master.c 监听来自53端口的反向链接，主要用于多个shell的协调和管理，可以输入c表示显示有几个client,然后输入个数字来选择一个，可以输入a来表示进入master mode所输入的命令会发送给每一个client-master.c monitor backlinks from 53 ports, mainly for the coordination and management of multiple shell, type c that shows several client, and then enter a number to select one, you can enter a past that into the master mode entered an order will be sent to each client