vesely at tana
Mar 30, 2008, 1:40 AM
Post #4 of 4
(1201 views)
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Lisa Casey wrote:
> Hi,
>
> I run an ISP. I would like to implement SPF but have a couple of
> questions.
>
> 1. We outsource our dialups, so I have no control over the IP address
> space that our customers are using. We do force the use of SMTP Auth on
> our mail server. This question probably underscores my ignorance of
> SPF, but will the fact that our customers emails come from all sorts of
> IP addresses affect my ability to successfully implement SPF?
No, provided that your SPF filtering software is able to recognize
that a customer has been authenticated. You should skip SPF checking
for authenticated customers, which is consistent with granting relay
authorizations.
> 2. We have several domains, all of which are used by various customers
> as their email address (@jellico.com, @copperhill.com,
> @campbellcounty.com, etc.). I assume I need to setup SPF records for
> each of these. Is this correct?
Yes, it is. You need a good understanding of users mailout settings
and habits. I'd also recommend notifying them about SPF restrictions;
for example, using an email address with a published SPF record with a
non-compliant email postcard machine that a user may casually employ
might unexpectedly break the policy. IMHO, owners/users of a domain
should define its policy themselves; however, that implies they are
acknowledgeable enough for doing it.
Also, consider setting up an SPF record for each IPv4 A record in each
example.com domain of yours, in order to avoid senders like, say,
spammer-abused [at] dns, which is a valid address even if that
host has no MX record and no SMTP server running on it. You may want
to use a command like the following one liner, or variants thereof:
perl -n -e 'if
(m/^([a-z0-9]+)\s+IN\s+A\s+[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\s*$/)
{printf "%-16sIN TXT \"v=spf1 a -all\"\n", $1;}' /your/zone/file
and then append the result to the zone file.
HTH
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