DNS and sendmail
program uses DNS in four different ways:
first starts, it may use DNS to get the
canonical name for the local host. That name is then assigned
Section 31.10.20, $j
If DNS returns additional names for
the local host, those names are assigned to the class
Section 32.5.8, $=w
When another host connects to the local host to transfer mail,
looks up the other host with DNS to find
the other host's canonical name.
When delivering network SMTP mail,
to find the address (or addresses) to which it should
in the RHS of a rule,
it looks up the hostname (or IP number) between them.
We discuss each of these uses individually later in this chapter.
All versions of
use more or less the same logical process
to obtain the canonical name of the local host.
As illustrated in the sample program below,
(3) to obtain the local host's name.
That name may either be a short name
or a fully qualified one depending on which comes first in the
file. If the call
the name of the local host is set to
struct hostent *hp;
/* Get the local hostname */
if (gethostname(hostbuf, sizeof(hostbuf)) < 0)
printf("hostname = \"%s\"\n", hostbuf);
/* canonicalize it and get aliases */
if((hp = gethostbyname(hostbuf)) == NULL)
printf("canonical = \"%s\"\n", hp->h_name);
while (*hp->h_aliases != NULL)
printf("alias: \"%s\"\n", *hp->h_aliases);
The local hostname is then given to the
Section 37.5.186, -d61.10
) to obtain the canonical name for the local
host. That same routine also returns any aliases (other
names for the local host).
On some Sun and Ultrix machines that are set up to use NIS services, the
canonical name is the short name, and a fully qualified
name that should have been the canonical name
appears as an alias. For such systems you must link with
the BIND library (
) when compiling this program
. That library gets its information from
DNS rather than from NIS.
But note that V8.7 and above versions of
do the intelligent thing and use
the canonical name that was found as the aliases if it exists.
If a good BIND library is not available, or if it is not convenient
to compile and install a new version of
you can circumvent the short name assigned to
your domain here
is your site's hostname with a dot and
your domain name appended. These two lines cause
to have your host's fully qualified (and canonical)
name assigned to it.
The canonical name found by
(3) is assigned as the
value of the
macro. The short name and any aliases
are added to the class
The result of all these lookups can be viewed by running
debugging switch (see
Section 37.5.2, -d0.4
The actual DNS lookups can be watched with the
Section 37.5.35, -d8.8
begins to run as a daemon, it creates a socket,
binds to that socket, and listens for incoming SMTP connections.
When a remote host connects to the local host,
(2) library routine to accept the connection.
(2) routine provides the IP
address of the remote machine to
(2) to convert that IP address to
a canonical (official) hostname.
program needs the canonical hostname for four
The remote hostname is compared to the local hostname to prevent
from connecting to itself.
The remote hostname claimed in the HELO SMTP line is compared
to the canonical name. If they differ,
is assigned the canonical hostname as
The canonical name is included in many log messages produced
by the setting of the
) option (see
Section 34.8.33, LogLevel (L)
and is available for inclusion
Section 35.10.25, Received:
Section 18.104.22.168, "Timeout.ident"
) is greater than zero,
the local host also connects to the
at the sending host to discover who opened the connection. If available,
that user and host information is assigned to the
Section 31.10.1, $-
prepares to connect to a remote host for transfer
of mail, it first performs a series of checks that vary from version to
version. All versions accept an IP address surrounded with square brackets
as a literal address and use it as is.
Beginning with V8.1,
checks to see whether the host part of the address is surrounded
with square brackets. If so, it skips looking up MX records.
(We'll elaborate on MX records soon.)
Beginning with V8.8,
first checks to see
Section 30.8.1, F=0
) is set for the selected
If it is set,
skips looking up MX records.
is allowed to look up MX records,
it calls the
(3) BIND library routine
to find all the MX records for the host.
If it finds any MX records, it sorts them in order of cost, selecting
the least cost first.
finds two costs that are the same,
it randomizes the selection between the two when sorting.
After all MX records are found, or if
no MX records were found,
adds the host
specified by the
) option (see
Section 34.8.25, FallbackMXhost (V)
if there was one, to that list.
program then tries to deliver the message to each
host in the list of MX hosts,
one at a time, until one of them succeeds or until they all fail.
Beginning with V8.8
, any host in the list that returns a
SMTP code (permanent failure) causes all subsequent
MX hosts to be ignored (but temporary and connect failures
continue to the next MX host as usual).
If no MX records are found,
tries to deliver
the message to the single original host.
If all else fails,
attempts to deliver to the host
listed with the
tries to connect to the original host
or to a list of MX hosts, it calls
get the network address for each. It then opens a network connection
to that address and attempts to send SMTP mail.
Section 28.6.6, "Canonicalize Hostname: $[ and $]"
are used to canonicalize
Here is a simplified description of the process.
Each lookup is actually composed of many lookups that occur in the form
of a loop within a loop.
In the outermost loop, the following logic is used:
If the address has at least one dot somewhere in it,
looks up that address unmodified first.
If the unmodified address is not found and the RES_DNSRCH
bit is set (see the
looks up variations on the domain part
of the address. The default domain is tried first (for a host
that would be
thus looking up
If that fails, BIND 4.8 then throws away the lowest part of
the domain and tries again (looks up
BIND 4.9 uses the
attribute, if given,
and tries that list of possible domains.
If the address has no dots and the RES_DEFNAMES bit is set
tries the single default domain (looks up
). This is for compatibility
with older versions of DNS.
Each lookup described above is performed by using the following steps:
Try the hostname with a T_ANY query that requests all the cached
DNS records for that host.
If it succeeds, A records
and/or MX records may be among those returned.
However, success is not guaranteed,
because sometimes only NS (
erver) records are returned.
In that instance the following steps are also taken.
Try the hostname with a T_A query that requests the A
record for that host.
Try the hostname with a T_MX query that requests MX records
for the host.
Each query searches the data returned as follows:
Search for a CNAME (alias) record.
If one is found, replace the initial hostname (the alias) with
the canonical name returned and start over.
Search for an A record (the IP address).
If one is found, the hostname that was just used to query
is considered the canonical address.
Search for an MX record.
If one is found and a default domain has not been added,
treat the MX record like an A record. For example, if the
input hostname is
MX record is found the MX record is considered official.
If, on the other hand, the input hostname has no domain
and the query happens to stumble across
the MX record, the following searches are also tried.
If an MX record is found and no MX record has been previously
found, the looked-up hostname is saved for future use.
For example, if the query was for
MX records were returned (
is saved for future use.
If no MX record is found, but one was found previously,
the previous one is used. This assumes that the search is normally
from most to least complex (
All this apparent complexity is necessary to deal with wildcard MX records
Section 21.3.4, "Wildcard MX Records"
) in a reasonable and successful way.