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Use Active Directory’s DNS sites

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Problem Statement

In larger Active Directory environments there is typically more than one domain controller. Some of them are used for redundancy, others to build different administrative domains. But in environments with multiple physical locations each location often has at least one local domain controller to reduce latency and network load between the locations.

Now clients have to find the local or nearest domain controller. For this the concept of sites was introduce where each physical location can be seen as an individual site with a unique name. The naming scheme for DNS service records was extended (see e.g. so that clients can first try to find the needed service in the local site and can fall back to look in the whole domain if there is no local service available.

Additionally clients have to find out about which site they belong to. This must be done dynamically because clients might move from one location to a different one on regular basis (roaming users). For this a special LDAP request, the (C)LDAP ping (, was introduced.

Overview view of the solution

General considerations

The solution in SSSD should take into account that other types of domains, e.g. a FreeIPA domain, want to implement their own scheme to discover the nearest service of a certain type. A plugin interface where the configured ID provider can implement methods to determine the location of the client looks like the most flexible solution here.

Since the currently available (AD sites) or discussed schemes ( use DNS SRV lookups the plugin will be called in this code path. Since network lookups will be needed the plugin interface must allow asynchronous operations. SSSD prefers the tevent_req style for asynchronous operations where the plugin has to provide a *_send and a *_recv method. Besides a list of server names which will be handled as primary servers, like the servers currently returned by DNS SRV lookups, the *_recv method can additionally return a list of fallback servers to make full use of the current fallback infrastructure on SSSD.

Sites specific details

The plugin of the AD provider will do the following steps:

  1. do a DNS lookup to find any DC
  2. send a CLDAP ping to the first DC returned to get the client’s site
  3. after a timeout send a CLDAP ping to the next DC on the list
  4. if after an overall timeout no response is received the CLDAP lookups will be terminated and the client’s site is unknown
  5. if the clients site is known a DNS SRV for primary server and for backup server is send, otherwise only one with is done
  6. if primary and backup server lists are available all primary servers are removed from the backup list

The results of the different step should be available with one of the debug levels reserved for tracing to make debugging easier and to allow acceptance tests to validate the behavior with the help of the debug logs.

Implementation details

struct resolv_ctx should get 3 new members, two function pointer to hold the *_send and *_recv method of the plugin and a pointer to private data for the plugin. Since most of the structs related to the fail-over and resolver code are private a setter method to add the pointers should be added as well. This is more flexible than adding additional arguments to resolv_init().

Besides the the service type and protocol and domain, which are all available in struct srv_data, the plugin should get a tevent context and its private data as arguments. With this the plugin interface might look like: :

typedef struct tevent_req *(*location_plugin_send_t)(TALLOC_CTX *mem_ctx, struct tevent_context *ev, const char *service, const char *protocol, const char *domain, void *private_data);

typedef int (*location_plugin_recv_t)(TALLOC_CTX *mem_ctx, struct tevent_req *req, int *status, int *timeouts, struct ares_srv_reply **primary_reply_list, struct ares_srv_reply **backup_reply_list);

If a plugin is defined it can then be called in resolve_srv_cont() instead of get_srv_query(). If it is not defined either the result of get_srv_query() can be used or a default request with the same interface as the plugin can be used. I think the latter one would make the code flow more easy to follow.

Additionally, if s backup server list is returned the results must be added to the server list in resolve_srv_done().

Finding a DC for the CLDAP ping

To find any DC in the domain samba look for a I would suggest to use as well for the SSSD implementation.

Sending the CLDAP ping

The CLDAP ping is a LDAP search request with a filter like :


and the attribute “NetLogon”. The flags given with the NtVer component of the search filter will be different for a domain member (AD provider) and an IPA server in an environment with trusts (IPA provider).

A domain member will belong to a site and the following flags from /usr/include/samba-4.0/gen_ndr/nbt.h should be used ‘NETLOGON_NT_VERSION_5 NETLOGON_NT_VERSION_5EX NETLOGON_NT_VERSION_IP’. A trusted server does not belong to one of the sites of trusting domain so it can only ask for the closest site with ‘NETLOGON_NT_VERSION_5 NETLOGON_NT_VERSION_5EX NETLOGON_NT_VERSION_WITH_CLOSEST_SITE’. Maybe NETLOGON_NT_VERSION_WITH_CLOSEST_SITE is useful for a domain member as well if e.g. the services on the local site are not available.

Parsing the server response

The server response is a single attribute “NetLogon” which is a binary blob containing multiple NDR encoded values. This value can be decoded with ndr_pull_netlogon_samlogon_response() from the Samba library libndr-nbt.

Side note about struct resolv_ctx and the usage of resolv_init()

In previous discussions it was said that resolv_init() should be only called once during the initialization of a provider, preferable from the common responder code. This means that there is only one instance of the resolv_ctx for the whole provider.

Currently resolv_init() is called at two other places as well, in ipa_dyndns.c and sdap_async_sudo_hostinfo.c. I think the only reason for calling resolv_init() at those two place is, that both needed to call some low level resolve routines which need a resolv_ctx as parameter and that there is no easy way to get the resolv_ctx because it is hidden in a private struct. Instead of adding an appropriate getter method which returns the current resolve_ctx resolv_init() was called for a second time.

If the resolv_init() calls are removed from those two places with the help of a getter method or similar, I think the prev and next members can be removed from struct resolv_ctx as well. Because there will not be a list of resolver contexts, but only one.

How to test

If this feature is tested the following scenarios can be considered:

AD domain does only has a single site

  • site name might be ‘Default-First-Site-Name’ but it can be renamed or localized as well
  • SSSD should be able to discover the site, e.g. ‘Default-First-Site-Name’
  • SSSD should connect to any DC.

AD domain has sites but the local site of the SSSD client has no domain controller

  • SSSD should be able to discover the local site
  • SSSD should connect to a any DC

AD domain has sites and the local site of the SSSD client has a domain controller

  • SSSD should be able to discover the local site
  • SSSD should connect to a DC from the local site

Besides inspection the log files with a high debug level to connection to the domain controller can also be verified with the netstat or ss utilities.


Sumit Bose <>