IMG 1010 - Feature - DNS Query using SRV Record
The IMG supports utilizing an A-Record, SRV or Service Record, or both to specify which sip server to route a call to. The determination of which record to use is configured in the object. DNS Queries on SRV records will be described below. SRV Records are defined in RFC 2782 and identifies the server(s) that will support a particular service. In this case the IMG will be looking for the best way to route a call to a server which employs the sip service. An IMG Channel Group can be configured so that a query is sent to a DNS server and the DNS response would be an SRV record including all the sip servers that match the initial DNS service request. From the SRV Record the IMG will handle load balancing from the priority/weight of each server using Random number method. The final result will be that the IMG will send the outbound INVITE message to the server with highest priority/weight and attempt to connect to that SIP server. If however after a few Re-INVITES the initial SIP Server cannot be reached, the IMG will then send an outbound INVITE to the SIP server with the next highest priority/weight. This will continue until all responses in the dns query has been exhausted at which time the call will fail. The IMG supports up to five DNS responses in each SRV Record.
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Right Click on Dialogic IMG EMS object and select New External Network Elements.
Right Click on External Network Elements object and select New External Gateways.
Right Click on External Gateways object and select New External Gateway. Below is screen capture on how to configure it. Use the screen capture and the field
Name- Enter a name into the 'Name' field
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Right Click on Dialogic IMG EMS object in object tree and select New Routing Configuration. See IMG 1010 - Routing Configuration Object link for more info.
Right Click on Routing Configuration Object just created and select New Channel Groups. See IMG 1010 - Channel Groups Link for more info.
Right Click on the Channel Groups Object just created and select New Channel Group. Verify that Signaling Type is SIP. See IMG 1010 - Channel Group Link for more information on how to configure Channel Groups.
Right Click on the Channel Group created and select New IP Network Element and select which gateway will be associated with this Channel Group. See screen capture below
You can create multiple channel groups as needed and select a different IP Network Elements for each. As long as the Gateway Host Name field in the External Gateway of the IP Network Element selected is of the format _<service>._<protocol>.<Domainname> this channel group will initiate a DNS lookup with that format. For Example: A DNS Query formatted as _sip._udp.jsmith.dialogic.com would be sent to the DNS server if Channel Group named DNS_Config from above is being used.
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When receiving multiple responses in the SRV records in the DNS response, the weight and priority fields are accessed for each SRV record. From here the Random Number Method is used to select the priority of which SIP server to send call to first.
IMG will store a maximum of 5 SRV record results per query.
The port number received in the SRV record will be provided to sip server. The SRV record will override any port numbers configured on each gateway.
The SRV target is to be used during SIP PDU encoding (Protocol Data Unit Logging), not the service name.
SIP is to handle a failure to resolve a service query in the same way as an existing query. A 480 response will be returned to the initiating gateway
CDR's are not affected by this feature. The resolved hostname will be used in the CDR record.
If an outbound proxy is enabled for an external gateway that uses a Service Name, the SRV query is resolved prior to determining the proxy parameters during a call. A SRV query will execute only on an external gateway and will not be executed on an outbound proxy hostname.
If user wants to assign an SRV record name to another IMG the user must create an external gateway equivalent to the IMG and select it from drop down list in the external network element object created under the SIP channel group.
Related Topics:
IMG 1010 - Fully Qualified Domain Name Support