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To: Chris Roehrig <croehrig AT house DOT org>,
        "cygwin AT cygwin DOT com" <cygwin AT cygwin DOT com>
Subject: RE: scp and ssh 'cat' stalls at 64k bytes
Thread-Topic: scp and ssh 'cat' stalls at 64k bytes
Thread-Index: AQHZpgrjOPCW5W29O0imgUDbhF6UfK+ZAZ/w
Date: Fri, 23 Jun 2023 23:07:56 +0000
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From: "Voris, Ben via Cygwin" <cygwin AT cygwin DOT com>
Reply-To: "Voris, Ben" <ben DOT voris AT hpe DOT com>
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Using scp from OpenSSH_9.3p1, 30 May 2023, copied a 8317351936 byte file from a remote to Cygwin. sha1sum says it arrived correctly.

Windows Firewall is running.

> -----Original Message-----
> From: Chris Roehrig <croehrig AT house DOT org>
> Sent: 23 June 2023 13:42
> To: cygwin AT cygwin DOT com
> Subject: Re: scp and ssh 'cat' stalls at 64k bytes
> 
> 
> On 2023-06-23 08:28, Brian Inglis wrote:
> > On 2023-06-23 00:26, Chris Roehrig via Cygwin wrote:
> >> I've upgraded cygwin recently (from a much older version) and am
> >> encountering a new problem on all my Win10/WIn11 machines.
> >>
> >> With openssh and pv installed on cygwin (3.4.7-1):
> >>
> >> dd if=/dev/zero bs=1 count=65536 | ssh localhost 'cat > /dev/null'
> >> # works
> >> dd if=/dev/zero bs=1 count=65537 | ssh localhost 'cat > /dev/null'
> >> # stalls (and anything larger)
> >> dd if=/dev/zero bs=1 count=65537 | ssh localhost 'pv > /dev/null'
> >> # replace 'cat' with 'pv' and it works
> >>
> >> This happens with or without Windows Firewall enabled, with any input
> >> > 64k, and also remotely from Linux.
> >> It also seems to affect scp to cygwin which stalls if the file is >= 64k
> >
> > In case equivalent Windows services are running, I check for, shut
> > down, and disable the following Windows services I have seen, or seen
> > warnings about, before trying to start Cygwin services:
> >
> >     ssh ssh-agent sshbroker sshproxy sshd sshdbroker sshdproxy
> >
> > Do you take similar precautions on the Windows systems with issues?
> 
> The only service listed is ssh-agent which is disabled and stopped.
> TaskManager did show many zombie/stuck cat.exe  and scp.exe which I
> terminated, but that did not help.   Nor did rebooting.
> 
> I use ssh, rsync, slogin frequently on these machines to/from windows
> and linux and rsync (over ssh) gigabytes of stuff daily to/from Linux
> and other windows machines with no issues.   It is only this case of
> reading more than 64K from stdin over ssh (and scp) that is hanging.
> 
> > Please show at least the output of `uname -srvmo`,
> CYGWIN_NT-10.0-19045 3.4.7-1.x86_64 2023-06-16 14:04 UTC x86_64 Cygwin
> 
> > and if possible provide a text attachment of the output log from
> > running `cygcheck -hrsv > cygcheck.out`, as suggested under the
> > Problem reports link below.
> I've attached the file.
> 
> 
> I take it then you can't replicate this?    Can you confirm whether this
> command works successfully (i.e. exits without hanging):
> 
> dd if=/dev/zero bs=1 count=65537 | ssh localhost 'cat > /dev/null'
> 
> Are you able to 'scp' large files to cygwin?
> 
> 
> It also hangs if I replace 'cat' with 'sed {}' or 'awk {print}', but not
> with 'pv'.   Those all passthrough stdin>stdout and should behave the
> same as cat.  (pv is basically cat with a progress bar which is omitted
> if there's  no controlling terminal like in this case).
> 
> 
> The curious fact that 'pv' works makes me think it is a Cygwin library
> problem and not anything related to firewall/network/Windows.
> 
> I've verified that cat, pv, and sshd all have the same ACLs.  e.g.
> aquila[77]% icacls pv.exe
> pv.exe AQUILA\croehrig:(F)
>          BUILTIN\Administrators:(RX)
>         Everyone:(RX)

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