Re: [nznog] Continued problems with xtra email

On Sat, September 1, 2007 01:35, Hamish MacEwan wrote:

On 8/31/07, Joe Abley wrote:

...

[A] --------- [B] \ / X / X / \ / [C]

[A], [B] and [C] are different networks on the Internet. I have a priority-0 MX installed at [A], and another one at [B]. The MX at [A] has a lower number in its RDATA than the one at [B] (i.e. the one at [B] is the backup).

Suppose some device at [C] tries to send me mail, and at the time it chooses to attempt delivery, there's a network problem which prevents traffic from getting through. It instead delivers to the backup MX at [B]. There is no network problem between [B] and [A], so mail is forwarded on straight away.

I'm just curious what reasons, and they may be myriad, explain why SMTP will route around the damage using the MX priority, but TCP/IP won't? Ie, I can send mail where I can't send a packet?

smtp isn't routing around the damage per se...c cannot reach a, perhaps because it is blackholed/filtered/etc...so it times-out, and tries the backup, which is b. c can reach b, and delivers the mail to b...b then tries, and delivers the mail to a /joshua -- A common mistake that people make when trying to design something completely foolproof is to underestimate the ingenuity of complete fools. - Douglas Adams -