The domain name system (DNS) server used on a site runs the SUN operating system and uses BIND 9.4.2. When the SGSN initiates a recursive access point name (APN) resolution request for a roaming mobile station (MS), the resolution time is sometimes excessively long and even exceeds the timeout interval. This results in resolution failures. The APN can be resolved only after multiple recursive resolution requests are sent.

Possible Causes

The format of packets is not supported by the DNS server.

Fault Diagnosis

EDNS0 packets are sent from BIND 9.0 or BIND 8.3. The default packets supported by the root DNS server conform to RFC1035 that restricts the size of a UDP packet to 512 bytes. EDNS0, however, can be used to transmit UDP packets whose sizes exceed 512 bytes.

The first APN resolution request sent by the local DNS server is in the EDNS0 format that is not supported by the root DNS server. As a result, the root DNS serverreturns a "Format Error" message to the local DNS server. When sending the second APN resolution request, the local DNS server initiates an RFC1035 packet that is supported by the root DNS server. The first APN resolution request of the local DNS server may time out and it is possible that the resolution can be performed only after more than one APN resolution request is sent.

Procedure

1. Ping the local DNS server from the SGSN and check the ping response time. Ping the root DNS server from the DNS server and check the ping response time.

   If the response time is several milliseconds, this cause can be excluded.

2. Check whether a resolution request has been sent to the local DNS server or the local DNS server has sent a resolution request to the root DNS server.

   Packets captured on the local switch and the switch on the user side indicate that every request sent by the SGSN arrives at the local DNS server rapidly and the local DNS server sends the request to the root DNS server rapidly. Therefore, this cause can also be excluded.

3. Check whether captured packets from the root DNS server indicate "Format Error."

   You can find that the problem is caused by the fact that the packets sent by the local DNS server are not supported by the root DNS server.

4. Add the following commands to the named.conf file on the Master and Slave DNS servers:

   Server 202.123.213.94 {             edns no; }; Server 203.208.133.110 {             edns no; };

5. Disable the EDNS0 format forcibly and make the local DNS server send recursive resolution requests that are in the RF1035 format. In the command, the IP address is the IP address of the root DNS server. Then, check whether the problem persists.

It will enrich my knowledge.Thanks

This is good post.Thank you............

sound good.....Thanks

Very Technical.......

Technically very sound and this is very rich doc.Thanks a lot for such type of doc and share more the same type of doc pls.

DNS expert..........