On Mon, Jan 13, 2014 at 03:18:12PM +0100, Peer Heinlein wrote:
> > We want to load balance mails from the intranet to the
> > postfix-relayserver-farm for outgoing traffic.
> > Can we abuse A-records to load-balance in the same way MX-records
> > have been designed?
>
> No, because in that case MX-Records would be useless.
MX records can redirect mail delivery across organizational
boundaries, where using A records that track the IP addresses of
remote hosts would be impractical and allow for backup-MX hosts,
which round-robin A records cannot. When all MX preferences are
equal one may be able to achieve a similar effect with "round-robin"
DNS A records, provided the DNS server or client actually randomizes
the address list.
Unfortunately, as Wietse notes, the text in RFC 5321, Section 5.1:
The destination host (perhaps taken from the preferred MX record) may
be multihomed, in which case the domain name resolver will return a
list of alternative IP addresses. It is the responsibility of the
domain name resolver interface to have ordered this list by
decreasing preference if necessary, and the SMTP sender MUST try them
in the order presented.
(incorrectly) discourages address randomization by the SMTP client.
What kind of address RRset ordering, if any, is performed by
nameservres is configuration and implementation dependent.
When the client is Postfix, it sensibly ignores the advice in RFC
5321 and randomizes the addresses. The randomization algorithm
was made more uniform in Postfix 2.11-20130513. When the reply
order from DNS is not uniformly random, previous versions of Postfix
exhibit a bias in the first selected host when shuffling more than
2 records. There was a noticeable bias to choose the second element
more frequently than the rest and the last element less frequently
than the rest. The Perl code below computes the relative frequencies
for 3 to 7 address records and the ratio of least to most frequent:
$ perl -e '
for ($n = 3; $n < 8; ++$n) {
$N = $n**$n;
my %c;
for ($i = 0; $i < $N; ++$i) {
@x = (1 .. $n);
$k = $i;
for ($j = 0; $j < $n; ++$j) {
$m = $k % $n; $k = ($k - $m) / $n;
($x[$j], $x[$m]) = ($x[$m], $x[$j]);
}
++$c{$x[0]};
}
for (sort {$c{$b}<=>$c{$a}} keys %c) {printf "%d:%d ", $_, $c{$_}}
printf "%4.2f\n", $c{$n}/$c{2};
}'
2:10 1:9 3:8 0.80
2:75 1:64 3:63 4:54 0.72
2:756 3:656 1:625 4:576 5:512 0.68
2:9605 3:8525 1:7776 4:7625 5:6875 6:6250 0.65
2:147498 3:133092 4:120744 1:117649 5:110160 6:101088 7:93312 0.63
The more mathematically astute among you might guess that "0.63"
is close to the limiting ratio, and that the limit is the ever
common "1 - 1/e". So the least frequent host is used at least 63%
as often as the most frequent.
If your DNS server returns address RRs in a fixed order, and you
want Postfix SMTP clients to impose a more uniform load on a pool
of hosts specified via a "round-robin" A record, upgrade to 2.11.0
which will be released this month! Alternatively, configure your
DNS nameserver to shuffle A records.
As noted by others, various SMTP implementations will process
multi-homed (aka round-robin) relay hosts and MX hosts as they see
fit.
--
Viktor.
