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 == -1)
            continue;
        switch (local_family) {
#ifdef DHCPv6 
        case AF_INET6:
            status = omapi_register_io_object((omapi_object_t *)tmp,
                              if_readsocket, 
                              0, got_one_v6, 0, 0);
            break;
#endif /* DHCPv6 */
        case AF_INET:
        default:
            status = omapi_register_io_object((omapi_object_t *)tmp,
                              if_readsocket, 
                              0, got_one, 0, 0);
            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 = 4;
        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 */