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TCP Socket Establish；UDP Send Package Process In Kernel Sourcecode Learning
目录
0. 引言1. TCP握手流程2. TCP connect() API原理3. TCP listen() API原理4. UDP交互过程5. UDP send() API原理6. UDP bind() API原理
0. 引言
Monitoring project
1. outer link connect: connect()->inet_stream_connect 2. Suspicious port monitor: listen()->inet_listen 3. SuspiciousUDP Package Send: sendto()->inet_sendmsg4. Suspicious port monitor: bind()->inet_bind()
we need monitor the systemcall is:
1. inet_stream_connect2. inet_listen3. inet_sendmsg4. inet_bind()
1. TCP握手流程
在和网络有关的编程中，我们常常会使用到Socket库提供的API进行开发，本节将给出一个最基本的socket tcp的ring3代码，在之后的章节，我们将一起从内核源代码的角度学习一下lixnu的网络连接在内核协议栈是怎么实现的
client:
#include <sys/socket.h>#include <sys/types.h>#include <netinet/in.h>#include <arpa/inet.h>#include <netdb.h>#include <unistd.h> //以下头文件是为了使样例程序正常运行#include <string.h>#include <stdio.h>#include <stdlib.h>int main(int argc, char const *argv[]){ struct sockaddr_in pin; struct hostent *nlp_host; int sd; char host_name[256]; int port; //初始化主机名和端口。主机名可以是IP，也可以是可被解析的名称 strcpy(host_name,"127.0.0.1"); port=8000; //解析域名，如果是IP则不用解析，如果出错，显示错误信息 while ((nlp_host=gethostbyname(host_name))==0) { printf("Resolve Error!\n"); } //设置pin变量，包括协议、地址、端口等，此段可直接复制到自己的程序中 bzero(&pin,sizeof(pin)); pin.sin_family = AF_INET; //AF_INET表示使用IPv4 pin.sin_addr.s_addr=htonl(INADDR_ANY); pin.sin_addr.s_addr=((struct in_addr *)(nlp_host->h_addr))->s_addr; pin.sin_port=htons(port); //建立socket sd=socket(AF_INET,SOCK_STREAM,0); //建立连接 while (connect(sd,(struct sockaddr*)&pin,sizeof(pin))==-1) { printf("Connect Error!\n"); } return 0;}
server.c
#include <sys/socket.h>#include <sys/types.h>#include <netinet/in.h>#include <arpa/inet.h>#include <netdb.h>#include <unistd.h> //以下头文件是为了使样例程序正常运行#include <string.h>#include <stdio.h>#include <stdlib.h>int main(int argc, char const *argv[]){ int serverSocket; struct sockaddr_in serverAddr; struct sockaddr_in clientAddr; // 用port保存使用的端口 int port=8000; // 建立Socket，并设置 serverSocket = socket(AF_INET, SOCK_STREAM, 0); // 设置socket选项，这是可选的，可以避免服务器程序结束后无法快速重新运行 int val=1; setsockopt(serverSocket, SOL_SOCKET, SO_REUSEADDR, &val, sizeof(val)); // 定义端口和监听的地址 serverAddr.sin_family = AF_INET; serverAddr.sin_port = htons(port); serverAddr.sin_addr.s_addr = htonl(INADDR_ANY); memset(&(serverAddr.sin_zero), 0, 8); int rc = bind(serverSocket, (struct sockaddr*) &serverAddr, sizeof(struct sockaddr)); if (rc == -1) { printf("Bad bind\n"); exit(1); } // 让serverSocket开始监听，客户队列长为5 rc = listen(serverSocket, 5); if (rc == -1) { printf("Bad listen\n"); exit(1); } // 等待客户连接 int sock; int clientAddrSize = sizeof(struct sockaddr_in); sock = accept(serverSocket, (struct sockaddr*) &clientAddr, (socklen_t*) &clientAddrSize); if(sock != -1) { printf("tcp connection established ok! \n"); } else { printf("error!!\n"); } return 0;}
从代码中可以看到，在一个Tcp握手连接的过程中，最核心的两个API就是connect()、accept()。我们接下来逐一学习
2. connect() API原理
我们知道，对于每一个ring3的C库中的API，都有一个对应的系统调用(C库的API只是对系统调用的一个封装wraper)来实现该接口的功能
整个流程如下:
/*在ring3层调用socket的API: connect()*/1. ring3: 1) connect()->/*进入ring0层*/2. ring0: /* SYSCALL_DEFINE2(socketcall..等效于调用sys_socketcall(.. */ 1) SYSCALL_DEFINE2(socketcall, int, call, unsigned long __user *, args) /* SYSCALL_DEFINE3(connect..等效于调用sys_connect(.. */ 2) SYSCALL_DEFINE3(connect, int, fd, struct sockaddr __user *, uservaddr, int, addrlen)-> 3) inet_stream_connect()-> 4) tcp_v4_connect()-> 5) tcp_connect()-> 5.1) tcp_connect_init()-> 5.2) tcp_transmit_skb()-> 6) 回到inet_stream_connect()-> 7) inet_wait_for_connect()进入SYN等待状态
我们来详细跟踪一下内核源代码，分析的过程中涉及到很多的.c文件和很多的函数嵌套调用，希望朋友们能耐心的结合注释跟着代码的逻辑理解socket tcp的建立过程
\linux-2.6.32.63\net\socket.c
/*进行调用派发
\linux-2.6.32.63\net\socket.c*/SYSCALL_DEFINE2(socketcall, int, call, unsigned long __user *, args){ unsigned long a[6]; unsigned long a0, a1; int err; unsigned int len; if (call < 1 || call > SYS_ACCEPT4) return -EINVAL; len = nargs[call]; if (len > sizeof(a)) return -EINVAL; /* copy_from_user should be SMP safe. */ if (copy_from_user(a, args, len)) return -EFAULT; audit_socketcall(nargs[call] / sizeof(unsigned long), a); a0 = a[0]; a1 = a[1]; /* 进入内核的sys_socketcall()系统调用后，linux系统需要将特定的调用分路发送到指定目标，例如 1) sockcet()调用将由sys_socket()完成 2) send()调用将由sys_send()完成 系统在include/linux/net.h中定义了以下17种类型socketcall 1) #define SYS_SOCKET 1: sys_socket(2) 2) #define SYS_BIND 2: sys_bind(2) 3) #define SYS_CONNECT 3: sys_connect(2) 4) #define SYS_LISTEN 4: sys_listen(2) 5) #define SYS_ACCEPT 5: sys_accept(2) 6) #define SYS_GETSOCKNAME 6: sys_getsockname(2) 7) #define SYS_GETPEERNAME 7: sys_getpeername(2) 8) #define SYS_SOCKETPAIR 8: sys_socketpair(2) 9) #define SYS_SEND 9: sys_send(2) 10) #define SYS_RECV 10: sys_recv(2) 11) #define SYS_SENDTO 11: sys_sendto(2) 12) #define SYS_RECVFROM 12: sys_recvfrom(2) 13) #define SYS_SHUTDOWN 13: sys_shutdown(2) 14) #define SYS_SETSOCKOPT 14: sys_setsockopt(2) 15) #define SYS_GETSOCKOPT 15: sys_getsockopt(2) 16) #define SYS_SENDMSG 16: sys_sendmsg(2) 17) #define SYS_RECVMSG 17: sys_recvmsg(2) 18) #define SYS_ACCEPT4 18: sys_accept4(2) */ switch (call) SYS_SOCKET { case : err = sys_socket(a0, a1, a[2]); break; case SYS_BIND: err = sys_bind(a0, (struct sockaddr __user *)a1, a[2]); break; case SYS_CONNECT: /* sys_connect()函数的声明和实现在当前文件的 SYSCALL_DEFINE3(connect, int, fd, struct sockaddr __user *, uservaddr, int, addrlen) 中实现 */ err = sys_connect(a0, (struct sockaddr __user *)a1, a[2]); break; case SYS_LISTEN: err = sys_listen(a0, a1); break; case SYS_ACCEPT: err = sys_accept4(a0, (struct sockaddr __user *)a1, (int __user *)a[2], 0); break; case SYS_GETSOCKNAME: err = sys_getsockname(a0, (struct sockaddr __user *)a1, (int __user *)a[2]); break; case SYS_GETPEERNAME: err = sys_getpeername(a0, (struct sockaddr __user *)a1, (int __user *)a[2]); break; case SYS_SOCKETPAIR: err = sys_socketpair(a0, a1, a[2], (int __user *)a[3]); break; case SYS_SEND: err = sys_send(a0, (void __user *)a1, a[2], a[3]); break; case SYS_SENDTO: err = sys_sendto(a0, (void __user *)a1, a[2], a[3], (struct sockaddr __user *)a[4], a[5]); break; case SYS_RECV: err = sys_recv(a0, (void __user *)a1, a[2], a[3]); break; case SYS_RECVFROM: err = sys_recvfrom(a0, (void __user *)a1, a[2], a[3], (struct sockaddr __user *)a[4], (int __user *)a[5]); break; case SYS_SHUTDOWN: err = sys_shutdown(a0, a1); break; case SYS_SETSOCKOPT: err = sys_setsockopt(a0, a1, a[2], (char __user *)a[3], a[4]); break; case SYS_GETSOCKOPT: err = sys_getsockopt(a0, a1, a[2], (char __user *)a[3], (int __user *)a[4]); break; case SYS_SENDMSG: err = sys_sendmsg(a0, (struct msghdr __user *)a1, a[2]); break; case SYS_RECVMSG: err = sys_recvmsg(a0, (struct msghdr __user *)a1, a[2]); break; case SYS_ACCEPT4: err = sys_accept4(a0, (struct sockaddr __user *)a1, (int __user *)a[2], a[3]); break; default: err = -EINVAL; break; } return err;}/*调用sys_connect()
\linux-2.6.32.63\net\socket.c*/SYSCALL_DEFINE3(connect, int, fd, struct sockaddr __user *, uservaddr, int, addrlen){ struct socket *sock; struct sockaddr_storage address; int err, fput_needed; /* 找到文件描述符对应的BSD socket结构，在前面的socket调用中建立 */ sock = sockfd_lookup_light(fd, &err, &fput_needed); if (!sock) { goto out; } /* copy对端的地址到内核空间 */ err = move_addr_to_kernel(uservaddr, addrlen, (struct sockaddr *)&address); if (err < 0) { goto out_put; } err = security_socket_connect(sock, (struct sockaddr *)&address, addrlen); if (err) { goto out_put; } /* 调用该BSD socket对应的connect调用 这里的sock是在前面的socket调用的时候就初始化的，对应于TCP的BSD socket的操作符集是inet_stream_ops 要注意的是: 网络子模块在内核初始化的时候(linux-2.6.32.63\net\ipv4\af_inet.c->inet_init()中)就注册了TCP，UDP和RAW3中协议 linux-2.6.32.63\net\ipv4\af_inet.c const struct proto_ops inet_stream_ops = { .family = PF_INET, .owner = THIS_MODULE, .release = inet_release, .bind = inet_bind, .connect = inet_stream_connect, .socketpair = sock_no_socketpair, .accept = inet_accept, .getname = inet_getname, .poll = tcp_poll, .ioctl = inet_ioctl, .listen = inet_listen, .shutdown = inet_shutdown, .setsockopt = sock_common_setsockopt, .getsockopt = sock_common_getsockopt, .sendmsg = tcp_sendmsg, .recvmsg = sock_common_recvmsg, .mmap = sock_no_mmap, .sendpage = tcp_sendpage, .splice_read = tcp_splice_read, #ifdef CONFIG_COMPAT .compat_setsockopt = compat_sock_common_setsockopt, .compat_getsockopt = compat_sock_common_getsockopt, #endif };
EXPORT_SYMBOL(inet_stream_ops); 从上面的结构中可以看到connect对应的函数是inet_stream_connect，我们继续分析inet_stream_connect函数
\linux-2.6.32.63\net\ipv4\af_inet.c */ err = sock->ops->connect(sock, (struct sockaddr *)&address, addrlen, sock->file->f_flags);out_put: /* 释放文件的引用 */ fput_light(sock->file, fput_needed);out: return err;}/*调用inet_stream_connect()
\linux-2.6.32.63\net\ipv4\af_inet.c*/int inet_stream_connect(struct socket *sock, struct sockaddr *uaddr, int addr_len, int flags){ struct sock *sk = sock->sk; int err; long timeo; lock_sock(sk); if (uaddr->sa_family == AF_UNSPEC) { err = sk->sk_prot->disconnect(sk, flags); sock->state = err ? SS_DISCONNECTING : SS_UNCONNECTED; goto out; } switch (sock->state) { default: err = -EINVAL; goto out; /* 该BSD socket已连接 */ case SS_CONNECTED: err = -EISCONN; goto out; /* 该BSD socket正在连接 */ case SS_CONNECTING: err = -EALREADY; /* Fall out of switch with err, set for this state */ break; case SS_UNCONNECTED: err = -EISCONN; if (sk->sk_state != TCP_CLOSE) goto out; /* INET SOCKET调用协议特有connect操作符 对于INET socket中的tcp连接，协议特有操作符集为tcp_prot linux-2.6.32.63\net\ipv4\tcp_ipv4.c struct proto tcp_prot = { .name = "TCP", .owner = THIS_MODULE, .close = tcp_close, .connect = tcp_v4_connect, .disconnect = tcp_disconnect, .accept = inet_csk_accept, .ioctl = tcp_ioctl, .init = tcp_v4_init_sock, .destroy = tcp_v4_destroy_sock, .shutdown = tcp_shutdown, .setsockopt = tcp_setsockopt, .getsockopt = tcp_getsockopt, .recvmsg = tcp_recvmsg, .backlog_rcv = tcp_v4_do_rcv, .hash = inet_hash, .unhash = inet_unhash, .get_port = inet_csk_get_port, .enter_memory_pressure = tcp_enter_memory_pressure, .sockets_allocated = &tcp_sockets_allocated, .orphan_count = &tcp_orphan_count, .memory_allocated = &tcp_memory_allocated, .memory_pressure = &tcp_memory_pressure, .sysctl_mem = sysctl_tcp_mem, .sysctl_wmem = sysctl_tcp_wmem, .sysctl_rmem = sysctl_tcp_rmem, .max_header = MAX_TCP_HEADER, .obj_size = sizeof(struct tcp_sock), .slab_flags = SLAB_DESTROY_BY_RCU, .twsk_prot = &tcp_timewait_sock_ops, .rsk_prot = &tcp_request_sock_ops, .h.hashinfo = &tcp_hashinfo, #ifdef CONFIG_COMPAT .compat_setsockopt = compat_tcp_setsockopt, .compat_getsockopt = compat_tcp_getsockopt, #endif }; 可以看出，对于TCP，流程进入tcp_v4_connect函数(.connect对应的是tcp_v4_connect()函数)，我们继续跟进tcp_v4_connect
\linux-2.6.32.63\net\ipv4\tcp_ipv4.c */ err = sk->sk_prot->connect(sk, uaddr, addr_len); if (err < 0) goto out; /* 上面的调用完成后，连接并没有完成 */ sock->state = SS_CONNECTING; /* Just entered SS_CONNECTING state; the only * difference is that return value in non-blocking * case is EINPROGRESS, rather than EALREADY. */ err = -EINPROGRESS; break; } /* 获取连接超时时间 */ timeo = sock_sndtimeo(sk, flags & O_NONBLOCK); if ((1 << sk->sk_state) & (TCPF_SYN_SENT | TCPF_SYN_RECV)) { /* Error code is set above 进入定时等待 */ if (!timeo || !inet_wait_for_connect(sk, timeo)) goto out; err = sock_intr_errno(timeo); if (signal_pending(current)) goto out; } /* Connection was closed by RST, timeout, ICMP error * or another process disconnected us. */ if (sk->sk_state == TCP_CLOSE) goto sock_error; /* sk->sk_err may be not zero now, if RECVERR was ordered by user * and error was received after socket entered established state. * Hence, it is handled normally after connect() return successfully. */ sock->state = SS_CONNECTED; err = 0;out: release_sock(sk); return err;sock_error: err = sock_error(sk) ? : -ECONNABORTED; sock->state = SS_UNCONNECTED; if (sk->sk_prot->disconnect(sk, flags)) sock->state = SS_DISCONNECTING; goto out;}EXPORT_SYMBOL(inet_stream_connect);/*调用tcp_v4_connect()
\linux-2.6.32.63\net\ipv4\tcp_ipv4.c该函数的主要功能是:1. 准备好路由，如果源端口没有指定，还要选择一个端口，然后再次更新路由信息2. 代表该连接的sk结构加入到对应的hash表中(已连接ehash)3. 获取一个write_seq，以及当 sysctl_tw_recycle设置时，读取上次连接(如果存在)进入time-wait时保存的时戳，赋给当前连接的rx_opt结构中最后该函数调用tcp_connect来完成连接*/int tcp_v4_connect(struct sock *sk, struct sockaddr *uaddr, int addr_len){ struct inet_sock *inet = inet_sk(sk); struct tcp_sock *tp = tcp_sk(sk); struct sockaddr_in *usin = (struct sockaddr_in *)uaddr; struct rtable *rt; __be32 daddr, nexthop; int tmp; int err; struct ip_options_rcu *inet_opt; if (addr_len < sizeof(struct sockaddr_in)) return -EINVAL; if (usin->sin_family != AF_INET) return -EAFNOSUPPORT; /* 开始准备路由 */ nexthop = daddr = usin->sin_addr.s_addr; inet_opt = inet->inet_opt; if (inet_opt && inet_opt->opt.srr) { if (!daddr) return -EINVAL; nexthop = inet_opt->opt.faddr; } /* 调用路由模块获取出口信息 */ tmp = ip_route_connect(&rt, nexthop, inet->saddr, RT_CONN_FLAGS(sk), sk->sk_bound_dev_if, IPPROTO_TCP, inet->sport, usin->sin_port, sk, 1); if (tmp < 0) { if (tmp == -ENETUNREACH) IP_INC_STATS(sock_net(sk), IPSTATS_MIB_OUTNOROUTES); return tmp; } /* 如果获取的路由是广播或多播域， 返回网络不可达，tcp不支持多播与广播 */ if (rt->rt_flags & (RTCF_MULTICAST | RTCF_BROADCAST)) { ip_rt_put(rt); return -ENETUNREACH; } if (!inet_opt || !inet_opt->opt.srr) daddr = rt->rt_dst; if (!inet->saddr) inet->saddr = rt->rt_src; inet->rcv_saddr = inet->saddr; if (tp->rx_opt.ts_recent_stamp && inet->daddr != daddr) { /* Reset inherited state */ tp->rx_opt.ts_recent = 0; tp->rx_opt.ts_recent_stamp = 0; tp->write_seq = 0; } if (tcp_death_row.sysctl_tw_recycle && !tp->rx_opt.ts_recent_stamp && rt->rt_dst == daddr) { struct inet_peer *peer = rt_get_peer(rt); /* * VJ‘s idea. We save last timestamp seen from * the destination in peer table, when entering state * TIME-WAIT * and initialize rx_opt.ts_recent from it, * when trying new connection. */ if (peer != NULL && peer->tcp_ts_stamp + TCP_PAWS_MSL >= get_seconds()) { tp->rx_opt.ts_recent_stamp = peer->tcp_ts_stamp; tp->rx_opt.ts_recent = peer->tcp_ts; } } inet->dport = usin->sin_port; inet->daddr = daddr; inet_csk(sk)->icsk_ext_hdr_len = 0; if (inet_opt) inet_csk(sk)->icsk_ext_hdr_len = inet_opt->opt.optlen; tp->rx_opt.mss_clamp = 536; /* Socket identity is still unknown (sport may be zero). * However we set state to SYN-SENT and not releasing socket * lock select source port, enter ourselves into the hash tables and * complete initialization after this. */ tcp_set_state(sk, TCP_SYN_SENT); err = inet_hash_connect(&tcp_death_row, sk); if (err) goto failure; err = ip_route_newports(&rt, IPPROTO_TCP, inet->sport, inet->dport, sk); if (err) goto failure; /* OK, now commit destination to socket. */ sk->sk_gso_type = SKB_GSO_TCPV4; sk_setup_caps(sk, &rt->u.dst); if (!tp->write_seq) tp->write_seq = secure_tcp_sequence_number(inet->saddr, inet->daddr, inet->sport, usin->sin_port); /* id是IP包头的id域 */ inet->id = tp->write_seq ^ jiffies; /* 调用tcp_connect来完成连接 我们继续跟踪tcp_connect() \linux-2.6.32.63\net\ipv4\tcp_output.c */ err = tcp_connect(sk); rt = NULL; if (err) goto failure; return 0;failure: /* * This unhashes the socket and releases the local port, * if necessary. */ tcp_set_state(sk, TCP_CLOSE); ip_rt_put(rt); sk->sk_route_caps = 0; inet->dport = 0; return err;}/*在tcp_v4_connect()的最后，调用tcp_connect来完成连接
\linux-2.6.32.63\net\ipv4\tcp_output.c该函数的流程是1. 初始化该次连接的sk结构2. 分配一个skb数据包3. 初始化skb4 skb加入发送队列后，调用tcp_transmit_skb发送该数据包5. 更新snd_nxt，并启动超时定时器*/int tcp_connect(struct sock *sk){ struct tcp_sock *tp = tcp_sk(sk); struct sk_buff *buff; /* 初始化连接对应的INET socket结构的参数，为连接做准备
下面跟进tcp_connect_init()
\linux-2.6.32.63\net\ipv4\tcp_output.c */ tcp_connect_init(sk); /* 获取一个skb，由于是syn包，没有数据，所以大小是MAX_TCP_HEADER的16位对齐 */ buff = alloc_skb_fclone(MAX_TCP_HEADER + 15, sk->sk_allocation); if (unlikely(buff == NULL)) return -ENOBUFS; /* Reserve space for headers. */ skb_reserve(buff, MAX_TCP_HEADER); tp->snd_nxt = tp->write_seq; /* 设置skb相关参数 */ tcp_init_nondata_skb(buff, tp->write_seq++, TCPCB_FLAG_SYN); /* 设置ECN */ TCP_ECN_send_syn(sk, buff); /* Send it off. */ /* 保存该数据包的发送时间 */ TCP_SKB_CB(buff)->when = tcp_time_stamp; tp->retrans_stamp = TCP_SKB_CB(buff)->when; skb_header_release(buff); /* 加入发送队列，待确认后在丢弃 */ __tcp_add_write_queue_tail(sk, buff); sk->sk_wmem_queued += buff->truesize; sk_mem_charge(sk, buff->truesize); tp->packets_out += tcp_skb_pcount(buff); /* 发送数据包，调用tcp_transimit_skb函数
下面跟进tcp_transmit_skb进行分析
\linux-2.6.32.63\net\ipv4\tcp_output.c */ tcp_transmit_skb(sk, buff, 1, sk->sk_allocation); /* We change tp->snd_nxt after the tcp_transmit_skb() call * in order to make this packet get counted in tcpOutSegs. */ tp->snd_nxt = tp->write_seq; tp->pushed_seq = tp->write_seq; TCP_INC_STATS(sock_net(sk), TCP_MIB_ACTIVEOPENS); /* Timer for repeating the SYN until an answer. */ inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS, inet_csk(sk)->icsk_rto, TCP_RTO_MAX); return 0;}/*接下来跟进一下tcp_connect_init函数，它负责sk结构的初始化(在tcp_connect()的一开始)
\linux-2.6.32.63\net\ipv4\tcp_output.c*/static void tcp_connect_init(struct sock *sk){ struct dst_entry *dst = __sk_dst_get(sk); struct tcp_sock *tp = tcp_sk(sk); __u8 rcv_wscale; /* We‘ll fix this up when we get a response from the other end. * See tcp_input.c:tcp_rcv_state_process case TCP_SYN_SENT. */ tp->tcp_header_len = sizeof(struct tcphdr) + (sysctl_tcp_timestamps ? TCPOLEN_TSTAMP_ALIGNED : 0);#ifdef CONFIG_TCP_MD5SIG if (tp->af_specific->md5_lookup(sk, sk) != NULL) tp->tcp_header_len += TCPOLEN_MD5SIG_ALIGNED;#endif /* If user gave his TCP_MAXSEG, record it to clamp */ if (tp->rx_opt.user_mss) tp->rx_opt.mss_clamp = tp->rx_opt.user_mss; tp->max_window = 0; /* 初始化MTU probe */ tcp_mtup_init(sk); /* 设置mss */ tcp_sync_mss(sk, dst_mtu(dst)); if (!tp->window_clamp) tp->window_clamp = dst_metric(dst, RTAX_WINDOW); tp->advmss = dst_metric(dst, RTAX_ADVMSS); if (tp->rx_opt.user_mss && tp->rx_opt.user_mss < tp->advmss) tp->advmss = tp->rx_opt.user_mss; tcp_initialize_rcv_mss(sk); /* 根据接收空间大小初始化一个通告窗口 */ tcp_select_initial_window(tcp_full_space(sk), tp->advmss - (tp->rx_opt.ts_recent_stamp ? tp->tcp_header_len - sizeof(struct tcphdr) : 0), &tp->rcv_wnd, &tp->window_clamp, sysctl_tcp_window_scaling, &rcv_wscale); tp->rx_opt.rcv_wscale = rcv_wscale; tp->rcv_ssthresh = tp->rcv_wnd; sk->sk_err = 0; sock_reset_flag(sk, SOCK_DONE); tp->snd_wnd = 0; /* 更新一些滑动窗口的成员 */ tcp_init_wl(tp, 0); tp->snd_una = tp->write_seq; tp->snd_sml = tp->write_seq; tp->snd_up = tp->write_seq; tp->rcv_nxt = 0; tp->rcv_wup = 0; tp->copied_seq = 0; inet_csk(sk)->icsk_rto = TCP_TIMEOUT_INIT; inet_csk(sk)->icsk_retransmits = 0; tcp_clear_retrans(tp);}/*我们继续分析，tcp_connect()，在tcp_connect()的最后，函数调用了tcp_transmit_skb()进行数据报发送
\linux-2.6.32.63\net\ipv4\tcp_output.c*/static int tcp_transmit_skb(struct sock *sk, struct sk_buff *skb, int clone_it, gfp_t gfp_mask){ const struct inet_connection_sock *icsk = inet_csk(sk); struct inet_sock *inet; struct tcp_sock *tp; struct tcp_skb_cb *tcb; struct tcp_out_options opts; unsigned tcp_options_size, tcp_header_size; struct tcp_md5sig_key *md5; __u8 *md5_hash_location; struct tcphdr *th; int err; BUG_ON(!skb || !tcp_skb_pcount(skb)); /* If congestion control is doing timestamping, we must * take such a timestamp before we potentially clone/copy. */ if (icsk->icsk_ca_ops->flags & TCP_CONG_RTT_STAMP) __net_timestamp(skb); /* 因为发送时要保留skb以备重传，所以大部分时候都设置了clone_it，不过发送ack除外 */ if (likely(clone_it)) { if (unlikely(skb_cloned(skb))) skb = pskb_copy(skb, gfp_mask); else skb = skb_clone(skb, gfp_mask); if (unlikely(!skb)) return -ENOBUFS; } inet = inet_sk(sk); tp = tcp_sk(sk); tcb = TCP_SKB_CB(skb); memset(&opts, 0, sizeof(opts)); /* 获取tcp选项部分信息，分为syn包和普通包2部分，因为有的选项只在syn中设置 */ if (unlikely(tcb->flags & TCPCB_FLAG_SYN)) tcp_options_size = tcp_syn_options(sk, skb, &opts, &md5); else tcp_options_size = tcp_established_options(sk, skb, &opts, &md5); tcp_header_size = tcp_options_size + sizeof(struct tcphdr); /* 该函数判断是否网络中有数据存在，没有就通知拥塞控制模块 */ if (tcp_packets_in_flight(tp) == 0) tcp_ca_event(sk, CA_EVENT_TX_START); skb_push(skb, tcp_header_size); skb_reset_transport_header(skb); skb_set_owner_w(skb, sk); /* Build TCP header and checksum it. */ th = tcp_hdr(skb); th->source = inet->sport; th->dest = inet->dport; th->seq = htonl(tcb->seq); th->ack_seq = htonl(tp->rcv_nxt); *(((__be16 *)th) + 6) = htons(((tcp_header_size >> 2) << 12) | tcb->flags); if (unlikely(tcb->flags & TCPCB_FLAG_SYN)) { /* RFC1323: The window in SYN & SYN/ACK segments is never scaled.SYN包和SYN/ACK包中窗口不扩放 */ th->window = htons(min(tp->rcv_wnd, 65535U)); } else { th->window = htons(tcp_select_window(sk)); } th->check = 0; th->urg_ptr = 0; /* The urg_mode check is necessary during a below snd_una win probe */ if (unlikely(tcp_urg_mode(tp) && before(tcb->seq, tp->snd_up))) { if (before(tp->snd_up, tcb->seq + 0x10000)) { th->urg_ptr = htons(tp->snd_up - tcb->seq); th->urg = 1; } else if (after(tcb->seq + 0xFFFF, tp->snd_nxt)) { th->urg_ptr = 0xFFFF; th->urg = 1; } } /* 填充TCP 选项字段 */ tcp_options_write((__be32 *)(th + 1), tp, &opts, &md5_hash_location); if (likely((tcb->flags & TCPCB_FLAG_SYN) == 0)) /* 如果不是SYN包的话，就检查是否需要向对方发送ECN */ TCP_ECN_send(sk, skb, tcp_header_size);#ifdef CONFIG_TCP_MD5SIG /* Calculate the MD5 hash, as we have all we need now */ if (md5) { sk->sk_route_caps &= ~NETIF_F_GSO_MASK; tp->af_specific->calc_md5_hash(md5_hash_location, md5, sk, NULL, skb); }#endif /* 校验和相关计算 */ icsk->icsk_af_ops->send_check(sk, skb->len, skb); if (likely(tcb->flags & TCPCB_FLAG_ACK)) tcp_event_ack_sent(sk, tcp_skb_pcount(skb)); if (skb->len != tcp_header_size) tcp_event_data_sent(tp, skb, sk); if (after(tcb->end_seq, tp->snd_nxt) || tcb->seq == tcb->end_seq) TCP_INC_STATS(sock_net(sk), TCP_MIB_OUTSEGS); /* 调用IP层的发送函数 */ err = icsk->icsk_af_ops->queue_xmit(skb, 0); if (likely(err <= 0)) return err; tcp_enter_cwr(sk, 1); return net_xmit_eval(err);}/*skb发送后，connect并没有返回，因为此时连接还没有建立，tcp进入等待状态，此时回到前面的inet_stream_connect函数我们继续跟进分析inet_wait_for_connect()
\linux-2.6.32.63\net\ipv4\af_inet.c在发送syn后进入等待状态*/static long inet_wait_for_connect(struct sock *sk, long timeo){ DEFINE_WAIT(wait); /* sk_sleep保存此INET SOCKET的等待队列 */ prepare_to_wait(sk->sk_sleep, &wait, TASK_INTERRUPTIBLE); /* Basic assumption: if someone sets sk->sk_err, he _must_ * change state of the socket from TCP_SYN_*. * Connect() does not allow to get error notifications * without closing the socket. */ /* 定时等待知道状态变化 */ while ((1 << sk->sk_state) & (TCPF_SYN_SENT | TCPF_SYN_RECV)) { release_sock(sk); timeo = schedule_timeout(timeo); lock_sock(sk); if (signal_pending(current) || !timeo) break; prepare_to_wait(sk->sk_sleep, &wait, TASK_INTERRUPTIBLE); } finish_wait(sk->sk_sleep, &wait); return timeo;}
到此为止的代码，只是完成了connect()过程中的SYN包的发送过程的分析，要完成一个完整的TCP连接，还需要有被动方的SYN/ACK、以及发送方的ACK过程，这里就暂时不做深入分析了，因为我只关心发送方主动向外进行连接的"connect()动作"，即"inet_stream_connect"这个系统调用
有关TCP握手的完整过程，请参阅下面的链接
Relevant Link:
http://docs.huihoo.com/joyfire.net/6-1.htmlhttp://blog.csdn.net/bycy2005/article/details/3096861http://blog.csdn.net/chensichensi/article/details/5272346http://blog.csdn.net/chensichensi/article/details/5272696http://blog.csdn.net/chensichensi/article/details/5273481
3. listen() API原理
listen()从ring3到ring0的整个流程如下:
/*在ring3层调用socket的API: listen()*/1. ring3: 1) listen()->/*进入ring0层*/2. ring0: /* SYSCALL_DEFINE2(socketcall..等效于调用sys_socketcall(.. */ 1) SYSCALL_DEFINE2(socketcall, int, call, unsigned long __user *, args) /* SYSCALL_DEFINE2(listen..等效于调用sys_listen(.. */ 2) SYSCALL_DEFINE2(listen, int, fd, int, backlog)-> 3) inet_listen()-> /* 监听指定端口 */ 4) inet_csk_listen_start()->
内核源代码分析
\linux-2.6.32.63\net\socket.c
/*进行调用派发
\linux-2.6.32.63\net\socket.c*/ SYSCALL_DEFINE2(socketcall, int, call, unsigned long __user *, args){ unsigned long a[6]; unsigned long a0, a1; int err; unsigned int len; if (call < 1 || call > SYS_ACCEPT4) return -EINVAL; len = nargs[call]; if (len > sizeof(a)) return -EINVAL; /* copy_from_user should be SMP safe. */ if (copy_from_user(a, args, len)) return -EFAULT; audit_socketcall(nargs[call] / sizeof(unsigned long), a); a0 = a[0]; a1 = a[1]; /* 进入内核的sys_socketcall()系统调用后，linux系统需要将特定的调用分路发送到指定目标，例如 1) sockcet()调用将由sys_socket()完成 2) send()调用将由sys_send()完成 系统在include/linux/net.h中定义了以下17种类型socketcall 1) #define SYS_SOCKET 1: sys_socket(2) 2) #define SYS_BIND 2: sys_bind(2) 3) #define SYS_CONNECT 3: sys_connect(2) 4) #define SYS_LISTEN 4: sys_listen(2) 5) #define SYS_ACCEPT 5: sys_accept(2) 6) #define SYS_GETSOCKNAME 6: sys_getsockname(2) 7) #define SYS_GETPEERNAME 7: sys_getpeername(2) 8) #define SYS_SOCKETPAIR 8: sys_socketpair(2) 9) #define SYS_SEND 9: sys_send(2) 10) #define SYS_RECV 10: sys_recv(2) 11) #define SYS_SENDTO 11: sys_sendto(2) 12) #define SYS_RECVFROM 12: sys_recvfrom(2) 13) #define SYS_SHUTDOWN 13: sys_shutdown(2) 14) #define SYS_SETSOCKOPT 14: sys_setsockopt(2) 15) #define SYS_GETSOCKOPT 15: sys_getsockopt(2) 16) #define SYS_SENDMSG 16: sys_sendmsg(2) 17) #define SYS_RECVMSG 17: sys_recvmsg(2) 18) #define SYS_ACCEPT4 18: sys_accept4(2) */ switch (call) SYS_SOCKET { case : err = sys_socket(a0, a1, a[2]); break; case SYS_BIND: err = sys_bind(a0, (struct sockaddr __user *)a1, a[2]); break; case SYS_CONNECT: err = sys_connect(a0, (struct sockaddr __user *)a1, a[2]); break; case SYS_LISTEN: /* SYSCALL_DEFINE2(listen, int, fd, int, backlog) 等效于调用sys_listen 继续跟进SYSCALL_DEFINE2(listen, int, fd, int, backlog) */ err = sys_listen(a0, a1); break; case SYS_ACCEPT: err = sys_accept4(a0, (struct sockaddr __user *)a1, (int __user *)a[2], 0); break; case SYS_GETSOCKNAME: err = sys_getsockname(a0, (struct sockaddr __user *)a1, (int __user *)a[2]); break; case SYS_GETPEERNAME: err = sys_getpeername(a0, (struct sockaddr __user *)a1, (int __user *)a[2]); break; case SYS_SOCKETPAIR: err = sys_socketpair(a0, a1, a[2], (int __user *)a[3]); break; case SYS_SEND: err = sys_send(a0, (void __user *)a1, a[2], a[3]); break; case SYS_SENDTO: err = sys_sendto(a0, (void __user *)a1, a[2], a[3], (struct sockaddr __user *)a[4], a[5]); break; case SYS_RECV: err = sys_recv(a0, (void __user *)a1, a[2], a[3]); break; case SYS_RECVFROM: err = sys_recvfrom(a0, (void __user *)a1, a[2], a[3], (struct sockaddr __user *)a[4], (int __user *)a[5]); break; case SYS_SHUTDOWN: err = sys_shutdown(a0, a1); break; case SYS_SETSOCKOPT: err = sys_setsockopt(a0, a1, a[2], (char __user *)a[3], a[4]); break; case SYS_GETSOCKOPT: err = sys_getsockopt(a0, a1, a[2], (char __user *)a[3], (int __user *)a[4]); break; case SYS_SENDMSG: err = sys_sendmsg(a0, (struct msghdr __user *)a1, a[2]); break; case SYS_RECVMSG: err = sys_recvmsg(a0, (struct msghdr __user *)a1, a[2]); break; case SYS_ACCEPT4: err = sys_accept4(a0, (struct sockaddr __user *)a1, (int __user *)a[2], a[3]); break; default: err = -EINVAL; break; } return err;}/*跟进SYSCALL_DEFINE2(listen, int, fd, int, backlog)进行分析在sys_listen函数中，系统是调用"inet_listen"这个系统调用完成listen功能的
\linux-2.6.32.63\net\socket.c*/SYSCALL_DEFINE2(listen, int, fd, int, backlog){ struct socket *sock; int err, fput_needed; int somaxconn; sock = sockfd_lookup_light(fd, &err, &fput_needed); if (sock) { somaxconn = sock_net(sock->sk)->core.sysctl_somaxconn; if ((unsigned)backlog > somaxconn) backlog = somaxconn; err = security_socket_listen(sock, backlog); if (!err) /* 调用该BSD socket对应的listen调用 这里的sock是在前面的socket调用的时候就初始化的，对应于TCP的BSD socket的操作符集是inet_stream_ops 要注意的是: 网络子模块在内核初始化的时候(linux-2.6.32.63\net\ipv4\af_inet.c->inet_init()中)就注册了TCP，UDP和RAW3中协议 linux-2.6.32.63\net\ipv4\af_inet.c const struct proto_ops inet_stream_ops = { .family = PF_INET, .owner = THIS_MODULE, .release = inet_release, .bind = inet_bind, .connect = inet_stream_connect, .socketpair = sock_no_socketpair, .accept = inet_accept, .getname = inet_getname, .poll = tcp_poll, .ioctl = inet_ioctl, .listen = inet_listen, .shutdown = inet_shutdown, .setsockopt = sock_common_setsockopt, .getsockopt = sock_common_getsockopt, .sendmsg = tcp_sendmsg, .recvmsg = sock_common_recvmsg, .mmap = sock_no_mmap, .sendpage = tcp_sendpage, .splice_read = tcp_splice_read, #ifdef CONFIG_COMPAT .compat_setsockopt = compat_sock_common_setsockopt, .compat_getsockopt = compat_sock_common_getsockopt, #endif };
EXPORT_SYMBOL(inet_stream_ops); 从上面的结构中可以看到listen对应的函数是inet_listen，我们继续分析inet_listen函数 \linux-2.6.32.63\net\ipv4\af_inet.c */ err = sock->ops->listen(sock, backlog); fput_light(sock->file, fput_needed); } return err;}/*我们继续跟进inet_listen()\linux-2.6.32.63\net\ipv4\af_inet.c*/int inet_listen(struct socket *sock, int backlog){ struct sock *sk = sock->sk; unsigned char old_state; int err; /* 在进行listen初始化之前对sk结构体进行加锁 */ lock_sock(sk); err = -EINVAL; if (sock->state != SS_UNCONNECTED || sock->type != SOCK_STREAM) goto out; old_state = sk->sk_state; if (!((1 << old_state) & (TCPF_CLOSE | TCPF_LISTEN))) goto out; /* Really, if the socket is already in listen state we can only allow the backlog to be adjusted. */ if (old_state != TCP_LISTEN) { /* 调用inet_csk_listen_start()进行端口监听 我们继续跟进inet_csk_listen_start()进行分析 \linux-2.6.32.63\net\ipv4\inet_connection_sock.c */ err = inet_csk_listen_start(sk, backlog); if (err) goto out; } sk->sk_max_ack_backlog = backlog; err = 0;out: /* 解锁 */ release_sock(sk); return err;}EXPORT_SYMBOL(inet_listen);/*我们继续跟进inet_csk_listen_start()\linux-2.6.32.63\net\ipv4\inet_connection_sock.c*/int inet_csk_listen_start(struct sock *sk, const int nr_table_entries){ struct inet_sock *inet = inet_sk(sk); struct inet_connection_sock *icsk = inet_csk(sk); int rc = reqsk_queue_alloc(&icsk->icsk_accept_queue, nr_table_entries); if (rc != 0) return rc; sk->sk_max_ack_backlog = 0; sk->sk_ack_backlog = 0; inet_csk_delack_init(sk); /* There is race window here: we announce ourselves listening, * but this transition is still not validated by get_port(). * It is OK, because this socket enters to hash table only * after validation is complete. */ sk->sk_state = TCP_LISTEN; if (!sk->sk_prot->get_port(sk, inet->num)) { inet->sport = htons(inet->num); sk_dst_reset(sk); sk->sk_prot->hash(sk); return 0; } sk->sk_state = TCP_CLOSE; __reqsk_queue_destroy(&icsk->icsk_accept_queue); return -EADDRINUSE;}EXPORT_SYMBOL_GPL(inet_csk_listen_start);
4. UDP交互过程
UDP的数据报发送常见于一些隐蔽通道、后门端口(例如icmp通道、dns隐蔽通道等等)，和TCP不同的是，UDP是一种无状态的协议，这意味着它不需要建立三次握手连接即可在通信双方之间发送数据
client.c
#include <unistd.h>#include <sys/types.h>#include <sys/socket.h>#include <netinet/in.h>#include <arpa/inet.h>#include <stdlib.h>#include <stdio.h>#include <errno.h>#include <string.h>#define ERR_EXIT(m) do { perror(m); exit(EXIT_FAILURE); } while(0)void echo_cli(int sock){ struct sockaddr_in servaddr; memset(&servaddr, 0, sizeof(servaddr)); servaddr.sin_family = AF_INET; servaddr.sin_port = htons(5188); servaddr.sin_addr.s_addr = inet_addr("127.0.0.1"); int ret; char sendbuf[1024] = {0}; char recvbuf[1024] = {0}; while (fgets(sendbuf, sizeof(sendbuf), stdin) != NULL) { sendto(sock, sendbuf, strlen(sendbuf), 0, (struct sockaddr *)&servaddr, sizeof(servaddr)); ret = recvfrom(sock, recvbuf, sizeof(recvbuf), 0, NULL, NULL); if (ret == -1) { if (errno == EINTR) continue; ERR_EXIT("recvfrom"); } fputs(recvbuf, stdout); memset(sendbuf, 0, sizeof(sendbuf)); memset(recvbuf, 0, sizeof(recvbuf)); } close(sock);}int main(void){ int sock; if ((sock = socket(PF_INET, SOCK_DGRAM, 0)) < 0) ERR_EXIT("socket"); echo_cli(sock); return 0;}
server.c
#include<stdio.h>#include<stdlib.h>#include<unistd.h>#include<errno.h>#include<sys/types.h>#include<sys/socket.h>#include<netinet/in.h>#include<string.h>#define ERR_EXIT(m) do { perror(m); exit(EXIT_FAILURE); } while (0)void echo_ser(int sock){ char recvbuf[1024] = {0}; struct sockaddr_in peeraddr; socklen_t peerlen; int n; while (1) { peerlen = sizeof(peeraddr); memset(recvbuf, 0, sizeof(recvbuf)); n = recvfrom(sock, recvbuf, sizeof(recvbuf), 0, (struct sockaddr *)&peeraddr, &peerlen); if (n == -1) { if (errno == EINTR) continue; ERR_EXIT("recvfrom error"); } else if(n > 0) { fputs(recvbuf, stdout); sendto(sock, recvbuf, n, 0, (struct sockaddr *)&peeraddr, peerlen); } } close(sock);}int main(void){ int sock; if ((sock = socket(PF_INET, SOCK_DGRAM, 0)) < 0) ERR_EXIT("socket error"); struct sockaddr_in servaddr; memset(&servaddr, 0, sizeof(servaddr)); servaddr.sin_family = AF_INET; servaddr.sin_port = htons(5188); servaddr.sin_addr.s_addr = htonl(INADDR_ANY); if (bind(sock, (struct sockaddr *)&servaddr, sizeof(servaddr)) < 0) ERR_EXIT("bind error"); echo_ser(sock); return 0;}
Relevant Link:
http://blog.csdn.net/jnu_simba/article/details/9077455http://www.cplusplus.me/1070.html
5. UDP sendto() API原理
sendto()从ring3到ring0的整个流程如下:
/*在ring3层调用socket的API: sendto()*/1. ring3: 1) sendto()->/*进入ring0层*/2. ring0: /* SYSCALL_DEFINE2(socketcall..等效于调用sys_socketcall(.. */ 1) SYSCALL_DEFINE2(socketcall, int, call, unsigned long __user *, args) /* SYSCALL_DEFINE6(sendto..等效于调用sys_sendto(.. */ 2) SYSCALL_DEFINE6(sendto, int, fd, void __user *, buff, size_t, len, unsigned, flags, struct sockaddr __user *, addr, int, addr_len)-> 3) sock_sendmsg()-> 4) __sock_sendmsg()-> 5) inet_sendmsg()-> /* 发送UDP数据报 */ 6) udp_sendmsg()->
\linux-2.6.32.63\net\socket.c
/*进行调用派发
\linux-2.6.32.63\net\socket.c*/ SYSCALL_DEFINE2(socketcall, int, call, unsigned long __user *, args){ unsigned long a[6]; unsigned long a0, a1; int err; unsigned int len; if (call < 1 || call > SYS_ACCEPT4) return -EINVAL; len = nargs[call]; if (len > sizeof(a)) return -EINVAL; /* copy_from_user should be SMP safe. */ if (copy_from_user(a, args, len)) return -EFAULT; audit_socketcall(nargs[call] / sizeof(unsigned long), a); a0 = a[0]; a1 = a[1]; /* 进入内核的sys_socketcall()系统调用后，linux系统需要将特定的调用分路发送到指定目标，例如 1) sockcet()调用将由sys_socket()完成 2) send()调用将由sys_send()完成 系统在include/linux/net.h中定义了以下17种类型socketcall 1) #define SYS_SOCKET 1: sys_socket(2) 2) #define SYS_BIND 2: sys_bind(2) 3) #define SYS_CONNECT 3: sys_connect(2) 4) #define SYS_LISTEN 4: sys_listen(2) 5) #define SYS_ACCEPT 5: sys_accept(2) 6) #define SYS_GETSOCKNAME 6: sys_getsockname(2) 7) #define SYS_GETPEERNAME 7: sys_getpeername(2) 8) #define SYS_SOCKETPAIR 8: sys_socketpair(2) 9) #define SYS_SEND 9: sys_send(2) 10) #define SYS_RECV 10: sys_recv(2) 11) #define SYS_SENDTO 11: sys_sendto(2) 12) #define SYS_RECVFROM 12: sys_recvfrom(2) 13) #define SYS_SHUTDOWN 13: sys_shutdown(2) 14) #define SYS_SETSOCKOPT 14: sys_setsockopt(2) 15) #define SYS_GETSOCKOPT 15: sys_getsockopt(2) 16) #define SYS_SENDMSG 16: sys_sendmsg(2) 17) #define SYS_RECVMSG 17: sys_recvmsg(2) 18) #define SYS_ACCEPT4 18: sys_accept4(2) */ switch (call) SYS_SOCKET { case : err = sys_socket(a0, a1, a[2]); break; case SYS_BIND: err = sys_bind(a0, (struct sockaddr __user *)a1, a[2]); break; case SYS_CONNECT: err = sys_connect(a0, (struct sockaddr __user *)a1, a[2]); break; case SYS_LISTEN: err = sys_listen(a0, a1); break; case SYS_ACCEPT: err = sys_accept4(a0, (struct sockaddr __user *)a1, (int __user *)a[2], 0); break; case SYS_GETSOCKNAME: err = sys_getsockname(a0, (struct sockaddr __user *)a1, (int __user *)a[2]); break; case SYS_GETPEERNAME: err = sys_getpeername(a0, (struct sockaddr __user *)a1, (int __user *)a[2]); break; case SYS_SOCKETPAIR: err = sys_socketpair(a0, a1, a[2], (int __user *)a[3]); break; case SYS_SEND: err = sys_send(a0, (void __user *)a1, a[2], a[3]); break; /* 等效于调用 SYSCALL_DEFINE6(sendto, int, fd, void __user *, buff, size_t, len, unsigned, flags, struct sockaddr __user *, addr, int, addr_len) */ case SYS_SENDTO: err = sys_sendto(a0, (void __user *)a1, a[2], a[3], (struct sockaddr __user *)a[4], a[5]); break; case SYS_RECV: err = sys_recv(a0, (void __user *)a1, a[2], a[3]); break; case SYS_RECVFROM: err = sys_recvfrom(a0, (void __user *)a1, a[2], a[3], (struct sockaddr __user *)a[4], (int __user *)a[5]); break; case SYS_SHUTDOWN: err = sys_shutdown(a0, a1); break; case SYS_SETSOCKOPT: err = sys_setsockopt(a0, a1, a[2], (char __user *)a[3], a[4]); break; case SYS_GETSOCKOPT: err = sys_getsockopt(a0, a1, a[2], (char __user *)a[3], (int __user *)a[4]); break; case SYS_SENDMSG: err = sys_sendmsg(a0, (struct msghdr __user *)a1, a[2]); break; case SYS_RECVMSG: err = sys_recvmsg(a0, (struct msghdr __user *)a1, a[2]); break; case SYS_ACCEPT4: err = sys_accept4(a0, (struct sockaddr __user *)a1, (int __user *)a[2], a[3]); break; default: err = -EINVAL; break; } return err;}/*调用SYSCALL_DEFINE6(sendto, int, fd, void __user *, buff, size_t, len, unsigned, flags, struct sockaddr __user *, addr, int, addr_len)
\linux-2.6.32.63\net\socket.c*/SYSCALL_DEFINE6(sendto, int, fd, void __user *, buff, size_t, len, unsigned, flags, struct sockaddr __user *, addr, int, addr_len){ struct socket *sock; struct sockaddr_storage address; int err; struct msghdr msg; struct iovec iov; int fput_needed; if (len > INT_MAX) len = INT_MAX; /* 通过函数sockfd_lookup_light和参数fd，来得到对应的sock sockfd_lookup_light的实现比较简单 1. fd就是进程的fdtable的索引 2. 通过这个fd索引就可以得到对应的file指针 3. 然后在从file指针中，得到sock的地址 static struct socket *sockfd_lookup_light(int fd, int *err, int *fput_needed) { struct file *file; struct socket *sock; *err = -EBADF; file = fget_light(fd, fput_needed); if (file) { sock = sock_from_file(file, err); if (sock) return sock; fput_light(file, *fput_needed); } return NULL; } */ sock = sockfd_lookup_light(fd, &err, &fput_needed); if (!sock) goto out; /* 初始化iov和msg，因为这里的消息传递方式采用的是BSD的消息传递方式 struct msghdr { void * msg_name; // Socket name int msg_namelen; // Length of name struct iovec * msg_iov; // Data blocks __kernel_size_t msg_iovlen; // Number of blocks void * msg_control; // Per protocol magic (eg BSD file descriptor passing) __kernel_size_t msg_controllen;// Length of cmsg list unsigned msg_flags; }; */ iov.iov_base = buff; iov.iov_len = len; msg.msg_name = NULL; msg.msg_iov = &iov; msg.msg_iovlen = 1; msg.msg_control = NULL; msg.msg_controllen = 0; msg.msg_namelen = 0; /* 1. 如果sendto指定了addr，那么首先将用户空间的地址addr复制到kernel空间的address中，并用内核空间的address来初始化msg */ if (addr) { err = move_addr_to_kernel(addr, addr_len, (struct sockaddr *)&address); if (err < 0) goto out_put; msg.msg_name = (struct sockaddr *)&address; msg.msg_namelen = addr_len; } /* 如果该socket指定了O_NONBLOCK，那么将flags设置上MSG_DONTWAIT，并将flags赋给msg.msg_flags */ if (sock->file->f_flags & O_NONBLOCK) flags |= MSG_DONTWAIT; msg.msg_flags = flags; /* 最后调用sock_sendmsg，将msg发送出去 我们继续跟进sock_sendmsg()分析
\linux-2.6.32.63\net\socket.c */ err = sock_sendmsg(sock, &msg, len);out_put: fput_light(sock->file, fput_needed);out: return err;}/*继续跟进sock_sendmsg()分析\linux-2.6.32.63\net\socket.c*/int sock_sendmsg(struct socket *sock, struct msghdr *msg, size_t size){ struct kiocb iocb; struct sock_iocb siocb; int ret; init_sync_kiocb(&iocb, NULL); iocb.private = &siocb; /* 调用__sock_sendmsg进行UDP数据报的发送 继续跟进__sock_sendmsg()进行分析
\linux-2.6.32.63\net\socket.c */ ret = __sock_sendmsg(&iocb, sock, msg, size); if (-EIOCBQUEUED == ret) ret = wait_on_sync_kiocb(&iocb); return ret;}/*继续跟进__sock_sendmsg()分析\linux-2.6.32.63\net\socket.c*/static inline int __sock_sendmsg(struct kiocb *iocb, struct socket *sock, struct msghdr *msg, size_t size){ struct sock_iocb *si = kiocb_to_siocb(iocb); int err; si->sock = sock; si->scm = NULL; si->msg = msg; si->size = size; err = security_socket_sendmsg(sock, msg, size); if (err) return err; /* const struct proto_ops inet_dgram_ops = { .family = PF_INET, .owner = THIS_MODULE, .release = inet_release, .bind = inet_bind, .connect = inet_dgram_connect, .socketpair = sock_no_socketpair, .accept = sock_no_accept, .getname = inet_getname, .poll = udp_poll, .ioctl = inet_ioctl, .listen = sock_no_listen, .shutdown = inet_shutdown, .setsockopt = sock_common_setsockopt, .getsockopt = sock_common_getsockopt, .sendmsg = inet_sendmsg, .recvmsg = sock_common_recvmsg, .mmap = sock_no_mmap, .sendpage = inet_sendpage, #ifdef CONFIG_COMPAT .compat_setsockopt = compat_sock_common_setsockopt, .compat_getsockopt = compat_sock_common_getsockopt, #endif }; EXPORT_SYMBOL(inet_dgram_ops); 从结构体中可以看出，sendmsg()对应的系统调用是inet_sendmsg() 我们继续跟进分析inet_sendmsg() \linux-2.6.32.63\net\ipv4\af_inet.c */ return sock->ops->sendmsg(iocb, sock, msg, size);}/*继续跟进分析inet_sendmsg()\linux-2.6.32.63\net\ipv4\af_inet.c*/int inet_sendmsg(struct kiocb *iocb, struct socket *sock, struct msghdr *msg, size_t size){ struct sock *sk = sock->sk; /* We may need to bind the socket. */ if (!inet_sk(sk)->num && inet_autobind(sk)) return -EAGAIN; /* INET SOCKET调用协议特有sendmsg操作符 对于INET socket中的udp发送，协议特有操作符集为udp_prot linux-2.6.32.63\net\ipv4\udp.c struct proto udp_prot = { .name = "UDP", .owner = THIS_MODULE, .close = udp_lib_close, .connect = ip4_datagram_connect, .disconnect = udp_disconnect, .ioctl = udp_ioctl, .destroy = udp_destroy_sock, .setsockopt = udp_setsockopt, .getsockopt = udp_getsockopt, .sendmsg = udp_sendmsg, .recvmsg = udp_recvmsg, .sendpage = udp_sendpage, .backlog_rcv = __udp_queue_rcv_skb, .hash = udp_lib_hash, .unhash = udp_lib_unhash, .get_port = udp_v4_get_port, .memory_allocated = &udp_memory_allocated, .sysctl_mem = sysctl_udp_mem, .sysctl_wmem = &sysctl_udp_wmem_min, .sysctl_rmem = &sysctl_udp_rmem_min, .obj_size = sizeof(struct udp_sock), .slab_flags = SLAB_DESTROY_BY_RCU, .h.udp_table = &udp_table, #ifdef CONFIG_COMPAT .compat_setsockopt = compat_udp_setsockopt, .compat_getsockopt = compat_udp_getsockopt, #endif }; EXPORT_SYMBOL(udp_prot); 可以看出，对于UDP，流程进入udp_sendmsg函数(.sendmsg对应的是udp_sendmsg()函数)，我们继续跟进udp_sendmsg() \linux-2.6.32.63\net\ipv4\udp.c */ return sk->sk_prot->sendmsg(iocb, sk, msg, size);}EXPORT_SYMBOL(inet_sendmsg);/*继续跟进udp_sendmsg()\linux-2.6.32.63\net\ipv4\udp.c*/int udp_sendmsg(struct kiocb *iocb, struct sock *sk, struct msghdr *msg, size_t len){ struct inet_sock *inet = inet_sk(sk); struct udp_sock *up = udp_sk(sk); int ulen = len; struct ipcm_cookie ipc; struct rtable *rt = NULL; int free = 0; int connected = 0; __be32 daddr, faddr, saddr; __be16 dport; u8 tos; int err, is_udplite = IS_UDPLITE(sk); int corkreq = up->corkflag || msg->msg_flags&MSG_MORE; int (*getfrag)(void *, char *, int, int, int, struct sk_buff *); struct ip_options_data opt_copy; if (len > 0xFFFF) return -EMSGSIZE; /* * Check the flags. */ if (msg->msg_flags & MSG_OOB) /* Mirror BSD error message compatibility */ return -EOPNOTSUPP; ipc.opt = NULL; ipc.shtx.flags = 0; if (up->pending) { /* * There are pending frames. * The socket lock must be held while it‘s corked. */ lock_sock(sk); if (likely(up->pending)) { if (unlikely(up->pending != AF_INET)) { release_sock(sk); return -EINVAL; } goto do_append_data; } release_sock(sk); } ulen += sizeof(struct udphdr); /* * Get and verify the address. */ if (msg->msg_name) { struct sockaddr_in * usin = (struct sockaddr_in *)msg->msg_name; if (msg->msg_namelen < sizeof(*usin)) return -EINVAL; if (usin->sin_family != AF_INET) { if (usin->sin_family != AF_UNSPEC) return -EAFNOSUPPORT; } daddr = usin->sin_addr.s_addr; dport = usin->sin_port; if (dport == 0) return -EINVAL; } else { if (sk->sk_state != TCP_ESTABLISHED) return -EDESTADDRREQ; daddr = inet->daddr; dport = inet->dport; /* Open fast path for connected socket. Route will not be used, if at least one option is set. */ connected = 1; } ipc.addr = inet->saddr; ipc.oif = sk->sk_bound_dev_if; err = sock_tx_timestamp(msg, sk, &ipc.shtx); if (err) return err; if (msg->msg_controllen) { err = ip_cmsg_send(sock_net(sk), msg, &ipc); if (err) return err; if (ipc.opt) free = 1; connected = 0; } if (!ipc.opt) { struct ip_options_rcu *inet_opt; rcu_read_lock(); inet_opt = rcu_dereference(inet->inet_opt); if (inet_opt) { memcpy(&opt_copy, inet_opt, sizeof(*inet_opt) + inet_opt->opt.optlen); ipc.opt = &opt_copy.opt; } rcu_read_unlock(); } saddr = ipc.addr; ipc.addr = faddr = daddr; if (ipc.opt && ipc.opt->opt.srr) { if (!daddr) return -EINVAL; faddr = ipc.opt->opt.faddr; connected = 0; } tos = RT_TOS(inet->tos); if (sock_flag(sk, SOCK_LOCALROUTE) || (msg->msg_flags & MSG_DONTROUTE) || (ipc.opt && ipc.opt->opt.is_strictroute)) { tos |= RTO_ONLINK; connected = 0; } if (ipv4_is_multicast(daddr)) { if (!ipc.oif) ipc.oif = inet->mc_index; if (!saddr) saddr = inet->mc_addr; connected = 0; } if (connected) rt = (struct rtable *)sk_dst_check(sk, 0); if (rt == NULL) { struct flowi fl = { .oif = ipc.oif, .mark = sk->sk_mark, .nl_u = { .ip4_u = { .daddr = faddr, .saddr = saddr, .tos = tos } }, .proto = sk->sk_protocol, .flags = inet_sk_flowi_flags(sk), .uli_u = { .ports = { .sport = inet->sport, .dport = dport } } }; struct net *net = sock_net(sk); security_sk_classify_flow(sk, &fl); err = ip_route_output_flow(net, &rt, &fl, sk, 1); if (err) { if (err == -ENETUNREACH) IP_INC_STATS(net, IPSTATS_MIB_OUTNOROUTES); goto out; } err = -EACCES; if ((rt->rt_flags & RTCF_BROADCAST) && !sock_flag(sk, SOCK_BROADCAST)) goto out; if (connected) sk_dst_set(sk, dst_clone(&rt->u.dst)); } if (msg->msg_flags&MSG_CONFIRM) goto do_confirm;back_from_confirm: saddr = rt->rt_src; if (!ipc.addr) daddr = ipc.addr = rt->rt_dst; lock_sock(sk); if (unlikely(up->pending)) { /* The socket is already corked while preparing it. */ /* ... which is an evident application bug. --ANK */ release_sock(sk); LIMIT_NETDEBUG(KERN_DEBUG "udp cork app bug 2\n"); err = -EINVAL; goto out; } /* * Now cork the socket to pend data. */ inet->cork.fl.fl4_dst = daddr; inet->cork.fl.fl_ip_dport = dport; inet->cork.fl.fl4_src =http://www.mamicode.com/ saddr; inet->cork.fl.fl_ip_sport = inet->sport; up->pending = AF_INET;do_append_data: up->len += ulen; getfrag = is_udplite ? udplite_getfrag : ip_generic_getfrag; err = ip_append_data(sk, getfrag, msg->msg_iov, ulen, sizeof(struct udphdr), &ipc, &rt, corkreq ? msg->msg_flags|MSG_MORE : msg->msg_flags); if (err) udp_flush_pending_frames(sk); else if (!corkreq) err = udp_push_pending_frames(sk); else if (unlikely(skb_queue_empty(&sk->sk_write_queue))) up->pending = 0; release_sock(sk);out: ip_rt_put(rt); if (free) kfree(ipc.opt); if (!err) return len; /* * ENOBUFS = no kernel mem, SOCK_NOSPACE = no sndbuf space. Reporting * ENOBUFS might not be good (it‘s not tunable per se), but otherwise * we don‘t have a good statistic (IpOutDiscards but it can be too many * things). We could add another new stat but at least for now that * seems like overkill. */ if (err == -ENOBUFS || test_bit(SOCK_NOSPACE, &sk->sk_socket->flags)) { UDP_INC_STATS_USER(sock_net(sk), UDP_MIB_SNDBUFERRORS, is_udplite); } return err;do_confirm: dst_confirm(&rt->u.dst); if (!(msg->msg_flags&MSG_PROBE) || len) goto back_from_confirm; err = 0; goto out;}EXPORT_SYMBOL(udp_sendmsg);
Relevant Link:
http://blog.chinaunix.net/uid-23629988-id-85912.html
6. UDP bind() API原理
对于UDP的服务端程序来说(很多隐藏通道、icmp、iis恶意模块...)都需要监听一个UDP端口，对于TCP服务端程序，可以使用listen监听指定的IP和端口，但是对于UDP，它必须使用bind()进行端口绑定
bind()从ring3到ring0的整个流程如下:
/*在ring3层调用socket的API: bind()*/1. ring3: 1) bind()->/*进入ring0层*/2. ring0: /* SYSCALL_DEFINE2(socketcall..等效于调用sys_socketcall(.. */ 1) SYSCALL_DEFINE2(socketcall, int, call, unsigned long __user *, args) /* SYSCALL_DEFINE3(bind..等效于调用sys_bind(.. */ 2) SYSCALL_DEFINE3(bind, int, fd, struct sockaddr __user *, umyaddr, int, addrlen)-> /* 绑定指定端口 */ 3) inet_bind()->
\linux-2.6.32.63\net\socket.c
/*进行调用派发\linux-2.6.32.63\net\socket.c*/ SYSCALL_DEFINE2(socketcall, int, call, unsigned long __user *, args){ unsigned long a[6]; unsigned long a0, a1; int err; unsigned int len; if (call < 1 || call > SYS_ACCEPT4) return -EINVAL; len = nargs[call]; if (len > sizeof(a)) return -EINVAL; /* copy_from_user should be SMP safe. */ if (copy_from_user(a, args, len)) return -EFAULT; audit_socketcall(nargs[call] / sizeof(unsigned long), a); a0 = a[0]; a1 = a[1]; /* 进入内核的sys_socketcall()系统调用后，linux系统需要将特定的调用分路发送到指定目标，例如 1) sockcet()调用将由sys_socket()完成 2) send()调用将由sys_send()完成 系统在include/linux/net.h中定义了以下17种类型socketcall 1) #define SYS_SOCKET 1: sys_socket(2) 2) #define SYS_BIND 2: sys_bind(2) 3) #define SYS_CONNECT 3: sys_connect(2) 4) #define SYS_LISTEN 4: sys_listen(2) 5) #define SYS_ACCEPT 5: sys_accept(2) 6) #define SYS_GETSOCKNAME 6: sys_getsockname(2) 7) #define SYS_GETPEERNAME 7: sys_getpeername(2) 8) #define SYS_SOCKETPAIR 8: sys_socketpair(2) 9) #define SYS_SEND 9: sys_send(2) 10) #define SYS_RECV 10: sys_recv(2) 11) #define SYS_SENDTO 11: sys_sendto(2) 12) #define SYS_RECVFROM 12: sys_recvfrom(2) 13) #define SYS_SHUTDOWN 13: sys_shutdown(2) 14) #define SYS_SETSOCKOPT 14: sys_setsockopt(2) 15) #define SYS_GETSOCKOPT 15: sys_getsockopt(2) 16) #define SYS_SENDMSG 16: sys_sendmsg(2) 17) #define SYS_RECVMSG 17: sys_recvmsg(2) 18) #define SYS_ACCEPT4 18: sys_accept4(2) */ switch (call) SYS_SOCKET { case : err = sys_socket(a0, a1, a[2]); break; /* 等效于调用 SYSCALL_DEFINE3(bind, int, fd, struct sockaddr __user *, umyaddr, int, addrlen) \linux-2.6.32.63\net\socket.c */ case SYS_BIND: err = sys_bind(a0, (struct sockaddr __user *)a1, a[2]); break; case SYS_CONNECT: err = sys_connect(a0, (struct sockaddr __user *)a1, a[2]); break; case SYS_LISTEN: err = sys_listen(a0, a1); break; case SYS_ACCEPT: err = sys_accept4(a0, (struct sockaddr __user *)a1, (int __user *)a[2], 0); break; case SYS_GETSOCKNAME: err = sys_getsockname(a0, (struct sockaddr __user *)a1, (int __user *)a[2]); break; case SYS_GETPEERNAME: err = sys_getpeername(a0, (struct sockaddr __user *)a1, (int __user *)a[2]); break; case SYS_SOCKETPAIR: err = sys_socketpair(a0, a1, a[2], (int __user *)a[3]); break; case SYS_SEND: err = sys_send(a0, (void __user *)a1, a[2], a[3]); break; case SYS_SENDTO: err = sys_sendto(a0, (void __user *)a1, a[2], a[3], (struct sockaddr __user *)a[4], a[5]); break; case SYS_RECV: err = sys_recv(a0, (void __user *)a1, a[2], a[3]); break; case SYS_RECVFROM: err = sys_recvfrom(a0, (void __user *)a1, a[2], a[3], (struct sockaddr __user *)a[4], (int __user *)a[5]); break; case SYS_SHUTDOWN: err = sys_shutdown(a0, a1); break; case SYS_SETSOCKOPT: err = sys_setsockopt(a0, a1, a[2], (char __user *)a[3], a[4]); break; case SYS_GETSOCKOPT: err = sys_getsockopt(a0, a1, a[2], (char __user *)a[3], (int __user *)a[4]); break; case SYS_SENDMSG: err = sys_sendmsg(a0, (struct msghdr __user *)a1, a[2]); break; case SYS_RECVMSG: err = sys_recvmsg(a0, (struct msghdr __user *)a1, a[2]); break; case SYS_ACCEPT4: err = sys_accept4(a0, (struct sockaddr __user *)a1, (int __user *)a[2], a[3]); break; default: err = -EINVAL; break; } return err;}/*继续跟进SYSCALL_DEFINE3(bind, int, fd, struct sockaddr __user *, umyaddr, int, addrlen)1. sys_bind()首先调用sockfd_lookup_light()查找套接字对应的socket实例，如果没有找到，则返回EBADF错误2. 在进行绑定操作之前，要先将用户传入的本地协议地址从用户空间拷贝到内核缓冲区中，在拷贝过程中会检查用户传入的地址是否正确。如果指定的长度参数小于0或者大于sockaddr_storage的大小，则返回EINVAL错误3. 如果在调用copy_from_user()执行拷贝操作过程中出现错误，则返回EFAULT错误4. 在上述的准备工作都完成后，调用inet_bind()函数(即sock->ops->bind指向的函数)来完成绑定操作\linux-2.6.32.63\net\socket.c*/SYSCALL_DEFINE3(bind, int, fd, struct sockaddr __user *, umyaddr, int, addrlen){ struct socket *sock; struct sockaddr_storage address; int err, fput_needed; /* 1. 以fd为索引从当前进程的文件描述符表中 2. 找到对应的file实例，然后从file实例的private_data中 3. 获取socket实例 */ sock = sockfd_lookup_light(fd, &err, &fput_needed); if (sock) { /* 将用户空间的地址拷贝到内核空间的缓冲区中。 */ err = move_addr_to_kernel(umyaddr, addrlen, (struct sockaddr *)&address); if (err >= 0) { /* SELinux相关 */ err = security_socket_bind(sock, (struct sockaddr *)&address, addrlen); /* 如果是TCP套接字，sock->ops指向的是inet_stream_ops， sock->ops是在inet_create()函数中初始化，所以bind接口调用的是inet_bind()函数。 继续跟进inet_bind() \linux-2.6.32.63\net\ipv4\af_inet.c */ if (!err) err = sock->ops->bind(sock, (struct sockaddr *)&address, addrlen); } fput_light(sock->file, fput_needed); } return err;}/*继续跟进inet_bind()\linux-2.6.32.63\net\ipv4\af_inet.c*/int inet_bind(struct socket *sock, struct sockaddr *uaddr, int addr_len){ struct sockaddr_in *addr = (struct sockaddr_in *)uaddr; struct sock *sk = sock->sk; struct inet_sock *inet = inet_sk(sk); unsigned short snum; int chk_addr_ret; int err; /* If the socket has its own bind function then use it. (RAW) 如果是TCP套接字，sk->sk_prot指向的是tcp_prot，在inet_create()中调用的sk_alloc()函数中初始化 由于tcp_prot中没有设置bind接口，因此判断条件不成立 */ if (sk->sk_prot->bind) { err = sk->sk_prot->bind(sk, uaddr, addr_len); goto out; } err = -EINVAL; if (addr_len < sizeof(struct sockaddr_in)) goto out; if (addr->sin_family != AF_INET) goto out; /* 判断传入的地址类型 */ chk_addr_ret = inet_addr_type(sock_net(sk), addr->sin_addr.s_addr); /* Not specified by any standard per-se, however it breaks too * many applications when removed. It is unfortunate since * allowing applications to make a non-local bind solves * several problems with systems using dynamic addressing. * (ie. your servers still start up even if your ISDN link * is temporarily down) */ err = -EADDRNOTAVAIL; /* 如果系统不支持绑定本地地址，或者 传入的地址类型有误，则返回EADDRNOTAVAIL错误 */ if (!sysctl_ip_nonlocal_bind && !(inet->freebind || inet->transparent) && addr->sin_addr.s_addr != htonl(INADDR_ANY) && chk_addr_ret != RTN_LOCAL && chk_addr_ret != RTN_MULTICAST && chk_addr_ret != RTN_BROADCAST) goto out; snum = ntohs(addr->sin_port); err = -EACCES; /* 如果绑定的端口号小于1024(保留端口号)，但是当前用户没有CAP_NET_BIND_SERVICE权限，则返回EACCESS错误 */ if (snum && snum < PROT_SOCK && !capable(CAP_NET_BIND_SERVICE)) goto out; /* We keep a pair of addresses. rcv_saddr is the one * used by hash lookups, and saddr is used for transmit. * * In the BSD API these are the same except where it * would be illegal to use them (multicast/broadcast) in * which case the sending device address is used. */ lock_sock(sk); /* 如果套接字状态不是TCP_CLOSE(套接字的初始状态，参见sock_init_data()函数) 或者已经绑定过，则返回EINVAL错误 */ err = -EINVAL; if (sk->sk_state != TCP_CLOSE || inet->num) goto out_release_sock; inet->rcv_saddr = inet->saddr = addr->sin_addr.s_addr; if (chk_addr_ret == RTN_MULTICAST || chk_addr_ret == RTN_BROADCAST) inet->saddr = 0; /* Use device */ /* 这里实际调用的是inet_csk_get_port()函数。 1. 检查要绑定的端口号是否已经使用，如果已经使用， 2. 则检查是否允许复用。如果检查失败，则返回EADDRINUSE错误 */ if (sk->sk_prot->get_port(sk, snum)) { inet->saddr = inet->rcv_saddr = 0; err = -EADDRINUSE; goto out_release_sock; } /* rcv_saddr存储的是已绑定的本地地址，接收数据时使用。 如果已绑定的地址不为0，则设置SOCK_BINDADDR_LOCK标志，表示已绑定本地地址 */ if (inet->rcv_saddr) sk->sk_userlocks |= SOCK_BINDADDR_LOCK; /* 如果绑定的端口号不为0，则设置SOCK_BINDPORT_LOCK标志，表示已绑定本地端口号。 */ if (snum) sk->sk_userlocks |= SOCK_BINDPORT_LOCK; inet->sport = htons(inet->num); inet->daddr = 0; inet->dport = 0; /* 重新初始化目的路由缓存项，如果之前已设置，则调用dst_release()释放老的路由缓存项 */ sk_dst_reset(sk); err = 0;out_release_sock: release_sock(sk);out: return err;}
Relevant Link:
http://blog.csdn.net/justlinux2010/article/details/8593539
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