Re: [secdir] Secdir review of draft-ietf-idr-bgp-flowspec-oid-13

Magnus Nyström <magnusn@gmail.com> Tue, 04 May 2021 00:48 UTC

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From: =?UTF-8?Q?Magnus_Nystr=C3=B6m?= <magnusn@gmail.com>
Date: Mon, 3 May 2021 17:48:35 -0700
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To: "Juan Alcaide (jalcaide)" <jalcaide@cisco.com>
Cc: "secdir@ietf.org" <secdir@ietf.org>, "draft-ietf-idr-bgp-flowspec-oid@ietf.org" <draft-ietf-idr-bgp-flowspec-oid@ietf.org>
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Subject: Re: [secdir] Secdir review of draft-ietf-idr-bgp-flowspec-oid-13
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Thanks Juan, yes, to me this is at least clearer about the risk. At this
point I leave it to the Security Area directors if they'd like to see any
other modifications.

Thanks!
Magnus

On Mon, May 3, 2021 at 5:17 PM Juan Alcaide (jalcaide) <jalcaide@cisco.com>
wrote:

> Magnus,
>
>
>
> Yes, they’ll be opening to risk. It’s the risk described in the draft. But
> this risk is necessary for route-servers to work with flowspec. And as
> mentioned, perhaps route servers can be configured to avoid that risk.
>
>
>
>
>
> I wanted to use the term ‘remain’ instead of ‘become’.. We went from
> RFC4271 (rule optional) -> RFC8955 (rule mandatory) -> this draft (rule
> again optional)
>
> Would the below make sense (I changed the first sentence):
>
>
>
>    This document makes the original AS_PATH validation rule ([RFC4271]
> section 6.3) again optional
>
>    (section 4.2) for Flow Specification Address Family (the rule is no
> longer mandatory as it was specified by [RFC8955]).
>
>    If that original rule is not enforced for Flow Specification it may
> introduce some new security risks.
>
>    A peer (or a client of a route server peer) in AS X could advertise a
> rogue Flow
>
>    Specification route whose first AS in AS_PATH was Y (assume Y is the
>
>    first AS in the AS_PATH of the best-match unicast route).  This risk
>
>    is impossible to prevent if the Flow Specification route is received
>
>    from a route server peer.  If that peer is known for a fact not to be
>
>    a route server, that optional rule SHOULD be enforced for Flow
>
>    Specification routes. Note that identifying those peers that are route
> servers may suppose an
>
>    operational challenge. If the condition of the peer is unknown, the
> rule SHOULD not be
>
>    enforced.
>
>
>
>    A route server itself may be in a good position to enforce the AS_PATH
> validation rule described
>
>   in the previous paragraph. If a route server knowns it’s not peering
> with any other route server,
>
>    it can enforce the AS_PATH validation rule across all its peers. If, in
> addition to that,
>
>    the route server is trusted, the security threat described above
> disappears.
>
>
>
>
>
>
>
>
>
> *From:* Magnus Nyström <magnusn@gmail.com>
> *Sent:* Monday, May 3, 2021 9:39 PM
> *To:* Juan Alcaide (jalcaide) <jalcaide@cisco.com>
> *Cc:* secdir@ietf.org; draft-ietf-idr-bgp-flowspec-oid@ietf.org
> *Subject:* Re: Secdir review of draft-ietf-idr-bgp-flowspec-oid-13
>
>
>
> Thanks Juan.
>
> For the editorial ones, agree on the first one and I think the second you
> could change to "with this memo, becomes optional." or similar.
>
> For the new text, perhaps I am misunderstanding something, but this
> sentence:
>
> Note that identifying those peers that are route servers may suppose an
> operational challenge. If the condition of the peer is unknown, the rule
> SHOULD not be enforced.
>
> Is it correctly understood, that you're saying that if a validator is
> unable to determine if the peer is a route server, they should not enforce
> the rule? If so, doesn't that mean "failing open, i.e., opening themselves
> to risk? Sorry if I misunderstand.
>
>
>
>
>
> On Mon, May 3, 2021 at 3:36 AM Juan Alcaide (jalcaide) <jalcaide@cisco.com>
> wrote:
>
> Thanks for your comments, Magnus
>
>
>
> Inline..
>
>
>
> *From:* Magnus Nyström <magnusn@gmail.com>
> *Sent:* Monday, May 3, 2021 6:44 AM
> *To:* secdir@ietf.org; draft-ietf-idr-bgp-flowspec-oid@ietf.org
> *Subject:* Secdir review of draft-ietf-idr-bgp-flowspec-oid-13
>
>
>
> I have reviewed this document as part of the security directorate's
> ongoing effort to review all IETF documents being processed by the IESG.
> These comments were written primarily for the benefit of the security area
> directors.  Document editors and WG chairs should treat these comments just
> like any other last call comments.
>
>
>
> This document describes "a modification to the validation procedure
> defined for the dissemination
>
> of BGP Flow Specifications." More specifically. the memo describes a
> mechanism which relaxes  the existing validation rule that requires Flow
> Specifications to be originating from the originator of the best-match
> unicast route, and now allows such specifications to be originated within
> the same AS as the validator. As a result. the Security Considerations
> section does call out: "The original AS_PATH validation rule ([RFC4271]
> section 6.3 <https://tools.ietf.org/html/rfc4271#section-6.3>) becomes
> hereby optional (section 4.2
> <https://tools.ietf.org/html/draft-ietf-idr-bgp-flowspec-oid-13#section-4.2>).
> If that original rule is actually not enforced it may introduce some
> security risks. A peer (or a client of a route server peer) in AS X could
> advertise a rogue Flow Specification route...." and "If t[the originator of
> a rule] is known for a fact not to be a route server, that optional rule
> SHOULD be enforced for Flow Specification routes."
>
>
>
> It is not clear to me how a validator would now "for a fact" that a peer
> isn't a route server, and thus that it would have to enforce the
> now-optional path validation rule. It seems some clarity on this would be
> good such that implementations have less of a risk of accepting flow
> specifications from unauthorized parties, even if they are on the same AS.
>
>
>
> [JUAN]: This paragraph was not intended to pressure the operator to know
> if the peer was a route server, it was just a ‘if’. Note it’s the same case
> for RFC4271 :
>
>
>
> If the UPDATE message is received from an external peer, the local
>
>    system MAY check whether the leftmost (with respect to the position
>
>    of octets in the protocol message) AS in the AS_PATH attribute is
>
>    equal to the autonomous system number of the peer that sent the
>
>    message.
>
>
>
>
>
> Note that the same challenge of identifying route servers applies for
> other address-families.
>
> Note also that the route-server itself may enforce the rule.
>
>
>
> What about for clarity:
>
>
>
>    The original AS_PATH validation rule ([RFC4271] section 6.3) remains
> hereby still optional
>
>    (section 4.2) for Flow Specification Address Family (changes introduced
> in [RFC5575] are cancelled).
>
>    If that original rule is not enforced for Flow Specification it may
> introduce some new security risks.
>
>    A peer (or a client of a route server peer) in AS X could advertise a
> rogue Flow
>
>    Specification route whose first AS in AS_PATH was Y (assume Y is the
>
>    first AS in the AS_PATH of the best-match unicast route).  This risk
>
>    is impossible to prevent if the Flow Specification route is received
>
>    from a route server peer.  If that peer is known for a fact not to be
>
>    a route server, that optional rule SHOULD be enforced for Flow
>
>    Specification routes. Note that identifying those peers that are route
> servers may suppose an
>
>    operational challenge. If the condition of the peer is unknown, the
> rule SHOULD not be
>
>    enforced.
>
>
>
>    A route server itself may be in a good position to enforce the AS_PATH
> validation rule described
>
>   in the previous paragraph. If a route server knowns it’s not peering
> with any other route server,
>
>    it can enforce the AS_PATH validation rule across all its peers. If, in
> addition to that,
>
>    the route server is trusted, the security threat described above
> disappears.
>
>
>
>
>
>
>
> Anybody feel free to reword the two paragraphs above if it helps them for
> clarity.
>
>
>
>
>
>
>
> Editorial:
>
>    - "Let's consider the particular case where the Flow Specification is
>    originated in any location within the same autonomous system than the
>    speaker performing the validation (for example by a centralized BGP route
>    controller), and the best-match unicast route is originated in another
>    autonomous system." - should the word "than" be replaced with "that" here?
>
> [JUAN]: Thanks for pointing that out. A few googling tells me the even
> better grammatical choice would be ‘same as’ in this case. I’ll be using
> ‘same as’ unless  you disagree.
>
>
>
>    - In the security considerations section, "becomes hereby optional"
>    could probably be simplified to "becomes optional" or similar, and
>    "actually" could be removed.
>
> [JUAN]:  Hmmm, I thought that it was important to emphasize it becomes
> optional because of **this** draft redefinition of rules. Don’t you think
> it’s important? (whatever wording you want to use). Regardless, refer to my
> new reworded 2 paragraphs above for that section .
>
>
>
>
>
>
>
> Thanks,
>
> -- Magnus
>
>
>
>
>
> --
>
> -- Magnus
>


-- 
-- Magnus