DHCP源码分析--主流程

DHCP 服务器，客户端代码都采用了统一的事件轮询(event loop)，包含了任务处理消息，定时器消息，socke收发消息等等。

 static struct {
     isc_appmethods_t methods;

     /*%
      * The following are defined just for avoiding unused static functions.
      */
 #ifndef BIND9
     void *run, *shutdown, *start, *onrun, *reload, *finish,
         *block, *unblock;
 #endif
 } appmethods = {
     {
         isc__appctx_destroy,
         isc__app_ctxstart,
         isc__app_ctxrun,
         isc__app_ctxsuspend,
         isc__app_ctxshutdown,
         isc__app_ctxfinish,
         isc__appctx_settaskmgr,
         isc__appctx_setsocketmgr,
         isc__appctx_settimermgr
     }
 #ifndef BIND9
     ,
     (void *)isc__app_run, (void *)isc__app_shutdown,
     (void *)isc__app_start, (void *)isc__app_onrun, (void *)isc__app_reload,
     (void *)isc__app_finish, (void *)isc__app_block,
     (void *)isc__app_unblock
 #endif
 };
         (void)isc__taskmgr_dispatch(ctx->taskmgr);

     result = evloop(ctx);
     if (result != ISC_R_SUCCESS)
         return (result);

网卡接口对象的收包处理函数注册：

 for (tmp = interfaces; tmp; tmp = tmp -> next) {
         /* not if it's been registered before */
         if (tmp -> flags & INTERFACE_RUNNING)
             continue;
         if (tmp -> rfdesc == -)
             continue;
         switch (local_family) {
 #ifdef DHCPv6
         case AF_INET6:
             status = omapi_register_io_object((omapi_object_t *)tmp,
                               if_readsocket,
                               , got_one_v6, , );
             break;
 #endif /* DHCPv6 */
         case AF_INET:
         default:
             status = omapi_register_io_object((omapi_object_t *)tmp,
                               if_readsocket,
                               , got_one, , );
             break;
         }

         if (status != ISC_R_SUCCESS)
             log_fatal ("Can't register I/O handle for %s: %s",
                    tmp -> name, isc_result_totext (status));

 #if defined(DHCPv6)
         /* Only register the first interface for V6, since they all
          * use the same socket.  XXX: This has some messy side
          * effects if we start dynamically adding and removing
          * interfaces, but we're well beyond that point in terms of
          * mess.
          */
         if (local_family == AF_INET6)
             break;
 #endif
     } /* for (tmp = interfaces; ... */

omapi_register_io_object函数将收包处理函数got_one与对应的接口对象指针关联：

 status = omapi_object_reference (&obj -> inner, h, MDL);
     if (status != ISC_R_SUCCESS) {
         omapi_io_dereference (&obj, MDL);
         return status;
     }

     status = omapi_object_reference (&h -> outer,
                      (omapi_object_t *)obj, MDL);
     if (status != ISC_R_SUCCESS) {
         omapi_io_dereference (&obj, MDL);
         return status;
     }

         ...
         obj -> reader = reader;

reader 就是指向 got_one 函数的函数指针参数，这样就可以从主流程main函数入口进入到收包处理流程了。

 isc_result_t got_one (h)
     omapi_object_t *h; {
     ....
     if (bootp_packet_handler) {
         ifrom.len = ;
         memcpy (ifrom.iabuf, &from.sin_addr, ifrom.len);

         (*bootp_packet_handler) (ip, &u.packet, (unsigned)result,
                      from.sin_port, ifrom, &hfrom);
     }
     ....
 }

bootp_packet_handler 即是 main函数设置的包处理函数：

 /* Set up various hooks. */
     dhcp_interface_setup_hook = dhcpd_interface_setup_hook;
     bootp_packet_handler = do_packet;
 #ifdef DHCPv6
     dhcpv6_packet_handler = do_packet6;
 #endif /* DHCPv6 */