When an SGSN initiates an access point name (APN) resolution request to the DNS server, the resolution can be performed successfully if the request is sent for a local mobile station (MS) but the resolution fails if the request is sent for a roaming MS.

http://localhost:7904/pages/31185199/02/31185199/02/resources/figure/ch/dns_04.png

The root DNS server has been connected to the network and recursive query is complete on the site. Log in to the DNS server by Telnet and run nslookup. You can find that the root DNS server is capable of resolving the APN requested by a roaming MS.

http://localhost:7904/pages/31185199/02/31185199/02/resources/public_sys-resources/icon-note.gif NOTE:

The BIND in use is BIND 9.4.2.

Possible Causes

The allow-query field is not configured on the DNS server.

Fault Diagnosis

• The packets captured on the Gn/Gp interface and DNS server trace results show that the DNS server does not support recursive query. In the APN resolution request sent by the SGSN to the DNS server, the value of the Recursion field is 0.
• When a PC is used to simulate an SGSN and is used to initiate an APN resolution request, the APN resolution is successful if the MS requesting the APN is a roaming MS. In addition, the value of the Recursion field is 1 which indicates that the DNS server supports recursive query. This indicates that the problem may be caused by the packets sent by the SGSN. The APN resolution requests initiated by the SGSN and PC are in fact the same and the SGSN and PC are the same from the perspective of the DNS server.
• Different from earlier BIND versions, BIND 9.4.2 does not accept any resolution request sent from an SGSN that is not in the same network segment as the DNS server if the allow-query { any; }; field is not available in the named.conf file. If the allow-query { any; }; field is not available in the file, only APNs sent from local MSs can be resolved. When the allow-query { any; }; field is available in the named.conf file, APNs sent from roaming MSs can also be resolved. If the DNS server is in the same network segment as the SGSN, the DNS server accepts the resolution requests sent from the SGSN.

Procedure

1. Add the allow-query { any; }; field to the named.conf file.
2. Make the SGSN initiate an APN resolution request for a roaming MS.

   The DNS server supports recursive query and successfully returns the GGSN address.

   Configuration example:

   options {      directory "/var/named";                //Specifies /var/named as the working directory of the named        pid-file   "/var/named/named.pid";   //Specifies named.pid as the named process ID allow-query { any; };                //Allows recursive query requests to be sent from a device in a network segment to a device in another network segment          forwarders  {                      //Root DNS in the case of recursive query                     211.136.17.107;                                            211.136.17.171;                     }; }; zone "ln.mnc000.mcc460.gprs" in {            //The zone is *.ln. type master;                                 //Defines the Master name server        file "named.ln";                          //Defines the domain name resolution file allow-query { any; };                    //Allows recursive query requests to be sent from a device in a network segment to a device in another network segment };

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What is the difference between  Gn/Gp interface & Gn interface?

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