How best to eliminate "domain mismatch" warning in mail clients when TLS is used

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How best to eliminate "domain mismatch" warning in mail clients when TLS is used

Ben Johnson
Hello,

We host mail services for a few dozen domains. We will eventually
require TLS for all client connections.

I have reviewed what seems to be the most comprehensive thread on this
subject (
http://postfix.1071664.n5.nabble.com/TLS-SNI-support-td25552.html ) and,
in light of that information, am trying to determine the best course of
action.

In essence, our clients wish to use their own SSL certificates for their
SMTP connections. Given that there are no plans to implement SNI in
Postfix (it seems not to be sufficiently useful to justify the work
involved, which I understand), I am left wondering what the alternative
might be.

Our clients will not accept the position, "You just have to ignore the
'domain mistmatch' warning and accept the certificate permanently when
you connect to the mail server." And I don't blame them.

Also, our clients don't want to create DNS records that contain our
hostname or IP address. The reasons vary, but, in general, our clients
don't want to look "unprofessional" by having a hosting company's domain
name in their DNS records. They want to maintain the appearance that
they handle all of their own I.T. needs. I know, it seems silly, but we
run into this often.

To quote Peter in the above-cited thread:

"I used google apps as an example of a provider that services what
probably amounts to tens or hundreds of thousands of domains for email,
and they do it all with one SSL certificate with only a single common
name.  smtp is not http and it does not work the same, you simply do not
need to have a separate SSL certificate for every domain you host, one
certificate will work for everything."

Sure, one certificate will "work", but won't using one certificate for
all domains cause a "domain mistmatch" warning if the client uses his
own hostname to send mail from within his mail client (and we do not
have a certificate that includes all of our clients' hostnames in the
SubjectAltNames field)? That has certainly been my experience.

I've read over the information at
http://www.postfix.org/TLS_README.html#client_tls_dane several times and
am still trying to digest it fully. The "gist" seems to be that DANE
would require our company's hostname and/or IP address to be present in
the client's DNS records. Some of our clients have stated that they do
not want rDNS look-ups to return records relating to our Web
Design/Development/Hosting company. Again, the rationale for this
usually relates to "maintaining a professional and independent I.T.
presence" (a euphemism for, "we don't want to appear incompetent by
outsourcing our I.T. needs to a third-party").

To quote Viktor from the same thread:

"If you want to host submission for large numbers of vanity domains
on a single MTA, why must the clients be configured to contact
"smtp.vanity-domain.com"? What's wrong with "smtp.provider.net"?"

I've explained the problem in this regard ("domain mismatch" warnings).

We have considered using SubjectAlternativeNames, but we would have to
change our SSL work-flow considerably and spend a lot of money with our
"trusted" friends in the SSL CA business.

Have I missed anything fundamental? What are others doing to address
similar client demands?

Thanks for any pointers,

-Ben
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Re: How best to eliminate "domain mismatch" warning in mail clients when TLS is used

Patrick Ben Koetter-2
* Ben Johnson <[hidden email]>:

> Hello,
>
> We host mail services for a few dozen domains. We will eventually
> require TLS for all client connections.
>
> I have reviewed what seems to be the most comprehensive thread on this
> subject (
> http://postfix.1071664.n5.nabble.com/TLS-SNI-support-td25552.html ) and,
> in light of that information, am trying to determine the best course of
> action.
>
> In essence, our clients wish to use their own SSL certificates for their
> SMTP connections. Given that there are no plans to implement SNI in
> Postfix (it seems not to be sufficiently useful to justify the work
> involved, which I understand), I am left wondering what the alternative
> might be.
>
> Our clients will not accept the position, "You just have to ignore the
> 'domain mistmatch' warning and accept the certificate permanently when
> you connect to the mail server." And I don't blame them.
>
> Also, our clients don't want to create DNS records that contain our
> hostname or IP address. The reasons vary, but, in general, our clients
> don't want to look "unprofessional" by having a hosting company's domain
> name in their DNS records. They want to maintain the appearance that
> they handle all of their own I.T. needs. I know, it seems silly, but we
> run into this often.

In absence of SNI either the MX of all domains point to one MX with a valid
cert or you bring up an instance per domain.

If your clients insist that a mail server is only professional if the TLS
session has their domain name written on it, then give them what they want at
the price it costs to implement it.

Those are the choices and don't mean to start a flame war.

p@rick



--
[*] sys4 AG
 
http://sys4.de, +49 (89) 30 90 46 64
Franziskanerstraße 15, 81669 München
 
Sitz der Gesellschaft: München, Amtsgericht München: HRB 199263
Vorstand: Patrick Ben Koetter, Axel von der Ohe, Marc Schiffbauer
Aufsichtsratsvorsitzender: Florian Kirstein
 
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Re: How best to eliminate "domain mismatch" warning in mail clients when TLS is used

Viktor Dukhovni
In reply to this post by Ben Johnson
On Mon, Jul 15, 2013 at 12:47:53PM -0400, Ben Johnson wrote:

> In essence, our clients wish to use their own SSL certificates for their
> SMTP connections.

Are these submission clients?  What does the above mean?

> Our clients will not accept the position, "You just have to ignore the
> 'domain mismatch' warning and accept the certificate permanently when
> you connect to the mail server." And I don't blame them.

Why are they each using a different name for the same submission
service?

> Also, our clients don't want to create DNS records that contain our
> hostname or IP address.

There is certainly no need for that.  The right server name lies
in your DNS zone.

> The reasons vary, but, in general, our clients
> don't want to look "unprofessional" by having a hosting company's domain
> name in their DNS records.

It would not be in their DNS.  It would be in their submission
client (MUA) configuration.

> They want to maintain the appearance that they handle all of their own I.T.
> needs. I know, it seems silly, but we run into this often.

To their own users or to people sending them email?  3rd party
senders don't see the name of the submission service used between
your clients and their provider.  Politely explain that this cosmetic
preference has a high cost for you and them, and they're better off
without this.

> To quote Peter in the above-cited thread:
>
> "I used google apps as an example of a provider that services what
> probably amounts to tens or hundreds of thousands of domains for email,
> and they do it all with one SSL certificate with only a single common
> name.  smtp is not http and it does not work the same, you simply do not
> need to have a separate SSL certificate for every domain you host, one
> certificate will work for everything."

Quote this to your clients.

> Sure, one certificate will "work", but won't using one certificate for
> all domains cause a "domain mistmatch" warning if the client uses his
> own hostname to send mail from within his mail client (and we do not
> have a certificate that includes all of our clients' hostnames in the
> SubjectAltNames field)? That has certainly been my experience.

The correct configuration of the MUA is to include the right MSA
name.  When in a decade or so, MUAs generally use SRV records to
locate the right MSA for a domain, they can find this MSA via SRV
records, and use DANE to authenticate it.  For now they set the
right server name.

> I've read over the information at
> http://www.postfix.org/TLS_README.html#client_tls_dane several times and
> am still trying to digest it fully. The "gist" seems to be that DANE
> would require our company's hostname and/or IP address to be present in
> the client's DNS records.

No.  And in any case MUAs don't yet do DANE.


> not want rDNS look-ups to return records relating to our Web
> Design/Development/Hosting company. Again, the rationale for this
> usually relates to "maintaining a professional and independent I.T.
> presence" (a euphemism for, "we don't want to appear incompetent by
> outsourcing our I.T. needs to a third-party").

Tell they look even more competent when they sensibly choose a well-reputed
competent provider!

> To quote Viktor from the same thread:
>
> "If you want to host submission for large numbers of vanity domains
> on a single MTA, why must the clients be configured to contact
> "smtp.vanity-domain.com"? What's wrong with "smtp.provider.net"?"
>
> I've explained the problem in this regard ("domain mismatch" warnings).

There is no mismatch when the MUAs are configured to use
"smtp.provider.net" and the MSA has the corresponding certificate.  You're
failing to explain what problem you're seeing.

> Have I missed anything fundamental? What are others doing to address
> similar client demands?

Publish a single client-independent name for your MSA.  Your client
MUAs must use that name.  This works with no domain mismatch or other
warnings.

--
        Viktor.
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Re: How best to eliminate "domain mismatch" warning in mail clients when TLS is used

Wietse Venema
In reply to this post by Ben Johnson
Ben Johnson:

> Hello,
>
> We host mail services for a few dozen domains. We will eventually
> require TLS for all client connections.
>
> I have reviewed what seems to be the most comprehensive thread on this
> subject (
> http://postfix.1071664.n5.nabble.com/TLS-SNI-support-td25552.html ) and,
> in light of that information, am trying to determine the best course of
> action.
>
> In essence, our clients wish to use their own SSL certificates for their
> SMTP connections. Given that there are no plans to implement SNI in
> Postfix (it seems not to be sufficiently useful to justify the work
> involved, which I understand), I am left wondering what the alternative
> might be.
>
> Our clients will not accept the position, "You just have to ignore the
> 'domain mistmatch' warning and accept the certificate permanently when
> you connect to the mail server." And I don't blame them.

You are creating massive confusion because you fail to explain

a) whether you're talking about MUA service or MTA service, and

b) what name is "mismatching" with your SMTP server name, and

c) why the customer is using that name.

        Wietse
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Re: How best to eliminate "domain mismatch" warning in mail clients when TLS is used

Jeffrey 'jf' Lim
In reply to this post by Ben Johnson
On Tue, Jul 16, 2013 at 12:47 AM, Ben Johnson <[hidden email]> wrote:

> Hello,
>
> We host mail services for a few dozen domains. We will eventually
> require TLS for all client connections.
>
> I have reviewed what seems to be the most comprehensive thread on this
> subject (
> http://postfix.1071664.n5.nabble.com/TLS-SNI-support-td25552.html ) and,
> in light of that information, am trying to determine the best course of
> action.
>
> In essence, our clients wish to use their own SSL certificates for their
> SMTP connections. Given that there are no plans to implement SNI in
> Postfix (it seems not to be sufficiently useful to justify the work
> involved, which I understand), I am left wondering what the alternative
> might be.
>
> Our clients will not accept the position, "You just have to ignore the
> 'domain mistmatch' warning and accept the certificate permanently when
> you connect to the mail server." And I don't blame them.
>
> Also, our clients don't want to create DNS records that contain our
> hostname or IP address. The reasons vary, but, in general, our clients
> don't want to look "unprofessional" by having a hosting company's domain
> name in their DNS records. They want to maintain the appearance that
> they handle all of their own I.T. needs. I know, it seems silly, but we
> run into this often.
>
> <snip>
>
> Sure, one certificate will "work", but won't using one certificate for
> all domains cause a "domain mistmatch" warning if the client uses his
> own hostname to send mail from within his mail client (and we do not
> have a certificate that includes all of our clients' hostnames in the
> SubjectAltNames field)? That has certainly been my experience.
>
> I've read over the information at
> http://www.postfix.org/TLS_README.html#client_tls_dane several times and
> am still trying to digest it fully. The "gist" seems to be that DANE
> would require our company's hostname and/or IP address to be present in
> the client's DNS records. Some of our clients have stated that they do
> not want rDNS look-ups to return records relating to our Web
> Design/Development/Hosting company. Again, the rationale for this
> usually relates to "maintaining a professional and independent I.T.
> presence" (a euphemism for, "we don't want to appear incompetent by
> outsourcing our I.T. needs to a third-party").
>

surely even using SNI would not help you in this regard? (rDNS lookup)


> To quote Viktor from the same thread:
>
> "If you want to host submission for large numbers of vanity domains
> on a single MTA, why must the clients be configured to contact
> "smtp.vanity-domain.com"? What's wrong with "smtp.provider.net"?"
>
> I've explained the problem in this regard ("domain mismatch" warnings).
>
> We have considered using SubjectAlternativeNames, but we would have to
> change our SSL work-flow considerably and spend a lot of money with our
> "trusted" friends in the SSL CA business.
>
> Have I missed anything fundamental? What are others doing to address
> similar client demands?
>

I would look into the possibility of using an SNI proxy.

-jf

--
He who settles on the idea of the intelligent man as a static entity
only shows himself to be a fool.

"Every nonfree program has a lord, a master --
and if you use the program, he is your master."
    --Richard Stallman
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Re: How best to eliminate "domain mismatch" warning in mail clients when TLS is used

Ben Johnson
In reply to this post by Patrick Ben Koetter-2


On 7/15/2013 1:03 PM, Patrick Ben Koetter wrote:
> In absence of SNI either the MX of all domains point to one MX with a valid
> cert or you bring up an instance per domain.
>

Bringing-up a Postfix instance per domain would require unique ports (or
a dedicated IP address) for each instance, correct? Seems like a
maintenance nightmare.

> If your clients insist that a mail server is only professional if the TLS
> session has their domain name written on it, then give them what they want at
> the price it costs to implement it.
>

Your position is perfectly reasonable, and is more or less the position
that I've taken on the matter. I just wanted to be sure that there isn't
some panacea that I had overlooked.

In order to give our clients what they want, what are our choices? To
use a SAN certificate that includes each client's domain name?

The most obvious problem with this seems to be that this would leak our
"client list" to the public. That is, it would be trivial to inspect the
certificate and discern the companies for which we host email services.

The second problem is adding new domains to the SAN field as new clients
come online. Presumably, this requires having the certificate re-issued.
Is anyone else using this approach, and if so, does the CA charge you
for each re-issue? Or are you able to add new domains at a whim without
incurring additional costs?

> Those are the choices and don't mean to start a flame war.
>

I appreciate the frankness of your reply; I was looking for a succinct
response, and you provided it.

As final point of note, I realize that it is impossible to avoid having
IP addresses that our company controls present in our clients' DNS
records, unless we issue unique IP addresses to each client. (I had made
a few conflicting statements in my initial post.)

Thank you!

-Ben
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Re: How best to eliminate "domain mismatch" warning in mail clients when TLS is used

Ben Johnson
In reply to this post by Wietse Venema
(Viktor, I'm going to reply to Wietse first, just because his questions
are fewer and I am hoping to clarify the points of confusion before
others reply.)

On 7/15/2013 1:24 PM, Wietse Venema wrote:

> Ben Johnson:
>> Hello,
>>
>> We host mail services for a few dozen domains. We will eventually
>> require TLS for all client connections.
>>
>> I have reviewed what seems to be the most comprehensive thread on this
>> subject (
>> http://postfix.1071664.n5.nabble.com/TLS-SNI-support-td25552.html ) and,
>> in light of that information, am trying to determine the best course of
>> action.
>>
>> In essence, our clients wish to use their own SSL certificates for their
>> SMTP connections. Given that there are no plans to implement SNI in
>> Postfix (it seems not to be sufficiently useful to justify the work
>> involved, which I understand), I am left wondering what the alternative
>> might be.
>>
>> Our clients will not accept the position, "You just have to ignore the
>> 'domain mistmatch' warning and accept the certificate permanently when
>> you connect to the mail server." And I don't blame them.
>
> You are creating massive confusion because you fail to explain
>
> a) whether you're talking about MUA service or MTA service, and
>

The "domain mismatch" occurs in the MUA (e.g., Thunderbird, Apple Mail,
etc.).

> b) what name is "mismatching" with your SMTP server name, and
>

Some of our clients insist that they access the the MTA that handles
their mail, which resides on one of our servers, via their own domain
names. So, these clients are using smtp.client.com instead of
smtp.provider.com. Whether or not this is a "necessary" or "a good idea"
is, unfortunately, largely irrelevant. I have tried to convince clients
that they should be connecting to the submission service via *our*
domain name, and not their own domain name, but have faced nothing but
resistance.

> c) why the customer is using that name.
>

Because the customer wants to "control" his own SSL certificate,
including its renewal, re-issuance, and revocation. This does not seem
like an entirely unreasonable request, to be fair. (I do realize that
this is a "false sense of security", given that the client must relay
all certificate changes through us, since we control the MTA.)

> Wietse
>

I really appreciate your time, Wietse. Thanks for the reply. Let me know
if anything else is unclear.

-Ben
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Re: How best to eliminate "domain mismatch" warning in mail clients when TLS is used

Ben Johnson
In reply to this post by Viktor Dukhovni
On 7/15/2013 1:10 PM, Viktor Dukhovni wrote:
> On Mon, Jul 15, 2013 at 12:47:53PM -0400, Ben Johnson wrote:
>
>> In essence, our clients wish to use their own SSL certificates for their
>> SMTP connections.
>
> Are these submission clients?  What does the above mean?
>

Yes, these are submission clients. To be clear, our clients want to be
able to configure their MUAs to use our MTA's submission service via
their own domain names. I know; it is not necessarily a rational or
reasonable request.

>> Our clients will not accept the position, "You just have to ignore the
>> 'domain mismatch' warning and accept the certificate permanently when
>> you connect to the mail server." And I don't blame them.
>
> Why are they each using a different name for the same submission
> service?
>

This is an excellent question, and a question to which I lack a "good
answer". I have no idea why there is such resistance to using our domain
name. The best I can discern is that our company's clients (as in
"customers" -- not "mail clients") want their own domain names and their
own SSL certificates to be used for TLS connections. As I said in my
reply to Wietse, this is not a logical position to take, given that our
clients must "trust" us, implicitly, as we control the server on which
plaintext copies of their mail are stored. For some reason, this
"control" seems to pacify the decision-makers, and falsely so, of course.

>> Also, our clients don't want to create DNS records that contain our
>> hostname or IP address.
>
> There is certainly no need for that.  The right server name lies
> in your DNS zone.
>
>> The reasons vary, but, in general, our clients
>> don't want to look "unprofessional" by having a hosting company's domain
>> name in their DNS records.
>
> It would not be in their DNS.  It would be in their submission
> client (MUA) configuration.
>

Right, but only provided that the customer can live with its own users
configuring their MUAs to use our (the hosting company's) domain for
submission.

>> They want to maintain the appearance that they handle all of their own I.T.
>> needs. I know, it seems silly, but we run into this often.
>
> To their own users or to people sending them email?  3rd party
> senders don't see the name of the submission service used between
> your clients and their provider.  Politely explain that this cosmetic
> preference has a high cost for you and them, and they're better off
> without this.

To their own users. I agree with you completely; and that's exactly what
it is: a cosmetic appearance that carries a high cost while providing no
real value.

>
>> To quote Peter in the above-cited thread:
>>
>> "I used google apps as an example of a provider that services what
>> probably amounts to tens or hundreds of thousands of domains for email,
>> and they do it all with one SSL certificate with only a single common
>> name.  smtp is not http and it does not work the same, you simply do not
>> need to have a separate SSL certificate for every domain you host, one
>> certificate will work for everything."
>
> Quote this to your clients.

I will do that.

>
>> Sure, one certificate will "work", but won't using one certificate for
>> all domains cause a "domain mistmatch" warning if the client uses his
>> own hostname to send mail from within his mail client (and we do not
>> have a certificate that includes all of our clients' hostnames in the
>> SubjectAltNames field)? That has certainly been my experience.
>
> The correct configuration of the MUA is to include the right MSA
> name.  When in a decade or so, MUAs generally use SRV records to
> locate the right MSA for a domain, they can find this MSA via SRV
> records, and use DANE to authenticate it.  For now they set the
> right server name.
>

Understood.

>> I've read over the information at
>> http://www.postfix.org/TLS_README.html#client_tls_dane several times and
>> am still trying to digest it fully. The "gist" seems to be that DANE
>> would require our company's hostname and/or IP address to be present in
>> the client's DNS records.
>
> No.  And in any case MUAs don't yet do DANE.
>
>
>> not want rDNS look-ups to return records relating to our Web
>> Design/Development/Hosting company. Again, the rationale for this
>> usually relates to "maintaining a professional and independent I.T.
>> presence" (a euphemism for, "we don't want to appear incompetent by
>> outsourcing our I.T. needs to a third-party").
>
> Tell they look even more competent when they sensibly choose a well-reputed
> competent provider!
>

Yours is a fair argument, indeed. :) I sympathize.

>> To quote Viktor from the same thread:
>>
>> "If you want to host submission for large numbers of vanity domains
>> on a single MTA, why must the clients be configured to contact
>> "smtp.vanity-domain.com"? What's wrong with "smtp.provider.net"?"
>>
>> I've explained the problem in this regard ("domain mismatch" warnings).
>
> There is no mismatch when the MUAs are configured to use
> "smtp.provider.net" and the MSA has the corresponding certificate.  You're
> failing to explain what problem you're seeing.
>

You're right; as long as the MUAs are configured to use
smtp.provider.net, there is no issue.

The challenge for me is explaining why MUAs must be configured with
ourhostingcompany.com and not the customer's own domain name.

>> Have I missed anything fundamental? What are others doing to address
>> similar client demands?
>
> Publish a single client-independent name for your MSA.  Your client
> MUAs must use that name.  This works with no domain mismatch or other
> warnings.
>

This seems to be the only course of action. And trust me, I do not
undertake this route begrudgingly. Everything that you and the other
respondents have said makes perfect sense, from a logical and technical
perspective. Unfortunately, the majority of folks who depend on people
in our professional is neither technical nor logical (at least where
technology is concerned), so this will be a bit of a "tough sell". But
this thread has given me additional ammo to throw at the challenge.

I appreciate all of the expert insight, Viktor, and everyone else who
responded.

Best regards,

-Ben
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Re: How best to eliminate "domain mismatch" warning in mail clients when TLS is used

Wietse Venema
Ben Johnson:

> On 7/15/2013 1:10 PM, Viktor Dukhovni wrote:
> > On Mon, Jul 15, 2013 at 12:47:53PM -0400, Ben Johnson wrote:
> >
> >> In essence, our clients wish to use their own SSL certificates for their
> >> SMTP connections.
> >
> > Are these submission clients?  What does the above mean?
> >
>
> Yes, these are submission clients. To be clear, our clients want to be
> able to configure their MUAs to use our MTA's submission service via
> their own domain names. I know; it is not necessarily a rational or
> reasonable request.

It's entirely reasonable if they want to be able to change email
provider without having to update all their clients.

Unfortunately there are not a lot of development cycles for adding
a decent SNI implementation to Postfix.

        Wietse
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Re: How best to eliminate "domain mismatch" warning in mail clients when TLS is used

Jeffrey 'jf' Lim

On 16 Jul 2013 03:15, "Wietse Venema" <[hidden email]> wrote:
>
> Ben Johnson:
> > On 7/15/2013 1:10 PM, Viktor Dukhovni wrote:
> > > On Mon, Jul 15, 2013 at 12:47:53PM -0400, Ben Johnson wrote:
> > >
> > >> In essence, our clients wish to use their own SSL certificates for their
> > >> SMTP connections.
> > >
> > > Are these submission clients?  What does the above mean?
> > >
> >
> > Yes, these are submission clients. To be clear, our clients want to be
> > able to configure their MUAs to use our MTA's submission service via
> > their own domain names. I know; it is not necessarily a rational or
> > reasonable request.
>
> It's entirely reasonable if they want to be able to change email
> provider without having to update all their clients.
>
> Unfortunately there are not a lot of development cycles for adding
> a decent SNI implementation to Postfix.
>

What about using an SNI proxy? Would u have any to recommend?

-jf

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Re: How best to eliminate "domain mismatch" warning in mail clients when TLS is used

Ben Johnson
In reply to this post by Wietse Venema


On 7/15/2013 3:14 PM, Wietse Venema wrote:

> Ben Johnson:
>> On 7/15/2013 1:10 PM, Viktor Dukhovni wrote:
>>> On Mon, Jul 15, 2013 at 12:47:53PM -0400, Ben Johnson wrote:
>>>
>>>> In essence, our clients wish to use their own SSL certificates for their
>>>> SMTP connections.
>>>
>>> Are these submission clients?  What does the above mean?
>>>
>>
>> Yes, these are submission clients. To be clear, our clients want to be
>> able to configure their MUAs to use our MTA's submission service via
>> their own domain names. I know; it is not necessarily a rational or
>> reasonable request.
>
> It's entirely reasonable if they want to be able to change email
> provider without having to update all their clients.
>

This is the strongest argument that I've seen for adding SNI support to
Postfix. I hadn't even considered this. Maybe this is the basis for our
customers' respective positions; I wish they had made it clearer to
begin with.

> Unfortunately there are not a lot of development cycles for adding
> a decent SNI implementation to Postfix.
>
> Wietse
>

I can't even imagine the complexities; I understand.

In the meantime, I am all ears, regarding jf's question about SNI
proxying via, for example, nginx. If that subject is best addressed to
the nginx mailing list, I am happy to take the discussion to the
appropriate list.

Thanks again,

-Ben
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Re: How best to eliminate "domain mismatch" warning in mail clients when TLS is used

Wietse Venema
Ben Johnson:
> In the meantime, I am all ears, regarding jf's question about SNI
> proxying via, for example, nginx. If that subject is best addressed to
> the nginx mailing list, I am happy to take the discussion to the
> appropriate list.

According to a thread in March 2013 they did not support SNI in the
MAIL feature.

http://list-archives.org/2013/03/29/nginx-nginx-org/mail-proxy-with-sni/f/1433768745

        Wietse
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Re: How best to eliminate "domain mismatch" warning in mail clients when TLS is used

Viktor Dukhovni
In reply to this post by Ben Johnson
On Mon, Jul 15, 2013 at 03:38:31PM -0400, Ben Johnson wrote:

> > It's entirely reasonable if they want to be able to change email
> > provider without having to update all their clients.
>
> This is the strongest argument that I've seen for adding SNI support to
> Postfix. I hadn't even considered this. Maybe this is the basis for our
> customers' respective positions; I wish they had made it clearer to
> begin with.

There's a lot more to SNI support than having a server that can
context-switch between multiple certificates.  You need a provisioning
system that allows clients to upload private keys and matching
certificates on a self-service basis via suitably authorized
administrator accounts.

You need to send the administrators reminders about iminent
certificate expiration, and alert your staff if they don't respond
promptly, so they ultimately get phone calls when they don't act
in a timely manner.

The whole thing is a major PITA for very little gain.

> > Unfortunately there are not a lot of development cycles for adding
> > a decent SNI implementation to Postfix.

I have no time for this.

--
        Viktor.
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Re: How best to eliminate "domain mismatch" warning in mail clients when TLS is used

Stan Hoeppner
On 7/15/2013 3:35 PM, Viktor Dukhovni wrote:

>>> Unfortunately there are not a lot of development cycles for adding
>>> a decent SNI implementation to Postfix.
>
> I have no time for this.

And this is precisely why an entire VPS industry has sprouted over the
past few years.  As someone stated down thread, give your customers what
they want and charge them accordingly.  This is trivially easy to do
with your choice of hypervisor with memory consolidation (same page
merging) and a guest OS template.  If your pool of IPv4 addresses is
limited charge them extra for that.  If they're exhausted, well, you can
go IPv6 only but that really has downsides.

Here's an even better idea.  Do what everyone else in your shoes does:
partner with a VPS provider and farm the bulk of this out.  There are
tons of small companies that do exactly this.  They buy X number of VPS
instances each with an IP address from a provider and rebrand them.  The
VPS provider does all the heavy lifting WRT provisioning.  You would
simply do the customization for your individual customers, i.e. DNS,
hostname, domain name, certificate, etc.  A basic VPS for this kind of
thing can normally be had for well less than $10/month.  The really
stripped down VPS services I see offered are $4.95/month.  All prices
USD.  If you already have a mailbox (IMAP/POP) server that currently
handles MX duty for all these customers, moving MX to these VPS
instances and relaying the mail to your mailbox server is easy as well.

--
Stan

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Re: How best to eliminate "domain mismatch" warning in mail clients when TLS is used

Peter Ajamian
In reply to this post by Ben Johnson
On 07/16/2013 05:30 AM, Ben Johnson wrote:
>> If your clients insist that a mail server is only professional if the TLS
>> session has their domain name written on it, then give them what they want at
>> the price it costs to implement it.
>
> Your position is perfectly reasonable, and is more or less the position
> that I've taken on the matter. I just wanted to be sure that there isn't
> some panacea that I had overlooked.
>
> In order to give our clients what they want, what are our choices?

Probably the best option is to go old tech here.  Get a separate IP for
each hostname that a client wants to connect to and set up separate
listeners in master.cf for each of those IPs with the appropriate TLS
options.  Then let the clients buy their own cert and provide it to you
to use on the server.  Up to you to come up with the additional pricing
for all of this.  The extra dedicated IP is the first and most obvious
cost, the rest is administrative.

Keep in mind that you'll have to configure dovecot (or whatever you use
for IMAP/POP3) to listen on these other IPs and use those
customer-supplied certs as well.

Personally I would ramp up the extra fee even more to account for the,
"I don't want to do this really stupid unnecessary vain thing" reason.
I would make sure the client knows that they are just spending extra
money to satisfy their own vanity and if they still want to go ahead
then do it for them.


Peter