8. Linux 网络

8.1 Linux 网络是如何管理的？

每个类型的网卡都存在一个设备驱动程序。在驱动程序内部，Linux 总是会调用一个标准的高级路由函数：“netif_rx [net/core/dev.c]”，该函数会控制帧属于哪个 3 层协议，并且会调用正确的 3 层函数（因此我们将使用函数指针来确定哪个是正确的函数）。

8.2 TCP 示例

现在我们将看到一个示例，说明当我们将 TCP 数据包发送到 Linux 时会发生什么，从调用 “netif_rx [net/core/dev.c]” 开始。

中断管理: "netif_rx"

|netif_rx
   |__skb_queue_tail
      |qlen++
      |* simple pointer insertion *    
   |cpu_raise_softirq
      |softirq_active(cpu) |= (1 << NET_RX_SOFTIRQ) // set bit NET_RX_SOFTIRQ in the BH vector
 

函数

• __skb_queue_tail [include/linux/skbuff.h]
• cpu_raise_softirq [kernel/softirq.c]

后中断管理: "net_rx_action"

一旦 IRQ 交互结束，我们需要继续跟踪帧生命周期的下一部分，并检查 NET_RX_SOFTIRQ 的作用。

接下来，我们将根据 "net_dev_init [net/core/dev.c]" 调用 “net_rx_action [net/core/dev.c]”。

|net_rx_action
   |skb = __skb_dequeue (the exact opposite of __skb_queue_tail)
   |for (ptype = first_protocol; ptype < max_protocol; ptype++) // Determine 
      |if (skb->protocol == ptype)                               // what is the network protocol
         |ptype->func -> ip_rcv // according to ''struct ip_packet_type [net/ipv4/ip_output.c]''
 
    **** NOW WE KNOW THAT PACKET IS IP ****
         |ip_rcv
            |NF_HOOK (ip_rcv_finish)
               |ip_route_input // search from routing table to determine function to call
                  |skb->dst->input -> ip_local_deliver // according to previous routing table check, destination is local machine
                     |ip_defrag // reassembles IP fragments
                        |NF_HOOK (ip_local_deliver_finish)
                           |ipprot->handler -> tcp_v4_rcv // according to ''tcp_protocol [include/net/protocol.c]''
 
     **** NOW WE KNOW THAT PACKET IS TCP ****
                           |tcp_v4_rcv   
                              |sk = __tcp_v4_lookup 
                              |tcp_v4_do_rcv
                                 |switch(sk->state) 

     *** Packet can be sent to the task which uses relative socket ***
                                 |case TCP_ESTABLISHED:
                                    |tcp_rcv_established
                                       |__skb_queue_tail // enqueue packet to socket
                                       |sk->data_ready -> sock_def_readable 
                                          |wake_up_interruptible
                                

     *** Packet has still to be handshaked by 3-way TCP handshake ***
                                 |case TCP_LISTEN:
                                    |tcp_v4_hnd_req
                                       |tcp_v4_search_req
                                       |tcp_check_req
                                          |syn_recv_sock -> tcp_v4_syn_recv_sock
                                       |__tcp_v4_lookup_established
                                 |tcp_rcv_state_process

                    *** 3-Way TCP Handshake ***
                                    |switch(sk->state)
                                    |case TCP_LISTEN: // We received SYN
                                       |conn_request -> tcp_v4_conn_request
                                          |tcp_v4_send_synack // Send SYN + ACK
                                             |tcp_v4_synq_add // set SYN state
                                    |case TCP_SYN_SENT: // we received SYN + ACK
                                       |tcp_rcv_synsent_state_process
                                          tcp_set_state(TCP_ESTABLISHED)
                                             |tcp_send_ack
                                                |tcp_transmit_skb
                                                   |queue_xmit -> ip_queue_xmit
                                                      |ip_queue_xmit2
                                                         |skb->dst->output
                                    |case TCP_SYN_RECV: // We received ACK
                                       |if (ACK)
                                          |tcp_set_state(TCP_ESTABLISHED)
                              

函数可以在以下位置找到

• net_rx_action [net/core/dev.c]
• __skb_dequeue [include/linux/skbuff.h]
• ip_rcv [net/ipv4/ip_input.c]
• NF_HOOK -> nf_hook_slow [net/core/netfilter.c]
• ip_rcv_finish [net/ipv4/ip_input.c]
• ip_route_input [net/ipv4/route.c]
• ip_local_deliver [net/ipv4/ip_input.c]
• ip_defrag [net/ipv4/ip_fragment.c]
• ip_local_deliver_finish [net/ipv4/ip_input.c]
• tcp_v4_rcv [net/ipv4/tcp_ipv4.c]
• __tcp_v4_lookup
• tcp_v4_do_rcv
• tcp_rcv_established [net/ipv4/tcp_input.c]
• __skb_queue_tail [include/linux/skbuff.h]
• sock_def_readable [net/core/sock.c]
• wake_up_interruptible [include/linux/sched.h]
• tcp_v4_hnd_req [net/ipv4/tcp_ipv4.c]
• tcp_v4_search_req
• tcp_check_req
• tcp_v4_syn_recv_sock
• __tcp_v4_lookup_established
• tcp_rcv_state_process [net/ipv4/tcp_input.c]
• tcp_v4_conn_request [net/ipv4/tcp_ipv4.c]
• tcp_v4_send_synack
• tcp_v4_synq_add
• tcp_rcv_synsent_state_process [net/ipv4/tcp_input.c]
• tcp_set_state [include/net/tcp.h]
• tcp_send_ack [net/ipv4/tcp_output.c]

描述

• 首先我们确定协议类型（IP，然后是 TCP）
• NF_HOOK (函数) 是一个包装例程，它首先管理网络过滤器（例如防火墙），然后调用 “function”。
• 之后我们管理 3 次 TCP 握手，包括：

SERVER (LISTENING)                       CLIENT (CONNECTING)
                           SYN 
                   <-------------------
 
 
                        SYN + ACK
                   ------------------->

 
                           ACK 
                   <-------------------

                    3-Way TCP handshake

• 最后，我们只需要启动 “tcp_rcv_established [net/ipv4/tcp_input.c]”，它将数据包传递给用户套接字并唤醒它。