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Kdcinfo files for trusted domains

Problem statement

When user authenticates using Kerberos, the KDCs that will actually be used are typically discovered by libkrb5 with the help of DNS SRV records, unless the KDCs are configured explicitly in /etc/krb5.conf.

Because the administrator typically expects that the servers they defined in sssd.conf would be used for both authentication through SSSD and for the Kerberos command line tools such as kinit, SSSD provides a locator plugin for libkrb5 that allows SSSD to inform libkrb5 the servers SSSD had configured. However, up until now, this plugin was only functional for the joined domain, not for the trusted domains. This means, that the administrator had to either rely on the DNS SRC records to be correct or manually configure krb5.conf.. The former is not true in many real-world deployments and the latter is non-obvious.and doesn’t allow the administrator to specify e.g. the Active Directory site.

Use cases

There are three separate, but interconnected use-cases:

  • An SSSD client joined to an Active Directory domain directly
  • SSSD running on an IPA master with a trust established towards an AD domain
  • SSSD running on an IPA client

In all three cases, the information provided by SSSD for the locator plugin would allow the administrator to pin the SSSD to a specific AD KDC for authentication using configuration options such as ad_server or ad_site. In addition, command line tools such as kinit would then connect to the DCs discovered by SSSD as well.

Overview of the solution

The Kerberos locator plugin reads the address(es) from text files written by SSSD located in the /var/lib/sss/pubconf directory.

On a high level, implementing this RFE requires several changes:

  • The code that sets up AD connections to trusted domains when SSSD is running on IPA masters must be augmented to create the kdcinfo files that the Kerberos locator reads
  • Similarly, the AD provider must be enhances to create the kdcinfo files for cases where the SSSD is joined to an AD domain directly
  • Becase the IPA provider on IPA clients is currently only able to perform the full fail over for the IPA domain (at the moment the fail over is bound to the back end, not the domain), the IPA provider must be extended to read a configured list of servers or an AD site from the configuration, resolve this input into a list of IP addresses and write the list into the kdcinfo files
  • Finally, the Kerberos locator plugin itself must be enhanced to allow reading multiple addresses and call the callback function provided by libkrb5 for each of the addresses

Implementation details

Writing the kdcinfo files on AD clients and IPA masters is a relatively straightforward change, the write_kdcinfo flag of the krb5_service must be set according to the value of the krb5_use_kdcinfo configuration option.

The scope of extending the locator plugin is changing the plugin to read a list of (host, port) tuples from a newline-separated list in the kdcinfo file and calling the provided callback function cbfunc in turn for each for the addresses.

The biggest change, at least code-wise is the support for the IPA clients. There, the subdomain provider must first read either the list of servers from the subdomain section (using the ad_server option value) or the AD site from the ad_site option. If neither of those is present, the subdomain is skipped. If servers are defined using the ad_server option, a new resolver request would resolve the host names from this list into a list of IP addresses and write those into the kdcinfo file. Care must be taken so that any unresolvable address is just skipped. If a site is configured, then the IPA subdomains provider would first resolve the servers in this site using a DNS SRV query and then call the same request to resolve the host names into addresses as in the case where the host names were configured. Finally, the addresses are also written to the kdcinfo files.

In the case where host names are specified using the ad_server option, all of the host names will be resolved and written into the kdcinfo file. In the case where the site is configured, the first 5 host names resolved from the DNS SRV query will be resolved. Using more host names wouldn’t make sense anyway, because either the SSSD authentication time out would abort the krb5_child helper or kinit itself would time out.

Configuration changes

No new configuration options are added. However, on IPA clients the following configuration:

[domain/ipa.domain/ad.domain]
ad_server = dc1.ad.domain, dc2.ad.doma

Or this configuration:

[domain/ipa.domain/ad.domain]
ad_site = localsite

Would now trigger the DNS resolution of either the host names or the site and then the host names in the site and populate the kdcinfo files.

How To Test

On an AD client or an IPA master, the kdcinfo files are populated in response to a request towards the trusted domain. Therefore a valid test is to make sure that after resolving a user from a trusted domain:

getent passwd user@child.domain

a valid kdcinfo file with an address of the DC that SSSD contacted in order to resolve the user is present. Given the example above, the file would be located at:

/var/lib/sss/pubconf/kdcinfo.CHILD.DOMAIN

In case of the IPA client, the kdcinfo files are generated after startup and then subsequently during each periodic invocation of the subdomains provider

In all cases, either shutting down SSSD or bringing SSSD into the offline mode (e.g with the SIGUSR1 signal) should remove all the kdcinfo files.

In all cases, calling a command line Kerberos tool such as kinit should read the kdcinfo file and contact the AD DC specified in the kdcinfo file. This can be verified using e.g. strace. or by using the KRB5_TRACE variable.

In all cases, disabling the krb5_use_kdcinfo option must prevent the kdcinfo files from being generated.

How To Debug

Information on which servers were discovered and written to the kdcinfo files are present in the SSSD debug logs.

The kdcinfo files are textual and can be inspected using e.g. cat.

Finally, whether the kdcinfo files are being used and which DCs are being contacted can be seen either in the krb5_child.log with a very high debug level (one that enables the KRB5_TRACE messages). Whether the kdcinfo files are being used from other command line utilities can be inspected by using the KRB5_TRACE environment variable, e.g.:

KRB5_TRACE=/dev/stderr kinit user@DOMAIN

Future development

At the moment, setting the AD DCs or site must be done on each and every IPA client by either modifying the sssd.conf file, or (better) dropping a configuration snippet into the /etc/sssd/conf.d directory. We would also like to, in one of the future releases, provide means of mapping an IPA host or a host group to an AD site so that this configuration can be done centrally.

Additionally, we would like to extend the kdcinfo file generation on AD clients as well so that the client can optionally write multiple addresses into the kdcinfo files in order to provide a way of failing over between multiple DCs from outside SSSD, for example from kinit.

Authors

  • Sumit Bose <<sbose@redhat.com>>
  • Jakub Hrozek <<jhrozek@redhat.com>>