[ale] more reverse DNS questions
Michael H. Warfield
mhw at WittsEnd.com
Mon Jan 9 14:52:02 EST 2012
On Mon, 2012-01-09 at 19:30 +0000, Lightner, Jeff wrote:
> If you notice I put "generic" in quotes - that was to signify it was
> their definition of the word rather than mine. I've refused to change
> my arpa entries just to satisfy one site's definition.
Noted and concur fully. Between your post and Bob's post I wanted to
make a point that there are several ways reverse DNS is being used and
abused in this way. Thanks for raising the point!
Regards,
Mike
> -----Original Message-----
> From: ale-bounces at ale.org [mailto:ale-bounces at ale.org] On Behalf Of Michael H. Warfield
> Sent: Monday, January 09, 2012 10:57 AM
> To: Atlanta Linux Enthusiasts
> Cc: mhw at wittsend.com
> Subject: Re: [ale] more reverse DNS questions
>
> On Mon, 2012-01-09 at 15:00 +0000, Lightner, Jeff wrote:
> > There are even some places (Paypal I think is one) that will reject
> > email if they consider the reverse to be "generic". (i.e. some folks
> > will put in a reverse range like xx.xx.xx.xx.arpa.domain and this is
> > considered "generic")
>
> Well, it's not exactly "generic". I think Paypal and others are looking
> for patterns that indicate broadband or dsl or dialup ranges. They'll
> look for static names (like ppp or dialup or dsl or broadband) where the
> simple host name is some common constant or autogenerated names where it
> appears the simple host name contains either some form of the address
> (like replacing dots with dashes or underscores) or has the last octet
> of the address.
>
> That's actually more common than the "must have a reverse and the
> reverse result must resolve back to the original" consistency check that
> Bob was mentioning. The later requires a second DNS lookup. I've seen
> a variety of spoofing and broadband checks in various packages like
> MailScanner and such and we've seen problems from various providers like
> AT&T, Yahoo, and PayPal / EBay. I've actually seen circumstances where
> a pattern name indicating a home connecting gets you rejected more often
> than if you have no reverse lookup.
>
> Best circumstance is if you have a valid reverse lookup that points back
> to your server AND the RRs for your server also includes and MX record
> that points at your server (not required but some sites are checking and
> adding to scoring +- and it's recommended as a best practice). Best
> circumstances are not common circumstances in practice. In the real
> world, they still accept things pretty commonly.
>
> An example is a site I take care of there near Emory, Callanwolde Fine
> Arts Center. Their primary E-Mail server gryphon.callanwolde.org on
> 65.15.91.131. Forward and reverse lookups are as follows...
>
> [mhw at canyon ~]$ host gryphon.callanwolde.org
> gryphon.callanwolde.org has address 65.15.91.131
> gryphon.callanwolde.org mail is handled by 10 gryphon.callanwolde.org.
> [mhw at canyon ~]$ host 65.15.91.131
> 131.91.15.65.in-addr.arpa domain name pointer adsl-065-015-091-131.sip.asm.bellsouth.net.
> [mhw at canyon ~]$ host adsl-065-015-091-131.sip.asm.bellsouth.net.
> adsl-065-015-091-131.sip.asm.bellsouth.net has address 65.15.91.131
>
> So that fits Bob's comments and still would identify it as a common dsl
> range. Yet, we very rarely have any problems. There have been problems
> with AT&T and Yahoo both, but they were unrelated to the reverse DNS and
> quickly resolved through their support contacts.
>
> BUT! I also have them set up with DKIM Domain Keys and their outbound
> E-Mail gets signed with their DKIM keys which can be validated under
> DNS. I can't say if the sites and services which validate and accept
> them now with a recognizable DSL based address would accept them if that
> level of verification didn't exist. Several of the antispam packages
> include DKIM in their scoring as well.
>
> Regards,
> Mike
>
> > -----Original Message-----
> > From: ale-bounces at ale.org [mailto:ale-bounces at ale.org] On Behalf Of Bob Toxen
> > Sent: Saturday, January 07, 2012 12:42 PM
> > To: mhw at wittsend.com; Atlanta Linux Enthusiasts
> > Subject: Re: [ale] more reverse DNS questions
> >
> > The important thing, if you are sending email out, is that there IS a
> > Reverse DNS record pointing to some Fully Qualified Host Name and that
> > there is a normal DNS record for that Fully Qualified Host Name that
> > points back to the IP of the original system.
> >
> > Many good spam filters will (and should) dump email without those two
> > DNS records!
> >
> > Thanks to MikeW for his detailed analysis!
> >
> > Bob Toxen
> > bob at verysecurelinux.com [Please use for email to me]
> > http://www.verysecurelinux.com [Network&Linux security consulting]
> > http://www.realworldlinuxsecurity.com [My book:"Real World Linux Security 2/e"]
> > Quality Linux & UNIX security and SysAdmin & software consulting since 1990.
> > Quality spam and virus filters.
> >
> > "One disk to rule them all, One disk to find them. One disk to bring
> > them all and in the darkness grind them. In the Land of Redmond where
> > the shadows lie...and the Eye is everwatching"
> > -- The Silicon Valley Tarot Henrique Holschuh with ... Bob
> >
> > On Fri, Jan 06, 2012 at 05:03:55PM -0500, Michael H. Warfield wrote:
> > > On Fri, 2012-01-06 at 15:00 -0600, John Heim wrote:
> > > > Last week I was asking about using godaddy for DNS. Well, I finally figured
> > > > out how to get godaddy to let us run our own DNS server. Toward the bottom
> > > > of the page for setting nameservers is a button called "Host summary". You
> > > > have to enter the names and IP addresses of your nameservers on that page
> > > > before you can enter them onto the list of nameservers for your domain. So I
> > > > did that and now we are up and running.
> > >
> > > > Well, except for one thing. you can't do a reverse lookup on the IP address
> > > > of our virtual machine. There is nothing I can do about that, right?
> > >
> > > Probably not.
> > >
> > > > This is
> > > > a vm that is donated to us by a local web services company.
> > >
> > > It's doubtful they even control their own reverse DNS.
> > >
> > > > Its one thing
> > > > to tell the world that www.iavit.org goes to 66.170.20.226. That can't mess
> > > > anything up. But you can't have just anybody doing it the other way around.
> > > > If that was something anybody could do, the internet could be severaly
> > > > messed up.
> > >
> > > No, that's not quite the situation. In fact, the reverse DNS is MORE
> > > often thoroughly messed it. In general, it's a morass. It's an
> > > entirely separate hierarchy and structure under the .arpa. TLD
> > > (in-addr.arpa. for IPv4 and ip6.arpa. for IPv6).
> > >
> > > No, the problem is in its structure and how it's organized and
> > > delegated. I have a large enough block of addresses that I have my own
> > > delegated nameservers for that namespace that are actually registered
> > > with ARIN.
> > >
> > > If you do a lookup on my web server you'll get this forward:
> > >
> > > [mhw at canyon ~]$ host www.wittsend.com
> > > www.wittsend.com has address 130.205.32.81
> > >
> > > If you do the reverse, you get this:
> > >
> > > [mhw at canyon ~]$ host 130.205.32.81
> > > 81.32.205.130.in-addr.arpa domain name pointer amethyst.wittsend.com.
> > >
> > > (You'll notice they don't match - Amethyst is the canonical name of that
> > > server - they don't have to match).
> > >
> > > Now you see what was actually looked up was "81.32.205.130.in-addr.arpa"
> > > looking for a pointer (PTR) record. But that record has to exist as an
> > > individual resource record in the "32.205.130.in-addr.arpa" zone (drop
> > > the 81. on the left). So I have a zone file for that entire block of
> > > 256 addresses. Here are a few of the records from that file:
> > >
> > > $ORIGIN 32.205.130.in-addr.arpa.
> > > ;
> > >
> > > ;
> > > 74 IN PTR slate.wittsend.com.
> > > 75 IN PTR ruby.wittsend.com.
> > > 76 IN PTR emerald.wittsend.com.
> > > 77 IN PTR perl.wittsend.com.
> > > 78 IN PTR basalt.wittsend.com.
> > > 79 IN PTR granite.wittsend.com.
> > > 80 IN PTR mercury.wittsend.com.
> > > 81 IN PTR amethyst.wittsend.com.
> > > 82 IN PTR mica.wittsend.com.
> > > 83 IN PTR dark-tower.wittsend.com.
> > >
> > > So, you see, that's all being managed in one spot under one zone shared
> > > for all those addresses. That would normally be delegated from the zone
> > > above it, 205.130.in-addr.arpa in this case, either by using NS
> > > delegation records in the parent zone or having the parent do zone
> > > transfers and be authoritative for the child, or reside on the same name
> > > server and have the child essentially inherit the parent's delegation.
> > > There are standards and ways of delegation out partial zones that don't
> > > fall on an 8 bit boundary but not all ISPs and block owners support that
> > > (I don't need to :-P).
> > >
> > > If you had a cooperative ISP, they could either update the PTR record
> > > for you (most common, if they are willing at all) OR change the PTR
> > > record to one or more NS records pointing at your name server like this:
> > >
> > > 81 IN NS ns1.wittsend.com.
> > > IN NS ns2.wittsend.com.
> > >
> > > Then you would have a zone file containing just the 81 PTR record and
> > > could update it to your hearts content. BUT! If you use that same name
> > > server as your caching name server (poor but very common practice) it
> > > won't know to refer the OTHER reverse records back to the parent and you
> > > could create a mess for yourself. Not a good idea and you probably
> > > couldn't get the ISP to agree to it without a very good reason and proof
> > > you had a good grasp on DNS configurations and gotchas.
> > >
> > > > So if I understand the way the internet works, I'm going to have to go to
> > > > the company that donated the virtual machine and get them to contact their
> > > > ISP on our behalf. Is that correct?
> > >
> > > More or less, correct. You have to go to the owner of that netblock,
> > > most likely the ISP, and possibly have to chase down who is responsible
> > > for the nameserver that is authoritative for the /24 containing your
> > > address. Most people will just shrug and go "why bother".
> > >
> > > Based on the address you listed in your A records below, I did a quick
> > > check on the SOA for the parent zone like this:
> > >
> > > [mhw at canyon ~]$ dig -t SOA 20.170.66.in-addr.arpa
> > >
> > > ; <<>> DiG 9.7.4-P1-RedHat-9.7.4-2.P1.fc14 <<>> -t SOA 20.170.66.in-addr.arpa
> > > ;; global options: +cmd
> > > ;; Got answer:
> > > ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 37261
> > > ;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 2, ADDITIONAL: 0
> > >
> > > ;; QUESTION SECTION:
> > > ;20.170.66.in-addr.arpa. IN SOA
> > >
> > > ;; ANSWER SECTION:
> > > 20.170.66.in-addr.arpa. 3600 IN SOA dns1.supranet.net. hostmaster.supranet.net. 2012010300 1800 900 604800 300
> > >
> > > ;; AUTHORITY SECTION:
> > > 20.170.66.in-addr.arpa. 86380 IN NS dns2.supranet.net.
> > > 20.170.66.in-addr.arpa. 86380 IN NS dns1.supranet.net.
> > >
> > > ;; Query time: 85 msec
> > > ;; SERVER: 127.0.0.1#53(127.0.0.1)
> > > ;; WHEN: Fri Jan 6 16:45:03 2012
> > > ;; MSG SIZE rcvd: 137
> > >
> > > Now... What you are REALLY interested in is in the "AUTHORITY SECTION"
> > > where you see two NS records. Those are the nameservers that have to be
> > > updated for the reverse DNS to be changed. You would have to contact
> > > Supranet.net. In the "ANSWER SECTION" also shows dns1.supranet.net. as
> > > the "primary" authoritative name server (but it can also be some slave
> > > off of a hidden master). You need to get to someone who can update the
> > > master authoritative nameserver for that zone.
> > >
> > > Doing a "whois 66.170.20.226" confirms Supranet as controlling that
> > > block and returns some contact information like this E-Mail address:
> > >
> > > hostmaster at supranet.net
> > >
> > > Which agrees with the pseudo E-Mail address in the SOA (the '@' is
> > > replaced by a '.' in the SOA).
> > >
> > > Honestly... Good luck with that. That block is a delegated /19. So,
> > > basically 32 of the /24 zone blocks like I was describing. Some ISPs
> > > will do it and some of the just couldn't be bothered, especially if it's
> > > an automatically generated zone and this would require manual
> > > intervention (not saying it is, not saying it's not).
> > >
> > > In you're particular case, they don't even have a PTR record in place
> > > for you:
> > >
> > > [mhw at canyon ~]$ host 66.170.20.226
> > > Host 226.20.170.66.in-addr.arpa. not found: 3(NXDOMAIN)
> > >
> > > So they would have to add a record and they may not even have a zone
> > > file for that /24 block to begin with.
> > >
> > > > PS: Here is my forward lookup zone file. Can anybody tell me if I've done
> > > > anything wrong?
> > > >
> > > > $TTL 86400 ; 24 hours could have been written as 24h or 1d
> > > > $ORIGIN iavit.org.
> > > > @ IN SOA iavit.iavit.org. hostmaster at iavit.org. (
> > > > 2011062601 ; serial
> > > > 3H ; refresh
> > > > 15 ; retry
> > > > 1w ; expire
> > > > 3h ; minimum
> > > > )
> > > > ;define name servers on domain
> > > > IN NS ns1
> > > > iavit.org. IN TXT "v=spf1 mx ~all"
> > > > IN MX 10 mailhost
> > > > IN A 66.170.20.226
> > > > iavit IN A 66.170.20.226
> > > > lists IN A 66.170.20.226
> > > > ns1 IN A 66.170.20.226
> > > > ns2 IN A 66.170.20.226
> > > > mailhost IN CNAME iavit
> > > > wiki IN CNAME iavit
> > > > www IN CNAME iavit
> > > >
> > >
> > > That "minimum" parameter really serves a different purpose than what it
> > > was originally defined for. In the distant past it was the default or
> > > minimum TTL applied to resource records (RR) but that is now handled by
> > > the $TTL directive, which you have there set to 1 day. Since every RR
> > > that is contained in a reply has a TTL specified, it wasn't necessary to
> > > also have a minimum TTL in the SOA. That was redefined years ago to be
> > > the negative TTL. That comes into play if someone requests a
> > > non-existent record and get an NXDOMAIN (no such domain or FQDN) error
> > > and basically returns no resource records for the query. The negative
> > > TTL in the SOA tells the cachers how long to remember that there was no
> > > records returned in the response. 3 hours is a rather low value for
> > > that unless you're planning on adding a lot of records as time goes on
> > > which may get queried and NX cached before you have them ready. I would
> > > set that to a day as well.
> > >
> > > Regards,
> > > Mike
> > > --
> > > Michael H. Warfield (AI4NB) | (770) 985-6132 | mhw at WittsEnd.com
> > > /\/\|=mhw=|\/\/ | (678) 463-0932 | http://www.wittsend.com/mhw/
> > > NIC whois: MHW9 | An optimist believes we live in the best of all
> > > PGP Key: 0x674627FF | possible worlds. A pessimist is sure of it!
> >
> >
> >
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> --
> Michael H. Warfield (AI4NB) | (770) 985-6132 | mhw at WittsEnd.com
> /\/\|=mhw=|\/\/ | (678) 463-0932 | http://www.wittsend.com/mhw/
> NIC whois: MHW9 | An optimist believes we live in the best of all
> PGP Key: 0x674627FF | possible worlds. A pessimist is sure of it!
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--
Michael H. Warfield (AI4NB) | (770) 985-6132 | mhw at WittsEnd.com
/\/\|=mhw=|\/\/ | (678) 463-0932 | http://www.wittsend.com/mhw/
NIC whois: MHW9 | An optimist believes we live in the best of all
PGP Key: 0x674627FF | possible worlds. A pessimist is sure of it!
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