On 5.1.2017 13:15, Sumit Bose wrote:
On Mon, Jan 02, 2017 at 08:06:04AM +0100, Jan Cholasta wrote:
On 19.12.2016 12:13, Sumit Bose wrote:
On Mon, Dec 19, 2016 at 10:02:58AM +0100, Jan Cholasta wrote:
I agree with *almost* everything Sumit said. See my inline comments below.

On 16.12.2016 11:53, Sumit Bose wrote:
On Tue, Dec 06, 2016 at 04:39:10PM +0100, Florence Blanc-Renaud wrote:

I have started a feature description for the Certificate Identity Mapping at
the following location:

This is a first step, focusing on the interface we would like to provide. It
still contains open questions, some of which are linked to the corresponding
design on SSSD side:

Comments, concerns and suggestions are welcome. Thanks!

Hi Flo,

thank you very much for setting up the page.

My comments are mostly about the commands.


* --enable=Boolean: if this option is 'False' SSSD will basically show
  the current behavior and just look up the certificates directly. But I
  wonder if the option is needed at all because not adding any mapping
  rules would have the same effect.

  What is the scope here, only the IPA domain, or all trusted domains as
  well? If it is for trusted domains as well will the certmappingrule-*
  commands and user-{add/remove}-certmapping return an error?

  So, in general I see an overlap with the mapping rules and I think it
  would be clearer to drop this option and do the lookups according to
  the mapping rules.

* --prompt-username=Boolean: the description implies that this option is
  synonymous to 1:1 mapping, but it is not. On Linux authentication in
  most cases use a user name either by directly asking (e.g. /bin/login)
  or using the current user name (e.g. sudo). So, according to its name
  it would only control if gdm is allowed to ask for an (optional) user

  If the option is renamed to e.g. --force-1-to-1-mapping to really
  enforce a 1:1 mapping then it would make sense to derived to gdm
  behavior. I.e. if 1:1 mapping is enforce it makes no sense for gdm to
  ask for a user name and if it is not enforced then it makes sense to
  offer and optional user name input field.

* --enable-username-mismatch=Boolean: I think this option can be
  dropped. My test so far show that if a non-matching hint is given on a
  Windows client authentication fails.

* --alternate-attribute=STRING: I think this option isn't needed as
  well. For IPA server-side we should decide on an attribute name and
  add it to the schema for user objects. On the client side the
  attribute name can be taken from the mapping rule.A


* ISSUERDN: it looks like you want to use issuerName here. In
  certificateRecord it it used with LDAP ordering and I would prefer
  LDAP ordering at all points where we have a choice. Unfortunately in the
  issuer-subject mapping AD dictates X.500 ordering.

LDAP ordering should indeed be preferred, as it is used everywhere else in
IPA. We can convert to/from X.500 ordering where necessary, when possible.

* DOMAINDN: does this refer to the nsslapd-certmap-basedn attribute in
  the example? My intention in the SSSD design-page was to specify the
  domain (as in DNS domain/IPA domain/trusted domain) where the matching
  user should be searched. Different domains might certificates from
  different issuers and some domains might not even use certificates.
  With this information SSSD does not have to search any domain trusted
  by IPA from a given certificate, but look only at domains listed here
  (the attribute should be a multi-value one).

  There are objects in the LDAP tree for each trusted domain which are
  used by SSSD so using a DN syntax would be valid here.

We use domain names rather than DNs to refer to domains everywhere else in
the framework. I don't think this place should be an exception.

I'm fine with domain names as well. In fact I didn't thought of using
DNs for this before I read DOMAINDN on the design page.

* LDAPSEARCHFILTER: I think a separate option is not need. LDAP search
  filters should just be a special kind of mapping rules. I can image in
  syntax like: <LDAPFILTER:(&(cn=%A)(email=%B)(authType=pkinit))>. I
  think the difficult part with the LDAP filters will to define sensible

I'm not sure I understand. Could you please elaborate a little bit?

A LDAP search filter which would cover the AD behavior would look like:



%A: must be replaced with the issuer of the certificate in X.500 order
%B: must be replaced with the subject of the certificate in X.500 order

it would be possible of course to use a specific template here which
would generate the complete search attribute value.

%C: must be replaced by the principal from AD's SAN
%D: must be replaced with only then name component (the part before the
    realm) of the principal from szOID_NT_PRINCIPAL_NAME

As %C and %D imply this filter will only work for certificates which
have szOID_NT_PRINCIPAL_NAME but for those it must be used to be
compatible with AD. For certificates without


is sufficient. It is possible to select the right filter with matching


So we have to find suitable names for the %A, %B, %C and %D templates
and also allow different representations (e.g. LDAP or X.500 order for

I would personally prefer if we used Python-style formatting strings for the
templates, I find them much more pleasant to work with than C-style

sure, I used %A etc as arbitrary templates, I didn't wanted to imply
that C-style syntax should be used.

formatting strings. The filter which covers AD behavior could be written as:


I think this is quite readable and understandable to an admin with basic
knowledge of LDAP search filters and certificate components. Are the
names used here (issuer_dn, subject_dn, subject_nt_principal) already
used by e.g. some Python modules or do we have to define the list of

They are not defined anywhere, it's just something I came up with.

If there a reason for using '!x500' to indicate X500 ordering? Wouldn't
e.g. issuer_dn.x500 work as well similar to Python's str.lower()? Having
only one symbol to indicate a special formatting of the value from the
certificate would make the syntax easier to learn.

I tried to follow the Python convention, where '.' is used for attribute access and '!' for value conversion. In other words, '.' selects a component of the value and '!' takes the whole value and formats it in a certain way.

  But as long as we keep the general mapping rule syntax
  flexible the LDAP filter rules can be added in a later version.

IMHO it should be the other way round and LDAP filters should be implemented
first, as they offer all the flexibility we need (all of the other fields
can be easily implemented on top of LDAP filters) and are by default
extensible without having to update servers and clients.

In general I agree, as long we can find a suitable scheme to handle the
templates to add content from the certificate in a specific format to
the search filters.

But from the user/admin perspective there should be special rules for
common use-cases which do not require to know too much details about
certificates and LDAP trees. E.g. for AD (either via direct or indirect
integration) there should be a <AD-LIKE> rule which just does all which
AD would do depending on the certificate type. For IPA something like
<ALT-SEC-ID-I-S> might be a good start for handling external
certificates which do not contain user specific data which can be mapped
to user object because the syntax is already known from AD.

This could be handled in the IPA plugin by converting from the user-friendly
representation to LDAP filter template internally when a mapping rule is
added or modified.

Yes, but it can also be expanded by the component/library which will
replace the templates with the actual values from the certificate. This
would have the befit that the user-friendly representation is visible
even with low-level LDAP commands?

That's true (although I can't imagine why would anyone look on the LDAP entries directly, especially if they didn't understand LDAP filters), but IMHO it lacks certain elegancy by having more than one way to define the same rule, more code paths to handle the rules, etc.

Additionally, LDAP filters provide better forward compatiblity - if we had to add support for a new mapping style in the future, with LDAP filters only the IPA server would have to be updated, without LDAP filters we would also have to worry about updating old clients which do not understand the new mapping style.


* enable/disable: I think this is a good idea and would be consistent
  with other rules like HBAC and sudo

* user-{add/mod} LOGIN --certmappingdata DATA: I think it might be
  better to not add this option and only implement the
  'user-{add/remove}-certmapping' commands

* user-{add/remove}-certmapping: you say '... almost any type of mapping,
  or a more user-friendly API ...'. I would not say 'or' but 'and' and
  implement both

* ipaCertMappingEnableMismatch and ipaCertMappingAlternateIdAttribute; I
  think both are note needed, see above

* altSecurityIdentities: I would prefer to use a different name and OID.
  Using the same definition as AD would imo imply that it can be used in
  the same way as in AD. But e.g. AD also supports other content like
  KERBEROS:alternative_user_principal@AD.DOMAIN which we will not

* issuerName vs ipaCAIssuerDN: I would prefer issuerName because it is
  general UTF-8 and not DN syntax ( Since
  the issuer DN in general will not be a DN from the local LDAP tree I
  think the UTF-8 version fits better.

I think it's worth mentioning that if the attribute used DN syntax and
matching, we wouldn't have to worry about normalizing the issuer name before
searching for it, as DS would do that for us.

Good point, but I think the main use case for this attribute is on the
client side to determine if a rule should be applied to a certificate or
not. So I guess LDAP searches with this attribute would be rare because
the client will load all rules in one run.

* nsslapd-certmap-basedn, see DOMAINDN above

* altSecurityIdentities example: X.500 ordering is used by AD here and
  unfortunately I think we have to adopt it at least for this specific
  usage, here is an ldapsearch output from AD:

 =ad,CN=Users,CN=t u,E=test.user@email.domain
altSecurityIdentities: X509:<I>O=Red Hat,OU=prod,CN=Certificate
 m,CN=Sumit Bose Sumit Bose

* Certificate Mapping Administrators or re-use Certificate
  Administrators: I would prefer a new 'Certificate Mapping

* Users can manage their own X.509 certificate mappings? I'm not sure
  here, at the first glance I would say no. How are OTP tokens handled?
  Maybe this would be a candidate for certmappingconfig-* option?

I think a better question is "How is userCertificate handled?"

Anyway, self-service permissions can be enabled/disabled, so there is really
no need for a new certmappingconfig option.

Yes, this makes sense.


That's all :-)


Jan Cholasta

Jan Cholasta

Jan Cholasta

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