How best to eliminate domain mismatch warning in mail clients when TLS is used
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
Re: How best to eliminate domain mismatch warning in mail clients when TLS is used
* Ben Johnson b...@indietorrent.org: 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
Re: How best to eliminate domain mismatch warning in mail clients when TLS is used
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.
Re: 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. 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
Re: How best to eliminate domain mismatch warning in mail clients when TLS is used
On Tue, Jul 16, 2013 at 12:47 AM, Ben Johnson b...@indietorrent.org 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
Re: How best to eliminate domain mismatch warning in mail clients when TLS is used
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
Re: How best to eliminate domain mismatch warning in mail clients when TLS is used
(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
Re: How best to eliminate domain mismatch warning in mail clients when TLS is used
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
Re: How best to eliminate domain mismatch warning in mail clients when TLS is used
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
Re: How best to eliminate domain mismatch warning in mail clients when TLS is used
On 16 Jul 2013 03:15, Wietse Venema wie...@porcupine.org 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
Re: How best to eliminate domain mismatch warning in mail clients when TLS is used
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
Re: How best to eliminate domain mismatch warning in mail clients when TLS is used
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
Re: How best to eliminate domain mismatch warning in mail clients when TLS is used
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.
Re: How best to eliminate domain mismatch warning in mail clients when TLS is used
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
Re: How best to eliminate domain mismatch warning in mail clients when TLS is used
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
