[uml-devel] Some firewalls require disabling ECN in the UML kernel

[Eric House <fixin@peak.org>]

This mail details the solution to a problem I had with UML networking.
My UML instance was able to ping any host on the LAN or internet, but
could only make TCP connections within the LAN.  On looking closer I
found that the initial packets were making it from the host to the
router and then to my cable modem but not reaching the internet
server.  I was unable to determine whether the cable modem was
dropping them (or why), or whether they were making it further.

Eventually I looked closely at the packets leaving the router, both
for (successful) telnet connections from non-UML hosts and for the
(doomed) attempt from the UML instance.  The only difference,
according to tcpdump running on the router, was that the
non-UML-sourced packets had only the S flag set while the UML-sourced
packets had three set: SWE.

The first hit when googling for "tcpdump SWE" is 

http://lists.debian.org/debian-user/2001/06/msg01577.html

a page that explains that some commercial firewalls block packets for
which TCP ECN is enabled.  And sure enough, the kernel that's part of
Debian's UML package has it enabled.  Once I turned it off using the
following command all was well.  I'm currently running apt-get to
bring the rootfs up to date.

sysctl -w net.ipv4.tcp_ecn=0

Of course I still don't know where the packets were getting blocked,
but my ActionTek DSL modem is the most likely suspect.

UML rocks!  Thanks!

--Eric House
-- 
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* From the desktop of: Eric House, fixin@peak.org                            *
*    Crosswords 4.0.6 for PocketPC is out!: <http://xwords.sourceforge.net>  *
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