Re: [Uta] General question on draft-ietf-uta-mta-sts-03
Daniel Margolis <dmargolis@google.com> Thu, 30 March 2017 12:45 UTC
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From: Daniel Margolis <dmargolis@google.com>
Date: Thu, 30 Mar 2017 14:45:14 +0200
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To: Gerard.DRAPER-GIL@ec.europa.eu
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Subject: Re: [Uta] General question on draft-ietf-uta-mta-sts-03
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Hi, The point is not to allow larger policy bodies but rather to leverage existing authentication of HTTPS server certificates. We want *some form* of authentication for two reasons: 1. We want it to be relatively hard to induce a DoS (as discussed in "Security Considerations"); requiring a policy to be authenticated helps mitigate that risk slightly. (Otherwise we're creating a variant of the inject-a-bad-MX-record DoS but with a really long TTL!) 2. More importantly, we want to allow recipient domains to update a policy on demand, outside of expirations, so that they can signal a new MX is trusted (and similar). The authentication mechanism need not be HTTPs, and in fact when I started drafting this originally I suggested we just stick the policy + signature + cert chain in a DNS record--but that implies big DNS records, and that people figure out how to fetch and validate them plus the whole cert chain, and ultimately, using HTTPS just seemed a lot easier and well-understood than that. Hopefully that makes sense, but I will note that there are some slight relevancies for the parallel thread going on about SAN vs MX host matching--namely that the motivator for HTTPS (to leverage well-understood existing protocols) is also an argument for host matching, as Alberto has pointed out. Dan On Thu, Mar 30, 2017 at 11:22 AM, <Gerard.DRAPER-GIL@ec.europa.eu> wrote: > After reading the MTA-STS drafts, and following the discussions, there's > still a question that I do not fully understand. > > Why using DNS to "announce" the use of MTA-STS but rely on HTTPS for > publishing the policy? So far, the policy is not that complex. > Wouldn't a DNS TXT record suffice in many cases? > I understand that in some cases the policy may be longer, depending on how > many MX records a domain may have. > Fetching the policy from an HTTPS service could be an option for those who > require longer policy descriptions. > Another option for longer policies could be to add an 'include' parameter, > like in SPF, therefore avoiding the need of another service (HTTPS) to > deliver the policy. > > It may not simplify the work for the implementation of MTA-STS (fetching > from HTTPS would still have to be implemented), but it would make it easier > to deploy (I think). > > As example, the TXT records could look like: > > MTA-STS just DNS: > _mta-sts.example.com IN TXT "v=STSv1; id=20173003110000Z; mode=enforce; > mx=*.example.com; max_age=12345600" > > MTA-STS with DNS and HTTPS: > _mta-sts.example.com IN TXT "v=STSv1; id=20173003110000Z; use=https" > > MTA-STS with DNS and 'include': > _mta-sts.example.com IN TXT "v=STSv1; id=20173003110000Z; max_age=12345600 > include=_mta-sts01,_mta-sts02;" > _mta-sts01.example.com IN TXT "mx=mx1.example.com,mx2.example.com;" > _mta-sts02.example.com IN TXT "mx=*.example1.com;" > > Gerard Draper Gil > > _______________________________________________ > Uta mailing list > Uta@ietf.org > https://www.ietf.org/mailman/listinfo/uta > >
- [Uta] General question on draft-ietf-uta-mta-sts-… Gerard.DRAPER-GIL
- Re: [Uta] General question on draft-ietf-uta-mta-… Daniel Margolis
- Re: [Uta] General question on draft-ietf-uta-mta-… Gerard.DRAPER-GIL
- Re: [Uta] General question on draft-ietf-uta-mta-… Viktor Dukhovni