wifidog 源码初分析（一）

wifidog 的核心还是依赖于 iptables 防火墙过滤规则来实现的，所以建议对 iptables 有了了解后再去阅读 wifidog 的源码。
在路由器上启动 wifidog 之后，wifidog 在启动时会初始化一堆的防火墙规则，如下：

/** Initialize the firewall rules
*/
int iptables_fw_init(void)
{
    const s_config *config;
    char * ext_interface = NULL;
    int gw_port = 0;
    t_trusted_mac *p;

    fw_quiet = 0;

    LOCK_CONFIG();
    config = config_get_config();
    gw_port = config->gw_port;
    if (config->external_interface) {
            ext_interface = safe_strdup(config->external_interface);
    } else {
            ext_interface = get_ext_iface();
    }

    if (ext_interface == NULL) {
            UNLOCK_CONFIG();
            debug(LOG_ERR, "FATAL: no external interface");
            return 0;
    }
    /*
     *
     * Everything in the MANGLE table
     *
     */

    /* Create new chains */
    iptables_do_command("-t mangle -N " TABLE_WIFIDOG_TRUSTED);
    iptables_do_command("-t mangle -N " TABLE_WIFIDOG_OUTGOING);
    iptables_do_command("-t mangle -N " TABLE_WIFIDOG_INCOMING);

    /* Assign links and rules to these new chains */
    iptables_do_command("-t mangle -I PREROUTING 1 -i %s -j " TABLE_WIFIDOG_OUTGOING, config->gw_interface);
    iptables_do_command("-t mangle -I PREROUTING 1 -i %s -j " TABLE_WIFIDOG_TRUSTED, config->gw_interface);//this rule will be inserted before the prior one
    iptables_do_command("-t mangle -I POSTROUTING 1 -o %s -j " TABLE_WIFIDOG_INCOMING, config->gw_interface);

    for (p = config->trustedmaclist; p != NULL; p = p->next)
            iptables_do_command("-t mangle -A " TABLE_WIFIDOG_TRUSTED " -m mac --mac-source %s -j MARK --set-mark %d", p->mac, FW_MARK_KNOWN);

    /*
     *
     * Everything in the NAT table
     *
     */

    /* Create new chains */
    iptables_do_command("-t nat -N " TABLE_WIFIDOG_OUTGOING);
    iptables_do_command("-t nat -N " TABLE_WIFIDOG_WIFI_TO_ROUTER);
    iptables_do_command("-t nat -N " TABLE_WIFIDOG_WIFI_TO_INTERNET);
    iptables_do_command("-t nat -N " TABLE_WIFIDOG_GLOBAL);
    iptables_do_command("-t nat -N " TABLE_WIFIDOG_UNKNOWN);
    iptables_do_command("-t nat -N " TABLE_WIFIDOG_AUTHSERVERS);

    /* Assign links and rules to these new chains */
    iptables_do_command("-t nat -A PREROUTING -i %s -j " TABLE_WIFIDOG_OUTGOING, config->gw_interface);

    iptables_do_command("-t nat -A " TABLE_WIFIDOG_OUTGOING " -d %s -j " TABLE_WIFIDOG_WIFI_TO_ROUTER, config->gw_address);
    iptables_do_command("-t nat -A " TABLE_WIFIDOG_WIFI_TO_ROUTER " -j ACCEPT");

    iptables_do_command("-t nat -A " TABLE_WIFIDOG_OUTGOING " -j " TABLE_WIFIDOG_WIFI_TO_INTERNET);
    iptables_do_command("-t nat -A " TABLE_WIFIDOG_WIFI_TO_INTERNET " -m mark --mark 0x%u -j ACCEPT", FW_MARK_KNOWN);
    iptables_do_command("-t nat -A " TABLE_WIFIDOG_WIFI_TO_INTERNET " -m mark --mark 0x%u -j ACCEPT", FW_MARK_PROBATION);
    iptables_do_command("-t nat -A " TABLE_WIFIDOG_WIFI_TO_INTERNET " -j " TABLE_WIFIDOG_UNKNOWN);

    iptables_do_command("-t nat -A " TABLE_WIFIDOG_UNKNOWN " -j " TABLE_WIFIDOG_AUTHSERVERS);
    iptables_do_command("-t nat -A " TABLE_WIFIDOG_UNKNOWN " -j " TABLE_WIFIDOG_GLOBAL);
    iptables_do_command("-t nat -A " TABLE_WIFIDOG_UNKNOWN " -p tcp --dport 80 -j REDIRECT --to-ports %d", gw_port);


    /*
     *
     * Everything in the FILTER table
     *
     */

    /* Create new chains */
    iptables_do_command("-t filter -N " TABLE_WIFIDOG_WIFI_TO_INTERNET);
    iptables_do_command("-t filter -N " TABLE_WIFIDOG_AUTHSERVERS);
    iptables_do_command("-t filter -N " TABLE_WIFIDOG_LOCKED);
    iptables_do_command("-t filter -N " TABLE_WIFIDOG_GLOBAL);
    iptables_do_command("-t filter -N " TABLE_WIFIDOG_VALIDATE);
    iptables_do_command("-t filter -N " TABLE_WIFIDOG_KNOWN);
    iptables_do_command("-t filter -N " TABLE_WIFIDOG_UNKNOWN);

    /* Assign links and rules to these new chains */

    /* Insert at the beginning */
    iptables_do_command("-t filter -I FORWARD -i %s -j " TABLE_WIFIDOG_WIFI_TO_INTERNET, config->gw_interface);


    iptables_do_command("-t filter -A " TABLE_WIFIDOG_WIFI_TO_INTERNET " -m state --state INVALID -j DROP");

    /* XXX: Why this? it means that connections setup after authentication
       stay open even after the connection is done... 
       iptables_do_command("-t filter -A " TABLE_WIFIDOG_WIFI_TO_INTERNET " -m state --state RELATED,ESTABLISHED -j ACCEPT");*/

    //Won't this rule NEVER match anyway?!?!? benoitg, 2007-06-23
    //iptables_do_command("-t filter -A " TABLE_WIFIDOG_WIFI_TO_INTERNET " -i %s -m state --state NEW -j DROP", ext_interface);

    /* TCPMSS rule for PPPoE */
    iptables_do_command("-t filter -A " TABLE_WIFIDOG_WIFI_TO_INTERNET " -o %s -p tcp --tcp-flags SYN,RST SYN -j TCPMSS --clamp-mss-to-pmtu", ext_interface);

    iptables_do_command("-t filter -A " TABLE_WIFIDOG_WIFI_TO_INTERNET " -j " TABLE_WIFIDOG_AUTHSERVERS);
    iptables_fw_set_authservers();

    iptables_do_command("-t filter -A " TABLE_WIFIDOG_WIFI_TO_INTERNET " -m mark --mark 0x%u -j " TABLE_WIFIDOG_LOCKED, FW_MARK_LOCKED);
    iptables_load_ruleset("filter", "locked-users", TABLE_WIFIDOG_LOCKED);

    iptables_do_command("-t filter -A " TABLE_WIFIDOG_WIFI_TO_INTERNET " -j " TABLE_WIFIDOG_GLOBAL);
    iptables_load_ruleset("filter", "global", TABLE_WIFIDOG_GLOBAL);
    iptables_load_ruleset("nat", "global", TABLE_WIFIDOG_GLOBAL);

    iptables_do_command("-t filter -A " TABLE_WIFIDOG_WIFI_TO_INTERNET " -m mark --mark 0x%u -j " TABLE_WIFIDOG_VALIDATE, FW_MARK_PROBATION);
    iptables_load_ruleset("filter", "validating-users", TABLE_WIFIDOG_VALIDATE);

    iptables_do_command("-t filter -A " TABLE_WIFIDOG_WIFI_TO_INTERNET " -m mark --mark 0x%u -j " TABLE_WIFIDOG_KNOWN, FW_MARK_KNOWN);
    iptables_load_ruleset("filter", "known-users", TABLE_WIFIDOG_KNOWN);

    iptables_do_command("-t filter -A " TABLE_WIFIDOG_WIFI_TO_INTERNET " -j " TABLE_WIFIDOG_UNKNOWN);
    iptables_load_ruleset("filter", "unknown-users", TABLE_WIFIDOG_UNKNOWN);
    iptables_do_command("-t filter -A " TABLE_WIFIDOG_UNKNOWN " -j REJECT --reject-with icmp-port-unreachable");

    UNLOCK_CONFIG();
    return 1;
}

在该 防火墙规则的初始化过程中，会首先清除掉已有的防火墙规则，重新创建新的过滤链，另外，除了通过iptables_do_command("-t nat -A "TABLE_WIFIDOG_UNKNOWN " -p tcp --dport 80 -j REDIRECT --to-ports %d",gw_port); 这个命令将 接入设备的 80 端口（HTTP）的访问重定向至网关自身的 HTTP 的端口之外，还通过iptables_fw_set_authservers(); 函数设置了 鉴权服务器(auth-server) 的防火墙规则：

void iptables_fw_set_authservers(void)
{
    const s_config *config;
    t_auth_serv *auth_server;

    config = config_get_config();

    for (auth_server = config->auth_servers; auth_server != NULL; auth_server = auth_server->next) {
        if (auth_server->last_ip && strcmp(auth_server->last_ip, "0.0.0.0") != 0) {
            iptables_do_command("-t filter -A " TABLE_WIFIDOG_AUTHSERVERS " -d %s -j ACCEPT", auth_server->last_ip);
            iptables_do_command("-t nat -A " TABLE_WIFIDOG_AUTHSERVERS " -d %s -j ACCEPT", auth_server->last_ip);
        }
    }
}

首先从上面的代码可以看出 wifidog 支持多个鉴权服务器，并且针对每一个鉴权服务器设置了如下两条规则：
1)在filter表中追加一条[任何访问鉴权服务器都被接受]的WiFiDog_$ID$AuthServers过滤链：iptables -t filter -A WiFiDog$ID$AuthServers -d auth-server地址 -j ACCEPT
2)在nat表中追加一条[任何访问鉴权服务器都被接受]的WiFiDog$ID$AuthServers过滤链：iptables -t nat -A WiFiDog$ID$_AuthServers -d auth-server地址 -j ACCEPT
这样确保可以访问鉴权服务器，而不是拒绝所有的出口访问。

本文章由 http://www.wifidog.pro/2014/12/08/wifidog%E6%BA%90%E7%A0%81%E5%88%86%E6%9E%90.html整理编辑，转载请注明出处

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