源设备以及源地址的选择

源设备的选择

源地址的选择

一个网卡可能配有多个ip地址,在发包流程中,如果源地址没有显示指定的话,会通过inet_select_addr选择源地址:

__be32 inet_select_addr(const struct net_device *dev, __be32 dst, int scope)
{
    __be32 addr = 0;
    struct in_device *in_dev;
    struct net *net = dev_net(dev);

    rcu_read_lock();
    in_dev = __in_dev_get_rcu(dev);
    if (!in_dev)
        goto no_in_dev;

    for_primary_ifa(in_dev) {
        if (ifa->ifa_scope > scope)
            continue;
        if (!dst || inet_ifa_match(dst, ifa)) {
            addr = ifa->ifa_local;
            break;
        }
        if (!addr)
            addr = ifa->ifa_local;
    } endfor_ifa(in_dev);

    if (addr)
        goto out_unlock;
no_in_dev:

    /* Not loopback addresses on loopback should be preferred
     in this case. It is importnat that lo is the first interface
     in dev_base list.
     */
    for_each_netdev_rcu(net, dev) {
        in_dev = __in_dev_get_rcu(dev);
        if (!in_dev)
            continue;

        for_primary_ifa(in_dev) {
            if (ifa->ifa_scope != RT_SCOPE_LINK &&
                ifa->ifa_scope <= scope) {
                addr = ifa->ifa_local;
                goto out_unlock;
            }
        } endfor_ifa(in_dev);
    }
out_unlock:
    rcu_read_unlock();
    return addr;
}

其中scope定义如下 :

enum rt_scope_t {
    RT_SCOPE_UNIVERSE=0,
/* User defined values */
    RT_SCOPE_SITE=200,
    RT_SCOPE_LINK=253,
    RT_SCOPE_HOST=254,
    RT_SCOPE_NOWHERE=255
}

网卡对应的ip信息存在in_device数据结构中：

struct in_device {
    struct net_device    *dev;
    atomic_t        refcnt;
    int            dead;
    struct in_ifaddr    *ifa_list;    /* IP ifaddr chain        */
    struct ip_mc_list __rcu    *mc_list;    /* IP multicast filter chain */
    int            mc_count;    /* Number of installed mcasts    */
    spinlock_t        mc_tomb_lock;
    struct ip_mc_list    *mc_tomb;
    unsigned long        mr_v1_seen;
    unsigned long        mr_v2_seen;
    unsigned long        mr_maxdelay;
    unsigned char        mr_qrv;
    unsigned char        mr_gq_running;
    unsigned char        mr_ifc_count;
    struct timer_list    mr_gq_timer;    /* general query timer */
    struct timer_list    mr_ifc_timer;    /* interface change timer */

    struct neigh_parms    *arp_parms;
    struct ipv4_devconf    cnf;
    struct rcu_head        rcu_head;
}

struct in_ifaddr {
    struct hlist_node    hash;
    struct in_ifaddr    *ifa_next;
    struct in_device    *ifa_dev;
    struct rcu_head        rcu_head;
    __be32            ifa_local;
    __be32            ifa_address;
    __be32            ifa_mask;
    __be32            ifa_broadcast;
    unsigned char        ifa_scope;
    unsigned char        ifa_flags;
    unsigned char        ifa_prefixlen;
    char            ifa_label[IFNAMSIZ];
}

选择源地址主要基于两条:范围要大于传入的参数，即值要小于；属于同一个子网
ip地址存在in_ifaddr 数据结构中，如下：

root@:/opt/module/net/udp# ip addr
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 16436 qdisc noqueue state UNKNOWN
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
    inet6 ::1/128 scope host
       valid_lft forever preferred_lft forever
2: eth0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc pfifo_fast state DOWN qlen 1000
    link/ether 60:eb:69:45:29:1b brd ff:ff:ff:ff:ff:ff
    inet 10.1.2.3/24 brd 10.1.2.255 scope global eth0
    inet 11.1.2.3/24 brd 11.1.2.255 scope global eth0:1
    inet 128.1.2.3/16 brd 128.1.255.255 scope global eth0:2
    inet6 fe80::62eb:69ff:fe45:291b/64 scope link
       valid_lft forever preferred_lft forever
3: wlan0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP qlen 1000
    link/ether 1c:65:9d:2c:fe:f7 brd ff:ff:ff:ff:ff:ff
    inet 192.168.1.109/24 brd 192.168.1.255 scope global wlan0
    inet6 fe80::1e65:9dff:fe2c:fef7/64 scope link
       valid_lft forever preferred_lft forever

for_each_netdev(&init_net, dev_tmp){
        struct in_device *in_dev;
        struct in_ifaddr *ifa;
        in_dev = __in_dev_get_rcu(dev_tmp);
        if (!in_dev)
            return 0;
          for (ifa = (in_dev)->ifa_list; ifa ; ifa = ifa->ifa_next)
              printk("dev=%s,ip=%x,name=%s,scope=%d\n",dev_tmp->name,ifa->ifa_address,ifa->ifa_label,ifa->ifa_scope);


}

输出：

[ 3452.341006] dev=lo,ip=100007f,name=lo,scope=254
[ 3452.341014] dev=eth0,ip=302010a,name=eth0,scope=0
[ 3452.341017] dev=eth0,ip=302010b,name=eth0:1,scope=0
[ 3452.341021] dev=eth0,ip=3020180,name=eth0:2,scope=0
[ 3452.341025] dev=wlan0,ip=6d01a8c0,name=wlan0,scope=0