sendmail/src/domain.c

/*
 * 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.
 *
 */

#pragma ident   "%Z%%M% %I% %E% SMI"

#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.
                */
                /* FALLTHROUGH */

# if NETINET6
              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 */