Re: [ietf-smtp] EHLO domain validation requirement in RFC 5321

Laura Atkins <> Mon, 28 September 2020 13:10 UTC

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From: Laura Atkins <>
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Date: Mon, 28 Sep 2020 14:10:06 +0100
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Subject: Re: [ietf-smtp] EHLO domain validation requirement in RFC 5321
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> On 28 Sep 2020, at 13:53, Keith Moore <> wrote:
> On 9/28/20 8:04 AM, Laura Atkins wrote:
>>> But if the RFC recommends poor practice, it will be harder to change that poor practice, because some people will say "but the RFC says...!"   So the RFC should not recommend poor practice.
>> What is your evidence that it is poor practice? 
>>> If, OTOH, the RFC recommends NOT filtering based on EHLO arguments, then it will be at least a bit easier for operators to stop doing that when they start               seeing that it's a bad idea.
>> What is your evidence that it’s a bad idea?
> I've made those arguments multiple times already.  "evidence" seems like the wrong thing to ask for because this is really a question of design - what choices should be made to allow the email network to continue operating seamlessly and efficiently in the event of widespread use of NAT within the network (either to gateway between IPv4 and IPv6 or to economize use of IPv4 space)?
I’ve seen lots of arguments, yes, but I’ve not seen any real evidence backing up your assertions. I’m trying to better understand your point of view and understand why this is so important. But it’s been hard to find that in the sniping. 

The design question is one that should be discussed, but is this the correct space for that? 
>>> I'm thinking long term here.   I expect 5321bis, if we do our jobs right, to be around for decades.   So its recommendations need to make sense in the long term rather than the short term.
>> That presumes that your recommendation makes sense and that allowing any random NATed machine to connect to the internet and send mail is a good thing. I think we have ample evidence that this is actually an abuse vector and a bad thing.
> I disagree.   At one time NAT was mostly associated with consumer grade routers, therefore NATted mail was unlikely to originate from a mail service provider, and more likely to originate from a compromised PC.   But "carrier grade" or "large scale" NATs are increasingly being used within the network (rather than only at the periphery) in order to maximize use of IPv4 address space in the face of increasing address scarcity.   Various flavors of NAT have also emerged as the likely best way to exchange traffic between IPv6 and IPv4, and their use is also increasing.
I understand carrier grade nat, thank you. 

I was struggling to come up with use cases. I understand the use case now, and better understand why you’re bringing this up thinking about the v6 to v4 NATing going on. 
>> What changes do you see happening that will make this currently good practice become a bad one. 
> The changes I see happening include the increasing scarcity of IPv4 address space and the consequent emergence of IPv6-only network providers using NAT to move packets between IPv4 and IPv6 addressing domains.  I'm also anticipating the need to eventually phase out the public IPv4 Internet altogether.    
Or we could write the BIS to recommend anyone running v4 services also provide v6 services, couldn’t we? Take the burden off the v6 only systems which are likely newer entrants and put it on the ‘old timers’ who’ve been around for decades. 
> (From operators' perspective: how long does it make sense for every network to maintain its IPv4 baggage, just so that email won't be blocked?   At the very least we need input from network operators.)
My experience is that folks running MTAs on IPv6 actually enact stricter technical requirements on those systems. For instance, many IPv4 servers will accept mail without valid rDNS on the sending IP or will accept mail without SPF records, while the IPv6 servers run by the same entities require those things directly. 
>>> It doesn't actually bother me that much if existing operators filter based on EHLO validation as long as they re-evaluate that policy over time.   I expect operators to be pragmatists.   But I really do expect use of NAT64 to increase, and I really think it's unhelpful to network operators if reliable email operation requires them all to maintain static IPv4 addresses and connections to the public IPv4 Internet.   It's silly for email to delay a transition away from IPv4 for this reason.
>> Can you explain this use case in more detail? 
> I'm not sure what I've left out.
This makes things clearer. 

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