netdev - Re: Fw: [Bug 102051] New: Unexpected TCP behavior

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Date:	Mon, 27 Jul 2015 23:35:14 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Stephen Hemminger <stephen@...workplumber.org>
Cc:	netdev@...r.kernel.org
Subject: Re: Fw: [Bug 102051] New: Unexpected TCP behavior

On Mon, 2015-07-27 at 14:02 -0700, Stephen Hemminger wrote:
> 
> Begin forwarded message:
> 
> Date: Mon, 27 Jul 2015 20:06:07 +0000
> From: "bugzilla-daemon@...zilla.kernel.org" <bugzilla-daemon@...zilla.kernel.org>
> To: "shemminger@...ux-foundation.org" <shemminger@...ux-foundation.org>
> Subject: [Bug 102051] New: Unexpected TCP behavior
> 
> 
> https://bugzilla.kernel.org/show_bug.cgi?id=102051
> 
>             Bug ID: 102051
>            Summary: Unexpected TCP behavior
>            Product: Networking
>            Version: 2.5
>     Kernel Version: 3.19
>           Hardware: All
>                 OS: Linux
>               Tree: Mainline
>             Status: NEW
>           Severity: low
>           Priority: P1
>          Component: IPV4
>           Assignee: shemminger@...ux-foundation.org
>           Reporter: vremetic@...il.com
>         Regression: No
> 
> While running nmap against localhost I started to see open ports in the dynamic
> range (>1024). Kind of odd, knowing netstat and ss did not show any listeners
> on the port.
> 
> With tcpdump, I confirmed system was returning S/ACK for ports that did not
> have a listener enabled. The "issue" or feature only occurs if source port
> matches the destination port.
> 
> # netstat -nap | grep 5000
> #
> 
> $ nc localhost -p 5000 5000
> a
> a
> 
> # tcpdump -i lo port 5000
> 14:28:18.059708 IP 127.0.0.1.5000 > 127.0.0.1.5000: Flags [S], seq 2005295207,
> win 43690, options [mss 65495,sackOK,TS val 4481790 ecr 0,nop,wscale 7], length
> 0
> 14:28:18.059721 IP 127.0.0.1.5000 > 127.0.0.1.5000: Flags [S.], seq 2005295207,
> ack 2005295208, win 43690, options [mss 65495,sackOK,TS val 4481790 ecr
> 4481790,nop,wscale 7], length 0

Nothing wrong here. This is well known TCP behavior ( cross syn ).

> 14:28:18.059729 IP 127.0.0.1.5000 > 127.0.0.1.5000: Flags [.], ack 2005295208,
> win 342, options [nop,nop,TS val 4481790 ecr 4481790], length 0
> 14:28:19.121392 IP 127.0.0.1.5000 > 127.0.0.1.5000: Flags [P.], seq
> 2005295208:2005295210, ack 2005295208, win 342, options [nop,nop,TS val 4482056
> ecr 4481790], length 2
> 14:28:19.121407 IP 127.0.0.1.5000 > 127.0.0.1.5000: Flags [.], ack 2005295210,
> win 342, options [nop,nop,TS val 4482056 ecr 4482056], length 0
> 
> # hping3 -S 127.0.0.1 -p 5000 -s 5000
> HPING 127.0.0.1 (lo 127.0.0.1): S set, 40 headers + 0 data bytes 
> len=40 ip=127.0.0.1 ttl=64 id=2036 sport=5000 flags=S seq=0 win=512 rtt=3.8 ms 
>            << SYN
> DUP! len=52 ip=127.0.0.1 ttl=64 DF id=670 sport=5000 flags=A seq=0 win=342
> rtt=3.8 ms      << SYN/ACK
> len=40 ip=127.0.0.1 ttl=64 DF id=43435 sport=5000 flags=RA seq=1 win=0 rtt=3.7
> ms
> 
> I confirmed it with nmap, nc, and hping3; granted they build on same c
> libraries, so I am not even sure if this should be filed as a kernel bug (or
> even a bug);
> just did not expect to see this behavior // 
> 
> Expected to see a RST instead.
> 

Sigh.

Wont fix.


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