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ip_forward参数对Linux内核转发影响分析

helloworld 发表于 2年前  | 评论(0 )| 阅读次数(1017 )|   0 人收藏此文章,   我要收藏

在进行Linux内核转发时,需要在proc文件系统的proc/sys目录设置转发的参数,可以使用下面的方法查看该参数的值 cat /proc/sys/net/ipv4/ip_forward,该参数的默认值为0,可以使用下面的方法进行修改该值,使能Linux内核的IP层的数据抓发,但是下面的方法在系统重启后不再生效。

echo 1 > /proc/sys/net/ipv4/ip_forward

         Linux系统中也提供了一个系统的配置工具sysctl,使用它可以读取和配置Linux内核的一些参数。但是该方法和proc文件系统相关,使用该工具Linux内核需要支持proc文件系统。下面是使用sysctl配置内核的转发参数。

# sysctl net.ipv4.ip_forward

net.ipv4.ip_forward = 0

/ #  sysctl -w net.ipv4.ip_forward=1

net.ipv4.ip_forward = 1

/ # sysctl net.ipv4.ip_forward

net.ipv4.ip_forward = 1

/ #

注意,参数 net.ipv4.ip_forward 实际是对应的 proc 目录/proc/sys/net/ipv4/ip_forward,选项 -w 表示配置该内核配置参数,没有选项表示读内核配置参数,不加任何选项信息,就表示读取操作。

通过上面的方法我们可以设置和读取IP转发的参数。但是本文重点不是讲该参数如何配置,而是在配置完成后,在内核的转发过程中如何生效的,以及如何配置到内核中。既然,该参数是配置使能IP层的转发,那应该在Linux内核的转发部分对该参数进行了判断,该参数的判断实际上是在查找路由时进行判断的,下面这张图显示了其中的调用关系,

在查路由的过程中,如果是转发的数据包调用下面的宏判断转发的参数是否开启。在函数ip_route_input_slow

if (!IN_DEV_FORWARD(in_dev))

            goto e_hostunreach;

         看一下该宏是如何进行定义的,下面的宏定义在include/linux/inetdevice.h文件中。

#define IN_DEV_FORWARD(in_dev)                 IN_DEV_CONF_GET((in_dev), FORWARDING)

在把IN_DEV_CONF_GET宏进一步展开了看:

#define IN_DEV_CONF_GET(in_dev, attr) \

         ipv4_devconf_get((in_dev), NET_IPV4_CONF_ ## attr)//这里的##表示连接两个字符串。

         下面是ipv4_devconf_get函数的定义:

static inline int ipv4_devconf_get(struct in_device *in_dev, int index)

{

         index--;//这里的index相当于NET_IPV4_CONF_FORWARDING

         return in_dev->cnf.data[index];// init_net->ipv4.devconf_dfl.data[0]

}

1)对于宏NET_IPV4_CONF_ FORWARDING定义在include/linux/sysctl.h文件中,是一个枚举类型的。

enum

{

         NET_IPV4_CONF_FORWARDING=1,

         NET_IPV4_CONF_MC_FORWARDING=2,

         NET_IPV4_CONF_PROXY_ARP=3,

         NET_IPV4_CONF_ACCEPT_REDIRECTS=4,

         NET_IPV4_CONF_SECURE_REDIRECTS=5,

         NET_IPV4_CONF_SEND_REDIRECTS=6,

         NET_IPV4_CONF_SHARED_MEDIA=7,

         NET_IPV4_CONF_RP_FILTER=8,

         NET_IPV4_CONF_ACCEPT_SOURCE_ROUTE=9,

         NET_IPV4_CONF_BOOTP_RELAY=10,

         NET_IPV4_CONF_LOG_MARTIANS=11,

         NET_IPV4_CONF_TAG=12,

         NET_IPV4_CONF_ARPFILTER=13,

         NET_IPV4_CONF_MEDIUM_ID=14,

         NET_IPV4_CONF_NOXFRM=15,

         NET_IPV4_CONF_NOPOLICY=16,

         NET_IPV4_CONF_FORCE_IGMP_VERSION=17,

         NET_IPV4_CONF_ARP_ANNOUNCE=18,

         NET_IPV4_CONF_ARP_IGNORE=19,

         NET_IPV4_CONF_PROMOTE_SECONDARIES=20,

         NET_IPV4_CONF_ARP_ACCEPT=21,

         NET_IPV4_CONF_ARP_NOTIFY=22,

         NET_IPV4_CONF_SRC_VMARK=24,

         __NET_IPV4_CONF_MAX

};

2)对于return in_dev->cnf.data[index];返回的相当于in_dev->cnf.data[0],那下面我们看一下该初始值是如何产生的。

         首先,in_dev是怎么获取到的,在ip_route_input_slow函数中通过struct in_device *in_dev = in_dev_get(dev);函数获取,在in_dev_get函数中调用__in_dev_get_rcu,通过下面的赋值语句进行赋值struct in_device *in_dev = dev->ip_ptr;

static inline struct in_device *__in_dev_get_rcu(const struct net_device *dev)

{

         struct in_device *in_dev = dev->ip_ptr;

         if (in_dev)

                   in_dev = rcu_dereference(in_dev);

         return in_dev;

}

 

static __inline__ struct in_device *

in_dev_get(const struct net_device *dev)

{

         struct in_device *in_dev;

 

         rcu_read_lock();

         in_dev = __in_dev_get_rcu(dev);

         if (in_dev)

                   atomic_inc(&in_dev->refcnt);

         rcu_read_unlock();

         return in_dev;

}

         dev->ip_ptr;又是什么时候赋值呢?答案是在net_device注册初始化函数inetdev_init中,

static struct in_device *inetdev_init(struct net_device *dev)

{

         struct in_device *in_dev;

 

         ASSERT_RTNL();

 

         in_dev = kzalloc(sizeof(*in_dev), GFP_KERNEL);

         if (!in_dev)

                   goto out;

         memcpy(&in_dev->cnf, dev_net(dev)->ipv4.devconf_dflt,

                            sizeof(in_dev->cnf));//这里对in_dev->cnt进行初始化操作,---1

         in_dev->cnf.sysctl = NULL;

         in_dev->dev = dev;

         if ((in_dev->arp_parms = neigh_parms_alloc(dev, &arp_tbl)) == NULL)

                   goto out_kfree;

         if (IPV4_DEVCONF(in_dev->cnf, FORWARDING))

                   dev_disable_lro(dev);

         /* Reference in_dev->dev */

         dev_hold(dev);

         /* Account for reference dev->ip_ptr (below) */

         in_dev_hold(in_dev);

 

         devinet_sysctl_register(in_dev);

         ip_mc_init_dev(in_dev);

         if (dev->flags & IFF_UP)

                   ip_mc_up(in_dev);

 

         /* we can receive as soon as ip_ptr is set -- do this last */

         rcu_assign_pointer(dev->ip_ptr, in_dev);//使用RCU保护锁机制对dev->ip_ptr进行赋值

out:

         return in_dev;

out_kfree:

         kfree(in_dev);

         in_dev = NULL;

         goto out;

}

1dev_net(dev)->ipv4.devconf_dfl 也就相当于init_net->ipv4.devconf_dfl,而devconf_dfl的初始化时在/net/ipv4/devinet.c文件中,devinet_init_net函数中,

static struct ipv4_devconf ipv4_devconf_dflt = {

         .data = {

                   [NET_IPV4_CONF_ACCEPT_REDIRECTS - 1] = 1,

                   [NET_IPV4_CONF_SEND_REDIRECTS - 1] = 1,

                   [NET_IPV4_CONF_SECURE_REDIRECTS - 1] = 1,

                   [NET_IPV4_CONF_SHARED_MEDIA - 1] = 1,

                   [NET_IPV4_CONF_ACCEPT_SOURCE_ROUTE - 1] = 1,

         },

};//这里并没有对FORWARDING进行赋值操作

static __net_init int devinet_init_net(struct net *net)

{

         int err;

         struct ipv4_devconf *all, *dflt;

#ifdef CONFIG_SYSCTL

         struct ctl_table *tbl = ctl_forward_entry;

         struct ctl_table_header *forw_hdr;

#endif

 

         err = -ENOMEM;

         all = &ipv4_devconf; //----------------------------进行初始化操作

         dflt = &ipv4_devconf_dflt;

 

         if (net != &init_net) {

                   all = kmemdup(all, sizeof(ipv4_devconf), GFP_KERNEL);

                   if (all == NULL)

                            goto err_alloc_all;

 

                   dflt = kmemdup(dflt, sizeof(ipv4_devconf_dflt), GFP_KERNEL);

                   if (dflt == NULL)

                            goto err_alloc_dflt;

 

#ifdef CONFIG_SYSCTL

                   tbl = kmemdup(tbl, sizeof(ctl_forward_entry), GFP_KERNEL);

                   if (tbl == NULL)

                            goto err_alloc_ctl;

 

                   tbl[0].data = &all->data[NET_IPV4_CONF_FORWARDING - 1];

                   tbl[0].extra1 = all;

                   tbl[0].extra2 = net;

#endif

         }

 

#ifdef CONFIG_SYSCTL

         err = __devinet_sysctl_register(net, "all",

                            NET_PROTO_CONF_ALL, all);

         if (err < 0)

                   goto err_reg_all;

 

         err = __devinet_sysctl_register(net, "default",

                            NET_PROTO_CONF_DEFAULT, dflt);

         if (err < 0)

                   goto err_reg_dflt;

 

         err = -ENOMEM;

         forw_hdr = register_net_sysctl_table(net, net_ipv4_path, tbl);

         if (forw_hdr == NULL)

                   goto err_reg_ctl;

         net->ipv4.forw_hdr = forw_hdr;

#endif

 

         net->ipv4.devconf_all = all;//这里对net->ipv4_devconfi_all进行了初始化

         net->ipv4.devconf_dflt = dflt;// //这里对net->devconf_dflt进行了初始化

         return 0;

………………………….

}

上面的函数对net相关功能的初始化,在devinet.c文件中还有一个和ipv4_devconf_dflt类似的变量ipv4_devconf,但是IN_DEV_FORWARD(in_dev)宏读取的是结构体ipv4_devconf_dflt中变量的值,所以,如果要在Linux内核中修改转发的参数时,需要在ipv4_devconf_dflt中添加才能生效。

static struct ipv4_devconf ipv4_devconf = {

         .data = {

                   [NET_IPV4_CONF_ACCEPT_REDIRECTS - 1] = 1,

                   [NET_IPV4_CONF_SEND_REDIRECTS - 1] = 1,

                   [NET_IPV4_CONF_SECURE_REDIRECTS - 1] = 1,

                   [NET_IPV4_CONF_SHARED_MEDIA - 1] = 1,

                   [NET_IPV4_CONF_FORCE_IGMP_VERSION-1]=2,

         },

};

3)下面看一下使用echo 1 > /proc/sys/net/ipv4/ip_forward配置语句如何是Linux内核IP转发生效的。

      在上面的devinet_init_net()函数中,有下面的两段代码

         struct ctl_table *tbl = ctl_forward_entry;

         forw_hdr = register_net_sysctl_table(net, net_ipv4_path, tbl);

其中ctl_forward_entry定义为下面的结构,

static struct ctl_table ctl_forward_entry[] = {

         {

                   .ctl_name         = NET_IPV4_FORWARD,//一个ID

                   .procname        = "ip_forward",//字符串,包含在proc/sys下目录项,实际为proc/sys目录下的文件名

                   .data                  = &ipv4_devconf.data[

                                               NET_IPV4_CONF_FORWARDING - 1],//回调函数设置的值

                   .maxlen             = sizeof(int),//设置值的最大长度

                   .mode                = 0644,//文件的权限,也就是ip_forward文件的权限

                   .proc_handler  = devinet_sysctl_forward,// /proc/sys下面的文件修改的时候调用该回调函数。

                   .strategy = devinet_conf_sysctl,// sysctl读写系统参数时候调用该回调函数

                   .extra1              = &ipv4_devconf,

                   .extra2              = &init_net,

         },

         { },

};

         forw_hdr = register_net_sysctl_table(net, net_ipv4_path, tbl);用于动态注册系统控制功能,其中net_ipv4_path定义为下面的形式。也就是proc/sys/下的目录名,tbl就是上面的ctl_forward_entry[]结构体。

static __net_initdata struct ctl_path net_ipv4_path[] = {

         { .procname = "net", .ctl_name = CTL_NET, },

         { .procname = "ipv4", .ctl_name = NET_IPV4, },

         { },

};

         使用echo 1 > /proc/sys/net/ipv4/ip_forward调用devinet_sysctl_forward函数进行处理,下面是该函数的定义实现。其中参数write1表示写配置,为0表示读取配置值,buffer是要写的值,lenpbuffer的大小,ppos为位置。这里的__user是告诉不应该解除该指针的引用,因为在当前地址空间中它是没有意义的,所以对于这种变量,在kernel中使用要用到copy_to_usercopy_from_user

static int devinet_sysctl_forward(ctl_table *ctl, int write,

                                       void __user *buffer,

                                       size_t *lenp, loff_t *ppos)

{

         int *valp = ctl->data;//获取&ipv4_devconf.data地址

         int val = *valp;

         loff_t pos = *ppos;

         int ret = proc_dointvec(ctl, write, buffer, lenp, ppos);//该函数处理传进来的int型,proc_dostring处理传过来的字符串。

/* ctl->data change  echo "0" >/proc/sys/net/ipv4/ip_forward  write = 1 *valp = 0 val = 1 */

         if (write && *valp != val) {

                   struct net *net = ctl->extra2;

 

                   if (valp != &IPV4_DEVCONF_DFLT(net, FORWARDING)) {

                            if (!rtnl_trylock()) {

                                     /* Restore the original values before restarting */

                                     *valp = val;

                                     *ppos = pos;

                                     return restart_syscall();

                            }

                            if (valp == &IPV4_DEVCONF_ALL(net, FORWARDING)) {

                                     inet_forward_change(net);//调用该函数进行配置in_dev->cnf.data

                            } else if (*valp) {

                                     struct ipv4_devconf *cnf = ctl->extra1;

                                     struct in_device *idev =

                                               container_of(cnf, struct in_device, cnf);

                                     dev_disable_lro(idev->dev);

                            }

                            rtnl_unlock();

                            rt_cache_flush(net, 0);

                   }

         }

 

         return ret;

}

下面是这个函数就是修改forward参数,

static void inet_forward_change(struct net *net)

{

         struct net_device *dev;

         int on = IPV4_DEVCONF_ALL(net, FORWARDING);//获取配置的值

 

         IPV4_DEVCONF_ALL(net, ACCEPT_REDIRECTS) = !on;

         IPV4_DEVCONF_DFLT(net, FORWARDING) = on;//设置ipv4_devconf_dflt结构体,

 

         read_lock(&dev_base_lock);

         for_each_netdev(net, dev) {

                   struct in_device *in_dev;

                   if (on)

                            dev_disable_lro(dev);

                   rcu_read_lock();

                   in_dev = __in_dev_get_rcu(dev);

                   if (in_dev)

                            IN_DEV_CONF_SET(in_dev, FORWARDING, on);//调用该宏设置in_dev->cnf.data

                   rcu_read_unlock();

         }

         read_unlock(&dev_base_lock);

}

 

static inline void ipv4_devconf_set(struct in_device *in_dev, int index,

                                         int val)

{

         index--;

         set_bit(index, in_dev->cnf.state);

         in_dev->cnf.data[index] = val;//设置in_devdata,这里的IndexNET_IPV4_CONF_FORWARDING

}

其调用关系如下图:

 

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