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

Keith Moore <moore@network-heretics.com> Mon, 28 September 2020 12:53 UTC

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To: Laura Atkins <laura@wordtothewise.com>
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From: Keith Moore <moore@network-heretics.com>
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Date: Mon, 28 Sep 2020 08:53:10 -0400
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Subject: Re: [ietf-smtp] EHLO domain validation requirement in RFC 5321
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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'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.

> 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.

(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.)

>> 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.

Keith