TCP链接主动关闭不发fin包奇怪行为分析

以前在写网络通讯模块的时候，抓包时候也发现这个问题：“TCP链接主动关闭不发fin包”；

今天看了以下文章，Mark. 转自：http://rdc.taobao.com/blog/cs/?p=1055

—
我们在1 8 . 2节中看到终止一个连接的正常方式是一方发送F I N。有时这也称为有序释放
（orderly release），因为在所有排队数据都已发送之后才发送F I N，正常情况下没有任何数据
丢失。但也有可能发送一个复位报文段而不是F I N来中途释放一个连接。有时称这为异常释放
（abortive release）。
异常终止一个连接对应用程序来说有两个优点：（1）丢弃任何待发数据并立即发送复位
报文段；（2）R S T的接收方会区分另一端执行的是异常关闭还是正常关闭。应用程序使用的
A P I必须提供产生异常关闭而不是正常关闭的手段。

二

问题描述：
多隆同学在做网络框架的时候，发现一条tcp链接在close的时候，对端会收到econnrest,而不是正常的fin包. 通过抓包发现close系统调用的时候，我端发出rst报文, 而不是正常的fin。这个问题比较有意思，我们来演示下：


$ erl
Erlang R14B03 (erts-5.8.4) [64-bit] [smp:16:16] [rq:16] [async-threads:0] [hipe] [kernel-poll:false]

Eshell V5.8.4 (abort with ^G)
1> {ok,Sock} = gen_tcp:connect("baidu.com", 80, [{active,false}]).
{ok,#Port<0.582>}
2> gen_tcp:send(Sock, "GET / HTTP/1.1\r\n\r\n").
ok
3> gen_tcp:close(Sock).
ok

我们往baidu的首页发了个http请求，百度会给我们回应报文的,我们send完立即调用close.

然后我们在另外一个终端开tcpdump抓包确认：

$ sudo tcpdump port 80 -i bond0
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on bond0, link-type EN10MB (Ethernet), capture size 96 bytes
17:22:38.246507 IP my031089.sqa.cm4.tbsite.net.19500 > 220.181.111.86.http: S 2228211568:2228211568(0) win 5840 
17:22:38.284602 IP 220.181.111.86.http > my031089.sqa.cm4.tbsite.net.19500: S 3250338304:3250338304(0) ack 2228211569 win 8190 
17:22:38.284624 IP my031089.sqa.cm4.tbsite.net.19500 > 220.181.111.86.http: . ack 1 win 5840
17:22:52.748468 IP my031089.sqa.cm4.tbsite.net.19500 > 220.181.111.86.http: P 1:19(18) ack 1 win 5840
17:22:52.786855 IP 220.181.111.86.http > my031089.sqa.cm4.tbsite.net.19500: . ack 19 win 5840
17:22:52.787194 IP 220.181.111.86.http > my031089.sqa.cm4.tbsite.net.19500: P 1:179(178) ack 19 win 5840
17:22:52.787203 IP my031089.sqa.cm4.tbsite.net.19500 > 220.181.111.86.http: . ack 179 win 6432
17:22:52.787209 IP 220.181.111.86.http > my031089.sqa.cm4.tbsite.net.19500: P 179:486(307) ack 19 win 5840
17:22:52.787214 IP my031089.sqa.cm4.tbsite.net.19500 > 220.181.111.86.http: . ack 486 win 7504
17:23:01.564358 IP my031089.sqa.cm4.tbsite.net.19500 > 220.181.111.86.http: R 19:19(0) ack 486 win 7504
...

我们可以清楚的看到 R 19:19(0) ack 486 win 7504,发了个rst包，通过strace系统调用也确认erlang确实调用了close系统调用。
那为什么呢？ @淘宝雕梁，tcp协议栈专家回答了这个问题：

在net/ipv4/tcp.c:1900附近

...
/* As outlined in RFC 2525, section 2.17, we send a RST here because
* data was lost. To witness the awful effects of the old behavior of
* always doing a FIN, run an older 2.1.x kernel or 2.0.x, start a bulk
* GET in an FTP client, suspend the process, wait for the client to
* advertise a zero window, then kill -9 the FTP client, wheee...
* Note: timeout is always zero in such a case.
*/
if (data_was_unread) {
/* Unread data was tossed, zap the connection. */
NET_INC_STATS_USER(sock_net(sk), LINUX_MIB_TCPABORTONCLOSE);
tcp_set_state(sk, TCP_CLOSE);
tcp_send_active_reset(sk, sk->sk_allocation);
..

代码里面写的很清楚，如果你的接收缓冲去还有数据，协议栈就会发rst代替fin.
我们再来验证一下：


$ erl
Erlang R14B03 (erts-5.8.4) [64-bit] [smp:16:16] [rq:16] [async-threads:0] [hipe] [kernel-poll:false]

Eshell V5.8.4 (abort with ^G)
1> {ok,Sock} = gen_tcp:connect("baidu.com", 80, [{active,false}]).
{ok,#Port<0.582>}
2> gen_tcp:send(Sock, "GET / HTTP/1.1\r\n\r\n").
ok
3> gen_tcp:recv(Sock,0).
{ok,"HTTP/1.1 400 Bad Request\r\nDate: Fri, 01 Jul 2011 09:24:37 GMT\r\nServer: Apache\r\nConnection: Keep-Alive\r\nTransfer-Encoding: chunked\r\nContent-Type: text/html; charset=iso-8859-1\r\n\r\n127\r\n\n\n\n\n

Bad Request

\nYour browser sent a request that this server could not understand.

\nclient sent HTTP/1.1 request without hostname (see RFC2616 section 14.23): /

\n\n\r\n0\r\n\r\n"}
4> gen_tcp:close(Sock).
ok
5>

这次我们把接收缓冲区里的东西拉干净了。

再看下tcpdump：

...
17:36:07.236627 IP my031089.sqa.cm4.tbsite.net.9405 > 123.125.114.144.http: S 3086473299:3086473299(0) win 5840 
17:36:07.274661 IP 123.125.114.144.http > my031089.sqa.cm4.tbsite.net.9405: S 738551248:738551248(0) ack 3086473300 win 8190 
17:36:07.274685 IP my031089.sqa.cm4.tbsite.net.9405 > 123.125.114.144.http: . ack 1 win 5840
17:36:10.295795 IP my031089.sqa.cm4.tbsite.net.9405 > 123.125.114.144.http: P 1:19(18) ack 1 win 5840
17:36:10.334280 IP 123.125.114.144.http > my031089.sqa.cm4.tbsite.net.9405: . ack 19 win 5840
17:36:10.334547 IP 123.125.114.144.http > my031089.sqa.cm4.tbsite.net.9405: P 1:179(178) ack 19 win 5840
17:36:10.334554 IP my031089.sqa.cm4.tbsite.net.9405 > 123.125.114.144.http: . ack 179 win 6432
17:36:10.334563 IP 123.125.114.144.http > my031089.sqa.cm4.tbsite.net.9405: P 179:486(307) ack 19 win 5840
17:36:10.334566 IP my031089.sqa.cm4.tbsite.net.9405 > 123.125.114.144.http: . ack 486 win 7504
17:36:19.671374 IP my031089.sqa.cm4.tbsite.net.9405 > 123.125.114.144.http: F 19:19(0) ack 486 win 7504
17:36:19.709619 IP 123.125.114.144.http > my031089.sqa.cm4.tbsite.net.9405: . ack 20 win 5840
17:36:19.709643 IP 123.125.114.144.http > my031089.sqa.cm4.tbsite.net.9405: F 486:486(0) ack 20 win 5840
17:36:19.709652 IP my031089.sqa.cm4.tbsite.net.9405 > 123.125.114.144.http: . ack 487 win 7504
...

这次是发fin包了。

多隆同学再进一步，找出来之前squid client代码中不能理解的一句话：
client_side.c

...
/* prevent those nasty RST packets */
{
char buf[SQUID_TCP_SO_RCVBUF];
while (FD_READ_METHOD(fd, buf, SQUID_TCP_SO_RCVBUF) > 0);
}
...

总算明白了这句话的意思了！

小结：认真学习协议栈太重要了。

玩得开心！