Internet DRAFT - draft-eddy-syn-flood
draft-eddy-syn-flood
Network Working Group W. Eddy
Internet-Draft Verizon Federal Network Systems
Expires: October 13, 2006 April 11, 2006
TCP SYN Flooding Attacks and Common Mitigations
draft-eddy-syn-flood-02
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Abstract
This document describes TCP SYN flooding attacks, which have been
well-known to the community for several years. Various
countermeasures against these attacks, and the trade-offs of each,
are described. This document archives explanations of the attack and
defense techniques so that implementers and users may better evaluate
them.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Attack Description . . . . . . . . . . . . . . . . . . . . . . 4
2.1 History . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2 Theory of Operation . . . . . . . . . . . . . . . . . . . 4
3. Common Defenses . . . . . . . . . . . . . . . . . . . . . . . 8
3.1 Filtering . . . . . . . . . . . . . . . . . . . . . . . . 8
3.2 Increasing Backlog . . . . . . . . . . . . . . . . . . . . 8
3.3 Reducing SYN-RECEIVED Timer . . . . . . . . . . . . . . . 8
3.4 SYN Cache . . . . . . . . . . . . . . . . . . . . . . . . 8
3.5 SYN Cookies . . . . . . . . . . . . . . . . . . . . . . . 9
3.6 Hybrid Approaches . . . . . . . . . . . . . . . . . . . . 10
3.7 Firewalls and Proxies . . . . . . . . . . . . . . . . . . 10
4. Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
5. Security Considerations . . . . . . . . . . . . . . . . . . . 13
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 14
7. Informative References . . . . . . . . . . . . . . . . . . . . 14
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 15
A. SYN Cookies . . . . . . . . . . . . . . . . . . . . . . . . . 16
Intellectual Property and Copyright Statements . . . . . . . . 19
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1. Introduction
The SYN flooding attack is a denial of service method taking
advantage of the state retention TCP performs for some time after
receiving a SYN segment to a port with a TCB in the LISTEN state.
The basic idea is to exploit this behavior by causing a host to
retain enough state for bogus half-connections that there are no
resources left to establish new legitimate connections.
This SYN flooding attack has been well-known to the community for
many years, and has been observed in the wild by network operators
and end-hosts. A number of methods have been developed and deployed
to make SYN flooding less effective. Despite the notoriety of the
attack, and the widely available countermeasures, the RFC series has
not documented the vulnerability, nor suggested any mitigation
techniques for TCP implementations. The purpose of this document is
to satisfy both of these ends in an informational context. The
advancement (or need to advance) of mitigation techniques through the
standards track is left to be considered as further work.
This majority of this document consists of three sections. Section 2
explains the SYN flooding attack in greater detail. Several common
mitigation techniques are described in Section 3. An analysis and
discussion of these techniques and their use is presented in
Section 4. Further information on SYN cookies is contained in
Appendix A.
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2. Attack Description
This section describes both the history and the technical basis of
the SYN flooding attack.
2.1 History
The TCP SYN flooding weakness was discovered as early as 1994 by Bill
Cheswick and Steve Bellovin. They included, and then removed, a
paragraph on the attack in their book "Firewalls and Internet
Security: Repelling the Wily Hacker" [CB94]. Unfortunately, no
countermeasures were developed within the next two years.
The SYN flooding attack was first publicized in 1996, with the
release of a description and exploit tool in Phrack Magazine
[P48-13]. Aside from some minor inaccuracies, this article is of
high enough quality to be useful. This article contains code for the
attack and was widely distributed via the Internet.
By September of 1996, SYN flooding attacks had been observed in the
wild. Particularly, an attack against the Panix ISP's mail servers
caused well-publicized outages. CERT quickly released an advisory on
the attack [CA-96.21]. SYN flooding was particularly serious in
comparison to other known denial of service attacks at the time.
Rather than relying on the common brute-force tactic of simply
exhausting the network's resources, SYN flooding targets host
resources, which it requires fewer packets to deplete.
The community quickly developed many widely-differing techniques for
preventing or limiting the impact of SYN flooding attacks. Many of
these have been deployed to varying degrees on the Internet, in both
end hosts and intervening routers. Some of these techniques have
become important pieces of the TCP implementations in certain
operating systems, although some significantly diverge from the TCP
specification and have not yet been standardized or sanctioned by the
IETF process.
2.2 Theory of Operation
As described in RFC 793, a TCP implementation may allow the LISTEN
state to be entered with either all, some. or none of the pair of IP
addresses and port numbers given by the application. In many common
applications like web servers, none of the remote host's information
is pre-known or preconfigured, so that a connection can be
established with any client, and that client does not have to be
known to the server ahead of time. This type of "unbound" LISTEN is
the target of SYN flooding attacks, due to the way it is typically
implemented by operating systems.
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For success, the SYN flooding attack relies on the victim host TCP
implementation's behavior. In particular, it assumes that the victim
allocates state for every TCP SYN segment when it is received, and
that there is a limit on the amount of such state than can be kept at
any time. The current TCP specification, RFC 793 [RFC0793],
describes the standard processing of incoming SYN segments. RFC 793
describes the concept of a Transmission Control Block (TCB) data
structure to store all the state information for an individual
connection. In practice, operating systems may implement this
concept rather differently, but the key is that each TCP connection
requires some memory space.
Per RFC 793, when a SYN is received for a local TCP port where a
connection is in the LISTEN state, then the state transitions to SYN-
RECEIVED and some of the TCB is initialized with information from the
header fields of the received SYN segment. In practice, this is not
really how things work. Many operating systems do not alter the TCB
in LISTEN, but instead make a copy of the TCB and perform the state
transition and update on the copy. This is done so that the local
TCP port may be shared amongst several distinct connections. This
TCB-copying behavior is not actually essential for this purpose, but
influences the way in which applications that wish to handle multiple
simultaneous connections through a single TCP port are written. The
crucial result of this behavior is that instead of updating already-
allocated memory, new (or unused) memory must be devoted to the
copied TCB.
As an example, in the Linux 2.6.10 networking code, a "sock"
structure is used to implement the TCB concept. By examination, this
structure takes over 1300 bytes to store in memory. In other systems
that implement less complex TCP algorithms and options, the overhead
may be less, although typically exceeds 280 bytes [SKK+97]. While
the exact size of the TCP connection data structures may vary between
implementations, it is always true that a received segment elicits
some allocation of resources.
To protect the host's memory from being exhausted by connection
requests, the number of TCB structures that can be resident at any
time is usually limited by operating system kernels. Systems vary on
whether limits are globally applied or local to a particular port
number. There is also variation on whether the limits apply to
fully-established connections as well as those in SYN-RECEIVED.
Commonly, systems implement a parameter to the typical listen()
system call that allows the application to suggest a value for this
limit, called the backlog. When the backlog limit is reached, then
either incoming SYN segments are ignored, or uncompleted connections
in the backlog are replaced. The concept of using a backlog is not
described in the standards documents, so the failure behavior when
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the backlog is reached may vary (for instance, TCP RSTs might be
generated). The exact failure behavior will determine whether
initiating hosts continue to retransmit SYN segments over time, or
quickly cease.
The SYN flooding attack neither attempts to overload the network's
resources, nor the end host's memory, but merely to exhaust an
application's backlog of half-open connections. The goal is to send
a quick barrage of SYN segments from spoofed IP addresses that will
not generate replies to the SYN-ACKs that are produced. By keeping
the backlog full of bogus half-opened connections, legitimate
requests will be rejected. Three important attack parameters for
success are the size of the barrage, the frequency with which
barrages are generated, and the means of selecting IP addresses to
spoof.
Barrage Size
To be effective, the size of the barrage must be made large enough
to reach the backlog. Ideally, the barrage size is no larger than
the backlog, minimizing the volume of traffic the attacker must
source. Typical default backlog values vary from a half-dozen to
several dozen, so the attack might be tailored to the particular
value determined by the victim host and application.
Barrage Frequency
To limit the lifetime of half-opened connection state, TCP
implementations commonly reclaim memory from half-opened
connections if they do not become fully-opened after some time
period. For instance, a timer of 75 seconds [SKK+97] might be set
when the first SYN-ACK is sent, and on expiration cause SYN-ACK
retransmissions to cease and the TCB to be released. The TCP
specifications do not include this behavior of giving up on
connection establishment after an arbitrary time. Some purists
have expressed that the TCP implementation should continue
retransmitting SYN and SYN-ACK segments without artificial bounds
(but with exponential backoff to some conservative rate) until the
application gives up. Despite this, common operating systems
today do implement some artificial limit on half-open TCB
lifetime. For instance, backing off and stopping after a total of
511 seconds can be observed in 4.4 BSD-Lite [Ste95].
To remain effective, a SYN flooding attack needs to send new
barrages of bogus connection requests as soon as the TCBs from the
previous barrage begin to be reclaimed. The frequency of barrages
are tailored to the victim TCP implementation's TCB reclamation
timer. Frequencies higher than needed source more packets,
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potentially drawing more attention, and frequencies that are too
low will allow windows of time where legitimate connections can be
established.
IP Address Selection
For an effective attack, it is important that the spoofed IP
addresses be unresponsive to the SYN-ACK segments that the victim
will generate. If addresses of normal connected hosts are used,
then those hosts will send the victim a TCP reset segment that
will immediately free the corresponding TCB and allow room in the
backlog for legitimate connections to be made. The code
distributed in the original Phrack article used a single source
address for all spoofed SYN segments. This makes the attack
segments somewhat easier to identify and filter. A strong
attacker will have a list of unresponsive and unrelated addresses
that it chooses spoofed source addresses from.
It is important to note that this attack is directed at particular
listening applications on a host, and not the host itself or the
network. The attack also prevents only the establishment of new
incoming connections to the victim port, and does not impact outgoing
connection requests, nor previously established connections to the
victim port.
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3. Common Defenses
3.1 Filtering
Since the ability to send packets with spoofed source IP addresses is
required for this attack to work, removing an attacker's ability to
send spoofed IP packets is an effective solution that requires no
modifications to TCP. The filtering techniques described in RFCs
2827, 3013, and 3704 represent the best current practices for packet
filtering based on IP addresses [RFC2827][RFC3013][RFC3704]. While
perfectly effective, end hosts should not rely on filtering policies
to prevent attacks from spoofed segments, as global deployment of
filters is neither guaranteed nor likely. An attacker with the
ability to use a group of compromised hosts or to move around in the
network will also make filtering an impotent solution.
3.2 Increasing Backlog
An obvious attempt at defense is for end hosts to use a larger
backlog. Lemon has shown that in FreeBSD 4.4, this tactic has some
serious negative aspects as the size of the backlog grows [Lem02].
The implementation has not been designed to scale past backlogs of a
few hundred, and the data structures and search algorithms that it
uses are inefficient with larger backlogs. It is reasonable to
assume that other TCP implementations have similar design factors
that limit their performance with large backlogs, and there seems to
be no compelling reason why stacks should be re-engineered to support
extremely large backlogs, since other solutions are available.
However, experiments with large backlogs using efficient data
structures and search algorithms have not been conducted, to our
knowledge.
3.3 Reducing SYN-RECEIVED Timer
Decreasing the timer that limits the lifetime of TCBs in SYN-RECEIVED
is also flawed. While a shorter timer will keep bogus connection
attempts from persisting for as long in the backlog, and thus free up
space for legitimate connections sooner, it can prevent some fraction
of legitimate connections from becoming fully established. This
tactic is also ineffective because it only requires the attacker to
increase the barrage frequency by a linearly proportional amount.
3.4 SYN Cache
The SYN cache, best described by Lemon [Lem02], is based on
minimizing the amount of state that a SYN allocates, i.e. not
immediately allocating a full TCB. The full state allocation is
delayed until the connection has been fully established. Hosts
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implementing a SYN cache have some secret bits that they select from
the incoming SYN segments. The secret bits are hashed along with the
IP addresses and TCP ports of a segment, and the hash value
determines the location in a global hash table where the incomplete
TCB is stored. There is a bucket limit for each hash value, and when
this limit is reached, the oldest entry is dropped.
The SYN cache technique is effective because the secret bits prevent
an attacker from being able to target specific hash values for
overflowing the bucket limit, and it bounds both the CPU time and
memory requirements. Lemon's evaluation of the SYN cache shows that
even under conditions where a SYN flooding attack is not being
performed, due to the modified processing path, connection
establishment is slightly more expedient. Under active attack, SYN
cache performance was observed to approximately linearly shift the
distribution of times to establish legitimate connections to about
15% longer than when not under attack.
If data accompanies the SYN segment, then this data is not
acknowledged or stored by the receiver, and will require
retransmission. This does not affect the reliability of TCP's data
transfer service, but it does affect its performance to some small
extent.
3.5 SYN Cookies
SYN cookies go a step further and allocate no state at all for
connections in SYN-RECEIVED. Instead, they encode most of the state
(and all of the strictly required) state that they would normally
keep into the sequence number transmitted on the SYN-ACK. If the SYN
was not spoofed, then the acknowledgement number (along with several
other fields) in the ACK that completes the handshake can be used to
reconstruct the state to be put into the TCB. To date, one of the
best references on SYN cookies can be found on Dan Bernstein's web
site [cr.yp.to]. This technique exploits the long-understood low
entropy in TCP header fields [RFC1144][WM04]. In Appendix A, we
describe the SYN cookie technique, to avoid the possibility that the
web page will become unavailable.
The exact mechanism for encoding state into the SYN-ACK sequence
number can be implementation dependent. A common consideration is
that to prevent replay, some time-dependent random bits must be
embedded in the sequence number. One technique used 7 bits for these
bits and 25 bits for the other data [Lem02]. One way to encode these
bits has been to XOR the initial sequence number received with a
truncated cryptographic hash of the IP address and TCP port number
pairs, and secret bits. In practice, this hash has been generated
using MD5.
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The problem with SYN cookies is that current schemes are incompatible
with some TCP options, if the cookie generation scheme does not
consider them. For example, an encoding of the MSS advertised on the
SYN has been accommodated by using 2 sequence number bits to
represent 4 predefined common MSS values. Similar techniques would
be required for some other TCP options, while negotiated use of other
TCP options can be detected implicitly. A timestamp on the ACK, as
an example, indicates that Timestamp use was successfully negotiated
on the SYN and SYN-ACK, while the reception of a SACK option at some
point during the connection implies that SACK was negotiated. Note
that SACK blocks should normally not be sent by a host using TCP
cookies unless they are first received. For the common
unidirectional data flow in many TCP connections, this can be a
problem, as it limits SACK usage. For this reason, SYN cookies
typically default to off on systems that implement them, and are only
enabled either under high-stress conditions indicative of an attack,
or via adminstrative action.
Similarly to SYN caches, SYN cookies do not handle application data
piggybacked on the SYN segment.
3.6 Hybrid Approaches
The SYN cache and SYN cookie techniques can be combined. For
example, in the event that the cache becomes full, then SYN cookies
can be sent instead of purging cache entries upon the arrival of new
SYNs. Such hybrid approaches may provide a strong combination of the
positive aspects of each approach. Lemon has demonstrated the
utility of this hybrid.
3.7 Firewalls and Proxies
Firewall-baed tactics may also be used to defend end-hosts from SYN
flooding attacks. The basic concept is to offload the connection
establishment proceedures onto a firewall that screens connection
attempts until they are completed and then proxies them back to
protected end hosts. This moves the problem away from end-hosts to
become the firewall's or proxy's problem, and may introduce other
problems related to altering TCP's expected end-to-end semantics.
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4. Analysis
Several of the defenses discussed in the previous section rely on
changes to behavior inside the network; via router filtering,
firewalls, and proxies. These may be highly effective, and often
require no modification or configuration of end host software. Given
the mobile nature and dynamic connectivity of many end hosts, it is
optimistic for TCP implementers to assume the presence of such
protective devices. TCP implementers should provide some means of
defense to SYN flooding attacks in end host implementations.
Among end host modifications, the SYN cache and SYN cookie approaches
seem to be the only viable techniques discoverd. Increasing the
backlog and reducing the SYN-RECEIVED timer are measurably
problematic. The SYN cache implies a higher memory footprint than
SYN cookies, however, SYN cookies may not be fully compatible with
some TCP options, and may hamper development of future TCP extensions
that require state. For these reasons, SYN cookies should not be
enabled by default on systems that provide them. SYN caches do not
have the same negative implications and may be enabled as a default
mode of processing.
In October of 1996, Dave Borman implemented a SYN cache at BSDi for
BSD/OS, which was given to the community with no restrictions. This
code seems to be the basis for the SYN cache implementations adopted
later in other BSD variants. The cache was used when the backlog
became full, rather than by default, as we have described. A note to
the tcp-impl mailing list explains that this code does not retransmit
SYN-ACKs, which is a practice we would not encourage.
In 1997, NetBSD incorporated a modified version of Borman's code.
Two notable differences from the original code stem from the decision
to use of the cache by default (for all connections). This implied
the need to perform retransmissions for SYN-ACKs, and to use larger
structures to keep more complete data. The original structure was 32
bytes long for IPv4 connections and 56 bytes with IPv6 support, while
the current FreeBSD structure is 196 bytes long. As previously
cited, Lemon implemented the SYN cache and cookie techniques in
FreeBSD 4.4. Lemon notes that a SYN cache structure took up 160
bytes compared to 736 for the full TCB (now 196 bytes for the cache
structure). We have examined the OpenBSD 3.6 code and determined
that it includes a similar SYN cache.
Linux 2.6.5 code, also by examination, contains a SYN cookie
implementation that encodes 8 MSS values, and does not use SYN
cookies by default. This functionality has been present in the Linux
kernel for several years previous to 2.6.5.
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Beginning with Windows 2000, Microsoft's Windows operating systems
have had a "TCP SYN attack protection" feature which can be toggled
on or off in the registry. This defaulted to off, until Windows 2003
SP1, in which it is on by default. With this feature enabled, when
the number of half-open connections and half-open connections with
retransmitted SYN-ACKs exceeds configurable thresholds, then the
number of times which SYN-ACKs are retransmitted before giving up is
reduced, and the "Route Cache Entry" creation is delayed, which
prevents some features (e.g. window scaling) from being used .
Several vendors of commercial firewall products sell devices that can
mitigate SYN flooding's effects on end hosts by proxying connections.
Discovery and exploitation of the SYN flooding vulnerability in TCP's
design provided a valuable lesson for protocol designers. The Stream
Control Transmission Protocol [RFC2960], which was designed more
recently, incorporated a 4-way handshake with a stateless cookie-
based component for the listening end. In this way, the passive-
opening side has better evidence that the initiator really exists at
the given address before it allocates any state. The Host Identity
Protocol base exchange [MNJH04] is similarly designed as a 4-way
handshake, but also involves a puzzle sent to the initiator which
must be solved before any state is reserved by the responder. The
general concept of designing statelessness into protocol setup to
avoid denial of service attacks has been discussed by Aura and
Nikander [AN97].
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5. Security Considerations
The SYN flooding attack on TCP has been described in numerous other
publications, and the details and code needed to perform the attack
have been easily available for years. Describing the attack in this
document does not pose any danger of further publicizing this
weakness in unmodified TCP stacks. Several widely-deployed operating
systems implement the mitigation techniques that this document
discusses for defeating SYN flooding attacks. In at least some
cases, these operating systems do not enable these countermeasures by
default, however, the mechanisms for defeating SYN flooding are well
deployed, and easily enabled by end-users. The publication of this
document should not influence the number of SYN flooding attacks
observed, and might increase the robustness of the Internet to such
attacks by encouraging use of the commonly available mitigations.
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6. Acknowledgements
A conversation with Ted Faber was the impetus for writing this
document. Comments and suggestions from Joe Touch, Dave Borman,
Fernando Gont, and Jean-Baptiste Marchand were useful in
strengthening this document.
Work on this document was performed at NASA's Glenn Research Center.
Funding was partially provided by a combination of NASA's Advanced
Communications, Navigation, and Surveillance Architectures and System
Technologies (ACAST) project, and the Sensis Corporation.
7. Informative References
[AN97] Aura, T. and P. Nikander, "Stateless Connections",
Proceedings of the First International Conference on
Information and Communication Security 1997.
[CA-96.21]
CERT, "CERT Advisory CA-1996-21 TCP SYN Flooding and IP
Spoofing Attacks", September 1996.
[CB94] Cheswick, W. and S. Bellovin, "Firewalls and Internet
Security", ISBN: 0201633574, January 1994.
[Lem02] Lemon, J., "Resisting SYN Flood DoS Attacks with a SYN
Cache", BSDCON 2002, February 2002.
[MNJH04] Moskowitz, R., Nikander, P., Jokela, P., and T. Henderson,
"Host Identity Protocol", (draft-ietf-hip-base-03),
June 2005.
[P48-13] daemon9, "Project Neptune", Phrack Magazine, Volume 7,
Issue 48, File 13 of 18, July 1996.
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, September 1981.
[RFC1144] Jacobson, V., "Compressing TCP/IP headers for low-speed
serial links", RFC 1144, February 1990.
[RFC2827] Ferguson, P. and D. Senie, "Network Ingress Filtering:
Defeating Denial of Service Attacks which employ IP Source
Address Spoofing", BCP 38, RFC 2827, May 2000.
[RFC2960] Stewart, R., Xie, Q., Morneault, K., Sharp, C.,
Schwarzbauer, H., Taylor, T., Rytina, I., Kalla, M.,
Zhang, L., and V. Paxson, "Stream Control Transmission
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Protocol", RFC 2960, October 2000.
[RFC3013] Killalea, T., "Recommended Internet Service Provider
Security Services and Procedures", BCP 46, RFC 3013,
November 2000.
[RFC3704] Baker, F. and P. Savola, "Ingress Filtering for Multihomed
Networks", BCP 84, RFC 3704, March 2004.
[SKK+97] Schuba, C., Krsul, I., Kuhn, M., Spafford, E., Sundaram,
A., and D. Zamboni, "Analysis of a Denial of Service
Attack on TCP", Proceedings of the 1997 IEEE Symposium on
Security and Privacy 1997.
[Ste95] Stevens, W. and G. Wright, "TCP/IP Illustrated, Volume 2:
The Implementation", January 1995.
[WM04] West, M. and S. McCann, "TCP/IP Field Behavior",
(draft-ietf-rohc-tcp-field-behavior), October 2004.
[cr.yp.to]
Bernstein, D., "URL: http://cr.yp.to/syncookies.html",
visited in December 2005.
[win2k3-wp]
Microsoft Corporation, "Microsoft Windows Server 2003
TCP/IP Implementation Details", White Paper, July 2005.
Author's Address
Wesley M. Eddy
Verizon Federal Network Systems
21000 Brookpark Rd, MS 54-5
Cleveland, OH 44135
Phone: 216-433-6682
Email: weddy@grc.nasa.gov
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Appendix A. SYN Cookies
This information is taken from Bernstein's web page on SYN cookies .
This is a rewriting of the technical information on that web page and
not a full replacement. There are other slightly different ways of
implementing the SYN cookie concept than the exact means described
here, although the basic idea of encoding data into the SYN-ACK
sequence number is constant.
A SYN cookie is an initial sequence number sent in the SYN-ACK, that
is chosen based on the connection initiator's initial sequence
number, MSS, a time counter, and the relevent addresses and port
numbers. The actual bits comprising the SYN cookie are chosen to be
the bitwise difference (exclusive-or) between the SYN's sequence
number and a 32 bit quantity computed so that the top five bits come
from a 32-bit counter value modulo 32, where the counter increases
every 64 seconds, the next 3 bits encode a usable MSS near to the one
in the SYN, and the bottom 24 bits are a server-selected secret
function of pair of IP addresses, the pair of port numbers, and the
32-bit counter used for the first 5 bits. This means of selecting an
initial sequence number for use in the SYN-ACK complies with the rule
that TCP sequence numbers increase slowly.
When a connection in LISTEN receives a SYN segment, it can generate a
SYN cookie and send it in the sequence number of a SYN-ACK, without
allocating any other state. If an ACK comes back, the difference
between the acknowledged sequence number and the sequence number of
the ACK segment can be checked against recent values of the counter
and the secret function's output given those counter values and the
IP addresses and port numbers in the ACK segment. If there is a
match, the connection can be accepted, since it is statistically very
likely that the other side received the SYN cookie and did not simply
guess a valid cookie value. If there is not a match, the connection
can be rejected under the heuristic that it is probably not in
response to a recently sent SYN-ACK.
With SYN cookies enabled, a host will be able to maintain responsive
even when under a SYN flooding attack. The largest price to be paid
for using SYN cookies is in the disabling of the window scaling
option, which disables high performance.
Bernstein's web page contains more information about the initial
conceptualization and implementation of SYN cookies, and archives of
emails documenting this history. It also lists some false negative
claims that have been made about SYN cookies, and discusses reducing
the vulnerability of SYN cookie implementations to blind connection
forgery by an attacker guessing valid cookies.
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The best description of the exact SYN cookie algorithms is in part of
an email from Bernstein, that is archived on the web site (notice it
does not set the top five bits from the counter modulo 32, as the
previous description did, but instead uses 29 bits from the second
MD5 operation and 3 bits for the index into the MSS table;
establishing the secret values is also not discussed):
Here's what an implementation would involve:
Maintain two (constant) secret keys, sec1 and sec2.
Maintain a (constant) sorted table of 8 common MSS values,
msstab[8].
Keep track of a ``last overflow time.''
Maintain a counter that increases slowly over time and never
repeats, such as ``number of seconds since 1970, shifted right
6 bits.''
When a SYN comes in from (saddr,sport) to (daddr,dport) with
ISN x, find the largest i for which msstab[i] <= the incoming
MSS. Compute
z = MD5(sec1,saddr,sport,daddr,dport,sec1)
+ x
+ (counter << 24)
+ (MD5(sec2,counter,saddr,sport,daddr,dport,sec2) % (1 <<
24))
and then
y = (i << 29) + (z % (1 << 29))
Create a TCB as usual, with y as our ISN. Send back a SYNACK.
Exception: _If_ we're out of memory for TCBs, set the ``last
overflow time'' to the current time. Send the SYNACK anyway,
with all fancy options turned off.
When an ACK comes back, follow this procedure to find a TCB:
(1) Look for a (saddr,sport,daddr,dport) TCB. If it's
there, done.
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(2) If the ``last overflow time'' is earlier than a few
minutes ago, give up.
(3) Figure out whether our alleged ISN makes sense. This
means recomputing y as above, for each of the counters that
could have been used in the last few minutes (say, the last
four counters), and seeing whether any of the y's match the
ISN in the bottom 29 bits. If none of them do, give up.
(4) Create a new TCB. The top three bits of our ISN give a
usable MSS. Turn off all fancy options.
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