写入关闭的本地 TCP 套接字不会失败 [英] Writing to a closed, local TCP socket not failing
问题描述
我的套接字似乎有问题.下面，您将看到一些分叉服务器和客户端的代码.服务器打开一个 TCP 套接字，客户端连接到它然后关闭它.睡眠用于协调时间.在客户端 close() 之后，服务器尝试 write() 到它自己的 TCP 连接端.根据 write(2) 手册页，这 应该 给我一个 SIGPIPE 和一个 EPIPE errno.但是，我没有看到这一点.从服务器的角度来看，写入本地关闭的套接字成功，并且没有 EPIPE 我看不出服务器应该如何检测到客户端已经关闭了套接字.
I seem to be having a problem with my sockets. Below, you will see some code which forks a server and a client. The server opens a TCP socket, and the client connects to it and then closes it. Sleeps are used to coordinate the timing. After the client-side close(), the server tries to write() to its own end of the TCP connection. According to the write(2) man page, this should give me a SIGPIPE and an EPIPE errno. However, I don't see this. From the server's point of view, the write to a local, closed socket succeeds, and absent the EPIPE I can't see how the server should be detecting that the client has closed the socket.
在客户端关闭端和服务器尝试写入之间的间隙中，调用 netstat 将显示连接处于 CLOSE_WAIT/FIN_WAIT2 状态，因此服务器端肯定能够拒绝写入.
In the gap between the client closing its end and the server attempting to write, a call to netstat will show that the connection is in a CLOSE_WAIT/FIN_WAIT2 state, so the server end should definitely be able to reject the write.
作为参考，我使用的是 Debian Squeeze，uname -r 是 2.6.39-bpo.2-amd64.
For reference, I'm on Debian Squeeze, uname -r is 2.6.39-bpo.2-amd64.
这是怎么回事?
#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <sys/socket.h>
#include <sys/select.h>
#include <netinet/tcp.h>
#include <errno.h>
#include <string.h>
#include <stdlib.h>
#include <fcntl.h>
#include <netdb.h>
#define SERVER_ADDRESS "127.0.0.7"
#define SERVER_PORT 4777
#define myfail_if( test, msg ) do { if((test)){ fprintf(stderr, msg "
"); exit(1); } } while (0)
#define myfail_unless( test, msg ) myfail_if( !(test), msg )
int connect_client( char *addr, int actual_port )
{
int client_fd;
struct addrinfo hint;
struct addrinfo *ailist, *aip;
memset( &hint, ' ', sizeof( struct addrinfo ) );
hint.ai_socktype = SOCK_STREAM;
myfail_if( getaddrinfo( addr, NULL, &hint, &ailist ) != 0, "getaddrinfo failed." );
int connected = 0;
for( aip = ailist; aip; aip = aip->ai_next ) {
((struct sockaddr_in *)aip->ai_addr)->sin_port = htons( actual_port );
client_fd = socket( aip->ai_family, aip->ai_socktype, aip->ai_protocol );
if( client_fd == -1) { continue; }
if( connect( client_fd, aip->ai_addr, aip->ai_addrlen) == 0 ) {
connected = 1;
break;
}
close( client_fd );
}
freeaddrinfo( ailist );
myfail_unless( connected, "Didn't connect." );
return client_fd;
}
void client(){
sleep(1);
int client_fd = connect_client( SERVER_ADDRESS, SERVER_PORT );
printf("Client closing its fd... ");
myfail_unless( 0 == close( client_fd ), "close failed" );
fprintf(stdout, "Client exiting.
");
exit(0);
}
int init_server( struct sockaddr * saddr, socklen_t saddr_len )
{
int sock_fd;
sock_fd = socket( saddr->sa_family, SOCK_STREAM, 0 );
if ( sock_fd < 0 ){
return sock_fd;
}
myfail_unless( bind( sock_fd, saddr, saddr_len ) == 0, "Failed to bind." );
return sock_fd;
}
int start_server( const char * addr, int port )
{
struct addrinfo *ailist, *aip;
struct addrinfo hint;
int sock_fd;
memset( &hint, ' ', sizeof( struct addrinfo ) );
hint.ai_socktype = SOCK_STREAM;
myfail_if( getaddrinfo( addr, NULL, &hint, &ailist ) != 0, "getaddrinfo failed." );
for( aip = ailist; aip; aip = aip->ai_next ){
((struct sockaddr_in *)aip->ai_addr)->sin_port = htons( port );
sock_fd = init_server( aip->ai_addr, aip->ai_addrlen );
if ( sock_fd > 0 ){
break;
}
}
freeaddrinfo( aip );
myfail_unless( listen( sock_fd, 2 ) == 0, "Failed to listen" );
return sock_fd;
}
int server_accept( int server_fd )
{
printf("Accepting
");
int client_fd = accept( server_fd, NULL, NULL );
myfail_unless( client_fd > 0, "Failed to accept" );
return client_fd;
}
void server() {
int server_fd = start_server(SERVER_ADDRESS, SERVER_PORT);
int client_fd = server_accept( server_fd );
printf("Server sleeping
");
sleep(60);
printf( "Errno before: %s
", strerror( errno ) );
printf( "Write result: %d
", write( client_fd, "123", 3 ) );
printf( "Errno after: %s
", strerror( errno ) );
close( client_fd );
}
int main(void){
pid_t clientpid;
pid_t serverpid;
clientpid = fork();
if ( clientpid == 0 ) {
client();
} else {
serverpid = fork();
if ( serverpid == 0 ) {
server();
}
else {
int clientstatus;
int serverstatus;
waitpid( clientpid, &clientstatus, 0 );
waitpid( serverpid, &serverstatus, 0 );
printf( "Client status is %d, server status is %d
",
clientstatus, serverstatus );
}
}
return 0;
}
推荐答案
这是 Linux 手册页关于 write 和 EPIPE 的说明:
This is what the Linux man page says about write and EPIPE:
EPIPE fd is connected to a pipe or socket whose reading end is closed.
When this happens the writing process will also receive a SIG-
PIPE signal. (Thus, the write return value is seen only if the
program catches, blocks or ignores this signal.)
当 Linux 使用 pipe 或 socketpair 时，它可以并且会检查对的读端，因为这两个程序会证明:
When Linux is using a pipe or a socketpair, it can and will check the reading end of the pair, as these two programs would demonstrate:
void test_socketpair () {
int pair[2];
socketpair(PF_LOCAL, SOCK_STREAM, 0, pair);
close(pair[0]);
if (send(pair[1], "a", 1, MSG_NOSIGNAL) < 0) perror("send");
}
void test_pipe () {
int pair[2];
pipe(pair);
close(pair[0]);
signal(SIGPIPE, SIG_IGN);
if (write(pair[1], "a", 1) < 0) perror("send");
signal(SIGPIPE, SIG_DFL);
}
Linux 能够这样做，因为内核具有关于管道或连接对的另一端的先天知识.但是，当使用connect时，socket的状态是由协议栈维护的.您的测试演示了这种行为，但下面是一个在单个线程中完成所有操作的程序，类似于上面的两个测试:
Linux is able to do so, because the kernel has innate knowledge about the other end of the pipe or connected pair. However, when using connect, the state about the socket is maintained by the protocol stack. Your test demonstrates this behavior, but below is a program that does it all in a single thread, similar to the two tests above:
int a_sock = socket(PF_INET, SOCK_STREAM, 0);
const int one = 1;
setsockopt(a_sock, SOL_SOCKET, SO_REUSEADDR, &one, sizeof(one));
struct sockaddr_in a_sin = {0};
a_sin.sin_port = htons(4321);
a_sin.sin_family = AF_INET;
a_sin.sin_addr.s_addr = htonl(INADDR_LOOPBACK);
bind(a_sock, (struct sockaddr *)&a_sin, sizeof(a_sin));
listen(a_sock, 1);
int c_sock = socket(PF_INET, SOCK_STREAM, 0);
fcntl(c_sock, F_SETFL, fcntl(c_sock, F_GETFL, 0)|O_NONBLOCK);
connect(c_sock, (struct sockaddr *)&a_sin, sizeof(a_sin));
fcntl(c_sock, F_SETFL, fcntl(c_sock, F_GETFL, 0)&~O_NONBLOCK);
struct sockaddr_in s_sin = {0};
socklen_t s_sinlen = sizeof(s_sin);
int s_sock = accept(a_sock, (struct sockaddr *)&s_sin, &s_sinlen);
struct pollfd c_pfd = { c_sock, POLLOUT, 0 };
if (poll(&c_pfd, 1, -1) != 1) perror("poll");
int erropt = -1;
socklen_t errlen = sizeof(erropt);
getsockopt(c_sock, SOL_SOCKET, SO_ERROR, &erropt, &errlen);
if (erropt != 0) { errno = erropt; perror("connect"); }
puts("P|Recv-Q|Send-Q|Local Address|Foreign Address|State|");
char cmd[256];
snprintf(cmd, sizeof(cmd), "netstat -tn | grep ':%hu ' | sed 's/ */|/g'",
ntohs(s_sin.sin_port));
puts("before close on client"); system(cmd);
close(c_sock);
puts("after close on client"); system(cmd);
if (send(s_sock, "a", 1, MSG_NOSIGNAL) < 0) perror("send");
puts("after send on server"); system(cmd);
puts("end of test");
sleep(5);
如果你运行上面的程序，你会得到类似这样的输出:
If you run the above program, you will get output similar to this:
P|Recv-Q|Send-Q|Local Address|Foreign Address|State|
before close on client
tcp|0|0|127.0.0.1:35790|127.0.0.1:4321|ESTABLISHED|
tcp|0|0|127.0.0.1:4321|127.0.0.1:35790|ESTABLISHED|
after close on client
tcp|0|0|127.0.0.1:35790|127.0.0.1:4321|FIN_WAIT2|
tcp|1|0|127.0.0.1:4321|127.0.0.1:35790|CLOSE_WAIT|
after send on server
end of test
这表明套接字转换到 CLOSED 状态需要一次 write.要找出发生这种情况的原因，事务的 TCP 转储可能很有用:
This shows it took one write for the sockets to transition to the CLOSED states. To find out why this occurred, a TCP dump of the transaction can be useful:
16:45:28 127.0.0.1 > 127.0.0.1
.809578 IP .35790 > .4321: S 1062313174:1062313174(0) win 32792 <mss 16396,sackOK,timestamp 3915671437 0,nop,wscale 7>
.809715 IP .4321 > .35790: S 1068622806:1068622806(0) ack 1062313175 win 32768 <mss 16396,sackOK,timestamp 3915671437 3915671437,nop,wscale 7>
.809583 IP .35790 > .4321: . ack 1 win 257 <nop,nop,timestamp 3915671437 3915671437>
.840364 IP .35790 > .4321: F 1:1(0) ack 1 win 257 <nop,nop,timestamp 3915671468 3915671437>
.841170 IP .4321 > .35790: . ack 2 win 256 <nop,nop,timestamp 3915671469 3915671468>
.865792 IP .4321 > .35790: P 1:2(1) ack 2 win 256 <nop,nop,timestamp 3915671493 3915671468>
.865809 IP .35790 > .4321: R 1062313176:1062313176(0) win 0
前三行代表3次握手.第四行是客户端发送给服务器的FIN包，第五行是来自服务器的ACK，确认收到.第六行是服务器尝试向客户端发送 1 个字节的数据，并设置 PUSH 标志.最后一行是客户端 RESET 数据包，它导致连接的 TCP 状态被释放，这也是为什么第三个 netstat 命令在上面测试.
The first three lines represent the 3-way handshake. The fourth line is the FIN packet the client sends to the server, and the fifth line is the ACK from the server, acknowledging receipt. The sixth line is the server trying to send 1 byte of data to the client with the PUSH flag set. The final line is the client RESET packet, which causes the TCP state for the connection to be freed, and is why the third netstat command did not result in any output in the test above.
因此，服务器不知道客户端会在尝试向其发送一些数据之前重置连接.重置的原因是客户端调用了close，而不是别的东西.
So, the server doesn't know the client will reset the connection until after it tries to send some data to it. The reason for the reset is because the client called close, instead of something else.
服务器无法确定客户端实际发出了什么系统调用，它只能遵循 TCP 状态.例如，我们可以将 close 调用替换为对 shutdown 的调用.
The server cannot know for certain what system call the client has actually issued, it can only follow the TCP state. For example, we could replace the close call with a call to shutdown instead.
//close(c_sock);
shutdown(c_sock, SHUT_WR);
shutdown 和 close 的区别在于 shutdown 只管理连接的状态，而 close还控制表示套接字的文件描述符的状态.shutdown 不会close 套接字.
The difference between shutdown and close is that shutdown only governs the state of the connection, while close also governs the state of the file descriptor that represents the socket. A shutdown will not close a socket.
输出将随着 shutdown 变化而不同:
The output will be different with the shutdown change:
P|Recv-Q|Send-Q|Local Address|Foreign Address|State|
before close on client
tcp|0|0|127.0.0.1:4321|127.0.0.1:56355|ESTABLISHED|
tcp|0|0|127.0.0.1:56355|127.0.0.1:4321|ESTABLISHED|
after close on client
tcp|1|0|127.0.0.1:4321|127.0.0.1:56355|CLOSE_WAIT|
tcp|0|0|127.0.0.1:56355|127.0.0.1:4321|FIN_WAIT2|
after send on server
tcp|1|0|127.0.0.1:4321|127.0.0.1:56355|CLOSE_WAIT|
tcp|1|0|127.0.0.1:56355|127.0.0.1:4321|FIN_WAIT2|
end of test
TCP 转储也会显示不同的内容:
The TCP dump will show also show something different:
17:09:18 127.0.0.1 > 127.0.0.1
.722520 IP .56355 > .4321: S 2558095134:2558095134(0) win 32792 <mss 16396,sackOK,timestamp 3917101399 0,nop,wscale 7>
.722594 IP .4321 > .56355: S 2563862019:2563862019(0) ack 2558095135 win 32768 <mss 16396,sackOK,timestamp 3917101399 3917101399,nop,wscale 7>
.722615 IP .56355 > .4321: . ack 1 win 257 <nop,nop,timestamp 3917101399 3917101399>
.748838 IP .56355 > .4321: F 1:1(0) ack 1 win 257 <nop,nop,timestamp 3917101425 3917101399>
.748956 IP .4321 > .56355: . ack 2 win 256 <nop,nop,timestamp 3917101426 3917101425>
.764894 IP .4321 > .56355: P 1:2(1) ack 2 win 256 <nop,nop,timestamp 3917101442 3917101425>
.764903 IP .56355 > .4321: . ack 2 win 257 <nop,nop,timestamp 3917101442 3917101442>
17:09:23
.786921 IP .56355 > .4321: R 2:2(0) ack 2 win 257 <nop,nop,timestamp 3917106464 3917101442>
请注意，在最后一个 ACK 数据包之后 5 秒，最后的重置发生了.此重置是由于程序在没有正确关闭套接字的情况下关闭.与之前不同的是重置前从客户端到服务器的ACK数据包.这表明客户端没有使用 close.在 TCP 中，FIN 指示实际上是没有更多数据要发送的指示.但是由于 TCP 连接是双向的，接收 FIN 的服务器假定客户端仍然可以接收数据.在上述情况下，客户端实际上确实接受了数据.
Notice the reset at the end comes 5 seconds after the last ACK packet. This reset is due to the program shutting down without properly closing the sockets. It is the ACK packet from the client to the server before the reset that is different than before. This is the indication that the client did not use close. In TCP, the FIN indication is really an indication that there is no more data to be sent. But since a TCP connection is bi-directional, the server that receives the FIN assumes the client can still receive data. In the case above, the client in fact does accept the data.
无论客户端使用close还是SHUT_WR发出FIN，都可以检测到FIN的到来 通过在服务器套接字上轮询可读事件.如果调用 read 后的结果是 0，那么您就知道 FIN 已经到达，您可以使用该信息做您想做的事情.
Whether the client uses close or SHUT_WR to issue a FIN, in either case you can detect the arrival of the FIN by polling on the server socket for a readable event. If after calling read the result is 0, then you know the FIN has arrived, and you can do what you wish with that information.
struct pollfd s_pfd = { s_sock, POLLIN|POLLOUT, 0 };
if (poll(&s_pfd, 1, -1) != 1) perror("poll");
if (s_pfd.revents|POLLIN) {
char c;
int r;
while ((r = recv(s_sock, &c, 1, MSG_DONTWAIT)) == 1) {}
if (r == 0) { /*...FIN received...*/ }
else if (errno == EAGAIN) { /*...no more data to read for now...*/ }
else { /*...some other error...*/ perror("recv"); }
}
现在，如果服务器在尝试写入之前发出 SHUT_WR 和 shutdown，它实际上会得到 EPIPE 错误.
Now, it is trivially true that if the server issues SHUT_WR with shutdown before it tries to do a write, it will in fact get the EPIPE error.
shutdown(s_sock, SHUT_WR);
if (send(s_sock, "a", 1, MSG_NOSIGNAL) < 0) perror("send");
相反，如果您希望客户端向服务器指示立即重置，您可以通过启用 linger 选项强制在大多数 TCP 堆栈上发生这种情况，延迟超时为 0 之前调用close.
If, instead, you want the client to indicate an immediate reset to the server, you can force that to happen on most TCP stacks by enabling the linger option, with a linger timeout of 0 prior to calling close.
struct linger lo = { 1, 0 };
setsockopt(c_sock, SOL_SOCKET, SO_LINGER, &lo, sizeof(lo));
close(c_sock);
经过上面的改动，程序的输出变成了:
With the above change, the output of the program becomes:
P|Recv-Q|Send-Q|Local Address|Foreign Address|State|
before close on client
tcp|0|0|127.0.0.1:35043|127.0.0.1:4321|ESTABLISHED|
tcp|0|0|127.0.0.1:4321|127.0.0.1:35043|ESTABLISHED|
after close on client
send: Connection reset by peer
after send on server
end of test
send 在这种情况下会立即出错，但它不是 EPIPE，而是 ECONNRESET.TCP 转储也反映了这一点:
The send gets an immediate error in this case, but it is not EPIPE, it is ECONNRESET. The TCP dump reflects this as well:
17:44:21 127.0.0.1 > 127.0.0.1
.662163 IP .35043 > .4321: S 498617888:498617888(0) win 32792 <mss 16396,sackOK,timestamp 3919204411 0,nop,wscale 7>
.662176 IP .4321 > .35043: S 497680435:497680435(0) ack 498617889 win 32768 <mss 16396,sackOK,timestamp 3919204411 3919204411,nop,wscale 7>
.662184 IP .35043 > .4321: . ack 1 win 257 <nop,nop,timestamp 3919204411 3919204411>
.691207 IP .35043 > .4321: R 1:1(0) ack 1 win 257 <nop,nop,timestamp 3919204440 3919204411>
RESET 数据包在 3 次握手完成后立即出现.但是，使用此选项有其危险.当RESET到达时，如果另一端socket缓冲区中有未读数据，则该数据将被清除，导致数据丢失.强制发送 RESET 通常用于请求/响应样式协议.请求的发送方在收到对其请求的整个响应时，可以知道不会丢失任何数据.然后，请求发送方强制在连接上发送 RESET 是安全的.
The RESET packet comes right after the 3-way handshake completes. However, using this option has its dangers. If the other end has unread data in the socket buffer when the RESET arrives, that data will be purged, causing the data to be lost. Forcing a RESET to be sent is usually used in request/response style protocols. The sender of the request can know there can be no data lost when it receives the entire response to its request. Then, it is safe for the request sender to force a RESET to be sent on the connection.
这篇关于写入关闭的本地 TCP 套接字不会失败的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！
