 | Chapter 21 DNS and sendmail |  |
| Chapter 21 DNS and sendmail | |
An MX record is simply the method used by DNS to route mail bound for one machine to another instead. An MX record is created by a single line in one of your named(8) files:
hostA IN MX 10 hostB
This line says that all mail destined for hostA in your
domain should instead be delivered to hostB in your domain.
The IN says that this is an Internet-type record,
and the 10 is the cost
[11]
for using this MX record.
[11] Technically, this field is called the preference. We use cost to clarify that lower values are preferable, whereas preference wrongly connotes that higher values are preferable.
An MX record may point to another host or to the original host:
hostA IN MX 0 hostA
This line says that mail for hostA will be delivered
to hostA. Such records may seem redundant, but they
are not. We'll cover why shortly.
A host can have multiple MX records (one pointing to itself or not):
hostA IN MX 0 hostA
IN MX 10 hostBHere, hostA has the lowest cost (0
versus 10 for hostB), so
delivery will be attempted to itself first. If hostA is
down, delivery will be attempted to hostB host instead.
Usually, MX records point to hosts inside the same domain. Therefore managing them does not require the cooperation of others. But it is legal for MX records to point to hosts in different domains:
hostA IN MX 0 hostA
IN MX 10 host.other.domain.Here, you must contact the administrator at other.domain
and obtain permission before creating this MX record. We cover
this concept in more detail when we discuss disaster preparation
later in this chapter.
Although MX records are usually straightforward, there can be a few problems associated with them.
The A record for a host is a line that gives the host's IP address.
hostC IN A 123.45.67.8
Here, hostC is the host's name. The IN says this
is an Internet-type record. The A marks this as an A
record. The 123.45.67.8 is the IP address
for the host hostC.
An MX record must point to a hostname that has an A record. To illustrate, consider the following:
hostA IN MX 10 hostBillegal IN MX 20 hostC hostB IN MX 10 hostC hostC IN A 123.45.67.8
Note that hostB lacks an A record but
hostC has one. It is illegal to point an MX record
at a host that lacks an A record.
Therefore the first line above is illegal, whereas the second line
is legal.
Although such a mistake is difficult to make when maintaining your own domain tables, it can easily happen to you if you rely on a name server in someone else's domain, as shown:
hostA IN MX 10 mail.other.domain.
The other administrator might, for example, retire the machine
mail and replace its A record with an MX record that
points to a different machine.
Unless you are notified of the change, your MX record
will suddenly become illegal.
The sendmail program is frequently more forgiving than other MTAs because it accepts an MX record that points to a CNAME record. The presumption is that, eventually, the CNAME will correctly point to an A record. But beware, this kind of indirection can cost additional DNS lookups. Consider this example of an exceptionally bad setup:
hostA IN MX 10 mailhub mailhub IN CNAME nfsmast nfsmast IN CNAME hostB hostB IN A 123.45.67.89
First, sendmail looks up hostA and gets an MX record
pointing to mailhub. Since there is only a single MX record,
sendmail considers mailhub to be official. Next, mailhub
is looked up to find an A record (IP address), but instead a
CNAME (nfsmast) is returned. Now sendmail must
look up the CNAME nfsmast to find
its A record.
But again a CNAME is returned instead of an A record. So sendmail
must again look for an A record (this time with hostB).
Finally, sendmail succeeds by finding the A record for hostB,
but only after a few too many lookups.
The correct way to form the above DNS file entries is as follows:
hostA IN MX 10 hostB mailhub IN CNAME hostB nfsmast IN CNAME hostB hostB IN A 123.45.67.89
In general, try to construct DNS records in such a way that the fewest lookups are required to resolve any A or MX records.
Consider the following MX setup, which causes all mail for hostA
to be sent to hostB and all mail for hostB
to be sent to hostB, or to hostC if hostB is down:
[12]
[12] We are fudging for the sake of simplicity. Here, we assume that all the hosts also have A records.
hostA IN MX 10 hostB
hostB IN MX 10 hostB
IN MX 20 hostCOne might expect sendmail to be smart and deliver mail
for hostA
to hostC if hostB is down.
But sendmail won't do that.
It does not try to
recursively look for additional MX records.
If it did, it could get hopelessly entangled in MX loops.
Consider the following:
hostA IN MX 10 hostB
hostB IN MX 10 hostB
IN MX 20 hostC
hostC IN MX 10 hostA potential loopIf your intention is to have hostA MX to two other hosts,
then you must state that explicitly:
hostA IN MX 10 hostB
IN MX 20 hostC
hostB IN MX 10 hostB
IN MX 20 hostCAnother reason sendmail refuses to follow MX records beyond
the target host is that costs in such a situation are undefined.
Consider the example with the potential loop above. What is the cost
of hostA when MX'd by hostB to hostC? Should it be the
minimum of 10, the maximum of 20, the mean of 15, or the sum of 30?
Wildcard MX records provide a shorthand for MX'ing many hosts with a single MX record:
*.dc.gov. IN MX 10 hostB
This says that any host in the domain .dc.gov (where that host doesn't
have any record of its own) should have its mail forwarded to
hostB.
; domain is .dc.gov *.dc.gov. IN MX 10 hostB hostA IN MX 10 hostC hostB IN A 123.45.67.8
Here, mail to hostD (no record at all) will be forwarded
to hostB. But the wildcard MX record will be ignored
for hostA and hostB, because each has its own record.
Care must be exercised in setting up wildcard MX records. It is easy to create ambiguous situations that DNS may not be be able to handle correctly. Consider the following, for example:
; domain is sub.dc.gov *.dc.gov. IN MX 10 hostB.dc.gov. *.sub.dc.gov. IN MX 10 hostC.dc.gov.
Here, an unqualified name such as just plain hostD
matches both wildcard records. This is ambiguous, so DNS
automatically picks the most complete one (*.sub.dc.gov.)
and supplies that MX record to sendmail.
One compelling weakness of wildcard MX records is that they match any hostname at all, even for machines that don't exist:
; domain is sub.dc.gov *.dc.gov. IN MX 10 hostB.dc.gov.
Here, mail to foo.dc.gov will be forwarded to hostB.dc.gov,
even if there is no host foo in that domain.
Wildcard MX records almost never have any appropriate use on the Internet. They are often misunderstood and are often used just to save the effort of typing hundreds of MX records. They do, however, have legitimate uses behind firewall machines and on non-Internet networks.
Many older MTAs on the network ignore MX records, and some Sun sites wrongly run the non-MX version of sendmail (that is, they should use /usr/lib/sendmail.mx). Because of this, you will occasionally find some sites that insist on sending mail to a host even though that host has been explicitly MX'd to another.
To illustrate why this is bad, consider a UUCP host that has only an MX record. It has no A record because it is not on the network:
uuhost IN MX 10 uucpserver
Here, mail to uuhost will be sent to uucpserver, which
will forward the message to uuhost with UUCP software.
An attempt to ignore this MX record will fail because uuhost
has no other records. Similar problems can arise for printers with
direct network connections, terminal servers, and even workstations
that don't run an SMTP daemon such as sendmail.
If you believe in DNS and disdain sites that don't, you can simply ignore the offending sites. In this case the mail will fail if your MX'd host doesn't run a sendmail daemon (or another MTA). This is not as nasty as it sounds. There is actually considerable support for this approach; failure to obey MX records is a clear violation of published network protocols. RFC1123, Host Requirements, section 5.3.5, notes that obeying MX records is mandatory.
On the other hand, if you want to ensure that all mail is received, even on a workstation whose mail is MX'd elsewhere, you can run the sendmail daemon on every machine.
Although you are not required to have MX records for all hosts, there is good reason to consider doing so. To illustrate, consider the following host that only has an A record:
hostB IN A 123.45.67.8
When sendmail first looks up this host, it asks the local name server for all records. Because there is only an A record, that is all it gets.
But note that asking for all records caused the local name server to cache the information. The next time sendmail looks up this same host, the local name server will return the A record from its cache. This is faster and reduces Internet traffic. The cached information is "nonauthoritative" (because it is a copy) and includes no MX records (because there are none).
When sendmail gets a nonauthoritative reply that lacks MX records, it is forced to do another DNS lookup. This time, it specifically asks for MX records. In this case there are none, so it gets none.
Because hostB lacks an MX record, sendmail
performs a DNS lookup each and every time mail is sent to that host.
If hostB were a major mail-receiving site, its lack
of an MX record would be causing many sendmail programs,
all over the world, to waste network bandwidth with otherwise
useless DNS lookups.
We strongly recommend that every host on the Internet have at least one MX record. As a minimum, it can simply point to itself with a 0 cost:
hostB IN A 123.45.67.8
IN MX 0 hostBThis will not change how mail is routed to hostB
but will reduce the number of DNS lookups required.
RFC974 leaves the treatment of ambiguous MX records to the implementor's discretion. This has generated much debate in sendmail circles. Consider the following:
foo IN MX 10 hostA foo IN MX 20 hostB
When mail is sent from a host (hostB) that is an MX record
for the receiving host (foo), all MX records that have a cost
equal to or greater than that of hostB must be discarded.
The mail is then delivered to
the remaining MX host with the lowest cost
(hostA).
This is a sensible rule, because it prevents hostB from wrongly
trying to deliver to itself.
It is possible to configure hostB so that it views the
name foo as a synonym for its own name.
Such a configuration results in
hostB never looking up any MX records because it
recognizes mail to foo as local.
But what should happen if hostB does not recognize foo
as local and if there is no hostA?
Again, RFC974 says that when mail is being sent from a host (hostB)
that is an MX record for the receiving host (foo),
all MX records that have a cost equal to or greater than that of hostB
must be discarded.
In this example that leaves zero MX records.
Three courses of action are now open to sendmail, but RFC974 doesn't
say which it should use:
Assume that this is an error condition. Clearly, hostB
should have been configured to recognize foo as local.
It didn't (hence the MX lookup and discarding in the first place),
so it must not have known what it was doing. V8 sendmail with
the TryNullMXList (w) option (see Section 34.8.71, TryNullMXList (w))
not set (undeclared or declared as false) will bounce the mail message.
Look to see whether foo has an
A record. If it does, go ahead and try to deliver the mail message
directly to foo. If it lacks an A record, bounce the message.
This approach runs the risk that foo
may not be configured to properly accept mail (thus causing mail to
disappear down a black hole). Still, this approach may be desirable in
some circumstances. V8 sendmail with the
TryNullMXList (w) option (see Section 34.8.71)
set always tries to connect to foo.
[13]
[13] As does the UIUC version of IDA sendmail. Other versions of IDA (such as KJS) do not.
Assume (even though it has not been configured to do so) that foo should be treated as local to hostB. No version of sendmail makes this assumption.
This situation is not an idle exercise. Consider the MX record for
uuhost presented in the previous section:
uuhost IN MX 10 uucpserver
Here, uuhost has no A record, because it is connected
to uucpserver via a dial-up line.
If uucpserver is not configured to recognize
uuhost as one of its UUCP clients, and if mail is sent from
uucpserver to uuhost, it will query DNS and get
itself as the MX record for uuhost. As we have shown, that
MX record is discarded, and an ambiguous situation has developed.
|  | |
| 21.2 How sendmail Uses DNS |  | 21.4 How to Use nslookup |