[Mimedefang] Potential for Business mail servers to not havereverse DNS
john at rudd.cc
Fri Sep 22 11:37:31 EDT 2006
Kevin A. McGrail wrote:
> The consensus, IMO at least but largely driven by AOL's policy, has
> been that a reverse ptr that isn't blank and others as suspect is not
> a completely bad idea. Here is AOL's full policy. The emphasis is mine.
> a.. AOL does *require* that all connecting Mail Transfer Agents have
> established reverse DNS, regardless of whether it matches the domain.
> b.. Reverse DNS must be in the form of a fully-qualified domain name.
> Reverse DNS containing in-addr.arpa are not acceptable, as these are
> merely placeholders for a valid PTR record. Reverse DNS consisting of
> IP addresses are also not acceptable, as they do not correctly
> establish the relationship between domain and IP address.
It's not just AOL policy, it's a strict interpretation of RFC 1912,
# 2.1 Inconsistent, Missing, or Bad Data
# Every Internet-reachable host should have a name. The consequences
# of this are becoming more and more obvious. Many services available
# on the Internet will not talk to you if you aren't correctly
# registered in the DNS.
# Make sure your PTR and A records match. For every IP address, there
# should be a matching PTR record in the in-addr.arpa domain. If a
# host is multi-homed, (more than one IP address) make sure that all IP
# addresses have a corresponding PTR record (not just the first one).
# Failure to have matching PTR and A records can cause loss of Internet
# services similar to not being registered in the DNS at all. Also,
# PTR records must point back to a valid A record, not a alias defined
# by a CNAME. It is highly recommended that you use some software
# which automates this checking, or generate your DNS data from a
# database which automatically creates consistent data.
1) every host on the internet should have a name
2) every IP address should have a PTR record
3) PTR records must point to a valid A record, and not a CNAME record
4) it's a good idea for the A record to match the PTR record (resolve
back to the IP addr)
By "strict interpretation", I mean "enforce all of these as MUST
directives, instead of mere SHOULD directives/suggestions".
My mimedefang-filter enforces these in filter_sender, along with
something like the "c" requirement from AOL's policy (about names that
look dynamic). And I interpret #1 as meaning "must have a name which is
listed in DNS" (in otherwords, it must be the key for an A or CNAME
record). This translates into:
Every system which wishes to deliver email to me must have:
* A PTR record for its IP address ("the connecting IP address").
Failure leads to a temporary rejection of the message.
* The PTR record must resolve to a name which is they key for an A
record. Failure leads to a temporary rejection of the message.
* The A record must have at least 1 IP address which matches the
connecting IP address. Failure leads to a permanent failure of the message.
* The name from the PTR record must not look dynamic. Failure is a
permanent rejection of the message.
* The above restrictions can be avoided via SMTP-AUTH.
I have yet to find out that any such rejection was a valid email message.
More information about the MIMEDefang