Re: Possible bug in `tcp(7)` regarding `TCP_USER_TIMEOUT`

["Alejandro Colomar (man-pages)" <alx.manpages@gmail.com>]

Hello João,

On 1/15/22 13:23, João Sampaio wrote:
> Hello,
> 
> The manpage for TCP (`man 7 tcp`, [1]) reads, in the paragraphs about
> `TCP_USER_TIMEOUT`:
> 
> [1] https://man7.org/linux/man-pages/man7/tcp.7.html
> 
> ```
> This option can be set during any state of a TCP
> connection, but is effective only during the synchronized
> states of a connection (ESTABLISHED, FIN-WAIT-1, FIN-
> WAIT-2, CLOSE-WAIT, CLOSING, and LAST-ACK).
> ```
> 
> When I read that, I understand that that option should only apply to
> established TCP connections, and should not apply when trying to
> establish a TCP connection.
> 
> I wrote a simple program to test that:
> 
> ```
> #include <netdb.h>
> #include <stdio.h>
> #include <stdlib.h>
> #include <unistd.h>
> #include <string.h>
> #include <time.h>
> #include <sys/socket.h>
> #include <netinet/in.h>
> #include <netinet/tcp.h>
> 
> // We'll skip error-checking since this is just for testing purposes.
> 
> void do_connect(int with_tcp_user_timeout) {
>     struct addrinfo hint;
>     struct addrinfo *ai;
>     int sock;
>     time_t start;
>     time_t end;
>     unsigned int timeout = 10000;
> 
>     memset(&hint, 0, sizeof(hint));
>     hint.ai_family = AF_INET;
>     hint.ai_socktype = SOCK_STREAM;
>     hint.ai_protocol = IPPROTO_TCP;
>     getaddrinfo("10.0.42.16", "5432", &hint, &ai);
> 
>     sock = socket(ai->ai_family, ai->ai_socktype, ai->ai_protocol);
>     if (with_tcp_user_timeout) {
>         setsockopt(
>             sock, IPPROTO_TCP, TCP_USER_TIMEOUT, &timeout,
>             sizeof(timeout)
>         );
>     }
> 
>     printf(
>         "Attempting to connect, with_user_tcp_timeout = %d... ",
>         with_tcp_user_timeout
>     );
>     fflush(stdout);
> 
>     start = time(NULL);
>     connect(sock, ai->ai_addr, ai->ai_addrlen);
>     end = time(NULL);
>     printf("Done!\n");
> 
>     freeaddrinfo(ai);
> 
>     printf(
>         "with_tcp_user_timeout = %d, took %ld seconds\n",
>         with_tcp_user_timeout, end - start
>     );
> }
> 
> int main(void) {
>     // Let's connect with TCP_USER_TIMEOUT first for faster useful
>     // first data.
>     do_connect(1);
>     do_connect(0);
> 
>     exit(EXIT_SUCCESS);
> }
> ```
> 
> This program gives me the following output:
> 
> ```
> $ gcc main.c -Wall -Wextra -Werror && ./a.out
> Attempting to connect, with_user_tcp_timeout = 1... Done!
> with_tcp_user_timeout = 1, took 10 seconds
> Attempting to connect, with_user_tcp_timeout = 0... Done!
> with_tcp_user_timeout = 0, took 130 seconds
> ```
> 
> So it seems like that option does apply when trying to establish a
> connection! This above output was obtained with Linux 5.15.8 and gcc
> 9.3.0.
> 
> Am I reading the manpage wrong, or is the manpage wrong? Or is this
> just an "undocumented behavior" that shouldn't be relied on and can
> be removed without warning in future releases?

I never used TCP_USER_TIMEOUT, so can't answer.  I added Jerry and David
(and netdev@) to the thread, which I guess should know how to answer you
better.

Thanks,

Alex

-- 
Alejandro Colomar
Linux man-pages maintainer; https://www.kernel.org/doc/man-pages/
http://www.alejandro-colomar.es/