Subject: [IPsec] Issue #26: Missing treatment of error cases

From: Yoav Nir <[email-masked]>
Date: Tue, 1 Sep 2009 09:43:26 +0300

Hello all.

Issue #26 was submitted by Tero Kivinen. It concerns section 2.21  
("error handling") and states that several things are missing:

- handling of errors before authentication
- listing what error conditions cause the IKE SA to be deleted entirely
- listing how errors are handled in the piggybacked exchanges.

Following is our suggested new text. Please let us know what you  
think. Also, please take a look at the description of  
"AUTHENTICATION_FAILED" in section 3.10.1. "response to an IKE_AUTH  
message" means either an IKE_AUTH response to an IKE_AUTH request, or  
an INFORMATIONAL request that describes an error in the IKE_AUTH  
response. Do you think this phrasing is clear enough?

2.21.  Error Handling

    There are many kinds of errors that can occur during IKE processing.
    If a request is received that is badly formatted, or unacceptable  
for
    reasons of policy (e.g., no matching cryptographic algorithms), the
    response MUST contain a Notify payload indicating the error.  If an
    error occurs in the processing of a response, then the initiator
    SHOULD initiate an INFORMATIONAL exchange with a Notify payload
    describing the problem.  If an error occurs outside the context of  
an
    IKE request (e.g., the node is getting ESP messages on a nonexistent
    SPI), the node SHOULD initiate an INFORMATIONAL exchange with a
    Notify payload describing the problem.

    Errors that occur before a cryptographically protected IKE SA is
    established must be handled very carefully.  There is a trade-off
    between wanting to be helpful in diagnosing a problem and responding
    to it and wanting to avoid being a dupe in a denial of service  
attack
    based on forged messages.

    If a node receives a message on UDP port 500 or 4500 outside the
    context of an IKE SA known to it (and not a request to start one),  
it
    may be the result of a recent crash of the node.  If the message is
    marked as a response, the node MAY audit the suspicious event but
    MUST NOT respond.  If the message is marked as a request, the node
    MAY audit the suspicious event and MAY send a response.  If a
    response is sent, the response MUST be sent to the IP address and
    port from whence it came with the same IKE SPIs and the Message ID
    copied.  The response MUST NOT be cryptographically protected and
    MUST contain a Notify payload indicating INVALID_IKE_SPI.  The
    INVALID_IKE_SPI notification indicates an IKE message was received
    with an unrecognized destination SPI; this usually indicates that  
the
    recipient has rebooted and forgotten the existence of an IKE SA.

    A node receiving such an unprotected Notify payload MUST NOT respond
    and MUST NOT change the state of any existing SAs.  The message  
might
    be a forgery or might be a response, the genuine correspondent was
    tricked into sending.  A node should treat such a message (and  
also a
    network message like ICMP destination unreachable) as a hint that
    there might be problems with SAs to that IP address and should
    initiate a liveness check for any such IKE SA.  An implementation
    SHOULD limit the frequency of such tests to avoid being tricked into
    participating in a denial of service attack.

    A node receiving a suspicious message from an IP address (and port,
    if NAT traversal is used) with which it has an IKE SA MAY send an  
IKE
    Notify payload in an IKE INFORMATIONAL exchange over that SA.  The
    recipient MUST NOT change the state of any SAs as a result, but may
    wish to audit the event to aid in diagnosing malfunctions.  A node
    MUST limit the rate at which it will send messages in response to
    unprotected messages.

    All errors that occur in an IKE_AUTH exchange, causing the
    authentication to fail for whatever reason (invalid shared secret,
    unrecognized ID, untrusted certificate issuer, revoked or expired
    certificate, etc.)  MUST result in an AUTHENTICATION_FAILED
    notification.  If the error occurred on the responder, the
    notification MUST be returned in the protected response, and MUST be
    the only payload in that response.  If the error occurs on the
    initiator, the notification MUST be returned in a separate
    INFORMATIONAL exchange, with no other payloads.  Note, however, that
    messages that contain an unsupported critical payload, or that are
    otherwise malformed, MUST be rejected in their entirety, and only
    lead to an UNSUPPORTED_CRITICAL_PAYLOAD or INVALID_SYNTAX
    Notification.  The receiver MUST NOT verify the payloads related to
    authentication in this case.

    If authentication has succeeded in the IKE_AUTH exchange, the IKE SA
    is established, provided that there are no unsupported critical
    payloads.  Establishing the child SA, or requesting configuration
    information may still fail, but they do not automatically cause the
    IKE SA to be deleted.  Specifically, a responder may include all the
    payloads associated with authentication (IDr, Cert and AUTH) while
    sending error notifications for the piggybacked exchanges
    (FAILED_CP_REQUIRED, INVALID_SELECTORS, NO_PROPOSAL_CHOSEN, etc.),
    and the initiator MUST NOT fail the authentication because of this.
    The initiator MAY, of course, for reasons of policy later delete  
such
    an IKE SA.

    Only authentication failures and malformed packets lead to a  
deletion
    of the IKE SA without requiring an explicit DELETE payload.  Other
    error conditions require such a payload.  In an IKE_SA_INIT  
exchange,
    any error notification causes the exchange to fail.  In an IKE_AUTH
    exchange, or in the subsequent INFORMATIONAL exchnage, only the
    following notifications cause the IKE SA to be deleted or not
    created, without a DELETE payload:
    o  UNSUPPORTED_CRITICAL_PAYLOAD
    o  INVALID_SYNTAX
    o  AUTHENTICATION_FAILED

    Extension documents may define new error notifications with these
    semantics, but MUST NOT use them unless the peer is known to
    understand them.