构造一个DNS报文，查了好些资料，好象许多朋友对UDP报文的校验和老是出问题，自己试了一些例子代码，也有问题。所以花了一些时间，做了一个成功的例子，在Linux下GCC通过并验证成功-Construct a DNS packet, to check better information, and if many friends UDP packet checksum always problems, his trial some example code, there are problems. So I spent some time doing a successful example under Linux GCC through and verify success