[OpenAFS] Windows client and firewall/antivirus/MTU conflict - possibility of file corruption

Jeffrey Altman jaltman@secure-endpoints.com
Wed, 13 Jan 2010 14:59:45 -0500


This is a cryptographically signed message in MIME format.

--------------ms050109040102050208000905
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: quoted-printable

On 1/12/2010 12:40 PM, Richard Brittain wrote:
> The combination of properties we have investigated most thoroughly is:
>=20
>  Windows XP (32-bit)
>  Symantec EndPoint Protection 11.0.4 firewall (Network Threat Protectio=
n)
>  OpenAFS 1.5.34 or later
>  Windows network interface with a non-default MTU set

[...]

> HKLM\System\CurrentControlSet\Services\TransarcAFSDaemon\Parameters\RxM=
axMTU
>=20
> (default value is 0)

To clarify, the problem is unrelated to the version of OpenAFS for
Windows being used.  What matters is the "RxMaxMTU" value that is set in
the registry.  Beginning in the 1.3 series the RxMaxMTU value was
changed from 0 to 1260 octets in order to avoid communication failures
that ensued when the then current version of Cisco VPN IPSec client
software was in use.  The Cisco IPSec tunnel would not transfer UDP
packets that required fragmentation when the effective MTU size for the
network was reduced by the size of the IPSec headers.

The 1.5.34 release of OpenAFS for Windows restored the RxMaxMTU value to
0 which permits the Rx library to use the largest MTU size that can be
negotiated between the two Rx peers (in most cases 1444 octets.)

When the network interface MTU size is hard coded to a value smaller
than 1500 octets, the UDP packets are fragmented.  When the Symantec
EndPoint Protection 11.0.4 firewall is added, data corruption ensues.

Removing the artificial limit on network interface MTU or forcing the Rx
library to use a RxMaxMTU smaller than 1272 octets prevents the UDP
fragmentation from taking place and the data corruption is avoided.

----

Other observations:

1. I produced a modified cache manager that computed and tracked MD5
hash values for all writes accepted via the SMB server and verified them
prior to passing data into the rx_Write() function.  The corruption is
not occurring to the cache manager file chunk buffers.

2. Data corruption occurs even when "fs setcrypt -on" is active.  This
indicates that the corruption is occurring prior to the rxkad checksum
being computed and added to the packet header.

3. I built a special version of an Rx library from 2008 to use with the
1.5.68 afsd_service.exe.  The data corruption still occurs so the
problem is not something that was introduced recently into the code base.=


4. Even when the network interface MTU is manually set from the
registry, the API calls that report the MTU to afsd_service.exe indicate
the configured MTU for the network subnet router.

----

Conclusions:

For the time being at least, set the RxMaxMTU value back to 1260 octets.
 This will result in a performance penalty but will avoid this data
corruption.   Hopefully one night soon I will wake up in the middle of
the night understanding where and how this data corruption is taking plac=
e.

Jeffrey Altman




--------------ms050109040102050208000905
Content-Type: application/pkcs7-signature; name="smime.p7s"
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename="smime.p7s"
Content-Description: S/MIME Cryptographic Signature
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--------------ms050109040102050208000905--