【转载请注明出处：钱国正专栏 http://blog.csdn.net/qianguozheng/article/details/37666829】

起因

为了解决客户反映的openwrt系统上不了网的问题，我们对其进行了远程协助，发现ip可以访问，只是域名访问不了，这就帮助我们定位了问题---DNS.

网上搜寻

找到了个有同样的问题，说是域名服务器不能用，同样是20M光纤拨号，真是巧合啊，到网上搜了这个哥们的网名，找到不少，但是最近发表的不多哦，无果，自己看代码查查。

代码分析

从拨号进行找了/lib/netifd脚本,无果，自己看看在线进程，发现竟然存在这么一个进程 netifd, 既然这样，那就看看源代码吧。

至此，又发现了【http://xinliang.me/blog/?p=149】，这位兄弟的博客，绝对是先驱，看他的东西很有条理，学习到了，openwrt下果然是使用ubus类似Linux发行版上的dbus， ubus主要是用于系统进程间通信，关于详情，请访问这个哥的博客。

nfetifd的ubus RPC接口如下：

<span style="font-size:18px;">static struct ubus_method main_object_methods[] = {
	{ .name = "restart", .handler = netifd_handle_restart },
	{ .name = "reload", .handler = netifd_handle_reload },
	UBUS_METHOD("add_host_route", netifd_add_host_route, route_policy),
	{ .name = "get_proto_handlers", .handler = netifd_get_proto_handlers },
	UBUS_METHOD("add_dynamic", netifd_add_dynamic, dynamic_policy),
};

static struct ubus_object_type main_object_type =
	UBUS_OBJECT_TYPE("netifd", main_object_methods);

static struct ubus_object main_object = {
	.name = "network",
	.type = &main_object_type,
	.methods = main_object_methods,
	.n_methods = ARRAY_SIZE(main_object_methods),
};

</span>

<span style="font-size:18px;">root@YSWiFi:/etc/config# ubus list -v
'dhcp' @8f0c907e
        "ipv4leases":{}
        "ipv6leases":{}
'log' @92083360
        "read":{"lines":"Integer"}
        "write":{"event":"String"}
<span style="color:#000099;">'network' @36acc569
        "restart":{}
        "reload":{}
        "add_host_route":{"target":"String","v6":"Boolean","interface":"String"}
        "get_proto_handlers":{}
        "add_dynamic":{"name":"String"}</span>
'network.device' @7b6892b7
        "status":{"name":"String"}
        "set_alias":{"alias":"Array","device":"String"}
        "set_state":{"name":"String","defer":"Boolean"}
'network.interface' @e62dccc3
        "up":{}
        "down":{}
        "status":{}
        "prepare":{}
        "dump":{}
        "add_device":{"name":"String"}
        "remove_device":{"name":"String"}
        "notify_proto":{}
        "remove":{}
        "set_data":{}
'network.interface.lan' @817ffe0f
        "up":{}
        "down":{}
        "status":{}
        "prepare":{}
        "dump":{}
        "add_device":{"name":"String"}
        "remove_device":{"name":"String"}
        "notify_proto":{}
        "remove":{}
        "set_data":{}
'network.interface.lan2' @137330bd
        "up":{}
        "down":{}
        "status":{}
        "prepare":{}
        "dump":{}
        "add_device":{"name":"String"}
        "remove_device":{"name":"String"}
        "notify_proto":{}
        "remove":{}
        "set_data":{}
'network.interface.loopback' @44806906
        "up":{}
        "down":{}
        "status":{}
        "prepare":{}
        "dump":{}
        "add_device":{"name":"String"}
        "remove_device":{"name":"String"}
        "notify_proto":{}
        "remove":{}
        "set_data":{}
'network.interface.wan' @4ebe9d3b
        "up":{}
        "down":{}
        "status":{}
        "prepare":{}
        "dump":{}
        "add_device":{"name":"String"}
        "remove_device":{"name":"String"}
        "notify_proto":{}
        "remove":{}
        "set_data":{}
'network.wireless' @9a257aa5
        "up":{}
        "down":{}
        "status":{}
        "notify":{}
'service' @582b527e
        "set":{"name":"String","script":"String","instances":"Table","triggers":"Array","validate":"Array"}
        "add":{"name":"String","script":"String","instances":"Table","triggers":"Array","validate":"Array"}
        "list":{"name":"String"}
        "delete":{"name":"String","instance":"String"}
        "update_start":{"name":"String"}
        "update_complete":{"name":"String"}
        "event":{"type":"String","data":"Table"}
        "validate":{"package":"String","type":"String","service":"String"}
'system' @d8c56a4b
        "board":{}
        "info":{}
        "upgrade":{}
        "watchdog":{"frequency":"Integer","timeout":"Integer","stop":"Boolean"}
        "signal":{"pid":"Integer","signum":"Integer"}
root@YSWiFi:/etc/config# </span>

netifd.h

<span style="font-size:18px;">#ifdef DUMMY_MODE
#define DEFAULT_MAIN_PATH	"./examples"
#define DEFAULT_CONFIG_PATH	"./config"
#define DEFAULT_HOTPLUG_PATH	"./examples/hotplug-cmd"
#define DEFAULT_RESOLV_CONF	"./tmp/resolv.conf"
#else
#define DEFAULT_MAIN_PATH	"/lib/netifd"
#define DEFAULT_CONFIG_PATH	NULL /* use the default set in libuci */
#define DEFAULT_HOTPLUG_PATH	"/sbin/hotplug-call"
<span style="color:#000099;">#define DEFAULT_RESOLV_CONF	"/tmp/resolv.conf.auto"</span>
#endif</span>

interface-ip.c

<span style="font-size:18px;">void
interface_write_resolv_conf(void)
{
	struct interface *iface;
	char *path = alloca(strlen(resolv_conf) + 5);
	FILE *f;
	uint32_t crcold, crcnew;

	sprintf(path, "%s.tmp", resolv_conf);
	unlink(path);
	f = fopen(path, "w+");
	if (!f) {
		D(INTERFACE, "Failed to open %s for writing\n", path);
		return;
	}

	vlist_for_each_element(&interfaces, iface, node) {
		if (iface->state != IFS_UP)
			continue;

		if (vlist_simple_empty(&iface->proto_ip.dns_search) &&
		    vlist_simple_empty(&iface->proto_ip.dns_servers) &&
			vlist_simple_empty(&iface->config_ip.dns_search) &&
		    vlist_simple_empty(&iface->config_ip.dns_servers))
			continue;

		fprintf(f, "# Interface %s\n", iface->name);
		write_resolv_conf_entries(f, &iface->config_ip);
		if (!iface->proto_ip.no_dns)
			write_resolv_conf_entries(f, &iface->proto_ip);
	}
	fflush(f);
	rewind(f);
	crcnew = crc32_file(f);
	fclose(f);

	crcold = crcnew + 1;
	f = fopen(resolv_conf, "r");
	if (f) {
		crcold = crc32_file(f);
		fclose(f);
	}

	if (crcold == crcnew) {
		unlink(path);
	} else if (rename(path, resolv_conf) < 0) {
		D(INTERFACE, "Failed to replace %s\n", resolv_conf);
		unlink(path);
	}
}
</span>

总结：

虽然找到了在哪里生产的文件，要一下子解决或者是重现这个问题可不容易。所以我采取了一种折中的方法，指定每个客户端的DNS服务器为google的DNS服务器8.8.8.8 , 8.8.4.4.

【转载请注明出处：钱国正专栏 http://blog.csdn.net/qianguozheng/article/details/37666829】