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FreeIPAv2:DNS Location Discovery

FreeIPAv2 Development Documentation

Please note that this content was made during FreeIPAv2 development and it may no longer be relevant for current versions of FreeIPA. We left the content here for study and archaeological purposes.

Please check our Documentation for a recent list of topics.

This design is obsolete and was replaced by V3/DNS_Location_Mechanism.


Simo Sorce, Red Hat Geert Jansen, Red Hat


Service Discovery is a process whereby a client uses information stored in the network to find servers that offer a specific service. It is useful for two reasons. First, it places the information required for the location of servers in the network, lessening the burden of client configuration. Second, it gives system and network administrators a central point of control that can be used to to define an optimized policy that determines which clients should use which server and in what order.

A standard for service discovery is defined in [RFC2782]. This standard defines a DNS RR with the mnemonic SRV and usage rules around it. It allows client to discover servers that offer a given service over a given protocol.

A drawback of [RFC2782] is that it assumes clients know the correct DNS domain to use as the query target. Without any further information, the client's options include using their own DNS domain and the name of the security domain in which the client operates (such as the AD domain name or the Kerberos REALM). Neither option is likely to yield optimal results however. One key service discovery requirement, especially in large distributed enterprises, is to choose a server that is close to a client. Doing this correctly can make the difference between a working and a non-working service. However, different considerations underlie the grouping of systems for the purpose of DNS domains and security contexts, and for the purpose of deciding network locality. For example, for locality purposes system administrators may want to group systems into groups corresponding to network subnets, but for security purposes they may want to group systems across organisational boundaries. Whatever option the client decides to take for a target for SRV based service discovery, if this target is not specifically destined for service discovery, an undesirable coupling is created between service discovery and the other use for the target.

The incompleteness of [RFC2782] is acknowledged by systems such as [AD] that contain extra functionality to augment SRV lookups to make them site aware. The basic idea is to use a target in SRV service discovery that is specific to a location or "site" as AD parlance. This introduces location awareness into the SRV service discovery. Unfortunately though, the protocol that AD uses to determine a site for a client is not submitted as an Internet standard and also contains Microsoft specific features.

This document specifies a protocol for location discovery based on DNS. It introduces two new RRs for this, with the mnemonics SUBNET and SUBNET6. Clients can use the protocol to find out their location, based on their IP address and DNS domain name. The result of the location discovery is a DNS name that can then be used as a target for a subsequent, location aware, DNS SRV based service discovery.

The SUBNET and SUBNET6 records

The format of the SUBNET and SUBNET6 RR is as follows.

   <network> SUBNET  TTL CLASS <address> <prefix> <location>
   <network> SUBNET6 TTL CLASS <address> <prefix> <location>

The components are explained below.


A name for the network that is being described.


The start IP address for the network. This must be an IPv4 address for the SUBNET RR and an IPv6 address for the SUBNET6 RR.


The length of the network prefix in bits. Together, <address> and <prefix> form a CIDR subnet.


If <location> does not start with an underscore ('_') then this is the name of the location that corresponds to the subnet. The location is a plain DNS name that can be used as the target for a subsequent SRV query.

If the location starts with an underscore, then this is not a location but a reference to an intermediate SUBNET instead. SUBNET references are used to build index structures that make lookups more efficient in the case of a large number of subnets.

The Discovery Protocol

Clients use a DNS domain for location discovery. A SUBNET aware DNS domain MUST have a SUBNET record published under the name "_network". This record directly or indirectly points to all locations that are known for the domain. The name "_network" MUST resolve into one or more SUBNET records. Each record MUST NOT have overlap with any of the other records. In case the <location> part of the SUBNET RR does not start with an underscore, it points to the final location for this subnet. This location is used as the target for a SRV lookup.

If <location> starts with an underscore then it MUST be a DNS name that resolves to further SUBNET records. We call the SUBNET that refers to the other subnets the parent and the subnets that are referred to by the parent the children. Based on this parent/child relationship the SUBNET records in a DNS domain form a multiway tree that is rooted under "_network". This tree is called the network tree. All parents in the network tree MUST have a CIDR subnet that is equal to the envelope of the CIDR subnets all its children. Each parent MUST have at least two children but SHOULD have more children up to an implementation defined optimum to ensure the tree forms an efficient search index.

Locations SHOULD be stored under the _locations node in the same domain as the network tree. Locations SHOULD have human readable names.

An example zone file that implements a network with 3 locations and 4 subnets is given below. One location (Boston) is the main location and contains the ldap server ldap.example.net. The two other locations are both Satellite locations and contain an ldap proxy server and a fallback to the main location.

   $TTL 1d
   $ORIGIN example.com.
   _network        SUBNET IN 24 boston._locations
   _network        SUBNET IN 24 boston._locations
   _network        SUBNET IN 24 newyork._locations
   _network        SUBNET IN 24 thehague._locations
   _ldap._tcp.boston._locations        SRV IN 10 10 389 ldap.example.net.
   _ldap._tcp.newyork._locations       SRV IN 10 10 389 proxy-ny.examle.net.
   _ldap._tcp.newyork._locations       SRV IN 20 10 389 ldap.example.net.
   _ldap._tcp.thehague._locations      SRV IN 10 10 389 proxy-hag.example.net.
   _ldap._tcp.thehague._locations      SRV IN 20 10 389 ldap.example.net.

A SUBNET aware client SHOULD use the following procedure to determine its location on the network. The location SHOULD be used as input into a subsequently SRV query.

  1. Set the SUBNET query target to the client DNS domain name.
  2. The client makes a DNS query with QNAME=_network.<target>, QCLASS=IN and QTYPE=SUBNET. Here, <target> is the current SUBNET query target.
  3. For each entry in the query result:
    1. Determine whether the client IP address is in the CIDR network formed by the <address> and <prefix> parts of the SUBNET RR. If it is not, continue with the next entry.
    2. Check if <location> starts with an underscore ('_'). If it does not, <location> is the result of our location discovery and exit succesfully.
    3. If we were called recursively, ensure that <length> is smaller than <length> in our caller. If not, abort with an error. This step protects from infinite loops that can arise in wrongly configured DNS zones.
    4. Set the current SUBNET query target to <location>, and jump to step 2.


Advice for Server Implementors

Nothing special is required to support the SUBNET and SUBNET6 RRs in a DNS server software.

For larger networks, it becomes ineffecient to list all SUBNET records under the signle "_network" node in a DNS domain, and a tree should be constructured. Current thinking suggest that no more than 20 SUBNET RRs should be added to a single node in the tree, which makes this also the limit for the root node.

Manually creating the network tree is inefficient and error-prone and therefore we suggest that server implementors provide functionality to facilitate this. Here we describe how such functionality can look like.

The input to the tree building software is a flat database containing (subnet, location) tuples. The procedure to create the tree is given below.

  1. First, all overlaps need to be removed from the subnet. This can be done by splitting overlapping regions into multiple regions and by deciding for each region what location will be the final location. Typically, this will be the location of the smallest (= most specific) subnet for that region.
  2. All non-overlapping regions are inserted in a data structure that is similar to a B+ tree but instead of working with keys and pointers it works with intervals. Each node contains a maximum number of intervals and intervals can point to other nodes. The standard B-tree procedures for splitting and merging nodes are trivially ported to the interval based approach. Using a B+ tree ensures that the tree will be balanced (ensuring efficient lookup) and that all leaves will be on the same, bottom level (because we cannot store both a location and a pointer in a node).
  3. The tree is dumped. Each internal and leaf node get a random name starting with an underscore assigned to it. All pointers are resolved using these names. All nodes are published under _network (the root node as _network).
  4. All locations are published under _locations.

Advice for DNS Administrators

Because of efficience considerations, DNS administrators are encouraged to publish the network tree only once under a DNS domain of their choice. Each other DNS (sub)domain that needs to use the network and location tree can be pointed to this using a "pointer" SUBNET record. The example below illustates a domain "corp.example.com" that uses the network tree and location databsea of the domain "example.com".

   _network.corp.example.com.   SUBNET IN 0 _network.example.net.

This requires one DNS record to be published in each DNS zone in the network which in our view is a manageable overhead.

Advice for Client Implementors

Location discovery requires a number of successive DNS queries to succeed. If efficient network trees are used with e.g. 20 subnets per node, the number of queries should not exceed 4 for even the largest networks. Nevertheless this discovery will take time and therefore we recommend that locator software caches the result of site discovery.

Another question is how to handle multi-homed sites. There is not a unique answer to this question and much depends on the context. Locator software could use the first network interface of the system to determine the IP address, or could accept a configuration setting indicating the system's primary IP address.

Alternative Solutions


DHCP could be extended to include an option that tells the client the site it is in. The granularity of such an approach would be reasonable as most subnets are contained to physical sites (the notable exception being strechted subnets for high availability purposes). Nevertheless, we don't think DHCP is a valid option because there are many systems that do not use it an use static IP configuration instead.

The resolver "sortlist" option

Some DNS resolvers recognize an option called "sortlist" that specifies a set of subnets that are "local" to the client. The resolve uses this list to order the results of queries that have multiple results so that matching IP addresses are put first. A location aware service discovery protocol could be constructed by agreeing that for a service everybody uses the same name, and let the resolver put the local server first. However, this option does not allow for the specification of the load-balancing parameters priority and weight which makes it unsuitable as a general purpose service discovery protocol in our view.

Another disadvantage is that the subnet as seen from he network may actually be different from the subnet that the administrator defined from a location point of view.

Use DNS subdomains

Each location could have its associated DNS subdomain, which could be used to publish SRV records. This approach has the drawbacks that many DNS domains are required, and that there is again an undesirable coupling between grouping for naming purposes and grouping for location purposes.

Use LDAP for location discovery

Instead of DNS, LDAP could be used to store the location and subnet information. In this case, the LDAP server could also take care of the indexing removing the burder of the creation of the network tree. Howver, as LDAP servers often contain interesting data, many deployments do not allow unauthenticated connections to it (apart from a few internal attributes on the LDAP root). This is a problem, as we'd like to use service discovery protocol to resolve servers for our identity service.

Use Remote Procedure Calls

Instead of a client resolving its site, a remote procedure call approach could be used. This would solve the unauthenicated access to the location database problem. This is also the approach taken in [AD] where clients make a connectionless LDAP request to a domain controller which is in fact is just an RPC. The disadvantage of this approach is that yet another protocol is introduce. the advantage is that the location and subnet database does not need to be public, it only needs to be available to the RPC server.

Transition Period

It is expected that it will take quite a while for DNS server to catch up and implement the SUBNET RR. Until this time, client implementations MAY use [RFC1464] style TXT records to store keys named "subnet" and "subnet6" in TXT records.

Security Considerations

Publishing the tree of SUBNET nodes under a well known location allows for anonymous discovery of all the subnets and location names. Although the data disclosed is not as relevant as what is discolsed via a zone transfer it may still be perceived as a security issue. An organization may use features in their DNS server to provide different results depending on the querying IP source address (views) so that this information is not available outside the internal organization networks.