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Specify the DNS site a client is using

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Problem statement

Even though the Active Directory provider is able to leverage DNS sites, the site discovery is always automatic. There is no way to pin a particular client into a particular site. This design document describes a way to do so.

Use cases

The site discovery relies on client being part of subnet. It is not always practical or even possible to assign GNU/Linux machines to the right subnet. Still, these clients should be able to leverage the nearest AD site, even at the expense of manual configuration in sssd.conf.

Overview of the solution

The SSSD will gain a new AD provider option that would, if AD sites are enabled, override any dynamically discovered sites. This option would pin the client to a particular site not only for primary domain but also for subdomains. The discovery search would only be used to find the forest we are enrolled with.

For Global Catalog service discovery of the primary and secondary domains would then be defined as follows:

  • primary domain - $HARDCODED_SITE._sites.$FOREST
  • backup domain - $FOREST

For pure LDAP searches, the domains would then be defined as:

  • primary domain - $HARDCODED_SITE._sites.$DOMAIN
  • backup domain - $DOMAIN

Above, $FOREST is auto-discovered and $DOMAIN is either the SSSD domain name as defined in the config file (for the main SSSD domain) or autodiscovered from object of class trustedDomain.

In both cases, the full DNS search consists of _$service._$port.$domain.

Especially for trusted domains, the overridden search domain might not return anything, but the DNS resolver code is built such that it iterates over search domains until the search yields some result.

Authentication against trusted domains

For trusted domains, we currently always talk to a local DC which gives libkrb5 a referral to a trusted-domain specific DC that handles authentication against a KDC from the trusted domain. This process is completely opaque to SSSD, which means that Kerberos authentication doesn’t take the sites into account at all.

Implementation details

The SSSD AD provider would gain a new option called ad_site. This option would be unset by default.

The SRV initialization function ad_srv_plugin_ctx_init() needs to be adjusted to accept a site override as a const char *site_override since the site name is just a string. In the default case, where the option is unset, this option would be set to NULL. In any case, the ad_get_client_site_send()/recv() request would run to completion since we need to learn the forest name anyway. If the new option is set, then the caller of ad_get_client_site_recv() would still read the forest value, but ignore the site value and use the value of the ad_site option instead.

Configuration changes

A new option ad_site as described above. The option would be both described in man pages and implemented in the configAPI.

How To Test

The best testing would be performed using an AD test environment consisting of at least two servers in the same domain. To test, join both DCs to the same domain. Create two sites such that the IP address of your SSSD client would set you in one of them.

Make sure that, by default, SSSD creates the kdcinfo file using the DC in the autodetected site and authenticates you against the DCs from the autodetected site. The latter can be verified using i.e. tcpdump and krb5_child log files.

Set the ad_site option to a non-default site. Verify, using tcpdump, kdcinfo file contents and SSSD debug logs that SSSD redirects communication to DCs in the non-default site.