Protecting Internet Key Exchange (IKE) Implementations from Distributed Denial of Service Attacks
draft-ietf-ipsecme-ddos-protection-00
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| Document | Type | Active Internet-Draft (ipsecme WG) | |
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| Author | Yoav Nir | ||
| Last updated | 2014-10-27 | ||
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draft-ietf-ipsecme-ddos-protection-00
IPSecME Working Group Y. Nir
Internet-Draft Check Point
Intended status: Standards Track October 27, 2014
Expires: April 30, 2015
Protecting Internet Key Exchange (IKE) Implementations from Distributed
Denial of Service Attacks
draft-ietf-ipsecme-ddos-protection-00
Abstract
This document recommends implementation and configuration best
practices for Internet-connected IPsec Responders, to allow them to
resist Denial of Service and Distributed Denial of Service attacks.
Additionally, the document introduces a new mechanism called "Client
Puzzles" that help accomplish this task.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 30, 2015.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Conventions Used in This Document . . . . . . . . . . . . 3
2. The Vulnerability . . . . . . . . . . . . . . . . . . . . . . 3
3. Puzzles . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Retention Periods for Half-Open SAs . . . . . . . . . . . . . 7
5. Rate Limiting . . . . . . . . . . . . . . . . . . . . . . . . 8
6. Plan for Defending a Responder . . . . . . . . . . . . . . . 9
7. Operational Considerations . . . . . . . . . . . . . . . . . 11
8. Security Considerations . . . . . . . . . . . . . . . . . . . 11
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
10.1. Normative References . . . . . . . . . . . . . . . . . . 11
10.2. Informative References . . . . . . . . . . . . . . . . . 12
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction
The IKE_SA_INIT Exchange described in section 1.2 of [RFC7296]
involves the Initiator sending a single message. The Responder
replies with a single message and also allocates memory for a
structure called a half-open IKE SA (Security Association). This
half-open SA is later authenticated in the IKE_AUTH Exchange, but if
that IKE_AUTH request never comes, the half-open SA is kept for an
unspecified amount of time. Depending on the algorithms used and
implementation, such a half-open SA will use from around 100 bytes to
several thousands bytes of memory.
This creates an easy attack vector against an Internet Key Exchange
(IKE) Responder. Generating the Initial request is cheap, and
sending multiple such requests can either cause the Responder to
allocate too much resources and fail, or else if resource allocation
is somehow throttled, legitimate Initiators would also be prevented
from setting up IKE SAs.
An obvious defense, which is described in Section 5, is limiting the
number of half-open SAs opened by a single peer. However, since all
that is required is a single packet, an attacker can use multiple
spoofed source IP addresses.
Section 2.6 of RFC 7296 offers a mechanism to mitigate this DoS
attack: the stateless cookie. When the server is under load, the
Responder responds to the Initial request with a calculated
"stateless cookie" - a value that can be re-calculated based on
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values in the Initial request without storing Responder-side state.
The Initiator is expected to repeat the Initial request, this time
including the stateless cookie.
Attackers that have multiple source IP addresses with return
routability, such as bot-nets can fill up a half-open SA table
anyway. The cookie mechanism limits the amount of allocated state to
the size of the bot-net, multiplied by the number of half-open SAs
allowed for one peer address, multiplied by the amount of state
allocated for each half-open SA. With typical values this can easily
reach hundreds of megabytes.
The mechanism described in Section 3 adds a proof of work for the
Initiator, by calculating a pre-image for a partial hash value. This
sets an upper bound, determined by the attacker's CPU to the number
of negotiations it can initiate in a unit of time.
1.1. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
2. The Vulnerability
If we break down what a responder has to do during an initial
exchange, there are three stages:
1. When the Initial request arrives, the responder:
* Generates or re-uses a D-H private part.
* Generates a responder SPI.
* Stores the private part and peer public part in a half-open SA
database.
2. When the Authentication request arrives, the responder:
* Derives the keys from the half-open SA.
* Decrypts the request.
3. If the Authentication request decrypts properly:
* Validates the certificate chain (if present) in the auth
request.
Yes, there's a stage 4 where the responder actually creates Child
SAs, but when talking about (D)DoS, we never get to this stage.
Stage #1 is pretty light on CPU power, but requires some storage, and
it's very light for the initiator as well. Stage #2 includes
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private-key operations, so it's much heavier CPU-wise, but it
releases the storage allocated in stage #1. Stage #3 includes a
public key operation, and possibly many of them.
To attack such a server, an attacker can attempt to either exhaust
memory or to exhaust CPU. Without any protection, the most efficient
attack is to send multiple Initial requests and exhaust memory. This
should be easy because those Initial requests are cheap.
There are obvious ways for the responder to protect itself even
without changes to the protocol. It can reduce the time that an
entry remains in the half-open SA database, and it can limit the
amount of concurrent half-open SAs from a particular address or
prefix. The attacker can overcome this by using spoofed source
addresses.
The stateless cookie mechanism from section 2.6 of RFC 7296 prevents
an attack with spoofed source addresses. This doesn't solve the
issue, but it makes the limiting of half-open SAs by address or
prefix work. Puzzles do the same thing only more of it. They make
it harder for an attacker to reach the goal of getting a half-open
SA. They don't have to be so hard that an attacker can't afford to
solve them - it's enough that they increase the cost of a half-open
SAs for the attacker.
Reducing the amount of time an abandoned half-open SA is kept attacks
the issue from the other side. It reduces the value the attacker
gets from managing to create a half-open SA. So if a half-open SA
takes 1 KB and it's kept for 1 minute and the capacity is 60,000
half-open SAs, an attacker would need to create 1,000 half-open SAs
per second. Reduce the retention time to 3 seconds, and the attacker
needs to create 20,000 half-open SAs per second. Make each of those
more expensive by introducing a puzzle, and you're likely to thwart
an exhaustion attack against responder memory.
At this point, filling up the half-open SA database in no longer the
most efficient DoS attack. The attacker has two ways to do better:
1. Go back to spoofed addresses and try to overwhelm the CPU that
deals with generating cookies, or
2. Take the attack to the next level by also sending an
Authentication request.
I don't think the first thing is something we can deal with at the
IKE level. It's probably better left to Intrusion Prevention System
(IPS) technology.
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Sending an Authentication request is surprisingly cheap. It requires
a proper IKE header with the correct IKE SPIs, and it requires a
single encrypted payload. The content of the payload might as well
be junk. The responder has to perform the relatively expensive key
derivation, only to find that the Authentication request does not
decrypt. Depending on the responder implementation, this can be
repeated with the same half-open SA (if the responder does not delete
the half-open SA following an unsuccessful decryption - see
discussion in Section 4).
Here too, the number of half-open SAs that the attacker can achieve
is crucial, because each one of them allows the attacker to waste
some CPU time. So making it hard to make many half-open SAs is
important.
A strategy against DDoS has to rely on at least 4 components:
1. Hardening the half-open SA database by reducing retention time.
2. Hardening the half-open SA database by rate-limiting single IPs/
prefixes.
3. Guidance on what to do when an Authentication request fails to
decrypt.
4. Increasing cost of half-open SA up to what is tolerable for
legitimate clients.
Puzzles have their place as part of #4.
3. Puzzles
The puzzle introduced here extends the cookie mechanism from RFC
7296. It is loosely based on the proof-of-work technique used in
BitCoins ([bitcoins]). Future versions of this document will have
the exact bit structure of the notification payloads, but for now, I
will only describe the semantics of the content.
A puzzle is sent to the Initiator in two cases:
o The Responder is so overloaded, than no half-open SAs are allowed
to be created without the puzzle, or
o The Responder is not too loaded, but the rate-limiting in
Section 5 prevents half-open SAs from being created with this
particular peer address or prefix without first solving a puzzle.
When the Responder decides to send the challenge notification in
response to a IKE_SA_INIT request, the notification includes two
fields:
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1. Cookie - this is calculated the same as in RFC 7296. As in RFC
7296, the process of generating the cookie is not specified, but
this specification does assume that it is fixed-length, meaning
that all cookies produced by a particular responder are of the
same length.
2. Zero Bit Count. This is a number between 8 and 255 that
represents the length of the zero-bit run at the end of the
SHA-256 hash of the Cookie payload that the Initiator is to send.
Since the mechanism is supposed to be stateless for the
Responder, the same value is sent to all Initiators who are
receiving this challenge. The values 0 and 1-8 are explicitly
excluded, because the value zero is meaningless, and the values
1-8 create a puzzle that is too easy to solve to make any
difference in mitigating DDoS attacks.
Upon receiving this challenge payload, the Initiator attempts to
append different strings to the Cookie field from the challenge, and
calculates the SHA-256 hash of the result. When a string is found
such that the resulting hash has a sufficient number of trailing zero
bits, that result is sent to the Responder in a Cookie notification,
similar to what is described in RFC 7296. The difference is that the
string in this Cookie notification is longer than the one
transmitted.
When receiving a request with an extended Cookie, the Responder
verifies two things:
o That the first bits of the transmitted cookie are indeed valid.
o That the hash of the transmitted cookie has a sufficient number of
trailing zero bits.
Example 1: Suppose the calculated cookie is
fdbcfa5a430d7201282358a2a034de0013cfe2ae (20 octets) and the required
number of zero bits is 16. After successively trying a bunch of
strings, the Initiator finds out that appending three octets: 022b3d
yields a 23-octet string whose SHA-256 hash is
3b4bdf201105e059e09f65219021738b8f6a148896b2e1be2fdc726aeb6e0000.
That has 17 trailing zero bits, so it is an acceptable cookie.
Example 2: Same cookie, but this time the required number of zero
bits is 22. The first string to satisfy that requirement is 5c2880,
which yields a hash with 23 trailing zero bits. Finding this
requires 6,105,472 hashes.
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+--------------+--------------------------+---------+---------------+
| Appended | Last 24 Hex Hash Digits | # | Time To |
| String | | 0-bits | Calculate |
+--------------+--------------------------+---------+---------------+
| 04 | 2817ae10f20f4e0b0739f5cc | 2 | 0.000 |
| 06 | e540cf315fff88c1c5f362a8 | 3 | 0.000 |
| 0d | 8c459376268f747d7ed40da0 | 5 | 0.000 |
| 1c | 398c49be1babe50576cdae40 | 6 | 0.000 |
| 00f0 | 3f523ad7c0e00252c51ad980 | 7 | 0.000 |
| 0182 | e284296e2ffffa256bdfa800 | 11 | 0.000 |
| 235c | 7dc74302dc8bd695821ab000 | 12 | 0.006 |
| 7186 | a4411c3df3661eff1d574000 | 14 | 0.019 |
| d836 | 498bcd04ab1ae0c2c3a08000 | 15 | 0.036 |
| 022b3d | 96b2e1be2fdc726aeb6e0000 | 17 | 0.136 |
| 0aa679 | 620f48af85428996c1f00000 | 20 | 0.512 |
| 4ffbad | f9ba0ece854cd0fa88e00000 | 21 | 3.602 |
| 5c2880 | d44e6467d8fc37723d800000 | 23 | 4.143 |
| cdafe1 | 0d4058660c3e67be62000000 | 25 | 9.245 |
| 022bffc8 | 5f2d874764a71e2948000000 | 27 | 36.169 |
| 181ac92a | c3b5449fa1019b0580000000 | 31 | 255.076 |
| a987978d | 95a5673968a9b37a00000000 | 33 | 1309.519 |
+--------------+--------------------------+---------+---------------+
Table 1: COOKIE=fdbcfa5a430d7201282358a2a034de0013cfe2ae
The figures above were obtained on a 2.4 GHz single core i5. Run
times can be halved or quartered with multi-core code, but would be
longer on mobile phone processors, even if those are multi-core as
well. With these figures I believe that 20 bits is a reasonable
choice for puzzle level difficulty for all Initiators, with 24 bits
acceptable for specific hosts/prefixes.
4. Retention Periods for Half-Open SAs
As a UDP-based protocol, IKEv2 has to deal with packet loss through
retransmissions. Section 2.4 of RFC 7296 recommends "that messages
be retransmitted at least a dozen times over a period of at least
several minutes before giving up". Retransmission policies in
practice wait at least one or two seconds before retransmitting for
the first time.
Because of this, setting the timeout on a half-open SA too low will
cause it to expire whenever even one IKE_AUTH request packet is lost.
When not under attack, the half-open SA timeout SHOULD be set high
enough that the Initiator will have enough time to send multiple
retransmissions, minimizing the chance of transient network
congestion causing IKE failure.
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When the system is under attack, as measured by the amount of half-
open SAs, it makes sense to reduce this lifetime. The Responder
should still allow enough time for the round-trip, enough time for
the Initiator to derive the Diffie-Hellman shared value, and enough
time to derive the IKE SA keys and the create the IKE_AUTH request.
Two seconds is probably as low a value as can realistically be used.
It could make sense to assign a shorter value to half-open SAs
originating from IP addresses or prefixes from which are considered
suspect because of multiple concurrent half-open SAs.
5. Rate Limiting
Even with DDoS, the attacker has only a limited amount of nodes
participating in the attack. By limiting the amount of half-open SAs
that are allowed to exist concurrently with each such node, the total
amount of half-open SAs is capped, as is the total amount of key
derivations that the Responder is forced to complete.
In IPv4 it makes sense to limit the number of half-open SAs based on
IP address. Most IPv4 nodes are either directly attached to the
Internet using a routable address or are hidden behind a NAT device
with a single IPv4 external address. IPv6 networks are currently a
rarity, so we can only speculate on what their wide deployment will
be like, but the current thinking is that ISP customers will be
assigned whole subnets, so we don't expect the kind of NAT deployment
that is common in IPv4. For this reason it makes sense to use a
64-bit prefix as the basis for rate limiting in IPv6.
The number of half-open SAs is easy to measure, but it is also
worthwhile to measure the number of failed IKE_AUTH exchanges. If
possible, both factors should be taken into account when deciding
which IP address or prefix is considered suspicious.
There are two ways to rate-limit a peer address or prefix:
1. Hard Limit - where the number of half-open SAs is capped, and any
further IKE_SA_INIT requests are rejected.
2. Soft Limit - where if a set number of half-open SAs exist for a
particular address or prefix, any IKE_SA_INIT request will
require solving a puzzle.
The advantage of the hard limit method is that it provides a hard cap
on the amount of half-open SAs that the attacker is able to create.
The downside is that it allows the attacker to block IKE initiation
from small parts of the Internet. For example, if a certain purveyor
of beverages resembling coffee provides Internet connectivity to its
customers through an IPv4 NAT device, a single malicious customer can
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create enough half-open SAs to fill the quota for the NAT device
external IP address. Legitimate Initiators on the same network will
not be able to initiate IKE.
The advantage of a soft limit is that legitimate clients can always
connect. The disadvantage is that a sufficiently resourceful (in the
sense that they have a lot of resources) adversary can still
effectively DoS the Responder.
Regardless of the type of rate-limiting used, there is a huge
advantage in blocking the DoS attack using rate-limiting in that
legitimate clients who are away from the attacking nodes should not
be adversely affected by either the attack or by the measures used to
counteract it.
6. Plan for Defending a Responder
This section outlines a plan for defending a Responder from a DDoS
attack based on the techniques described earlier. The numbers given
here are not normative, and their purpose is to illustrate the
configurable parameters needed for defeating the DDoS attack.
Implementations may be deployed in different environments, so it is
RECOMMENDED that the parameters be settable. As an example, most
commercial products are required to undergo benchmarking where the
IKE SA establishment rate is measured. Benchmarking is
indistinguishable from a DoS attack and the defenses described in
this document may defeat the benchmark by causing exchanges to fail
or take a long time to complete. Parameters should be tunable to
allow for benchmarking (if only by turning DDoS protection off).
Since all countermeasures may cause delays and work on the
initiators, they SHOULD NOT be deployed unless an attack is likely to
be in progress. To minimize the burden imposed on Initiators, the
Responder should monitor incoming IKE requests, searching for two
things:
1. A general DDoS attack. Such an attack is indicated by a high
number of concurrent half-open SAs, a high rate of failed
IKE_AUTH exchanges, or a combination of both. For example,
consider a Responder that has 10,000 distinct peers of which at
peak 7,500 concurrently have VPN tunnels. At the start of peak
time, 600 peers might establish tunnels at any given minute, and
tunnel establishment (both IKE_SA_INIT and IKE_AUTH) takes
anywhere from 0.5 to 2 seconds. For this Responder, we expect
there to be less than 20 concurrent half-open SAs, so having 100
concurrent half-open SAs can be interpreted as an indication of
an attack. Similarly, IKE_AUTH request decryption failures
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should never happen. Supposing the the tunnels are established
using EAP (see section 2.16 or RFC 7296), users enter the wrong
password about 20% of the time. So we'd expect 125 wrong
password failures a minute. If we get IKE_AUTH decryption
failures from multiple sources more than once per second, or EAP
failure more than 300 times per minute, that can also be an
indication of a DDoS attack.
2. An attack from a particular IP address or prefix. Such an attack
is indicated by an inordinate amount of half-open SAs from that
IP address or prefix, or an inordinate amount of IKE_AUTH
failures. A DDoS attack may be viewed as multiple such attacks.
If they are mitigated well enough, there will not be a need enact
countermeasures on all Initiators. Typical figures might be 5
concurrent half-open SAs, 1 decrypt failure, or 10 EAP failures
within a minute.
Note that using counter-measures against an attack from a particular
IP address may be enough to avoid the load on the half-open SA
database and the amount of failed IKE_AUTH exchanges to never exceed
the threshold of attack detection. This is a good thing as it
prevent Initiators that are not close to the attackers from being
affected.
When there is no general DDoS attack, it is suggested that no Cookie
or puzzles be used. At this point the only defensive measure is the
monitoring, and setting a soft limit per peer IP or prefix. The soft
limit can be set to 3-5, and the puzzle difficulty should be set to
such a level (number of zero-bits) that all legitimate clients can
handle it without degraded user experience.
As soon as any kind of attack is detected, either a lot of
initiations from multiple sources or a lot of initiations from a few
sources, it is best to begin by requiring stateless cookies from all
Initiators. This will force the attacker to use real source
addresses, and help avoid the need to impose a greater burden in the
form of cookies on the general population of initiators. This makes
the per-node or per-prefix soft limit more effective.
When Cookies are activated for all requests and the attacker is still
managing to consume too many resources, the Responder MAY increase
the difficulty of puzzles imposed on IKE_SA_INIT requests coming from
suspicious nodes/prefixes. It should still be doable by all
legitimate peers, but it can degrade experience, for example by
taking up to 10 seconds to calculate the cookie extension.
If the load on the Responder is still too great, and there are many
nodes causing multiple half-open SAs or IKE_AUTH failures, the
Responder MAY impose hard limits on those nodes.
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If it turns out that the attack is very widespread and the hard caps
are not solving the issue, a puzzle MAY be imposed on all Initiators.
Note that this is the last step, and the Responder should avoid this
if possible.
7. Operational Considerations
[This section needs a lot of expanding]
Not all Initiators support the puzzles, but all initiators are
supposed to support stateless cookies. If this notification is sent
to a non-supporting but legitimate initiator, the exchange will fail.
Responders are advised to first try to mitigate the DoS using
stateless cookies, even imposing them generally before resorting to
using puzzles.
The difficulty level should be set by balancing the requirement to
minimize the latency for legitimate initiators and making things
difficult for attackers. A good rule of thumb is for taking about 1
second to solve the puzzle. A typical initiator or bot-net member in
2014 can perform slightly less than a million hashes per second per
core, so setting the difficulty level to n=20 is a good compromise.
It should be noted that mobile initiators, especially phones are
considerably weaker than that. Implementations should allow
administrators to set the difficulty level, and/or be able to set the
difficulty level dynamically in response to load.
Initiators should set a maximum difficulty level beyond which they
won't try to solve the puzzle and log or display a failure message to
the administrator or user.
8. Security Considerations
To be added.
9. IANA Considerations
IANA is requested to assign a notify message type from the status
types range (16430-40959) of the "IKEv2 Notify Message Types - Status
Types" registry with name "PUZZLE".
10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
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[RFC7296] Kivinen, T., Kaufman, C., Hoffman, P., Nir, Y., and P.
Eronen, "Internet Key Exchange Protocol Version 2
(IKEv2)", RFC 7296, October 2014.
10.2. Informative References
[bitcoins]
Nakamoto, S., "Bitcoin: A Peer-to-Peer Electronic Cash
System", October 2008.
Author's Address
Yoav Nir
Check Point Software Technologies Ltd.
5 Hasolelim st.
Tel Aviv 6789735
Israel
Email: ynir.ietf@gmail.com
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