You left out yahoo, google, comcast, att, and the broad range of
non-US-based major ISPs.
Oh, sorry. Here you go. As a free bonus, I did Hotmail, too. I'm
modestly (but not hugely) surprised that every one of these uses at
least one level of CNAME. As you may recall, putting something at a
subdomain of a CNAME is rather naughty, so where do you plan to put
your TXT record?
$ dig pop.comcast.net a
;; ANSWER SECTION:
pop.comcast.net. 7200 IN CNAME mail.comcast.net.
mail.comcast.net. 900 IN CNAME mail.g.comcast.net.
mail.g.comcast.net. 300 IN A 76.96.30.119
; AT&T DSL POP server
; the first CNAME is also the name used for Yahoo's own POP service
$ dig pop.sbc.yahoo.com a
;; ANSWER SECTION:
pop.sbc.yahoo.com. 1798 IN CNAME pop.mail.yahoo.com.
pop.mail.yahoo.com. 598 IN CNAME pop.plus.mail.yahoo.com.
pop.plus.mail.yahoo.com. 298 IN CNAME pop.plus.mail.fy4.b.yahoo.com.
pop.plus.mail.fy4.b.yahoo.com. 300 IN A 206.190.53.11
$ dig pop.aol.com a
;; ANSWER SECTION:
pop.aol.com. 3600 IN CNAME open.imap.aol.com.
open.imap.aol.com. 3600 IN CNAME open.gopen.imap.aol.com.
open.gopen.imap.aol.com. 4 IN A 205.188.200.149
$ dig imap.aol.com a
;; ANSWER SECTION:
imap.aol.com. 501 IN CNAME open.imap.aol.com.
open.imap.aol.com. 3016 IN CNAME open.gopen.imap.aol.com.
open.gopen.imap.aol.com. 10 IN A 64.12.168.246
$ dig imap.gmail.com a
;; ANSWER SECTION:
imap.gmail.com. 146 IN CNAME gmail-imap.l.google.com.
gmail-imap.l.google.com. 146 IN A 74.125.93.109
gmail-imap.l.google.com. 146 IN A 74.125.93.111
; POP for Google, also for Virgin.net, the largest cableco in the UK
$ dig pop.googlemail.com a
;; ANSWER SECTION:
pop.googlemail.com. 600 IN CNAME googlemail-pop.l.google.com.
googlemail-pop.l.google.com. 300 IN A 74.125.91.16
; POP server for Hotmail
$ dig pop3.live.com a
;; ANSWER SECTION:
pop3.live.com. 2508 IN CNAME pop3.hot.glbdns.microsoft.com.
pop3.hot.glbdns.microsoft.com. 27 IN A 65.55.32.247
; POP server for BT Internet, the largest ISP in the UK, with mail hosted by
Yahoo
$ dig mail.btinternet.com a
;; ANSWER SECTION:
mail.btinternet.com. 600 IN CNAME pop-smtp.bt.mail.yahoo.com.
pop-smtp.bt.mail.yahoo.com. 1800 IN CNAME
pop-smtp.bt.mail.fy5.b.yahoo.com.
pop-smtp.bt.mail.fy5.b.yahoo.com. 300 IN A 217.12.13.134
pop-smtp.bt.mail.fy5.b.yahoo.com. 300 IN A 217.146.188.192
Here's two different domains at the largest ISP in France that use
different A records to point to the same set of POP servers. I don't
know whether these are only names that customers use to refer to those
servers. From what I've seen of France Telecom, I wouldn't count on
them knowing, either.
$ dig pop.orange.fr a
;; ANSWER SECTION:
pop.orange.fr. 514 IN A 193.252.23.65
pop.orange.fr. 514 IN A 80.12.242.8
pop.orange.fr. 514 IN A 80.12.242.14
pop.orange.fr. 514 IN A 80.12.242.51
pop.orange.fr. 514 IN A 80.12.242.60
pop.orange.fr. 514 IN A 80.12.242.90
pop.orange.fr. 514 IN A 80.12.242.143
pop.orange.fr. 514 IN A 80.12.242.149
pop.orange.fr. 514 IN A 193.252.22.85
pop.orange.fr. 514 IN A 193.252.22.90
$ dig pop.wanadoo.fr a
;; ANSWER SECTION:
pop.wanadoo.fr. 600 IN A 80.12.242.8
pop.wanadoo.fr. 600 IN A 80.12.242.14
pop.wanadoo.fr. 600 IN A 80.12.242.51
pop.wanadoo.fr. 600 IN A 80.12.242.60
pop.wanadoo.fr. 600 IN A 80.12.242.90
pop.wanadoo.fr. 600 IN A 80.12.242.143
pop.wanadoo.fr. 600 IN A 80.12.242.149
pop.wanadoo.fr. 600 IN A 193.252.22.85
pop.wanadoo.fr. 600 IN A 193.252.22.90
pop.wanadoo.fr. 600 IN A 193.252.23.65
And for reference, you are postulating that the DNS-based query is
hugely expensive for those providers using your scenario, but I'm not
convinced that it does.
No, I'm pointing out that the DNS configuration would often be much,
much more complicated than adding a single TXT record per server, if
it's even possible to do, and is therefore much, much harder to get
right.
You appear to be postulating that the DNS query would fail. Please
explain how and in what cases.
See many prior messages.
R's,
John
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