Cisco VPN3000 gateway MTU overflow

[<porte10 () free fr>]

Cisco VPN3000 gateway MTU overflow
==================================

Bug class: Conceptual/bad protocol implementation
Equipments affected: Cisco/VPN 3000 Concentrator with
  software vpn3000-3.5.Rel-k9.bin


FACTS
  The Cisco VPN3000 gateway lets remote client dictate
  which maximum MTU to use when sending back ESP
  frames, regardless of the transmitting capabilities
  of the physical medium.

IMPACT
* Oversized frames get silently discarded by
  equipements linked to the gateway's public
  interface and retransmissions occur.
* Other disturbances or DoS against neighboring
  equipements may occur, especially as many IP
  stacks on routers and sniffers etc ... are
  poorly implemented.

DETAILS
  We have witnessed this phenomena after establishing
  tunnels with the "VPN dialer" over a modem
  connexion: when the target sends back ethernet
  frames with size close to the max ethernet MTU
  (1500), the gateway encrypts the frames adding
  ESP headers and stupidly tries to send a
  1580-bytes frame back to the client.

RESOLUTION
-> From the official documentation there is no way
to enforce a maximum MTU on the VPN gateway.
-> Hence: a gateway software patch by Cisco is
necessary: if MTU negociation occurs, the gateway
should set a max-MTU threshold (the physical medium's !).

PSEUDO WORKAROUNDS

* client side: For Windows-based OS (likely Unix and
  Linux-based OS too), Cisco released a tool
  called setMTU.exe that can prevent ill MTU
  negociation from happening.

* target side: artificially lowering the max MTU
  on the interfaces.

->  But such a policy is not acceptable:
  The VPN client, as well as remote targets,
  should not have to be aware of
  the gateway's interface configuration !

    The bug does not lie in client software, but
  in the gateway's software.

Master Phi

---

Today's statement:
Networking software robustness isn't worth the tenth
of that of arcade game engines.
Let's call it junk software.