深入理解Linux网络技术内幕——L4层协议与Raw IP的处理

我们简单了解下L4层协议和Raw IP是如何与IP层进行交互的。

L4层协议

L4层协议可以通过静态编译和模块配置两种方式加入内核。

比较重要的协议如TCP、UDP、ICMP通常是静态编译至内核。

一些不常用的或者比较特殊的协议，则是通过内核配置加入内核。如IGMP，SCTP，IPIP等等。

L4层协议的注册

L4层协议有net_protocol结构定义：

/* This is used to register protocols. */
struct net_protocol {
    int         (*handler)(struct sk_buff *skb); //由协议注册的，用于处理入口封包的函数
    //由ICMP协议处理函数所用的函数，当收到ICMP UNREACHABLE 时，通知L4层
    void            (*err_handler)(struct sk_buff *skb, u32 info);
    int         (*gso_send_check)(struct sk_buff *skb);
    struct sk_buff         *(*gso_segment)(struct sk_buff *skb,
                           int features);
    struct sk_buff        **(*gro_receive)(struct sk_buff **head,
                           struct sk_buff *skb);
    int         (*gro_complete)(struct sk_buff *skb);
    unsigned int        no_policy:1, //使协议免于IPsec检查
                netns_ok:1;
};

/*
 *  Add a protocol handler to the hash tables
 */

int inet_add_protocol(const struct net_protocol *prot, unsigned char protocol)
{                                                                                                                                       
    int hash, ret;

    hash = protocol & (MAX_INET_PROTOS - 1); 

    spin_lock_bh(&inet_proto_lock);
    if (inet_protos[hash]) {
        ret = -1; 
    } else {
        inet_protos[hash] = prot;
        ret = 0;
    }   
    spin_unlock_bh(&inet_proto_lock);

    return ret;
}
/*
 *  Remove a protocol from the hash tables.
 */

int inet_del_protocol(const struct net_protocol *prot, unsigned char protocol)
{
    int hash, ret;

    hash = protocol & (MAX_INET_PROTOS - 1);

    spin_lock_bh(&inet_proto_lock);
    if (inet_protos[hash] == prot) {
        inet_protos[hash] = NULL;
        ret = 0;
    } else {
        ret = -1;
    }
    spin_unlock_bh(&inet_proto_lock);

    synchronize_net();

    return ret;
}
                                                                                                                                        
EXPORT_SYMBOL(inet_add_protocol);
EXPORT_SYMBOL(inet_del_protocol);

下面是TCP、UDP、ICMP协议结构体的初始化，这要先完成：

static const struct net_protocol tcp_protocol = {
    .handler =  tcp_v4_rcv,
    .err_handler =  tcp_v4_err,
    .gso_send_check = tcp_v4_gso_send_check,
    .gso_segment =  tcp_tso_segment,
    .gro_receive =  tcp4_gro_receive,
    .gro_complete = tcp4_gro_complete,
    .no_policy =    1,   
    .netns_ok = 1, 
};

static const struct net_protocol udp_protocol = {
    .handler =  udp_rcv,
    .err_handler =  udp_err,
    .gso_send_check = udp4_ufo_send_check,
    .gso_segment = udp4_ufo_fragment,
    .no_policy =    1,
    .netns_ok = 1,
};

static const struct net_protocol icmp_protocol = {
    .handler =  icmp_rcv,
    .no_policy =    1,
    .netns_ok = 1,
};

static int __init inet_init(void)
{
    struct sk_buff *dummy_skb;
    struct inet_protosw *q;
    struct list_head *r;
    int rc = -EINVAL;

    BUILD_BUG_ON(sizeof(struct inet_skb_parm) > sizeof(dummy_skb->cb));

    rc = proto_register(&tcp_prot, 1);
    if (rc)
        goto out;

    rc = proto_register(&udp_prot, 1);
    if (rc)
        goto out_unregister_tcp_proto; 

    rc = proto_register(&raw_prot, 1);
    if (rc)
        goto out_unregister_udp_proto; 

    /*
     *  Tell SOCKET that we are alive...
     */

    (void)sock_register(&inet_family_ops);
#ifdef CONFIG_SYSCTL
    ip_static_sysctl_init();
#endif

    /*
     *  Add all the base protocols.
     */

    if (inet_add_protocol(&icmp_protocol, IPPROTO_ICMP) < 0)
        printk(KERN_CRIT "inet_init: Cannot add ICMP protocol\n");
    if (inet_add_protocol(&udp_protocol, IPPROTO_UDP) < 0)
        printk(KERN_CRIT "inet_init: Cannot add UDP protocol\n");
    if (inet_add_protocol(&tcp_protocol, IPPROTO_TCP) < 0)
        printk(KERN_CRIT "inet_init: Cannot add TCP protocol\n");
#ifdef CONFIG_IP_MULTICAST
    if (inet_add_protocol(&igmp_protocol, IPPROTO_IGMP) < 0)
        printk(KERN_CRIT "inet_init: Cannot add IGMP protocol\n");
#endif

    /* Register the socket-side information for inet_create. */
    for (r = &inetsw[0]; r < &inetsw[SOCK_MAX]; ++r)
        INIT_LIST_HEAD(r);

    for (q = inetsw_array; q < &inetsw_array[INETSW_ARRAY_LEN]; ++q)
        inet_register_protosw(q);
                              
    /*
     *  Set the ARP module up
     */

    arp_init();

    /*
     *  Set the IP module up
     */

    ip_init();

    tcp_v4_init();

    /* Setup TCP slab cache for open requests. */
    tcp_init();

    /* Setup UDP memory threshold */
    udp_init();

    /* Add UDP-Lite (RFC 3828) */
    udplite4_register();
    /*
     *  Set the ICMP layer up
     */

    if (icmp_init() < 0)
        panic("Failed to create the ICMP control socket.\n");

    /*
     *  Initialise the multicast router
     */
#if defined(CONFIG_IP_MROUTE)
    if (ip_mr_init())
        printk(KERN_CRIT "inet_init: Cannot init ipv4 mroute\n");
#endif
    /*
     *  Initialise per-cpu ipv4 mibs
     */

    if (init_ipv4_mibs())
        printk(KERN_CRIT "inet_init: Cannot init ipv4 mibs\n");
                                                                                                                                        
    ipv4_proc_init();

    ipfrag_init();

    dev_add_pack(&ip_packet_type);

    rc = 0;
out:
    return rc;
out_unregister_udp_proto:
    proto_unregister(&udp_prot);
out_unregister_tcp_proto:
    proto_unregister(&tcp_prot);
    goto out;
}

L3到L4的封包传递：ip_local_deliver_finish

Raw套接字和Raw IP

我们要知道，并不是所有的L4层处理，都是在内核实现的。应用程序可以通过Raw 套接字和Raw IP跳过L4层协议，直接与IP层进行交互。