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RE: Question about SCP stalling over VPN



Matt,

To do full diagnosis, do captures at both ends:
tcpdump -w client.pcap -s0 host <IP> and port 22
## so that we don't capture unnecessary stuff, if you are at the client, the <IP> should be the server side

At server side,
tcpdump -w server.pcap -s0 host <IP> and port 22
## so that we don't capture unnecessary stuff, if you are at the server, the <IP> should be the client side

Upload the {client,server}.pcap somewhere and provide the links, that way someone might be able to take a look at it.

This is one thing you can give it a try easily:

Certain network devices I have seen have bugs dealing with TCP SACK.
CentOS 5.3 comes enabled with TCP SACK.
As root, try issuing "sysctl -w net.ipv4.tcp_sack=0", then scp to check if turning SACK off resolve your issues.

Robin



________________________________________
From: listbounce@xxxxxxxxxxxxxxxxx [listbounce@xxxxxxxxxxxxxxxxx] On Behalf Of Matthew Case [mattcase@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx]
Sent: Tuesday, March 09, 2010 12:44 PM
To: secureshell@xxxxxxxxxxxxxxxxx
Subject: Question about SCP stalling over VPN

Greetings,

I am attempting to SSH a series of relatively small files (tens of
megabytes) over a VPN connection to a remote linux server. I am able to
SSH between the servers without a problem, but when I use SCP to copy
the files I start seeing the following message more and more until
finally the copy grinds to a halt and eventually times out:

debug2: channel 0: rcvd adjust 131072
debug2: channel 0: rcvd adjust 131072
debug2: channel 0: rcvd adjust 131072
debug2: channel 0: rcvd adjust 131072

I've looked high and low and haven't really come up with anything
definitive. Someone somewhere had mentioned fiddling with MTU settings,
but I'm not really sure what that will accomplish as I am unfamiliar
with what MTU is and does. If this question has been answered
previously, I apologize ahead of time. Thanks!

I am running CentOS 5.3 with OpenSSH 4.3p2-29 on the server I am copying
the files from, and CentOS 5.4 with OpenSSH 4.3p2.36 on the server I am
copying to.
--

Matthew Case
Specialized Business Software
Software Engineer
SCJP 5 Certified
Phone: 440-542-9145
Fax: 440-542-9143




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