dns.c源代码：
Code:
/* dns.c * * char *getmxbyname(char *domain) - gets the DNS MX records for host/domain char *domain * it returns a colon delimited list of valid MXs. */ #include
#include
#include
#include
#include
#include
#include /* not always automatically included */
#include
#include
#include
#include
#undef NOERROR /* in on solaris 2.x */
#include
#include
/*
* Copyright (c) 1983 Eric P. Allman
* Copyright (c) 1988 Regents of the University of California.
* All rights reserved.
*
* Redistribution and use in source and binary forms are permitted provided
* that: (1) source distributions retain this entire copyright notice and
* comment, and (2) distributions including binaries display the following
* acknowledgement: ``This product includes software developed by the
* University of California, Berkeley and its contributors'' in the
* documentation or other materials provided with the distribution and in
* all advertising materials mentioning features or use of this software.
* Neither the name of the University nor the names of its contributors may
* be used to endorse or promote products derived from this software without
* specific prior written permission.
* THIS SOFTWARE IS PROVIDED ``AS IS'' AND WITHOUT ANY EXPRESS OR IMPLIED
* WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF
* MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.
*/
/*silly hackery */
#if defined(BIND_493)
typedef u_char qbuf_t;
#else
typedef char qbuf_t;
#endif
#if defined(BIND_493)
typedef char nbuf_t;
#else
typedef u_char nbuf_t;
#endif
/**/
/*
** GETMXBYNAME -- Fetch mx hosts for a domain
** ------------------------------------------
**
** Returns:
** Number of mx hosts found.
**
** Outputs:
** The contains the mx names.
*/
char *getmxbyname(char *domain) /*find mxs for this domain*/
{
//int verbose = 0;
//int debug = 0;
/* #define HFIXEDSZ sizeof(HEADER) actually 12 */
#define MAXPACKET 8192 /* max size of packet */
//#define MAXMXHOSTS 20 /* max num of mx records we want to see */
enum { MAXMXHOSTS = 20 };
//char _arr[MAXBUF][MAXBUF];
enum { MAXMXBUFSIZ = (MAXMXHOSTS * (MAXBUF+1)) };
typedef union {
HEADER hdr;
u_char buf[MAXPACKET];
} querybuf;
static char hostbuf[MAXMXBUFSIZ];
char *MxHosts[MAXMXHOSTS];
querybuf answer; /* answer buffer from nameserver */
HEADER *hp; /* answer buffer header */
int ancount, qdcount; /* answer count and query count */
u_char *msg, *eom, *cp; /* answer buffer positions */
int type, class, dlen; /* record type, class and length */
u_short pref; /* mx preference value */
u_short prefer[MAXMXHOSTS]; /* saved preferences of mx records */
char *bp; /* hostbuf pointer */
int nmx; /* number of mx hosts found */
register int i;
register int j;
register int n;
char *str; /* final answer string buffer. */
str = xmalloc(MAXBUF);
/*
* Query the nameserver to retrieve mx records for the given domain.
*/
errno = 0; /* reset before querying nameserver */
h_errno = 0;
n = res_search(domain, C_IN, T_MX, (u_char *)&answer, sizeof(answer));
if (n < 0)
{
if (_res.options & RES_DEBUG)
debug("sres_search failed\n");
return(0);
}
errno = 0; /* reset after we got an answer */
if (n < HFIXEDSZ)
{
h_errno = NO_RECOVERY;
return(0);
}
/* avoid problems after truncation in tcp packets */
if (n > sizeof(answer))
n = sizeof(answer);
/*
* Valid answer received. Skip the query record.
*/
hp = (HEADER *)&answer;
qdcount = ntohs((u_short)hp->qdcount);
ancount = ntohs((u_short)hp->ancount);
msg = (u_char *)&answer;
eom = (u_char *)&answer + n;
cp = (u_char *)&answer + HFIXEDSZ;
while (qdcount-- > 0 && cp < eom)
{
n = dn_skipname(cp, eom);
if (n < 0)
return(0);
cp += n;
cp += QFIXEDSZ;
}
/*
* Loop through the answer buffer and extract mx records.
*/
nmx = 0;
bp = hostbuf;
while (ancount-- > 0 && cp < eom && nmx < MAXMXHOSTS)
{
//if (verbose >= 4 || debug)
// (void) p_rr((qbuf_t *)cp, (qbuf_t *)msg, stdout);
n = dn_expand(msg, eom, cp, (nbuf_t *)bp, MAXBUF);
if (n < 0)
break;
cp += n;
type = _getshort(cp);
cp += INT16SZ;
class = _getshort(cp);
cp += INT16SZ;
/* ttl = _getlong(cp); */
cp += INT32SZ;
dlen = _getshort(cp);
cp += INT16SZ;
if (type != T_MX || class != C_IN)
{
cp += dlen;
continue;
}
pref = _getshort(cp);
cp += INT16SZ;
n = dn_expand(msg, eom, cp, (nbuf_t *)bp, MAXBUF);
if (n < 0)
break;
cp += n;
prefer[nmx] = pref;
MxHosts[nmx] = bp;
nmx++;
//n = strlength(bp) + 1;
n = strlen(bp) + 1;
bp += n;
}
/*
* Sort all records by preference.
*/
for (i = 0; i < nmx; i++)
{
for (j = i + 1; j < nmx; j++)
{
if (prefer[i] > prefer[j])
{
register u_short tmppref;
register char *tmphost;
tmppref = prefer[i];
prefer[i] = prefer[j];
prefer[j] = tmppref;
tmphost = MxHosts[i];
MxHosts[i] = MxHosts[j];
MxHosts[j] = tmphost;
}
}
}
for (i = 0; i< nmx; i++){
strcat(str, MxHosts[i]);
strcat(str, ":");
}
return(str);
}
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