternetChecksum(u_short *ptr, int nbytes)
33 {
34     int sum, oddbyte;
35 
36     sum = 0;
37     while (nbytes > 1)
38     {
39         sum += *ptr++;
40         nbytes -= 2;
41     }
42     if (nbytes == 1)
43     {
44         oddbyte = 0;
45         *((u_char *) &oddbyte) = *(u_char *) ptr;
46         sum += oddbyte;
47     }
48     sum = (sum >> 16) + (sum & 0xffff);
49     sum += (sum >> 16);
50     return(~sum);
51 }
52 
53 u_short
54 IpChecksum(struct ip *pip)
55 {
56     return( InternetChecksum((u_short *) pip, (pip->ip_hl << 2)) );
57 
58 }
59 
60 u_short
61 TcpChecksum(struct ip *pip)
62 {
63     u_short *ptr;
64     struct tcphdr *tc;
65     int nhdr, ntcp, nbytes;
66     int sum, oddbyte;
67 
68     nhdr = pip->ip_hl << 2;
69     ntcp = ntohs(pip->ip_len) - nhdr;
70 
71     tc = (struct tcphdr *) ((char *) pip + nhdr);
72     ptr = (u_short *) tc;
73 
74 /* Add up TCP header and data */
75     nbytes = ntcp;
76     sum = 0;
77     while (nbytes > 1)
78     {
79         sum += *ptr++;
80         nbytes -= 2;
81     }
82     if (nbytes == 1)
83     {
84         oddbyte = 0;
85         *((u_char *) &oddbyte) = *(u_char *) ptr;
86         sum += oddbyte;
87     }
88 
89 /* "Pseudo-header" data */
90     ptr = (u_short *) &(pip->ip_dst);
91     sum += *ptr++;
92     sum += *ptr;
93     ptr = (u_short *) &(pip->ip_src);
94     sum += *ptr++;
95     sum += *ptr;
96     sum += htons((u_short) ntcp);
97     sum += htons((u_short) pip->ip_p);
98 
99 /* Roll over carry bits */
100     sum = (sum >> 16) + (sum & 0xffff);
101     sum += (sum >> 16);
102 
103 /* Return checksum */
104     return((u_short) ~sum);
105 }
106 
107 
108 void
109 DifferentialChecksum(u_short *cksum, u_short *new, u_short *old, int n)
110 {
111     int i;
112     int accumulate;
113 
114     accumulate = *cksum;
115     for (i=0; i<n; i++)
116     {
117         accumulate -= *new++;
118         accumulate += *old++;
119     }
120 
121     if (accumulate < 0)
122     {
123         accumulate = -accumulate;
124         accumulate = (accumulate >> 16) + (accumulate & 0xffff);
125         accumulate += accumulate >> 16;
126         *cksum = (u_short) ~accumulate;
127     }
128     else
129     {
130         accumulate = (accumulate >> 16) + (accumulate & 0xffff);
131         accumulate += accumulate >> 16;
132         *cksum = (u_short) accumulate;
133     }
134 }
135 
136