North American Network Operators Group|
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Re: is reverse dns required? (policy question)
On Thu, 02 Dec 2004 16:03:55 +0100, Andre Oppermann said:Reverse zone file for 10.0.0.0/24: 188.8.131.52.in-addr.arpa. IN PTR mail.example.com. _send._smtp._srv.184.108.40.206.in-addr.arpa. IN TXT "1"The problem with that is that for *Steven* to benefit from it, *I'd* have to get the appropriate people here to stick in the appropriate stuff in the in-addr.arpa zones for 128.173/16 and 198.82/16. In other words, it suffers from the same deployment problem as SPF records. (Actually, locally, it's harder to deploy because SPF needs one TXT at the top of the zone, which is mostly static and amenable to hand-editing - those __srv records on the other hand are down in zones that are automagically written by software which then needs to be modified to support splatting out the additional TXT record each time...)
You would put in a global wildcard that says no smtp sender here. Only for those boxes being legitimate SMTP to outside senders you'd put in a more specific record as shown above. You probably have to enter some dozen to one hundred servers this way. Sure your reverse zone scripts need some changes but it's only two or three lines. Ideally you could tell your DNS server in the zone file this: _send._smtp._srv.*.*.173.128.in-addr.arpa. IN TXT "0" _send._smtp._srv.*.*.82.198.in-addr.arpa. IN TXT "0" being overidden by more specific information on single IP addresses.
In other news, we discovered that when we published our SPF record, it managed to push the DNS response over 512 bytes, as we already had several TXT records and 5 NS/A records got returned as well - and we got bit by the usual places that don't do TCP/53 or EDNS0. Anybody else hit that one accidentally? (We ended up jettisoning several TXT's and got it down to 410, so no problem now).
SPF and MTAMARK solve two entirely different problem sets. With SFP you designate that certain enumerated hosts are legitimate senders for emails from your *domain*. It does not de-legitimize some other random host on your network sending emails with a different domain (let's say "@merit.edu"). With MTAMARK you designate that certain IP's (hosts) are legitimate SMTP senders within your *network*. Domain doesn't matter here. That way you specify that all those 131'000 other IP's (hosts) on your network are *not* legitimate SMTP senders no matter for which domain. The nice thing with MTAMARK is that even if evil spammer uses SFP too for his $0.99 throw-away domain and puts the IP of one of the zombies of your network into his SFP record he will still get blocked because your MTAMARK record in the reverse zone will say this IP is not a designated SMTP sender. And since the ratio between non-SMTP senders and SMTP senders is very high you simply throw in a catch-all deny and only make a handful of exceptions for the real SMTP senders on your network. MTAMARK gives huge rewards for comparitative little work. The time you'd have to invest to solve the illegitimate SMTP sender problem for your *entire* network is measured in hours: changing the script that autogenerates the reverse zones and traking down all legitimate SMTP senders. But this you already have done and you can simply use the IP addresses from your SFP records. Like I said: as simple as it gets. -- Andre