On 2013-05-06 18:54:57 -0500, /dev/rob0 wrote:
> On Mon, May 06, 2013 at 11:13:20PM +0200, Vincent Lefevre wrote:
> > On 2013-05-06 01:10:59 -0500, Stan Hoeppner wrote:
> > > On 5/5/2013 8:10 PM, Vincent Lefevre wrote:
> > > > Received: from carotte.tilapin.org (unknown [22.214.171.124])
> > > > by ioooi.vinc17.net (Postfix) with ESMTPS id EFA4959
> > > > for <vincent@...>; Tue, 2 Oct 2012 03:15:23
> > > > +0200 (CEST)
> > > >
> > > > $ host 126.96.36.199
> > > > Host 188.8.131.52.in-addr.arpa. not found: 3(NXDOMAIN)
> > >
> > > ~$ host 184.108.40.206
> > > Host 220.127.116.11.in-addr.arpa. not found: 3(NXDOMAIN)
> > >
> > > ~$ host carotte.tilapin.org
> > > carotte.tilapin.org has address 18.104.22.168
> > >
> > > Not only is rDNS non-existent but the HELO name points to an IP
> > > different than the client IP. It's difficult to FUBAR this more
> > > than it is.
> > AFAIK, there's no requirement in the RFCs that the HELO name point
> > to the client IP, and there are good reasons to allow a mismatch, e.g.
> > due to several machines sharing the same IP with NAT, or a machine
> > having several interfaces (with several IPs), or a laptop that can
> > move between various networks.
> It's not usual, and definitely not ideal, to use NAT on a mail
> exchanger, although a load balancer (which is more common and
> sensible) can have similar effects. Also, a laptop as you describe
> would usually not be in the role of mail exchanger, so its HELO
> should only matter to its MSA.
There's no mail exchanger here. The machine in question
(carotte.tilapin.org) just sends the mail.
> > > > and this is from a Debian developer.
> > >
> > I just meant that
> > * his mail config is probably sane (the fact that the IP doesn't
> > have a rDNS is not his fault, but the ISP's);
> Don't try to run a mail exchanger on a dynamic IP address or one
> lacking FCrDNS. It's definitely his fault for doing so.
Except that the machine is just the client, not a mail exchanger.
> > * one can lose rather important mail (e.g. related to work).
> Yes. Reread Noel's post upthread. I was the one who originally said
> reject_unknown_reverse_client_hostname is safe, and Noel explained
> why: the mail you reject is also being rejected by most major
I don't think this is really true. This may depend on the country
and the people one communicates with. If users still send mail from
an IP without rDNS, there may be a reason...
Moreover some major receivers may support IPv4 only for their MX,
so that if the IPv4 address of the sender has a reverse hostname
but not the IPv6 address, this user may not notice the problem.
For instance, for two majors receivers in France:
$ host -t mx free.fr
free.fr mail is handled by 20 mx2.free.fr.
free.fr mail is handled by 10 mx1.free.fr.
$ host mx1.free.fr
mx1.free.fr has address 22.214.171.124
mx1.free.fr has address 126.96.36.199
$ host mx2.free.fr
mx2.free.fr has address 188.8.131.52
mx2.free.fr has address 184.108.40.206
$ host -t mx wanadoo.fr
wanadoo.fr mail is handled by 10 smtp-in.orange.fr.
$ host smtp-in.orange.fr
smtp-in.orange.fr has address 220.127.116.11
smtp-in.orange.fr has address 18.104.22.168
$ host -t mx vinc17.net
vinc17.net mail is handled by 10 ioooi.vinc17.net.
$ host ioooi.vinc17.net
ioooi.vinc17.net has address 22.214.171.124
ioooi.vinc17.net has IPv6 address 2001:4b98:dc0:45:216:3eff:fe9b:eb2f
So, the sender mentioned above would see no problems with the majors
receivers (free.fr, wanadoo.fr), where IPv4 will be used, but if I
configure Postfix with reject_unknown_reverse_client_hostname on my
domain, the sender in question will see his mail rejected because
IPv6 would be used and his IPv6 address doesn't have a reverse
> Your would-be correspondent has trouble corresponding with
> everyone. Eventually he should figure out that he can't run a mail
> server on a dynamic IP address.
> Sure, you might choose to open your floodgates to these clients. I
> guarantee the vast majority of them are spam zombies.
This is what I can observe, but I was thinking about using
reject_unknown_reverse_client_hostname-like filter with scoring.
> > Anyway one should be able to configure *client*-side mail software
> > without being a specialist of SMTP RFCs and things like that...
> Absolutely. You would have your MUA submit to a MSA. Your MSA would
> not care about FCrDNS.
I could do that since I have my own server.
But I don't see this as a final solution since most users use a
shared MSA and the outgoing mail server may be blacklisted more
or less often (this is the case of my ISP, which is frequently
blacklisted by spamcop) or not reliable (e.g. at my lab, which
has also been blacklisted several times due to some users with
compromised machines). And running a local MSA would yield the
same problems as not using a MSA.
Vincent Lefèvre <vincent@...
> - Web: <http://www.vinc17.net/
100% accessible validated (X)HTML - Blog: <http://www.vinc17.net/blog/
Work: CR INRIA - computer arithmetic / AriC project (LIP, ENS-Lyon)