DNS Support

The

supports domain-name resolution via external DNS servers. Each IP address context defines one or more DNS server groups, each containing up to eight DNS servers. The zone and/or SIP Trunk Group then indicates which DNS Server Group to use for requests which require DNS resolution.

Within a DNS server group, each server has both a priority and a weight. Requests are sent to the server with highest priority (lower value) first. Servers of a lower priority are only used when all servers of a higher priority are marked unavailable based on previous timeouts. If multiple servers of the same priority exist, requests are load-balanced across servers in proportion to their weights.

The

supports using DNS for either A record queries or full NAPTR+SRV+A record queries, and is configurable on a per SIP trunk group basis. Preferences for recursion and for name-server support are also configurable.

The

supports the following DNS queries to gather domain information:

• DNS NS—Redirects DNS query to other DNS servers
• DNS SRV—Gathers the transport information
• DNS A record—Gathers the address

The

maintains a DNS Cache, where it is able to store the DNS records received from DNS servers.

The

also supports ENUM queries to DNS servers for call routing.

Locating a SIP Server Using DNS

The

uses the DNS procedures RFC3263 to resolve a SIP Uniform Resource Identifier (URI) into the IP address, port, and transport protocol of the next hop to contact.

When a SIP endpoint (like a UAC, for example) needs to send a request to a resource identified by a SIP or Secure SIP (SIPS) URI, it needs to resolve that URI into the IP address, port, and transport protocol. This URI can identify the desired resource to which the request is targeted (in which case, the URI is found in the Request-URI), or it can identify an intermediate hop towards that resource (in which case, the URI is found in the Route header).

For a SIP call where the transport is not known, or cannot be derived from the URI, the SIP endpoint should perform a Naming Authority Pointer (NAPTR) query for the domain name in the URI. Once the transport protocol is found from the records returned by the NAPTR query, the client can then use Location of Services (SRV) query on the protocol to target host FQDN and port number. Finally, the client can then perform an Address (A) record query to resolve the domain names returned by the SRV query to obtain the IP address of the server.

Local Cache

For network configurations where SIP Server domain resolution is not available from external DNS servers, the

supports a local DNS cache. DNS queries can then be made against either external DNS servers or the local cache. The following DNS record types can be configured in the local cache:

• A (Address)
• SRV (Location of Services)
• NS (Name Server)
• NAPTR (Naming Authority Pointer) records.