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We found that one close-wait socket was reset by the other side due to a new connection reusing the same port which is beyond our expectation, so we have to investigate the underlying reason. The following experiment is conducted in the test environment. We limit the port range from 40000 to 40010 and delay the time to close() after receiving a fin from the active close side, which can help us easily reproduce like what happened in production. Here are three connections captured by tcpdump: 127.0.0.1.40002 > 127.0.0.1.9999: Flags [S], seq 2965525191 127.0.0.1.9999 > 127.0.0.1.40002: Flags [S.], seq 2769915070 127.0.0.1.40002 > 127.0.0.1.9999: Flags [.], ack 1 127.0.0.1.40002 > 127.0.0.1.9999: Flags [F.], seq 1, ack 1 // a few seconds later, within 60 seconds 127.0.0.1.40002 > 127.0.0.1.9999: Flags [S], seq 2965590730 127.0.0.1.9999 > 127.0.0.1.40002: Flags [.], ack 2 127.0.0.1.40002 > 127.0.0.1.9999: Flags [R], seq 2965525193 // later, very quickly 127.0.0.1.40002 > 127.0.0.1.9999: Flags [S], seq 2965590730 127.0.0.1.9999 > 127.0.0.1.40002: Flags [S.], seq 3120990805 127.0.0.1.40002 > 127.0.0.1.9999: Flags [.], ack 1 As we can see, the first flow is reset because: 1) client starts a new connection, I mean, the second one 2) client tries to find a suitable port which is a timewait socket (its state is timewait, substate is fin_wait2) 3) client occupies that timewait port to send a SYN 4) server finds a corresponding close-wait socket in ehash table, then replies with a challenge ack 5) client sends an RST to terminate this old close-wait socket. I don't think the port selection algo can choose a FIN_WAIT2 socket when we turn on tcp_tw_reuse because on the server side there remain unread data. In some cases, if one side haven't call close() yet, we should not consider it as expendable and treat it at will. Even though, sometimes, the server isn't able to call close() as soon as possible like what we expect, it can not be terminated easily, especially due to a second unrelated connection happening. After this patch, we can see the expected failure if we start a connection when all the ports are occupied in fin_wait2 state: "Ncat: Cannot assign requested address." Reported-by: Jade Dong <jadedong@tencent.com> Signed-off-by: Jason Xing <kernelxing@tencent.com> Reviewed-by: Eric Dumazet <edumazet@google.com> Link: https://patch.msgid.link/20240823001152.31004-1-kerneljasonxing@gmail.com Signed-off-by: Jakub Kicinski <kuba@kernel.org> |
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.