[GLLUG] DNS help please

[James Roberts]

I am really flaky/stale on DNS and so would much appreciate some advice 
before I shoot myself in the foot - if anyone has a mo.

I have a client with multiple *DSL/Cable connections for connection 
reliability, none of the connections being very reliable because of 
their location.

Their web is hosted by an Asian developer co on Hertzner, mail is 
self-hosted on a single Linux server running qmail mailfront etc.

The have a single A record, and MX, and a PTR at the DSL ISP. This works 
- but I want to retire that ISP.

Their current mail arrangement uses one ADSL connection which we intend 
to retire as they now have a fast (30Mbps+) cable connection, which I 
intend to use as the primary connection.

*I want to do two things*:

1 Move the email to the other ADSL connection they have as a first step

2 After a day or two of stable working set up the (tested working) cable 
connection

3 After a suitable settling period enable email in over both the 
connections.

I have thought of various ways of doing this wrong, and I hope two ways 
of doing it right.

The current entry can be thought of as:
*
theclient.co.uk MX (Mail Exchanger) Priority: 10 mail.theclient.co.uk
mail.theclient.co.uk A (Address) (say) ISP 0 000.000.000.000
*

The secondary existing service is on (say) ISP1 111.111.111.111
The new cable service is (say) ISP2 222.222.222.222

The transitional step would add a record as follows:

*
theclient.co.uk MX (Mail Exchanger) Priority: 10 mail.theclient.co.uk
mail.theclient.co.uk A (Address) 000.000.000.000
mail.theclient.co.uk A (Address) 111.111.111.111
*

Q1: But is it OK to have a PTR from each ISP back to the same A record?


Q2: However, if I want to make email reception failover work over both 
the ADSL (111...) and the cable (222...), which of these would be best? 
I think they achieve the same effect, but the B method allows me more 
control?

Method A


mail.theclient.co.uk A (Address) 222.222.222.222
mail.theclient.co.uk A (Address) 111.111.111.111

theclient.co.uk MX (Mail Exchanger) Priority: 10 mail.theclient.co.uk

Method B
mail.theclient.co.uk A (Address) 111.111.111.111
mail2.theclient.co.uk A (Address) 222.222.222.222

theclient.co.uk MX (Mail Exchanger) Priority: 5 mail2.theclient.co.uk
theclient.co.uk MX (Mail Exchanger) Priority: 10 mail.theclient.co.uk

Any comments from the DNS-literate would be appreciated... and don't 
hesitate to be rude if necessary :)

MeJ