named 的日志中带有 "network unreachable" 信息

Issue

在 /var/log/messages 或特定的 named 日志文件（如 /var/named/data/named.run）中有大量类似如下的错误消息：

error (network unreachable) resolving 'XX.113.112.192.in-addr.arpa/PTR/IN': 2001:418:3ff:4::21#53
error (network unreachable) resolving 'XY.113.112.192.in-addr.arpa/PTR/IN': 2001:218:2000:3005::202#53
error (network unreachable) resolving 'ns.example.com/A/IN': 2001:XXX:231d::2:30#53
error (network unreachable) resolving 'www.example.jp/A/IN': 2001:XXX:c000::35#53
error (network unreachable) resolving 'www.example.jp/A/IN': 2001:XXX:ad09::5#53
error (network unreachable) resolving 'www.example.jp/A/IN': 2001:YYY:0:100::153#53
error (network unreachable) resolving 'www.example.com/A/IN': 2001:XXX:a83e::2:30#53