不明白为什么我的DNS无法解释到www.sd.chinamobile.com，而网通ADSL提供的则可以？

dig @127.0.0.1 www.sd.chinamobile.com +trace输出如下：

; <<>> DiG 9.4.3-P1 <<>> www.sd.chinamobile.com +trace
;; global options: printcmd
. 3600000 IN NS M.ROOT-SERVERS.NET.
. 3600000 IN NS H.ROOT-SERVERS.NET.
. 3600000 IN NS E.ROOT-SERVERS.NET.
. 3600000 IN NS K.ROOT-SERVERS.NET.
. 3600000 IN NS I.ROOT-SERVERS.NET.
. 3600000 IN NS D.ROOT-SERVERS.NET.
. 3600000 IN NS A.ROOT-SERVERS.NET.
. 3600000 IN NS B.ROOT-SERVERS.NET.
. 3600000 IN NS C.ROOT-SERVERS.NET.
. 3600000 IN NS G.ROOT-SERVERS.NET.
. 3600000 IN NS J.ROOT-SERVERS.NET.
. 3600000 IN NS F.ROOT-SERVERS.NET.
. 3600000 IN NS L.ROOT-SERVERS.NET.
;; Received 228 bytes from 127.0.0.1#53(127.0.0.1) in 3 ms

com. 172800 IN NS A.GTLD-SERVERS.NET.
com. 172800 IN NS B.GTLD-SERVERS.NET.
com. 172800 IN NS C.GTLD-SERVERS.NET.
com. 172800 IN NS D.GTLD-SERVERS.NET.
com. 172800 IN NS E.GTLD-SERVERS.NET.
com. 172800 IN NS F.GTLD-SERVERS.NET.
com. 172800 IN NS G.GTLD-SERVERS.NET.
com. 172800 IN NS H.GTLD-SERVERS.NET.
com. 172800 IN NS I.GTLD-SERVERS.NET.
com. 172800 IN NS J.GTLD-SERVERS.NET.
com. 172800 IN NS K.GTLD-SERVERS.NET.
com. 172800 IN NS L.GTLD-SERVERS.NET.
com. 172800 IN NS M.GTLD-SERVERS.NET.
;; Received 500 bytes from 192.228.79.201#53(B.ROOT-SERVERS.NET) in 232 ms

chinamobile.com. 172800 IN NS ns.cnmobile.net.
chinamobile.com. 172800 IN NS ns1.cnmobile.net.
chinamobile.com. 172800 IN NS ns2.cnmobile.net.
;; Received 153 bytes from 192.26.92.30#53(C.GTLD-SERVERS.NET) in 290 ms

sd.chinamobile.com. 3600 IN NS ns1.sd.chinamobile.com.
sd.chinamobile.com. 3600 IN NS ns2.sd.chinamobile.com.
;; Received 108 bytes from 211.136.17.107#53(ns.cnmobile.net) in 8 ms

dig: couldn't get address for 'ns1.sd.chinamobile.com': not found


可能是网通的访问控制引起的，但不管是什么原因，使用bind的forward功能就可以满足我的需要，如下：

//deal with stupid chinamobile.com access control
zone "chinamobile.com" IN {
type forward;
forwarders {
202.106.195.68;
202.106.46.151;
};
};