1、创建ns_g_socketmgr：
首先，套接字管理器是全局唯一的，与有多少个网络接口无关，全局变量定义在/bin/named/include/named/globals.h:
EXTERN isc_socketmgr_t * ns_g_socketmgr INIT(NULL);
#0 isc__socketmgr_create2 (mctx=0x8742d0, managerp=0x8701f8, maxsocks=0) at socket.c:4143
#1 0x000000000041919e in create_managers () at ./main.c:604
#2 0x0000000000419727 in setup () at ./main.c:850
#3 0x0000000000419a2b in main (argc=4, argv=0x7fffffffe5c8) at ./main.c:1058
使用多线程时，isc__socketmgr_create2会创建管道、select\epoll线程，工作线程通过管道控制select\epoll线程的工作。
2、扫描网络接口：
bind9启动时会扫描一下网络接口，运行期间会定时扫描，扫描间隔可以设置相应定时器，这样网络环境发生变化，bind9可以及时感知。bind9会为每一个网络接口创建两个监听套接字，为lo网络接口创建控制套接字。所以只有一个物理网卡的机器，在启动时会创建3个套接字。
udp监听套接字：
#0 isc__socket_create (manager0=0x7ffff7fa9010, pf=2, type=isc_sockettype_udp, socketp=0x7fffec7870c8) at socket.c:2580 #1 0x00000000004861cc in open_socket (mgr=0x7ffff7fa9010, local=0x7ffff7fbe290, options=1, sockp=0x7fffec7872f8) at dispatch.c:1797 #2 0x0000000000489b27 in get_udpsocket (mgr=0x7ffff7fae270, sockmgr=0x7ffff7fa9010, taskmgr=0x7ffff7fa5010, localaddr=0x7ffff7fbe290, maxrequests=<value optimized out>, attributes=44, dispp=0x7fffec787418) at dispatch.c:2792 #3 dispatch_createudp (mgr=0x7ffff7fae270, sockmgr=0x7ffff7fa9010, taskmgr=0x7ffff7fa5010, localaddr=0x7ffff7fbe290, maxrequests=<value optimized out>, attributes=44, dispp=0x7fffec787418) at dispatch.c:2860 #4 0x000000000048a042 in dns_dispatch_getudp (mgr=0x7ffff7fae270, sockmgr=0x7ffff7fa9010, taskmgr=0x7ffff7fa5010, localaddr=0x7ffff7fbe290, buffersize=<value optimized out>, maxbuffers=<value optimized out>, maxrequests=32768, buckets=8219, increment=8237, attributes=44, mask=30, dispp=0x7ffff7fbe340) at dispatch.c:2714 #5 0x000000000041520b in ns_interface_listenudp (ifp=0x7ffff7fbe250) at interfacemgr.c:261 #6 0x00000000004155e5 in ns_interface_setup (mgr=0x7ffff7fb6f70, addr=0x7fffec787700, name=0x7fffec787570 "eth0", ifpret=0x7fffec787878, accept_tcp=isc_boolean_true) at interfacemgr.c:365 #7 0x0000000000416a16 in do_scan (mgr=0x7ffff7fb6f70, ext_listen=0x0, verbose=isc_boolean_true) at interfacemgr.c:844 #8 0x0000000000416bf2 in ns_interfacemgr_scan0 (mgr=0x7ffff7fb6f70, ext_listen=0x0, verbose=isc_boolean_true) at interfacemgr.c:897 #9 0x0000000000416c92 in ns_interfacemgr_scan (mgr=0x7ffff7fb6f70, verbose=isc_boolean_true) at interfacemgr.c:923 #10 0x0000000000435107 in scan_interfaces (server=0x7ffff7fae010, verbose=isc_boolean_true) at server.c:3604 #11 0x0000000000437d60 in load_configuration (filename=0x7fffffffe850 "/var/named/named.conf", server=0x7ffff7fae010, first_time=isc_boolean_true) at server.c:4638 #12 0x0000000000439fdf in run_server (task=0x7ffff7fba010, event=0x0) at server.c:5268 #13 0x00000000005b3b15 in dispatch (manager=0x7ffff7fa5010) at task.c:1012 #14 0x00000000005b3da1 in run (uap=0x7ffff7fa5010) at task.c:1157 #15 0x0000003817a07a51 in start_thread () from /lib64/libpthread.so.0 #16 0x00000038176e896d in clone () from /lib64/libc.so.6