Re: [webfinger] [apps-discuss] Mail client configuration via WebFinger

Marten Gajda <> Thu, 14 July 2016 22:09 UTC

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Subject: Re: [webfinger] [apps-discuss] Mail client configuration via WebFinger
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Hi Paul,

please see my comments below:

Am 14.07.2016 um 08:19 schrieb Paul E. Jones:
> Let's make sure the new draft is scoped to just mail configuration.
>  (If we want to go further with calendar, contacts, etc. then arguably
> those should be separate URLs.  My mail and calendar is not provided
> by the same provider, for example.)
If we only support mail configuration this will probably fail. I think
one of the strengths of this draft is that everything is in one place.
> Caching: HTTP allows one to dictate that how long cached data can
> live. I don't think we need to specify it further.  A client will pull
> the config when the user configures the client.  There's no reason to
> cache it beyond that point.  In the off chance it queries a second
> time, it's not a big deal.  It might get pulled from web cache or
> perhaps go back to the server.  It really does not matter, since this
> should not be queried over and over.  Only in the event that a config
> change is made, the client might need to re-query the config data.
>  I'm not sure if the client should automate that process or prompt the
> user "would you like me to re-validate the configuration settings?"
> Personally, I like the latter.
Sounds reasonable. I'm not sure why the current draft has this ttl
field. I'll think about that, but we probably can ditch that.
> Certificate pinning: Why? One fetches the mail config file.  The
> client will authenticate the certificate. Is this just to catch the
> very rare case where somebody hacks DNS and gets a fake certificate?
>  That's really a broader Internet infrastructure issue that I think we
> should solve generically.  (For example, use of notaries, DNSSEC/DANE,
> etc. Personally, I use DNSSEC/DANE, but somewhere close to zero people
> make use of that data.)
Yes, that's to prevent MITM attacks. While I believe that certificate
pinning is a good thing (if done right), I'm not sure if this document
is a good place for this. I guess there are better ways to bootstrap
certificate pinning and we probably should leave this out for now.
> Certificate Auth: I'm not sure what the problem is here, unless people
> want to use enterprise-issued certificates (i.e., those that are not
> from a public CA).  If that is the case, I would not want my mail
> client to insert those into my trusted certificate store.  So, would
> those be one-time uses?  Not very useful.  I'd say the IT department
> needs to install whatever root certs are needed.  A client should
> authenticate the certificates it sees, but we don't need more words
> about it other than to say the TLS connection must be authenticated.
No, this is about client authentication, i.e. the client has to provide
a client certificate signed by some specific authority. This is not
widely used and probably more for corporate or personal use cases, but
we have a significant number of users of this security feature.
At least in Java it's a real pain to distinguish a client certificate
request from any other SSLException. So it would be helpful to know in
advance if a specific client certificate is required or not.
> Document structure:  I think keeping this simple is really best.  I
> don't think we need multiple mail providers. If the email address is
>, <,%20>then I think it's
> reasonable to assume is the provider.  Yes, a user could
> go enter WebFinger information for some other provider, but the
> average person will not want to do that.  That's having the user
> configure the server so it can configure the client.  Not much gained
> there.  I would have something that's no more complex than something
> like this:
I fully agree that we should keep it as simple as possible. However, one
of the keys to success is that we support the services and use cases
that are not covered by the existing mechanisms. That's why I believe
that the document should have a generic structure that works with pretty
much every service out there. It shouldn't be designed around email.
This is one of the reasons for me to ditch the host/port/transport
notation and just use a single URI instead. I'm not aware of any service
that can't be addresses with a URI.

Another goal should be to revert the service discovery process to some
degree. Instead of having the client to probe the provider for each
service that might be supported, the current spec reverts that and
returns a list of everything that's supported. The local "account setup
agent" then can probe the local device software for client modules or
applications that might support a specific service. Ideally this would
be supported on OS level. So you run a generic account configuration on
OS level, the OS pulls the service configuration document and calls the
installed handlers for each supported service. It there is no client for
a specific service the OS could make suggestions like "Your provider
also supports <Some service>. Search the App Store for a suitable client
app now".

That's why I believe that we should put all services into a single
document, instead of using service specific rel-types that need to be
probed by the client.

If, like in your case, calendars and email are not hosted at the same
provider, the webfinger response for your account should just contain
two provider entries, each pointing to a document that lists all the
services provided by this provider.



> Paul
> ------ Original Message ------
> From: "Marten Gajda" < <>>
> To: "" < <>>
> Sent: 7/13/2016 6:32:01 PM
> Subject: Re: [webfinger] [apps-discuss] Mail client configuration via
> WebFinger
>> Hi Paul,
>> the "current draft" is still the one at
>> and the issues at GitHub are discussion items for the upcoming draft.
>> I wanted to clarify a few points before I start to work on the draft.
>> If you have anything to add to these points or if you have any other
>> concerns, please let me know, so we can address them.
>> Unless there is some more interest in the certificate stuff, I'm not
>> going to add this to the draft. We still can add that later on if it
>> turns out to be useful.
>> Cheers,
>> Marten
>> Am 11.07.2016 um 23:31 schrieb Paul E. Jones:
>>> Marten,
>>> Sorry to just be coming back to this after a whole month passed.
>>> What current draft?  Did you write one that I missed?  Or are these
>>> requirements for the draft you would like to see?
>>> Paul
>>> ------ Original Message ------
>>> From: "Marten Gajda" < <>>
>>> To: "" <
>>> <>>
>>> Sent: 6/8/2016 3:35:05 PM
>>> Subject: Re: [webfinger] [apps-discuss] Mail client configuration
>>> via WebFinger
>>>> All,
>>>> I've created a GitHub repository to track open issues with the
>>>> current draft, see
>>>> You're welcome to contribute to the discussion, either by creating
>>>> a new issue or by commenting on an existing one.
>>>> Thanks,
>>>> Marten
>>>> Am 05.06.2016 um 23:00 schrieb Marten Gajda:
>>>>> I think OpenID Connect already covers the discovery of everything
>>>>> you need to do OAuth2. That involves Webfinger, but there is no
>>>>> need to protect this request, because it doesn't contain personal
>>>>> information.
>>>>> So we don't need to reinvent the OAuth2 bootstrapping sequence.
>>>>> The only minor issue I see is that you may have to ask the user
>>>>> twice to grant access. Once to authorize the client to access the
>>>>> configuration document and another time to authorize the client to
>>>>> access the individual services. The second step is necessary,
>>>>> because the scope tokens of these services are not known when the
>>>>> first authorization request is presented to the user. The problem
>>>>> with that is, there doesn't seem to be a mechanism to broaden
>>>>> scope, without allowing the user to switch to a different account.
>>>>> To get access to the individual services, the client needs to
>>>>> start another authorization code grant. But the user is always
>>>>> free to log out and log in with a different account before
>>>>> granting access.
>>>>> Marten
>>>>> Am 03.06.2016 um 20:13 schrieb George Fletcher:
>>>>>> Did a quick scan of the draft document. Given that more and more
>>>>>> systems are trying to remove the need for passwords, it seems
>>>>>> like the final solution needs to support 2FA and biometric
>>>>>> mechanisms for "HTTP authentication". I definitely would not want
>>>>>> the webfinger instance releasing my config data without my
>>>>>> "authentication". I suppose OAuth2 could be used to protect the
>>>>>> webfinger APIs though there is a bit of a chicken-and-egg here:)
>>>>>> I kind of like the suggestion around returning a 401 with data in
>>>>>> the WWW-Authenticate header on where to get a token to use with
>>>>>> the API. This would allow the client to start and OAuth2 flow
>>>>>> with the Authorization Server specified and that would give the
>>>>>> user a clear indication of what password/credentials are being
>>>>>> requested. Once the client gets the token, it uses it with the
>>>>>> webfinger call and now the service-configuration data is returned
>>>>>> because the token is the authorization for the specified id.
>>>>>> Thanks,
>>>>>> George
>>>>>> On 6/3/16 10:44 AM, Marten Gajda wrote:
>>>>>>> Note that the idea behind
>>>>>>> is to put the service configuration for all services of a
>>>>>>> provider into a single document.
>>>>>>> So you would receive something like this:
>>>>>>> {
>>>>>>>     "rel " :  "service-configuration",
>>>>>>>     "href " : ""
>>>>>>> }
>>>>>>> If a user uses the same account identifier at another provider
>>>>>>> the Webfinger request could return their configuration too
>>>>>>> (given there is some mechanism to add it and the provider
>>>>>>> actually supports that).
>>>>>>> {
>>>>>>>     "rel " :  "service-configuration",
>>>>>>>     "href " : ""
>>>>>>> },
>>>>>>> {
>>>>>>>     "rel " :  "service-configuration",
>>>>>>>     "href " : ""
>>>>>>> }
>>>>>>> Without that it would be more difficult to setup your account at
>>>>>>> with your login "".
>>>>>>> would have to issue some kind of
>>>>>>> user-identifier like for
>>>>>>> auto-configuration purposes, even though you don't use it for
>>>>>>> authentication (because you use for
>>>>>>> authentication). I think that's the idea behind the `acct:` URI
>>>>>>> scheme, but I don't think that you can explain to users why they
>>>>>>> need another user identifier and when to use one or the other.
>>>>>>> But that also raises the privacy concerns I mentioned earlier.
>>>>>>> If the request is not authenticated, everyone could see that you
>>>>>>> have an account at
>>>>>>> Regarding SSO: There is an RFC that extends SASL based
>>>>>>> authentication to support the token authentication mechanisms as
>>>>>>> used by OAuth1 and OAuth2, see
>>>>>>> So SSO already works with IMAP, POP3 and SMTP.
>>>>>>> Cheers
>>>>>>> Marten
>>>>>>> Am 03.06.2016 um 15:40 schrieb Paul E. Jones:
>>>>>>>> Marten,
>>>>>>>>> So how would the UI work?
>>>>>>>>> 1) User enters user identifier, most likely an email address,
>>>>>>>>> like and hits "next"
>>>>>>>>> 2) Client sends a Webfinger request to
>>>>>>>>> to see if there
>>>>>>>>> is a configuration document
>>>>>>>>>   -> response 404 Not Found
>>>>>>>>>    a) Client falls back to "manual setup" or another
>>>>>>>>> auto-configuration mechanism
>>>>>>>>>   -> response 401 Unauthorized
>>>>>>>> One should not get 401 querying the WebFinger information for
>>>>>>>> the user.  What should happen is that the server should return
>>>>>>>> a JSON object that contains a link relation that might look
>>>>>>>> like this:
>>>>>>>> {
>>>>>>>>     "rel " :  "mail-config",
>>>>>>>>     "href " :
>>>>>>>> ""
>>>>>>>> }
>>>>>>>> Or something like that.  The mail client should query that URI
>>>>>>>> it is that one that should result in a potential 401.  The JSON
>>>>>>>> format that would come back here would need to be something we
>>>>>>>> define.  It could be based on JRD, but would not have to be.
>>>>>>>> Otherwise, I think the flow looks right.
>>>>>>>>>    b) Client asks for password at "" and goes back
>>>>>>>>> to step 2 (this time with authentication)
>>>>>>>>>   -> response 200 Ok
>>>>>>>>>    c) client moves on to next step
>>>>>>>>> 3) (optional) Client presents found services to the user to
>>>>>>>>> let him select the services to set up
>>>>>>>>> 4) Client runs the setup handler for each selected service
>>>>>>>>> I think in general that could work but it raises two questions:
>>>>>>>>> 1) How to make sure the user really understands that he's
>>>>>>>>> authenticating at in step 2b? If the user tries to
>>>>>>>>> configure calendar sync with "" where his
>>>>>>>>> login happens to be he might not be
>>>>>>>>> prepared to being asked for his password. Maybe
>>>>>>>>> I'm just paranoid or overcautious, but I think that it might
>>>>>>>>> easily happen that the users sends his
>>>>>>>>> password to in that case (since Basic
>>>>>>>>> authentication is still the most common mechanism, the client
>>>>>>>>> basically sends the password in plain text).
>>>>>>>> Yeah, that's a valid concern.  The only thing I can suggest is
>>>>>>>> that the Subject CN from the certificate is presented to the
>>>>>>>> user.  Alternatively, there could be two passwords: one that is
>>>>>>>> the "configuration password" and one that is the email
>>>>>>>> password.  However, I don't think two passwords will help us. 
>>>>>>>> If I want to hack somebody and can gain access to their WF
>>>>>>>> config, I can auto-config their email client to point to my own
>>>>>>>> IMAP server and just retrieve the password that way.
>>>>>>>> So, I think we have to rely on the certificate presented to the
>>>>>>>> mail client and the user will have to know to trust it.  The
>>>>>>>> mail provider can tell the user "when configuring email, ensure
>>>>>>>> that the configuration server is and do
>>>>>>>> not provide a password if that is not the name of the
>>>>>>>> configuration server indicated."
>>>>>>>> If the mail config information is not protected with a
>>>>>>>> password, we probably would want the customer to verify that
>>>>>>>> the SMTP server information is correct before the password is
>>>>>>>> provided.  These would be the steps users would take if
>>>>>>>> performing manual configuration, but the software presents that
>>>>>>>> information to the user without the user having to enter it.
>>>>>>>>> 2) How does the client know which credentials to use to set up
>>>>>>>>> the individual services in step 4? Should the client ask the
>>>>>>>>> user for the credentials for each service or can it assume
>>>>>>>>> that some of them share the same credentials? Is that
>>>>>>>>> something an auto-configuration document can help with?
>>>>>>>> It would be nice if there is a config indicator that says "SMTP
>>>>>>>> server and IMAP server passwords are the same", so the client
>>>>>>>> does not have to ask.
>>>>>>>> If we use a "config password" then we could even have the
>>>>>>>> server tell the client the password for services, which would
>>>>>>>> transparently allow those to be different.  Alternatively, the
>>>>>>>> client will have to ask each about each one.
>>>>>>>> For calendaring services, then yes: the client would have to
>>>>>>>> ask the user.  It could ask if it's the same or different than
>>>>>>>> the email password or the config password.  For some services,
>>>>>>>> the authentication mechanisms will be entirely different (like
>>>>>>>> Google Calendar).  The mail client will just have to know how
>>>>>>>> to access the service.  For that reason, I'm a little hesitant
>>>>>>>> to suggest including calendaring service config along with the
>>>>>>>> email config data.  We could have each of those services listed
>>>>>>>> in the users' WebFinger document.  For example, I might have
>>>>>>>> this entry in my WF document:
>>>>>>>> {
>>>>>>>>     "rel" : "calendar"
>>>>>>>>     "href" : "urn:whatever:google"
>>>>>>>> }
>>>>>>>> Note I did not provide a URL.  The reason is that this is an
>>>>>>>> entirely different service that has an entirely different
>>>>>>>> access method.  Maybe the client can ask the user "is
>>>>>>>> our user ID for your Google calendar?" 
>>>>>>>> In my case, I don't think it is.  Certainly, it's not my gmail
>>>>>>>> ID.  And, I would not want to advertise that to the world,
>>>>>>>> necessarily.  Anyway, I think we should limit the scope of what
>>>>>>>> we try to do to things that are standard OR we define a generic
>>>>>>>> URN that a client will just have to know how to deal with.
>>>>>>>>> Ideally the server supports some kind of SSO mechanism like
>>>>>>>>> OpenID Connect, so you don't need to enter your password
>>>>>>>>> multiple times. But a working auto-configuration is really the
>>>>>>>>> precondition for this, because an OpenID Connect
>>>>>>>>> implementations needs a way to discover the OAuth2 scope
>>>>>>>>> tokens to request from the server and auto-configuration is
>>>>>>>>> really the way to do that, IMO. For this it would be helpful
>>>>>>>>> to have some mechanism to request a broader scope from the
>>>>>>>>> user (without allowing him to switch to a different account),
>>>>>>>>> but that's a different topic I guess.
>>>>>>>> I definitely like the idea of SSO.  But, I struggle to see how
>>>>>>>> we would use this in practice with mail autoconfig since SMTP,
>>>>>>>> IMAP, and POP servers require a password, anyway.  If we use
>>>>>>>> that as a means to have those passwords provided behind the
>>>>>>>> scenes (as I indicated above), that might be a good argument
>>>>>>>> for using OpenID Connect.  In that way, the ISP can also ensure
>>>>>>>> that passwords are REALLY complex and unknown even to the
>>>>>>>> user.  Not a bad practice, in that we can view those passwords
>>>>>>>> as complex tokens.
>>>>>>>> Would that kind of use of OpenID Connect to retrieve the
>>>>>>>> password be workable?  (I'll admit I don't have so much
>>>>>>>> experience with OpenID Connect.  I implemented OpenID 2.0, but
>>>>>>>> that's not ideal for what we'd want to accomplish here.  I
>>>>>>>> don't have a good feel for how Connect might make this better.)
>>>>>>>> Paul
>>>>>>>> _______________________________________________
>>>>>>>> webfinger mailing list
>>>>>>> -- 
>>>>>>> Marten Gajda
>>>>>>> CEO
>>>>>>> dmfs GmbH
>>>>>>> Schandauer Straße 34
>>>>>>> 01309 Dresden
>>>>>>> GERMANY
>>>>>>> phone: +49 177 4427167
>>>>>>> email:
>>>>>>> Managing Director: Marten Gajda
>>>>>>> Registered address: Dresden
>>>>>>> Registered No.: AG Dresden HRB 34881
>>>>>>> VAT Reg. No.: DE303248743
>>>>>>> _______________________________________________
>>>>>>> webfinger mailing list
>>>>> -- 
>>>>> Marten Gajda
>>>>> CEO
>>>>> dmfs GmbH
>>>>> Schandauer Straße 34
>>>>> 01309 Dresden
>>>>> phone: +49 177 4427167
>>>>> email:
>>>>> Managing Director: Marten Gajda
>>>>> Registered address: Dresden
>>>>> Registered No.: AG Dresden HRB 34881
>>>>> VAT Reg. No.: DE303248743
>>>>> _______________________________________________
>>>>> webfinger mailing list
>>>> -- 
>>>> Marten Gajda
>>>> CEO
>>>> dmfs GmbH
>>>> Schandauer Straße 34
>>>> 01309 Dresden
>>>> phone: +49 177 4427167
>>>> email:
>>>> Managing Director: Marten Gajda
>>>> Registered address: Dresden
>>>> Registered No.: AG Dresden HRB 34881
>>>> VAT Reg. No.: DE303248743
>>> _______________________________________________
>>> webfinger mailing list
>> -- 
>> Marten Gajda
>> CEO
>> dmfs GmbH
>> Schandauer Straße 34
>> 01309 Dresden
>> phone: +49 177 4427167
>> email:
>> Managing Director: Marten Gajda
>> Registered address: Dresden
>> Registered No.: AG Dresden HRB 34881
>> VAT Reg. No.: DE303248743

Marten Gajda

dmfs GmbH
Schandauer Straße 34
01309 Dresden

phone: +49 177 4427167

Managing Director: Marten Gajda
Registered address: Dresden
Registered No.: AG Dresden HRB 34881
VAT Reg. No.: DE303248743