('binary' encoding is not supported, stored as-is) 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.
This archive was generated by hypermail 2b30 : Wed Jul 10 2002 - 20:37:26 PDT