/*
 * Copyright (c) 1998-2004, 2006 Sendmail, Inc. and its suppliers.
 *      All rights reserved.
 * Copyright (c) 1986, 1995-1997 Eric P. Allman.  All rights reserved.
 * Copyright (c) 1988, 1993
 *      The Regents of the University of California.  All rights reserved.
 *
 * By using this file, you agree to the terms and conditions set
 * forth in the LICENSE file which can be found at the top level of
 * the sendmail distribution.
 *
 */

#include <sendmail.h>
#include "map.h"

#if NAMED_BIND
SM_RCSID("@(#)$Id: domain.c,v 8.202 2006/12/19 01:15:07 ca Exp $ (with name server)")
#else /* NAMED_BIND */
SM_RCSID("@(#)$Id: domain.c,v 8.202 2006/12/19 01:15:07 ca Exp $ (without name server)")
#endif /* NAMED_BIND */

#if NAMED_BIND

# include <arpa/inet.h>


/*
**  The standard udp packet size PACKETSZ (512) is not sufficient for some
**  nameserver answers containing very many resource records. The resolver
**  may switch to tcp and retry if it detects udp packet overflow.
**  Also note that the resolver routines res_query and res_search return
**  the size of the *un*truncated answer in case the supplied answer buffer
**  it not big enough to accommodate the entire answer.
*/

# ifndef MAXPACKET
#  define MAXPACKET 8192        /* max packet size used internally by BIND */
# endif /* ! MAXPACKET */

typedef union
{
        HEADER          qb1;
        unsigned char   qb2[MAXPACKET];
} querybuf;

# ifndef MXHOSTBUFSIZE
#  define MXHOSTBUFSIZE (128 * MAXMXHOSTS)
# endif /* ! MXHOSTBUFSIZE */

static char     MXHostBuf[MXHOSTBUFSIZE];
#if (MXHOSTBUFSIZE < 2) || (MXHOSTBUFSIZE >= INT_MAX/2)
        ERROR: _MXHOSTBUFSIZE is out of range
#endif /* (MXHOSTBUFSIZE < 2) || (MXHOSTBUFSIZE >= INT_MAX/2) */

# ifndef MAXDNSRCH
#  define MAXDNSRCH     6       /* number of possible domains to search */
# endif /* ! MAXDNSRCH */

# ifndef RES_DNSRCH_VARIABLE
#  define RES_DNSRCH_VARIABLE   _res.dnsrch
# endif /* ! RES_DNSRCH_VARIABLE */

# ifndef NO_DATA
#  define NO_DATA       NO_ADDRESS
# endif /* ! NO_DATA */

# ifndef HFIXEDSZ
#  define HFIXEDSZ      12      /* sizeof(HEADER) */
# endif /* ! HFIXEDSZ */

# define MAXCNAMEDEPTH  10      /* maximum depth of CNAME recursion */

# if defined(__RES) && (__RES >= 19940415)
#  define RES_UNC_T     char *
# else /* defined(__RES) && (__RES >= 19940415) */
#  define RES_UNC_T     unsigned char *
# endif /* defined(__RES) && (__RES >= 19940415) */

static int      mxrand __P((char *));
static int      fallbackmxrr __P((int, unsigned short *, char **));

/*
**  GETFALLBACKMXRR -- get MX resource records for fallback MX host.
**
**      We have to initialize this once before doing anything else.
**      Moreover, we have to repeat this from time to time to avoid
**      stale data, e.g., in persistent queue runners.
**      This should be done in a parent process so the child
**      processes have the right data.
**
**      Parameters:
**              host -- the name of the fallback MX host.
**
**      Returns:
**              number of MX records.
**
**      Side Effects:
**              Populates NumFallbackMXHosts and fbhosts.
**              Sets renewal time (based on TTL).
*/

int NumFallbackMXHosts = 0;     /* Number of fallback MX hosts (after MX expansion) */
static char *fbhosts[MAXMXHOSTS + 1];

int
getfallbackmxrr(host)
        char *host;
{
        int i, rcode;
        int ttl;
        static time_t renew = 0;

#if 0
        /* This is currently done before this function is called. */
        if (host == NULL || *host == '\0')
                return 0;
#endif /* 0 */
        if (NumFallbackMXHosts > 0 && renew > curtime())
                return NumFallbackMXHosts;
        if (host[0] == '[')
        {
                fbhosts[0] = host;
                NumFallbackMXHosts = 1;
        }
        else
        {
                /* free old data */
                for (i = 0; i < NumFallbackMXHosts; i++)
                        sm_free(fbhosts[i]);

                /* get new data */
                NumFallbackMXHosts = getmxrr(host, fbhosts, NULL, false,
                                             &rcode, false, &ttl);
                renew = curtime() + ttl;
                for (i = 0; i < NumFallbackMXHosts; i++)
                        fbhosts[i] = newstr(fbhosts[i]);
        }
        return NumFallbackMXHosts;
}

/*
**  FALLBACKMXRR -- add MX resource records for fallback MX host to list.
**
**      Parameters:
**              nmx -- current number of MX records.
**              prefs -- array of preferences.
**              mxhosts -- array of MX hosts (maximum size: MAXMXHOSTS)
**
**      Returns:
**              new number of MX records.
**
**      Side Effects:
**              If FallbackMX was set, it appends the MX records for
**              that host to mxhosts (and modifies prefs accordingly).
*/

static int
fallbackmxrr(nmx, prefs, mxhosts)
        int nmx;
        unsigned short *prefs;
        char **mxhosts;
{
        int i;

        for (i = 0; i < NumFallbackMXHosts && nmx < MAXMXHOSTS; i++)
        {
                if (nmx > 0)
                        prefs[nmx] = prefs[nmx - 1] + 1;
                else
                        prefs[nmx] = 0;
                mxhosts[nmx++] = fbhosts[i];
        }
        return nmx;
}

/*
**  GETMXRR -- get MX resource records for a domain
**
**      Parameters:
**              host -- the name of the host to MX.
**              mxhosts -- a pointer to a return buffer of MX records.
**              mxprefs -- a pointer to a return buffer of MX preferences.
**                      If NULL, don't try to populate.
**              droplocalhost -- If true, all MX records less preferred
**                      than the local host (as determined by $=w) will
**                      be discarded.
**              rcode -- a pointer to an EX_ status code.
**              tryfallback -- add also fallback MX host?
**              pttl -- pointer to return TTL (can be NULL).
**
**      Returns:
**              The number of MX records found.
**              -1 if there is an internal failure.
**              If no MX records are found, mxhosts[0] is set to host
**                      and 1 is returned.
**
**      Side Effects:
**              The entries made for mxhosts point to a static array
**              MXHostBuf[MXHOSTBUFSIZE], so the data needs to be copied,
**              if it must be preserved across calls to this function.
*/

int
getmxrr(host, mxhosts, mxprefs, droplocalhost, rcode, tryfallback, pttl)
        char *host;
        char **mxhosts;
        unsigned short *mxprefs;
        bool droplocalhost;
        int *rcode;
        bool tryfallback;
        int *pttl;
{
        register unsigned char *eom, *cp;
        register int i, j, n;
        int nmx = 0;
        register char *bp;
        HEADER *hp;
        querybuf answer;
        int ancount, qdcount, buflen;
        bool seenlocal = false;
        unsigned short pref, type;
        unsigned short localpref = 256;
        char *fallbackMX = FallbackMX;
        bool trycanon = false;
        unsigned short *prefs;
        int (*resfunc) __P((const char *, int, int, u_char *, int));
        unsigned short prefer[MAXMXHOSTS];
        int weight[MAXMXHOSTS];
        int ttl = 0;
        extern int res_query(), res_search();

        if (tTd(8, 2))
                sm_dprintf("getmxrr(%s, droplocalhost=%d)\n",
                           host, droplocalhost);
        *rcode = EX_OK;
        if (pttl != NULL)
                *pttl = SM_DEFAULT_TTL;
        if (*host == '\0')
                return 0;

        if ((fallbackMX != NULL && droplocalhost &&
             wordinclass(fallbackMX, 'w')) || !tryfallback)
        {
                /* don't use fallback for this pass */
                fallbackMX = NULL;
        }

        if (mxprefs != NULL)
                prefs = mxprefs;
        else
                prefs = prefer;

        /* efficiency hack -- numeric or non-MX lookups */
        if (host[0] == '[')
                goto punt;

        /*
        **  If we don't have MX records in our host switch, don't
        **  try for MX records.  Note that this really isn't "right",
        **  since we might be set up to try NIS first and then DNS;
        **  if the host is found in NIS we really shouldn't be doing
        **  MX lookups.  However, that should be a degenerate case.
        */

        if (!UseNameServer)
                goto punt;
        if (HasWildcardMX && ConfigLevel >= 6)
                resfunc = res_query;
        else
                resfunc = res_search;

        errno = 0;
        n = (*resfunc)(host, C_IN, T_MX, (unsigned char *) &answer,
                       sizeof(answer));
        if (n < 0)
        {
                if (tTd(8, 1))
                        sm_dprintf("getmxrr: res_search(%s) failed (errno=%d, h_errno=%d)\n",
                                host, errno, h_errno);
                switch (h_errno)
                {
                  case NO_DATA:
                        trycanon = true;
                        /* FALLTHROUGH */

                  case NO_RECOVERY:
                        /* no MX data on this host */
                        goto punt;

                  case HOST_NOT_FOUND:
# if BROKEN_RES_SEARCH
                  case 0:       /* Ultrix resolver retns failure w/ h_errno=0 */
# endif /* BROKEN_RES_SEARCH */
                        /* host doesn't exist in DNS; might be in /etc/hosts */
                        trycanon = true;
                        *rcode = EX_NOHOST;
                        goto punt;

                  case TRY_AGAIN:
                  case -1:
                        /* couldn't connect to the name server */
                        if (fallbackMX != NULL)
                        {
                                /* name server is hosed -- push to fallback */
                                return fallbackmxrr(nmx, prefs, mxhosts);
                        }
                        /* it might come up later; better queue it up */
                        *rcode = EX_TEMPFAIL;
                        break;

                  default:
                        syserr("getmxrr: res_search (%s) failed with impossible h_errno (%d)",
                                host, h_errno);
                        *rcode = EX_OSERR;
                        break;
                }

                /* irreconcilable differences */
                return -1;
        }

        /* avoid problems after truncation in tcp packets */
        if (n > sizeof(answer))
                n = sizeof(answer);

        /* find first satisfactory answer */
        hp = (HEADER *)&answer;
        cp = (unsigned char *)&answer + HFIXEDSZ;
        eom = (unsigned char *)&answer + n;
        for (qdcount = ntohs((unsigned short) hp->qdcount);
             qdcount--;
             cp += n + QFIXEDSZ)
        {
                if ((n = dn_skipname(cp, eom)) < 0)
                        goto punt;
        }

        /* NOTE: see definition of MXHostBuf! */
        buflen = sizeof(MXHostBuf) - 1;
        SM_ASSERT(buflen > 0);
        bp = MXHostBuf;
        ancount = ntohs((unsigned short) hp->ancount);

        /* See RFC 1035 for layout of RRs. */
        /* XXX leave room for FallbackMX ? */
        while (--ancount >= 0 && cp < eom && nmx < MAXMXHOSTS - 1)
        {
                if ((n = dn_expand((unsigned char *)&answer, eom, cp,
                                   (RES_UNC_T) bp, buflen)) < 0)
                        break;
                cp += n;
                GETSHORT(type, cp);
                cp += INT16SZ;          /* skip over class */
                GETLONG(ttl, cp);
                GETSHORT(n, cp);        /* rdlength */
                if (type != T_MX)
                {
                        if (tTd(8, 8) || _res.options & RES_DEBUG)
                                sm_dprintf("unexpected answer type %d, size %d\n",
                                        type, n);
                        cp += n;
                        continue;
                }
                GETSHORT(pref, cp);
                if ((n = dn_expand((unsigned char *)&answer, eom, cp,
                                   (RES_UNC_T) bp, buflen)) < 0)
                        break;
                cp += n;
                n = strlen(bp);
# if 0
                /* Can this happen? */
                if (n == 0)
                {
                        if (LogLevel > 4)
                                sm_syslog(LOG_ERR, NOQID,
                                          "MX records for %s contain empty string",
                                          host);
                        continue;
                }
# endif /* 0 */
                if (wordinclass(bp, 'w'))
                {
                        if (tTd(8, 3))
                                sm_dprintf("found localhost (%s) in MX list, pref=%d\n",
                                        bp, pref);
                        if (droplocalhost)
                        {
                                if (!seenlocal || pref < localpref)
                                        localpref = pref;
                                seenlocal = true;
                                continue;
                        }
                        weight[nmx] = 0;
                }
                else
                        weight[nmx] = mxrand(bp);
                prefs[nmx] = pref;
                mxhosts[nmx++] = bp;
                bp += n;
                if (bp[-1] != '.')
                {
                        *bp++ = '.';
                        n++;
                }
                *bp++ = '\0';
                if (buflen < n + 1)
                {
                        /* don't want to wrap buflen */
                        break;
                }
                buflen -= n + 1;
        }

        /* return only one TTL entry, that should be sufficient */
        if (ttl > 0 && pttl != NULL)
                *pttl = ttl;

        /* sort the records */
        for (i = 0; i < nmx; i++)
        {
                for (j = i + 1; j < nmx; j++)
                {
                        if (prefs[i] > prefs[j] ||
                            (prefs[i] == prefs[j] && weight[i] > weight[j]))
                        {
                                register int temp;
                                register char *temp1;

                                temp = prefs[i];
                                prefs[i] = prefs[j];
                                prefs[j] = temp;
                                temp1 = mxhosts[i];
                                mxhosts[i] = mxhosts[j];
                                mxhosts[j] = temp1;
                                temp = weight[i];
                                weight[i] = weight[j];
                                weight[j] = temp;
                        }
                }
                if (seenlocal && prefs[i] >= localpref)
                {
                        /* truncate higher preference part of list */
                        nmx = i;
                }
        }

        /* delete duplicates from list (yes, some bozos have duplicates) */
        for (i = 0; i < nmx - 1; )
        {
                if (sm_strcasecmp(mxhosts[i], mxhosts[i + 1]) != 0)
                        i++;
                else
                {
                        /* compress out duplicate */
                        for (j = i + 1; j < nmx; j++)
                        {
                                mxhosts[j] = mxhosts[j + 1];
                                prefs[j] = prefs[j + 1];
                        }
                        nmx--;
                }
        }

        if (nmx == 0)
        {
punt:
                if (seenlocal)
                {
                        struct hostent *h = NULL;

                        /*
                        **  If we have deleted all MX entries, this is
                        **  an error -- we should NEVER send to a host that
                        **  has an MX, and this should have been caught
                        **  earlier in the config file.
                        **
                        **  Some sites prefer to go ahead and try the
                        **  A record anyway; that case is handled by
                        **  setting TryNullMXList.  I believe this is a
                        **  bad idea, but it's up to you....
                        */

                        if (TryNullMXList)
                        {
                                SM_SET_H_ERRNO(0);
                                errno = 0;
                                h = sm_gethostbyname(host, AF_INET);
                                if (h == NULL)
                                {
                                        if (errno == ETIMEDOUT ||
                                            h_errno == TRY_AGAIN ||
                                            (errno == ECONNREFUSED &&
                                             UseNameServer))
                                        {
                                                *rcode = EX_TEMPFAIL;
                                                return -1;
                                        }
# if NETINET6
                                        SM_SET_H_ERRNO(0);
                                        errno = 0;
                                        h = sm_gethostbyname(host, AF_INET6);
                                        if (h == NULL &&
                                            (errno == ETIMEDOUT ||
                                             h_errno == TRY_AGAIN ||
                                             (errno == ECONNREFUSED &&
                                              UseNameServer)))
                                        {
                                                *rcode = EX_TEMPFAIL;
                                                return -1;
                                        }
# endif /* NETINET6 */
                                }
                        }

                        if (h == NULL)
                        {
                                *rcode = EX_CONFIG;
                                syserr("MX list for %s points back to %s",
                                       host, MyHostName);
                                return -1;
                        }
# if NETINET6
                        freehostent(h);
                        h = NULL;
# endif /* NETINET6 */
                }
                if (strlen(host) >= sizeof(MXHostBuf))
                {
                        *rcode = EX_CONFIG;
                        syserr("Host name %s too long",
                               shortenstring(host, MAXSHORTSTR));
                        return -1;
                }
                (void) sm_strlcpy(MXHostBuf, host, sizeof(MXHostBuf));
                mxhosts[0] = MXHostBuf;
                prefs[0] = 0;
                if (host[0] == '[')
                {
                        register char *p;
# if NETINET6
                        struct sockaddr_in6 tmp6;
# endif /* NETINET6 */

                        /* this may be an MX suppression-style address */
                        p = strchr(MXHostBuf, ']');
                        if (p != NULL)
                        {
                                *p = '\0';

                                if (inet_addr(&MXHostBuf[1]) != INADDR_NONE)
                                {
                                        nmx++;
                                        *p = ']';
                                }
# if NETINET6
                                else if (anynet_pton(AF_INET6, &MXHostBuf[1],
                                                     &tmp6.sin6_addr) == 1)
                                {
                                        nmx++;
                                        *p = ']';
                                }
# endif /* NETINET6 */
                                else
                                {
                                        trycanon = true;
                                        mxhosts[0]++;
                                }
                        }
                }
                if (trycanon &&
                    getcanonname(mxhosts[0], sizeof(MXHostBuf) - 2, false, pttl))
                {
                        /* XXX MXHostBuf == "" ?  is that possible? */
                        bp = &MXHostBuf[strlen(MXHostBuf)];
                        if (bp[-1] != '.')
                        {
                                *bp++ = '.';
                                *bp = '\0';
                        }
                        nmx = 1;
                }
        }

        /* if we have a default lowest preference, include that */
        if (fallbackMX != NULL && !seenlocal)
        {
                nmx = fallbackmxrr(nmx, prefs, mxhosts);
        }
        return nmx;
}
/*
**  MXRAND -- create a randomizer for equal MX preferences
**
**      If two MX hosts have equal preferences we want to randomize
**      the selection.  But in order for signatures to be the same,
**      we need to randomize the same way each time.  This function
**      computes a pseudo-random hash function from the host name.
**
**      Parameters:
**              host -- the name of the host.
**
**      Returns:
**              A random but repeatable value based on the host name.
*/

static int
mxrand(host)
        register char *host;
{
        int hfunc;
        static unsigned int seed;

        if (seed == 0)
        {
                seed = (int) curtime() & 0xffff;
                if (seed == 0)
                        seed++;
        }

        if (tTd(17, 9))
                sm_dprintf("mxrand(%s)", host);

        hfunc = seed;
        while (*host != '\0')
        {
                int c = *host++;

                if (isascii(c) && isupper(c))
                        c = tolower(c);
                hfunc = ((hfunc << 1) ^ c) % 2003;
        }

        hfunc &= 0xff;
        hfunc++;

        if (tTd(17, 9))
                sm_dprintf(" = %d\n", hfunc);
        return hfunc;
}
/*
**  BESTMX -- find the best MX for a name
**
**      This is really a hack, but I don't see any obvious way
**      to generalize it at the moment.
*/

/* ARGSUSED3 */
char *
bestmx_map_lookup(map, name, av, statp)
        MAP *map;
        char *name;
        char **av;
        int *statp;
{
        int nmx;
        int saveopts = _res.options;
        int i;
        ssize_t len = 0;
        char *result;
        char *mxhosts[MAXMXHOSTS + 1];
#if _FFR_BESTMX_BETTER_TRUNCATION
        char *buf;
#else /* _FFR_BESTMX_BETTER_TRUNCATION */
        char *p;
        char buf[PSBUFSIZE / 2];
#endif /* _FFR_BESTMX_BETTER_TRUNCATION */

        _res.options &= ~(RES_DNSRCH|RES_DEFNAMES);
        nmx = getmxrr(name, mxhosts, NULL, false, statp, false, NULL);
        _res.options = saveopts;
        if (nmx <= 0)
                return NULL;
        if (bitset(MF_MATCHONLY, map->map_mflags))
                return map_rewrite(map, name, strlen(name), NULL);
        if ((map->map_coldelim == '\0') || (nmx == 1))
                return map_rewrite(map, mxhosts[0], strlen(mxhosts[0]), av);

        /*
        **  We were given a -z flag (return all MXs) and there are multiple
        **  ones.  We need to build them all into a list.
        */

#if _FFR_BESTMX_BETTER_TRUNCATION
        for (i = 0; i < nmx; i++)
        {
                if (strchr(mxhosts[i], map->map_coldelim) != NULL)
                {
                        syserr("bestmx_map_lookup: MX host %.64s includes map delimiter character 0x%02X",
                               mxhosts[i], map->map_coldelim);
                        return NULL;
                }
                len += strlen(mxhosts[i]) + 1;
                if (len < 0)
                {
                        len -= strlen(mxhosts[i]) + 1;
                        break;
                }
        }
        buf = (char *) sm_malloc(len);
        if (buf == NULL)
        {
                *statp = EX_UNAVAILABLE;
                return NULL;
        }
        *buf = '\0';
        for (i = 0; i < nmx; i++)
        {
                int end;

                end = sm_strlcat(buf, mxhosts[i], len);
                if (i != nmx && end + 1 < len)
                {
                        buf[end] = map->map_coldelim;
                        buf[end + 1] = '\0';
                }
        }

        /* Cleanly truncate for rulesets */
        truncate_at_delim(buf, PSBUFSIZE / 2, map->map_coldelim);
#else /* _FFR_BESTMX_BETTER_TRUNCATION */
        p = buf;
        for (i = 0; i < nmx; i++)
        {
                size_t slen;

                if (strchr(mxhosts[i], map->map_coldelim) != NULL)
                {
                        syserr("bestmx_map_lookup: MX host %.64s includes map delimiter character 0x%02X",
                               mxhosts[i], map->map_coldelim);
                        return NULL;
                }
                slen = strlen(mxhosts[i]);
                if (len + slen + 2 > sizeof(buf))
                        break;
                if (i > 0)
                {
                        *p++ = map->map_coldelim;
                        len++;
                }
                (void) sm_strlcpy(p, mxhosts[i], sizeof(buf) - len);
                p += slen;
                len += slen;
        }
#endif /* _FFR_BESTMX_BETTER_TRUNCATION */

        result = map_rewrite(map, buf, len, av);
#if _FFR_BESTMX_BETTER_TRUNCATION
        sm_free(buf);
#endif /* _FFR_BESTMX_BETTER_TRUNCATION */
        return result;
}
/*
**  DNS_GETCANONNAME -- get the canonical name for named host using DNS
**
**      This algorithm tries to be smart about wildcard MX records.
**      This is hard to do because DNS doesn't tell is if we matched
**      against a wildcard or a specific MX.
**
**      We always prefer A & CNAME records, since these are presumed
**      to be specific.
**
**      If we match an MX in one pass and lose it in the next, we use
**      the old one.  For example, consider an MX matching *.FOO.BAR.COM.
**      A hostname bletch.foo.bar.com will match against this MX, but
**      will stop matching when we try bletch.bar.com -- so we know
**      that bletch.foo.bar.com must have been right.  This fails if
**      there was also an MX record matching *.BAR.COM, but there are
**      some things that just can't be fixed.
**
**      Parameters:
**              host -- a buffer containing the name of the host.
**                      This is a value-result parameter.
**              hbsize -- the size of the host buffer.
**              trymx -- if set, try MX records as well as A and CNAME.
**              statp -- pointer to place to store status.
**              pttl -- pointer to return TTL (can be NULL).
**
**      Returns:
**              true -- if the host matched.
**              false -- otherwise.
*/

bool
dns_getcanonname(host, hbsize, trymx, statp, pttl)
        char *host;
        int hbsize;
        bool trymx;
        int *statp;
        int *pttl;
{
        register unsigned char *eom, *ap;
        register char *cp;
        register int n;
        HEADER *hp;
        querybuf answer;
        int ancount, qdcount;
        int ret;
        char **domain;
        int type;
        int ttl = 0;
        char **dp;
        char *mxmatch;
        bool amatch;
        bool gotmx = false;
        int qtype;
        int initial;
        int loopcnt;
        char nbuf[SM_MAX(MAXPACKET, MAXDNAME*2+2)];
        char *searchlist[MAXDNSRCH + 2];

        if (tTd(8, 2))
                sm_dprintf("dns_getcanonname(%s, trymx=%d)\n", host, trymx);

        if ((_res.options & RES_INIT) == 0 && res_init() == -1)
        {
                *statp = EX_UNAVAILABLE;
                return false;
        }

        *statp = EX_OK;

        /*
        **  Initialize domain search list.  If there is at least one
        **  dot in the name, search the unmodified name first so we
        **  find "vse.CS" in Czechoslovakia instead of in the local
        **  domain (e.g., vse.CS.Berkeley.EDU).  Note that there is no
        **  longer a country named Czechoslovakia but this type of problem
        **  is still present.
        **
        **  Older versions of the resolver could create this
        **  list by tearing apart the host name.
        */

        loopcnt = 0;
cnameloop:
        /* Check for dots in the name */
        for (cp = host, n = 0; *cp != '\0'; cp++)
                if (*cp == '.')
                        n++;

        /*
        **  Build the search list.
        **      If there is at least one dot in name, start with a null
        **      domain to search the unmodified name first.
        **      If name does not end with a dot and search up local domain
        **      tree desired, append each local domain component to the
        **      search list; if name contains no dots and default domain
        **      name is desired, append default domain name to search list;
        **      else if name ends in a dot, remove that dot.
        */

        dp = searchlist;
        if (n > 0)
                *dp++ = "";
        if (n >= 0 && *--cp != '.' && bitset(RES_DNSRCH, _res.options))
        {
                /* make sure there are less than MAXDNSRCH domains */
                for (domain = RES_DNSRCH_VARIABLE, ret = 0;
                     *domain != NULL && ret < MAXDNSRCH;
                     ret++)
                        *dp++ = *domain++;
        }
        else if (n == 0 && bitset(RES_DEFNAMES, _res.options))
        {
                *dp++ = _res.defdname;
        }
        else if (*cp == '.')
        {
                *cp = '\0';
        }
        *dp = NULL;

        /*
        **  Now loop through the search list, appending each domain in turn
        **  name and searching for a match.
        */

        mxmatch = NULL;
        initial = T_A;
# if NETINET6
        if (InetMode == AF_INET6)
                initial = T_AAAA;
# endif /* NETINET6 */
        qtype = initial;

        for (dp = searchlist; *dp != NULL; )
        {
                if (qtype == initial)
                        gotmx = false;
                if (tTd(8, 5))
                        sm_dprintf("dns_getcanonname: trying %s.%s (%s)\n",
                                host, *dp,
# if NETINET6
                                qtype == T_AAAA ? "AAAA" :
# endif /* NETINET6 */
                                qtype == T_A ? "A" :
                                qtype == T_MX ? "MX" :
                                "???");
                errno = 0;
                ret = res_querydomain(host, *dp, C_IN, qtype,
                                      answer.qb2, sizeof(answer.qb2));
                if (ret <= 0)
                {
                        int save_errno = errno;

                        if (tTd(8, 7))
                                sm_dprintf("\tNO: errno=%d, h_errno=%d\n",
                                           save_errno, h_errno);

                        if (save_errno == ECONNREFUSED || h_errno == TRY_AGAIN)
                        {
                                /*
                                **  the name server seems to be down or broken.
                                */

                                SM_SET_H_ERRNO(TRY_AGAIN);
                                if (**dp == '\0')
                                {
                                        if (*statp == EX_OK)
                                                *statp = EX_TEMPFAIL;
                                        goto nexttype;
                                }
                                *statp = EX_TEMPFAIL;

                                if (WorkAroundBrokenAAAA)
                                {
                                        /*
                                        **  Only return if not TRY_AGAIN as an
                                        **  attempt with a different qtype may
                                        **  succeed (res_querydomain() calls
                                        **  res_query() calls res_send() which
                                        **  sets errno to ETIMEDOUT if the
                                        **  nameservers could be contacted but
                                        **  didn't give an answer).
                                        */

                                        if (save_errno != ETIMEDOUT)
                                                return false;
                                }
                                else
                                        return false;
                        }

nexttype:
                        if (h_errno != HOST_NOT_FOUND)
                        {
                                /* might have another type of interest */
# if NETINET6
                                if (qtype == T_AAAA)
                                {
                                        qtype = T_A;
                                        continue;
                                }
                                else
# endif /* NETINET6 */
                                if (qtype == T_A && !gotmx &&
                                    (trymx || **dp == '\0'))
                                {
                                        qtype = T_MX;
                                        continue;
                                }
                        }

                        /* definite no -- try the next domain */
                        dp++;
                        qtype = initial;
                        continue;
                }
                else if (tTd(8, 7))
                        sm_dprintf("\tYES\n");

                /* avoid problems after truncation in tcp packets */
                if (ret > sizeof(answer))
                        ret = sizeof(answer);
                SM_ASSERT(ret >= 0);

                /*
                **  Appear to have a match.  Confirm it by searching for A or
                **  CNAME records.  If we don't have a local domain
                **  wild card MX record, we will accept MX as well.
                */

                hp = (HEADER *) &answer;
                ap = (unsigned char *) &answer + HFIXEDSZ;
                eom = (unsigned char *) &answer + ret;

                /* skip question part of response -- we know what we asked */
                for (qdcount = ntohs((unsigned short) hp->qdcount);
                     qdcount--;
                     ap += ret + QFIXEDSZ)
                {
                        if ((ret = dn_skipname(ap, eom)) < 0)
                        {
                                if (tTd(8, 20))
                                        sm_dprintf("qdcount failure (%d)\n",
                                                ntohs((unsigned short) hp->qdcount));
                                *statp = EX_SOFTWARE;
                                return false;           /* ???XXX??? */
                        }
                }

                amatch = false;
                for (ancount = ntohs((unsigned short) hp->ancount);
                     --ancount >= 0 && ap < eom;
                     ap += n)
                {
                        n = dn_expand((unsigned char *) &answer, eom, ap,
                                      (RES_UNC_T) nbuf, sizeof(nbuf));
                        if (n < 0)
                                break;
                        ap += n;
                        GETSHORT(type, ap);
                        ap += INT16SZ;          /* skip over class */
                        GETLONG(ttl, ap);
                        GETSHORT(n, ap);        /* rdlength */
                        switch (type)
                        {
                          case T_MX:
                                gotmx = true;
                                if (**dp != '\0' && HasWildcardMX)
                                {
                                        /*
                                        **  If we are using MX matches and have
                                        **  not yet gotten one, save this one
                                        **  but keep searching for an A or
                                        **  CNAME match.
                                        */

                                        if (trymx && mxmatch == NULL)
                                                mxmatch = *dp;
                                        continue;
                                }

                                /*
                                **  If we did not append a domain name, this
                                **  must have been a canonical name to start
                                **  with.  Even if we did append a domain name,
                                **  in the absence of a wildcard MX this must
                                **  still be a real MX match.
                                **  Such MX matches are as good as an A match,
                                **  fall through.
                                */

# if NETINET6
                                /* FALLTHROUGH */
                          case T_AAAA:
# endif /* NETINET6 */
                          case T_A:
                                /* Flag that a good match was found */
                                amatch = true;

                                /* continue in case a CNAME also exists */
                                continue;

                          case T_CNAME:
                                if (DontExpandCnames)
                                {
                                        /* got CNAME -- guaranteed canonical */
                                        amatch = true;
                                        break;
                                }

                                if (loopcnt++ > MAXCNAMEDEPTH)
                                {
                                        /*XXX should notify postmaster XXX*/
                                        message("DNS failure: CNAME loop for %s",
                                                host);
                                        if (CurEnv->e_message == NULL)
                                        {
                                                char ebuf[MAXLINE];

                                                (void) sm_snprintf(ebuf,
                                                        sizeof(ebuf),
                                                        "Deferred: DNS failure: CNAME loop for %.100s",
                                                        host);
                                                CurEnv->e_message =
                                                    sm_rpool_strdup_x(
                                                        CurEnv->e_rpool, ebuf);
                                        }
                                        SM_SET_H_ERRNO(NO_RECOVERY);
                                        *statp = EX_CONFIG;
                                        return false;
                                }

                                /* value points at name */
                                if ((ret = dn_expand((unsigned char *)&answer,
                                                     eom, ap, (RES_UNC_T) nbuf,
                                                     sizeof(nbuf))) < 0)
                                        break;
                                (void) sm_strlcpy(host, nbuf, hbsize);

                                /*
                                **  RFC 1034 section 3.6 specifies that CNAME
                                **  should point at the canonical name -- but
                                **  urges software to try again anyway.
                                */

                                goto cnameloop;

                          default:
                                /* not a record of interest */
                                continue;
                        }
                }

                if (amatch)
                {
                        /*
                        **  Got a good match -- either an A, CNAME, or an
                        **  exact MX record.  Save it and get out of here.
                        */

                        mxmatch = *dp;
                        break;
                }

                /*
                **  Nothing definitive yet.
                **      If this was a T_A query and we haven't yet found a MX
                **              match, try T_MX if allowed to do so.
                **      Otherwise, try the next domain.
                */

# if NETINET6
                if (qtype == T_AAAA)
                        qtype = T_A;
                else
# endif /* NETINET6 */
                if (qtype == T_A && !gotmx && (trymx || **dp == '\0'))
                        qtype = T_MX;
                else
                {
                        qtype = initial;
                        dp++;
                }
        }

        /* if nothing was found, we are done */
        if (mxmatch == NULL)
        {
                if (*statp == EX_OK)
                        *statp = EX_NOHOST;
                return false;
        }

        /*
        **  Create canonical name and return.
        **  If saved domain name is null, name was already canonical.
        **  Otherwise append the saved domain name.
        */

        (void) sm_snprintf(nbuf, sizeof(nbuf), "%.*s%s%.*s", MAXDNAME, host,
                           *mxmatch == '\0' ? "" : ".",
                           MAXDNAME, mxmatch);
        (void) sm_strlcpy(host, nbuf, hbsize);
        if (tTd(8, 5))
                sm_dprintf("dns_getcanonname: %s\n", host);
        *statp = EX_OK;

        /* return only one TTL entry, that should be sufficient */
        if (ttl > 0 && pttl != NULL)
                *pttl = ttl;
        return true;
}
#endif /* NAMED_BIND */