Internet DRAFT - draft-gont-tcpm-icmp-attacks
draft-gont-tcpm-icmp-attacks
TCP Maintenance and Minor F. Gont
Extensions (tcpm) UTN/FRH
Internet-Draft October 24, 2005
Expires: April 27, 2006
ICMP attacks against TCP
draft-gont-tcpm-icmp-attacks-05.txt
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Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This document discusses the use of the Internet Control Message
Protocol (ICMP) to perform a variety of attacks against the
Transmission Control Protocol (TCP) and other similar protocols. It
proposes several counter-measures to eliminate or minimize the impact
of these attacks.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Background . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1. The Internet Control Message Protocol (ICMP) . . . . . . . 5
2.1.1. ICMP for IP version 4 (ICMP) . . . . . . . . . . . . . 5
2.1.2. ICMP for IP version 6 (ICMPv6) . . . . . . . . . . . . 6
2.2. Handling of ICMP error messages . . . . . . . . . . . . . 6
3. Constraints in the possible solutions . . . . . . . . . . . . 7
4. General counter-measures against ICMP attacks . . . . . . . . 8
4.1. TCP sequence number checking . . . . . . . . . . . . . . . 8
4.2. Port randomization . . . . . . . . . . . . . . . . . . . . 9
4.3. Filtering ICMP error messages based on the ICMP payload . 9
5. Blind connection-reset attack . . . . . . . . . . . . . . . . 9
5.1. Description . . . . . . . . . . . . . . . . . . . . . . . 10
5.2. Attack-specific counter-measures . . . . . . . . . . . . . 11
5.2.1. Changing the reaction to hard errors . . . . . . . . . 11
5.2.2. Delaying the connection-reset . . . . . . . . . . . . 13
6. Blind throughput-reduction attack . . . . . . . . . . . . . . 13
6.1. Description . . . . . . . . . . . . . . . . . . . . . . . 14
6.2. Attack-specific counter-measures . . . . . . . . . . . . . 14
7. Blind performance-degrading attack . . . . . . . . . . . . . . 14
7.1. Description . . . . . . . . . . . . . . . . . . . . . . . 14
7.2. Attack-specific counter-measures . . . . . . . . . . . . . 16
8. Security Considerations . . . . . . . . . . . . . . . . . . . 19
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 19
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
10.1. Normative References . . . . . . . . . . . . . . . . . . . 20
10.2. Informative References . . . . . . . . . . . . . . . . . . 21
Appendix A. The counter-measure for the PMTUD attack in action . 22
A.1. Normal operation for bulk transfers . . . . . . . . . . . 23
A.2. Operation during Path-MTU changes . . . . . . . . . . . . 25
A.3. Idle connection being attacked . . . . . . . . . . . . . . 26
A.4. Active connection being attacked after discovery of
the Path-MTU . . . . . . . . . . . . . . . . . . . . . . . 27
A.5. TCP peer attacked when sending small packets just
after the three-way handshake . . . . . . . . . . . . . . 27
Appendix B. Pseudo-code for the counter-measure for the blind
performance-degrading attack . . . . . . . . . . . . 28
Appendix C. Additional considerations for the validation of
ICMP error messages . . . . . . . . . . . . . . . . . 32
Appendix D. Advice and guidance to vendors . . . . . . . . . . . 32
Appendix E. Changes from previous versions of the draft . . . . . 32
E.1. Changes from draft-gont-tcpm-icmp-attacks-04 . . . . . . . 32
E.2. Changes from draft-gont-tcpm-icmp-attacks-03 . . . . . . . 33
E.3. Changes from draft-gont-tcpm-icmp-attacks-02 . . . . . . . 33
E.4. Changes from draft-gont-tcpm-icmp-attacks-01 . . . . . . . 33
E.5. Changes from draft-gont-tcpm-icmp-attacks-00 . . . . . . . 34
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Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 35
Intellectual Property and Copyright Statements . . . . . . . . . . 36
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1. Introduction
ICMP [RFC0792] is a fundamental part of the TCP/IP protocol suite,
and is used mainly for reporting network error conditions. However,
the current specifications do not recommend any kind of validation
checks on the received ICMP error messages, thus leaving the door
open to a variety of attacks that can be performed against TCP
[RFC0793] by means of ICMP, which include blind connection-reset,
blind throughput-reduction, and blind performance-degrading attacks.
All of these attacks can be performed even being off-path, without
the need to sniff the packets that correspond to the attacked TCP
connection.
While the security implications of ICMP have been known in the
research community for a long time, there has never been an official
proposal on how to deal with these vulnerabiliies. Thus, only a few
implementations have implemented validation checks on the received
ICMP error messages to minimize the impact of these attacks.
Recently, a disclosure process has been carried out by the UK's
National Infrastructure Security Co-ordination Centre (NISCC), with
the collaboration of other computer emergency response teams. A
large number of implementations were found vulnerable to either all
or a subset of the attacks discussed in this document [NISCC][US-
CERT]. The affected systems ranged from TCP/IP implementations meant
for desktop computers, to TCP/IP implementations meant for core
Internet routers.
It is clear that implementations should be more cautious when
processing ICMP error messages, to eliminate or mitigate the use of
ICMP to perform attacks against TCP [I-D.iab-link-indications].
This document aims to raise awareness of the use of ICMP to perform a
variety of attacks against TCP, and proposes several counter-measures
that eliminate or minimize the impact of these attacks.
Section 2 provides background information on ICMP. Section 3
discusses the constraints in the general counter-measures that can be
implemented against the attacks described in this document.
Section 4 proposes several general validation checks and counter-
measures that can be implemented to mitigate any ICMP-based attack.
Finally, Section 5, Section 6, and Section 7, discuss a variety of
ICMP attacks that can be performed against TCP, and propose attack-
specific counter-measures that eliminate or greatly mitigate their
impact.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
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document are to be interpreted as described in RFC 2119 [RFC2119].
2. Background
2.1. The Internet Control Message Protocol (ICMP)
The Internet Control Message Protocol (ICMP) is used in the Internet
Architecture mainly to perform the fault-isolation function, that is,
the group of actions that hosts and routers take to determine that
there is some network failure [RFC0816]
When an intermediate router detects a network problem while trying to
forward an IP packet, it will usually send an ICMP error message to
the source host, to raise awareness of the network problem taking
place. In the same way, there are a number of scenarios in which an
end-system may generate an ICMP error message if it finds a problem
while processing a datagram. The received ICMP errors are handled to
the corresponding transport-protocol instance, which will usually
perform a fault recovery function.
It is important to note that ICMP error messages are unreliable, and
may be discarded due to data corruption, network congestion and rate-
limiting. Thus, while they provide useful information, upper layer
protocols cannot depend on ICMP for correct operation.
2.1.1. ICMP for IP version 4 (ICMP)
[RFC0792] specifies the Internet Control Message Protocol (ICMP) to
be used with the Internet Protocol version 4 (IPv4). It defines,
among other things, a number of error messages that can be used by
end-systems and intermediate systems to report network errors to the
sending host. The Host Requirements RFC [RFC1122] classifies ICMP
error messages into those that indicate "soft errors", and those that
indicate "hard errors", thus roughly defining the semantics of them.
The ICMP specification [RFC0792] also defines the ICMP Source Quench
message (type 4, code 0), which is meant to provide a mechanism for
flow control and congestion control.
[RFC1191] defines a mechanism called "Path MTU Discovery" (PMTUD),
which makes use of ICMP error messages of type 3 (Destination
Unreachable), code 4 (fragmentation needed and DF bit set) to allow
hosts to determine the MTU of an arbitrary internet path.
Appendix D of [RFC2401] provides information about which ICMP error
messages are produced by hosts, intermediate routers, or both.
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2.1.2. ICMP for IP version 6 (ICMPv6)
[RFC2463] specifies the Internet Control Message Protocol (ICMPv6) to
be used with the Internet Protocol version 6 (IPv6) [RFC2460].
[RFC2463] defines the "Packet Too Big" (type 2, code 0) error
message, that is analogous to the ICMP "fragmentation needed and DF
bit set" (type 3, code 4) error message. [RFC1981] defines the Path
MTU Discovery mechanism for IP Version 6, that makes use of these
messages to determine the MTU of an arbitrary internet path.
Appendix D of [RFC2401] provides information about which ICMPv6 error
messages are produced by hosts, intermediate routers, or both.
2.2. Handling of ICMP error messages
The Host Requirements RFC [RFC1122] states that a TCP MUST act on an
ICMP error message passed up from the IP layer, directing it to the
connection that elicited the error.
In order to allow ICMP messages to be demultiplexed by the receiving
host, part of the original packet that elicited the message is
included in the payload of the ICMP error message. Thus, the
receiving host can use that information to match the ICMP error to
the transport protocol instance that elicited it.
Neither the Host Requirements RFC [RFC1122] nor the original TCP
specification [RFC0793] recommend any validation checks on the
received ICMP messages. Thus, as long as the ICMP payload contains
the information that identifies an existing communication instance,
it will be processed by the corresponding transport-protocol
instance, and the corresponding action will be performed.
Therefore, in the case of TCP, an attacker could send a forged ICMP
message to the attacked host, and, as long as he is able to guess the
four-tuple that identifies the communication instance to be attacked,
he will be able to use ICMP to perform a variety of attacks.
As discussed in [Watson], there are a number of scenarios in which an
attacker may be able to know or guess the four-tuple that identifies
a TCP connection. If we assume the attacker knows the two systems
involved in the TCP connection to be attacked, both the client-side
and the server-side IP addresses will be known. Furthermore, as most
Internet services use the so-called "well-known" ports, only the
client port number would need to be guessed. This means that an
attacker would need to send, in principle, at most 65536 packets to
perform any of the attacks described in this document. However, most
systems choose the port numbers they use for outgoing connections
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from a subset of the whole port number space. Thus, in practice,
fewer packets are needed to perform any of the attacks discussed in
this document.
It is clear that TCP should be more cautious when processing received
ICMP error messages, in order to mitigate or eliminate the impact of
the attacks described in this document [I-D.iab-link-indications].
3. Constraints in the possible solutions
For ICMPv4, [RFC0792] states that the internet header plus the first
64 bits of the packet that elicited the ICMP message are to be
included in the payload of the ICMP error message. Thus, it is
assumed that all data needed to identify a transport protocol
instance and process the ICMP error message is contained in the first
64 bits of the transport protocol header. [RFC1122] states that "the
Internet header and at least the first 8 data octets of the datagram
that triggered the error" are to be included in the payload of ICMP
error messages, and that "more than 8 octets MAY be sent", thus
allowing implementations to include more data from the original
packet than those required by the original ICMP specification. The
Requirements for IP Version 4 Routers RFC [RFC1812] states that ICMP
error messages "SHOULD contain as much of the original datagram as
possible without the length of the ICMP datagram exceeding 576
bytes".
Thus, for ICMP messages generated by hosts, we can only expect to get
the entire IP header of the original packet, plus the first 64 bits
of its payload. For TCP, this means that the only fields that will
be included in the ICMP payload are: the source port number, the
destination port number, and the 32-bit TCP sequence number. This
clearly imposes a constraint on the possible validation checks that
can be performed, as there is not much information avalable on which
to perform them.
This means, for example, that even if TCP were signing its segments
by means of the TCP MD5 signature option [RFC2385], this mechanism
could not be used as a counter-measure against ICMP-based attacks,
because, as ICMP messages include only a piece of the TCP segment
that elicited the error, the MD5 [RFC1321] signature could not be
recalculated. In the same way, even if the attacked peer were
authenticating its packets at the IP layer [RFC2401], because only a
part of the original IP packet would be available, the signature used
for authentication could not be recalculated, and thus this mechanism
could not be used as a counter-measure aganist ICMP-based attacks
against TCP.
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For IPv6, the payload of ICMPv6 error messages includes as many
octets from the IPv6 packet that elicited the ICMPv6 error message as
will fit without making the resulting ICMPv6 packet exceed the
minimum IPv6 MTU (1280 octets) [RFC2463]. Thus, more information is
available than in the IPv4 case.
Hosts could require ICMP error messages to be authenticated
[RFC2401], in order to act upon them. However, while this
requirement could make sense for those ICMP error messages sent by
hosts, it would not be feasible for those ICMP error messages
generated by routers, as this would imply either that the attacked
host should have a security asociation [RFC2401] with every existing
intermediate system, or that it should be able to establish one
dynamically. Current levels of deployment of protocols for dynamic
establishment of security associations makes this unfeasible. Also,
there may be some cases, such as embedded devices, in which the
processing power requirements of authentication could not allow IPSec
authentication to be implemented effectively.
Additional considerations for the validation of ICMP error messages
can be found in Appendix C
4. General counter-measures against ICMP attacks
There are a number of counter-measures that can be implemented to
eliminate or mitigate the impact of the attacks discussed in this
document. Rather than being alternative counter-measures, they can
be implemented together to increase the protection against these
attacks. In particular, all TCP implementations SHOULD perform the
TCP sequence number checking described in Section 4.1.
4.1. TCP sequence number checking
TCP SHOULD check that the TCP sequence number contained in the
payload of the ICMP error message is within the range SND.UNA =<
SEG.SEQ < SND.NXT. This means that the sequence number should be
within the range of the data already sent but not yet acknowledged.
If an ICMP error message does not pass this check, it SHOULD be
discarded.
Even if an attacker were able to guess the four-tuple that identifies
the TCP connection, this additional check would reduce the
possibility of considering a spoofed ICMP packet as valid to
Flight_Size/2^^32 (where Flight_Size is the number of data bytes
already sent to the remote peer, but not yet acknowledged [RFC2581]).
For connections in the SYN-SENT or SYN-RECEIVED states, this would
reduce the possibility of considering a spoofed ICMP packet as valid
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to 1/2^^32. For a TCP endpoint with no data "in flight", this would
completely eliminate the possibility of success of these attacks.
This validation check has been implemented in Linux [Linux] for many
years, in OpenBSD [OpenBSD] since 2004, and in FreeBSD [FreeBSD] and
NetBSD [NetBSD] since 2005.
It is important to note that while this check greatly increases the
number of packets required to perform any of the attacks discussed in
this document, this may not be enough in those scenarios in which
bandwidth is easily available, and/or large TCP windows [RFC1323] are
in use. Therefore, implementation of the attack-specific counter-
measures discussed in this document is strongly RECOMMENDED.
4.2. Port randomization
As discussed in the previous sections, in order to perform any of the
attacks described in this document, an attacker would need to guess
(or know) the four-tuple that identifies the connection to be
attacked. Increasing the port number range used for outgoing TCP
connections, and randomizing the port number chosen for each outgoing
TCP conenctions would make it harder for an attacker to perform any
of the attacks discussed in this document.
[I-D.larsen-tsvwg-port-randomisation] discusses a number of
algorithms to randomize the ephemeral ports used by clients.
4.3. Filtering ICMP error messages based on the ICMP payload
The source address of ICMP error messages does not need to be spoofed
to perform the attacks described in this document. Therefore, simple
filtering based on the source address of ICMP error messages does not
serve as a counter-measure against these attacks. However, a more
advanced packet filtering could be implemented in firewalls as a
counter-measure. Firewalls implementing such advanced filtering
would look at the payload of the ICMP error messages, and would
perform ingress and egress packet filtering based on the source IP
address of the IP header contained in the payload of the ICMP error
message. As the source IP address contained in the payload of the
ICMP error message does need to be spoofed to perform the attacks
described in this document, this kind of advanced filtering would
serve as a counter-measure against these attacks.
5. Blind connection-reset attack
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5.1. Description
When TCP is handled an ICMP error message, it will perform its fault
recovery function, as follows:
o If the network problem being reported is a hard error, TCP will
abort the corresponding connection.
o If the network problem being reported is a soft error, TCP will
just record this information, and repeatedly retransmit its data
until they either get acknowledged, or the connection times out.
The Host Requirements RFC [RFC1122] states that a host SHOULD abort
the corresponding connection when receiving an ICMP error message
that indicates a "hard error", and states that ICMP error messages of
type 3 (Destination Unreachable) codes 2 (protocol unreachable), 3
(port unreachable), and 4 (fragmentation needed and DF bit set)
should be considered to indicate hard errors. While [RFC2463] did
not exist when [RFC1122] was published, one could extrapolate the
concept of "hard errors" to ICMPv6 error messages of type 1
(Destination unreachable) codes 1 (communication with destination
administratively prohibited), and 4 (port unreachable).
Thus, an attacker could use ICMP to perform a blind connection-reset
attack. That is, even being off-path, an attacker could reset any
TCP connection taking place. In order to perform such an attack, an
attacker would send any ICMP error message that indicates a "hard
error", to either of the two TCP endpoints of the connection.
Because of TCP's fault recovery policy, the connection would be
immediately aborted.
As discussed in Section 2.2, all an attacker needs to know to perform
such an attack is the socket pair that identifies the TCP connection
to be attacked. In some scenarios, the IP addresses and port numbers
in use may be easily guessed or known to the attacker [Watson].
Some stacks are known to extrapolate ICMP errors across TCP
connections, increasing the impact of this attack, as a single ICMP
packet could bring down all the TCP connections between the
corresponding peers.
It is important to note that even if TCP itself were protected
against the blind connection-reset attack described in [Watson] and
[I-D.ietf-tcpm-tcpsecure], by means authentication at the network
layer [RFC2401], by means of the TCP MD5 signature option [RFC2385],
or by means of the mechanism proposed in [I-D.ietf-tcpm-tcpsecure],
the blind connection-reset attack described in this document would
still succeed.
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5.2. Attack-specific counter-measures
5.2.1. Changing the reaction to hard errors
An analysis of the circumstances in which ICMP messages that indicate
hard errors may be received can shed some light to eliminate the
impact of ICMP-based blind connection-reset attacks.
ICMP type 3 (Destination Unreachable), code 2 (protocol unreachable)
This ICMP error message indicates that the host sending the ICMP
error message received a packet meant for a transport protocol it
does not support. For connection-oriented protocols such as TCP,
one could expect to receive such an error as the result of a
connection-establishment attempt. However, it would be strange to
get such an error during the life of a connection, as this would
indicate that support for that transport protocol has been removed
from the host sending the error message during the life of the
corresponding connection. Thus, it would be fair to treat ICMP
protocol unreachable error messages as soft errors (or completely
ignore them) if they are meant for connections that are in
synchronized states. For TCP, this means TCP should treat ICMP
protocol unreachable error messages as soft errors (or completely
ignore them) if they are meant for connections that are in the
ESTABLISHED, FIN-WAIT-1, FIN-WAIT-2, CLOSE-WAIT, CLOSING, LAST-ACK
or TIME-WAIT states.
ICMP type 3 (Destination Unreachable), code 3 (port unreachable)
This error message indicates that the host sending the ICMP error
message received a packet meant for a socket (IP address, port
number) on which there is no process listening. Those transport
protocols which have their own mechanisms for notifying this
condition should not be receiving these error messages. However,
the Host Requirements RFC [RFC1122] states that even those
transport protocols that have their own mechanism for notifying
the sender that a port is unreachable MUST nevertheless accept an
ICMP Port Unreachable for the same purpose. For security reasons,
it would be fair to treat ICMP port unreachable messages as soft
errors (or completely ignore them) when they are meant for
protocols that have their own mechanism for reporting this error
condition.
ICMP type 3 (Destination Unreachable), code 4 (fragmentation needed
and DF bit set)
This error message indicates that an intermediate node needed to
fragment a datagram, but the DF (Don't Fragment) bit in the IP
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header was set. Those systems that do not implement the PMTUD
mechanism should not be sending their IP packets with the DF bit
set, and thus should not be receiving these ICMP error messages.
Thus, it would be fair for them to treat this ICMP error message
as indicating a soft error, therefore not aborting the
corresponding connection when such an error message is received.
On the other hand, and for obvious reasons, those systems
implementing the Path-MTU Discovery (PMTUD) mechanism [RFC1191]
should not abort the corresponding connection when such an ICMP
error message is received.
ICMPv6 type 1 (Destination Unreachable), code 1 (communication with
destination administratively prohibited)
This error message indicates that the destination is unreachable
because of an administrative policy. For connection-oriented
protocols such as TCP, one could expect to receive such an error
as the result of a connection-establishment attempt. Receiving
such an error for a connection in any of the synchronized states
would mean that the administrative policy changed during the life
of the connection. Therefore, while it would be possible for a
firewall to be reconfigured during the life of a connection, it
would be fair, for security reasons, to ignore these messages for
connections that are in the ESTABLISHED, FIN-WAIT-1, FIN-WAIT-2,
CLOSE-WAIT, CLOSING, LAST-ACK or TIME-WAIT states.
ICMPv6 type 1 (Destination Unreachable), code 4 (port unreachable)
This error message is analogous to the ICMP type 3 (Destination
Unreachable), code 3 (Port unreachable) error message discussed
above. Therefore, the same considerations apply.
Therefore, TCP SHOULD treat all of the above messages as indicating
"soft errors", rather than "hard errors", and thus SHOULD NOT abort
the corresponding connection upon receipt of them. This is policy is
based on the premise that TCP should be as robust as possible. Also,
as discussed in Section 5.1, hosts SHOULD NOT extrapolate ICMP errors
across TCP connections.
In case the received message were legitimate, it would mean that the
"hard error" condition appeared during the life of the connection.
However, there is no reason to think that in the same way this error
condition appeared, it won't get solved in the near term. Therefore,
treating the received ICMP error messages as "soft errors" would make
TCP more robust, and could avoid TCP from aborting a TCP connection
unnecesarily. Aborting the connection would be to ignore the
valuable feature of the Internet that for many internal failures it
reconstructs its function without any disruption of the end points
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[RFC0816].
Also, it is important to note that the current specifications allow
this recommended counter-measure.
One scenario in which a host would benefit from treating the so-
called ICMP "hard errors" as "soft errors" would be that in which the
packets that correspond to a given TCP connection are routed along
multiple different paths. Some (but not all) of these paths may be
experiencing an error condition, and therefore generating the so-
called ICMP hard errors. However, communication should survive if
there is still a working path to the destination host [DClark].
Thus, treating the so-called "hard errors" as "soft errors" when a
connection is in any of the synchronized states would make TCP
achieve this goal.
It is interesting to note that, as ICMP error messages are
unreliable, transport protocols should not depend on them for correct
functioning. In the event one of these messages were legitimate, the
corresponding connection would eventually time out. Also,
applications may still be notified asynchronously about the received
error messages, and thus may still abort their connections on their
own if they consider it appropriate.
This counter-measure has been implemented in BSD-derived TCP/IP
implementations (e.g., [FreeBSD], [NetBSD], and [OpenBSD]) for more
than ten years [TCPv2][McKusick]. The Linux kernel has implemented
this policy for more than ten years, too [Linux].
5.2.2. Delaying the connection-reset
An alternative counter-measure could be to delay the connection
reset. Rather than immediately aborting a connection, a TCP would
abort a connection only after an ICMP error message indicating a hard
error has been received, and the corresponding data have already been
retransmitted more than some specified number of times.
The rationale behind this proposed fix is that if a host can make
forward progress on a connection, it can completely disregard the
"hard errors" being indicated by the received ICMP error messages.
While this counter-measure could be useful, we think that the
counter-measure discussed in Section 5.2.1 is easier to implement,
and provides increased protection against this type of attack.
6. Blind throughput-reduction attack
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6.1. Description
The Host requirements RFC [RFC1122] states that hosts MUST react to
ICMP Source Quench messages by slowing transmission on the
connection. Thus, an attacker could send ICMP Source Quench (type 4,
code 0) messages to a TCP endpoint to make it reduce the rate at
which it sends data to the other end-point of the connection.
[RFC1122] further adds that the RECOMMENDED procedure is to put the
corresponding connection in the slow-start phase of TCP's congestion
control algorithm [RFC2581]. In the case of those implementations
that use an initial congestion window of one segment, a sustained
attack would reduce the throughput of the attacked connection to
about SMSS (Sender Maximum Segment Size) [RFC2581] bytes per RTT
(round-trip time). The throughput achieved during attack might be a
little higher if a larger initial congestion window is in use
[RFC3390].
6.2. Attack-specific counter-measures
The Host Requirements RFC [RFC1122] states that hosts MUST react to
ICMP Source Quench messages by slowing transmission on the
connection. However, as discussed in the Requirements for IP Version
4 Routers RFC [RFC1812], research seems to suggest ICMP Source Quench
is an ineffective (and unfair) antidote for congestion. [RFC1812]
further states that routers SHOULD NOT send ICMP Source Quench
messages in response to congestion. On the other hand, TCP
implements its own congestion control mechanisms [RFC2581] [RFC3168],
that do not depend on ICMP Source Quench messages. Thus, hosts
SHOULD completely ignore ICMP Source Quench messages meant for TCP
connections.
This behavior has been implemented in Linux [Linux] since 2004, and
in FreeBSD [FreeBSD], NetBSD [NetBSD], and OpenBSD [OpenBSD] since
2005.
7. Blind performance-degrading attack
7.1. Description
When one IP host has a large amount of data to send to another host,
the data will be transmitted as a series of IP datagrams. It is
usually preferable that these datagrams be of the largest size that
does not require fragmentation anywhere along the path from the
source to the destination. This datagram size is referred to as the
Path MTU (PMTU), and is equal to the minimum of the MTUs of each hop
in the path [RFC1191].
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A technique called "Path MTU Discovery" (PMTUD) mechanism lets IP
hosts determine the Path MTU of an arbitrary internet path.
[RFC1191] and [RFC1981] specify the PMTUD mechanism for IPv4 and
IPv6, respectively.
The PMTUD mechanism for IPv4 uses the Don't Fragment (DF) bit in the
IP header to dynamically discover the Path MTU. The basic idea
behind the PMTUD mechanism is that a source host assumes that the MTU
of the path is that of the first hop, and sends all its datagrams
with the DF bit set. If any of the datagrams is too large to be
forwarded without fragmentation by some intermediate router, the
router will discard the corresponding datagram, and will return an
ICMP "Destination Unreachable" (type 3) "fragmentation neeed and DF
set" (code 4) error message to sending host. This message will
report the MTU of the constricting hop, so that the sending host can
reduce the assumed Path-MTU accordingly.
For IPv6, intermediate systems do not fragment packets. Thus,
there's an "implicit" DF bit set in every packet sent on a network.
If any of the datagrams is too large to be forwarded without
fragmentation by some intermediate router, the router will discard
the corresponding datagram, and will return an ICMPv6 "Packet Too
Big" (type 2, code 0) error message to sending host. This message
will report the MTU of the constricting hop, so that the sending host
can reduce the assumed Path-MTU accordingly.
As discussed in both [RFC1191] and [RFC1981], the Path-MTU Discovery
mechanism can be used to attack TCP. An attacker could send a forged
ICMP "Destination Unreachable, fragmentation needed and DF set"
packet (or their ICMPv6 counterpart) to the sending host, advertising
a small Next-Hop MTU. As a result, the attacked system would reduce
the size of the packets it sends for the corresponding connection
accordingly.
The effect of this attack is two-fold. On one hand, it will increase
the headers/data ratio, thus increasing the overhead needed to send
data to the remote TCP end-point. On the other hand, if the attacked
system wanted to keep the same throughput it was achieving before
being attacked, it would have to increase the packet rate. On
virtually all systems this will lead to an increase in the IRQ
(Interrrupt ReQuest) rate, thus increasing processor utilization, and
degrading the overall system performance.
A particular scenario that may take place is that in which an
attacker reports a Next-Hop MTU smaller than or equal to the amount
of bytes needed for headers (IP header, plus TCP header). For
example, if the attacker reports a Next-Hop MTU of 68 bytes, and the
amount of bytes used for headers (IP header, plus TCP header) is
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larger than 68 bytes, the assumed Path-MTU will not even allow the
attacked host to send a single byte of application data without
fragmentation. This particular scenario might lead to unpredictable
results. Another posible scenario is that in which a TCP connection
is being secured by means of IPSec. If the Next-Hop MTU reported by
the attacker is smaller than the amount of bytes needed for headers
(IP and IPSec, in this case), the assumed Path-MTU will not even
allow the attacked host to send a single byte of the TCP header
without fragmentation. This is another scenario that may lead to
unpredictable results.
For IPv4, the reported Next-Hop MTU could be as low as 68 octets, as
[RFC0791] requires every internet module to be able to forward a
datagram of 68 octets without further fragmentation. For IPv6, the
reported Next-Hop MTU could be as low as 1280 octets (the minimum
IPv6 MTU) [RFC2460].
7.2. Attack-specific counter-measures
Henceforth, we will refer to both ICMP "fragmentation needed and DF
bit set" and ICMPv6 "Packet Too Big" messages as "ICMP Packet Too
Big" messages.
In addition to the general validation check described in Section 4.1,
a counter-measure similar to that described in Section 5.2.2 could be
implemented to greatly minimize the impact of this attack.
This would mean that upon receipt of an ICMP "Packet Too Big" error
message, TCP would just record this information, and would honor it
only when the corresponding data had already been retransmitted a
specified number of times.
While this policy would greatly mitigate the impact of the attack
against the PMTUD mechanism, it would also mean that it might take
TCP more time to discover the Path-MTU for a TCP connection. This
would be particularly annoying for connections that have just been
established, as it might take TCP several transmission attempts (and
the corresponding timeouts) before it discovers the PMTU for the
corresponding connection. Thus, this policy would increase the time
it takes for data to begin to be received at the destination host.
We would like to protect TCP from the attack against the PMTUD
mechanism, while still allowing TCP to quickly determine the initial
Path-MTU for a connection.
To achieve both goals, we can divide the traditional PMTUD mechanism
into two stages: Initial Path-MTU Discovery, and Path-MTU Update.
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The Initial Path-MTU Discovery stage is when TCP tries to send
segments that are larger than the ones that have so far been sent and
acknowledged for this connection. That is, in the Initial Path-MTU
Discovery stage TCP has no record of these large segments getting to
the destination host, and thus it would be fair to believe the
network when it reports that these packets are too large to reach the
destination host without being fragmented.
The Path-MTU Update stage is when TCP tries to send segments that are
equal to or smaller than the ones that have already been sent and
acknowledged for this connection. During the Path-MTU Update stage,
TCP already has knowledge of the estimated Path-MTU for the given
connection. Thus, it would be fair to be more cautious with the
errors being reported by the network.
In order to allow TCP to distinguish segments between those
performing Initial Path-MTU Discovery and those performing Path-MTU
Update, two new variables should be introduced to TCP: maxsizeacked
and maxsizesent.
maxsizesent would hold the size (in octets) of the largest packet
that has so far been sent for this connection. It would be
initialized to 68 (the minimum IPv4 MTU) when the underlying internet
protocol is IPv4, and would be initialized to 1280 (the minimum IPv6
MTU) when the underlying internet protocol is IPv6. Whenever a
packet larger than maxsizesent octets is sent, maxsizesent should be
set to that value.
On the other hand, maxsizeacked would hold the size (in octets) of
the largest packet that has so far been acknowledged for this
connection. It would be initialized to 68 (the minimum IPv4 MTU)
when the underlying internet protocol is IPv4, and would be
initialized to 1280 (the minimum IPv6 MTU) when the underlying
internet protocol is IPv6. Whenever an acknowledgement for a packet
larger than maxsizeacked octets is received, maxsizeacked should be
set to the size of that acknowledged packet.
Upon receipt of an ICMP "Packet Too Big" error message, the Next-Hop
MTU claimed by the ICMP message (henceforth "claimedmtu") should be
compared with maxsizesent. If claimedmtu is equal to or larger than
maxsizesent, then the ICMP error message should be silently
discarded. The rationale for this is that the ICMP error message
cannot be legitimate if it claims to have been elicited by a packet
larger than the largest packet we have so far sent for this
connection.
If this check is passed, claimedmtu should be compared with
maxsizeacked. If claimedmtu is equal to or larger than maxsizeacked,
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TCP is supposed to be at the Initial Path-MTU Discovery stage, and
thus the ICMP "Packet Too Big" error message should be honored
immediately. That is, the assumed Path-MTU should be updated
according to the Next-Hop MTU claimed in the ICMP error message.
Also, maxsizesent should be reset to the minimum MTU of the internet
protocol in use (68 for IPv4, and 1280 for IPv6).
On the other hand, if claimedmtu is smaller than maxsizeacked, TCP is
supposed to be in the Path-MTU Update stage. At this stage, we
should be more cautious with the errors being reported by the
network, and should therefore just record the received error message,
and delay the update of the assumed Path-MTU.
To perform this delay, one new variable and one new parameter should
be introduced to TCP: nsegrto and MAXSEGRTO. nsegrto will hold the
number of times a specified segment has timed out. It should be
initialized to zero, and should be incremented by one everytime the
corresponding segment times out. MAXSEGRRTO should specify the
number of times a given segment must timeout before an ICMP "Packet
Too Big" error message can be honored, and can be set, in principle,
to any value greater than or equal to 0.
Thus, if nsegrto is greater than or equal to MAXSEGRTO, and there's a
pending ICMP "Packet Too Big" error message, the correspoing error
message should be processed. At that point, maxsizeacked should be
set to claimedmtu, and maxsizesent should be set to 68 (for IPv4) or
1280 (for IPv6).
If while there is a pending ICMP "Packet Too Big" error message the
TCP SEQ claimed by the pending message is acknowledged (i.e., an ACK
that acknowledges that sequence number is received), then the
"pending error" condition should be cleared.
The rationale behind performing this delayed processing of ICMP
"Packet Too Big" messages is that if there is progress on the
connection, the ICMP "Packet Too Big" errors must be a false claim.
MAXSEGRTO can be set, in principle, to any value greater than or
equal to 0. Setting MAXSEGRTO to 0 would make TCP perform the
traditional PMTUD mechanism defined in [RFC1191] and [RFC1981]. A
MAXSEGRTO of 1 should provide enough protection for most cases. In
any case, implementations are free to choose higher values for this
constant. MAXSEGRTO could be a function of the Next-Hop MTU claimed
in the received ICMP "Packet Too Big" message. That is, higher
values for MAXSEGRTO could be imposed when the received ICMP "Packet
Too Big" message claims a Next-Hop MTU that is smaller than some
specified value.
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In the event a higher level of protection is desired at the expense
of a higher delay in the discovery of the Path-MTU, an implementation
could consider TCP to always be in the Path-MTU Update stage, thus
always delaying the update of the assumed Path-MTU.
Appendix A shows the proposed counter-measure in action. Appendix B
shows the proposed counter-measure in pseudo-code.
This behavior has been implemented in NetBSD [NetBSD] and OpenBSD
[OpenBSD] since 2005.
It is important to note that the mechanism proposed in this section
is an improvement to the current Path-MTU discovery mechanism, to
mitigate its security implications. The current PMTUD mechanism, as
specified by [RFC1191] and [RFC1981], still suffers from some
functionality problems [RFC2923] that this document does not aim to
address. Thus, it does not aleviate the need for other improvements
to the current PMTUD mechanism or the introduction of an alternative
PMTUD that replaces the current one, to solve the remaining issues.
A mechanism that aims to address those remaining issues is described
in [I-D.ietf-pmtud-method].
8. Security Considerations
This document describes the use of ICMP error messages to perform a
number of attacks against the TCP protocol, and proposes a number of
counter-measures that either eliminate or reduce the impact of these
attacks.
9. Acknowledgements
This document was inspired by Mika Liljeberg, while discussing some
issues related to [I-D.gont-tcpm-tcp-soft-errors] by private e-mail.
The author would like to thank Ran Atkinson, James Carlson, Alan Cox,
Theo de Raadt, Ted Faber, Juan Fraschini, Markus Friedl, Guillermo
Gont, John Heffner, Vivek Kakkar, Michael Kerrisk, Mika Liljeberg,
David Miller, Miles Nordin, Eloy Paris, Kacheong Poon, Andrew Powell,
Pekka Savola, Joe Touch, and Andres Trapanotto, for contributing many
valuable comments.
Markus Friedl, Chad Loder, and the author of this document, produced
and tested in OpenBSD [OpenBSD] the first implementation of the
counter-measure described in Section 7.2. This first implementation
helped to test the effectiveness of the ideas introduced in this
document, and has served as a reference implementation for other
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operating systems.
The author would like to thank the UK's National Infrastructure
Security Co-ordination Centre (NISCC) for coordinating the disclosure
of these issues with a large number of vendors and CSIRTs (Computer
Security Incident Response Teams).
The author wishes to express deep and heartfelt gratitude to Jorge
Oscar Gont and Nelida Garcia, for their precious motivation and
guidance.
10. References
10.1. Normative References
[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791,
September 1981.
[RFC0792] Postel, J., "Internet Control Message Protocol", STD 5,
RFC 792, September 1981.
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, September 1981.
[RFC1122] Braden, R., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122, October 1989.
[RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
November 1990.
[RFC1812] Baker, F., "Requirements for IP Version 4 Routers",
RFC 1812, June 1995.
[RFC1981] McCann, J., Deering, S., and J. Mogul, "Path MTU Discovery
for IP version 6", RFC 1981, August 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2401] Kent, S. and R. Atkinson, "Security Architecture for the
Internet Protocol", RFC 2401, November 1998.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[RFC2463] Conta, A. and S. Deering, "Internet Control Message
Protocol (ICMPv6) for the Internet Protocol Version 6
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(IPv6) Specification", RFC 2463, December 1998.
10.2. Informative References
[DClark] Clark, D., "The Design Philosophy of the DARPA Internet
Protocols", Computer Communication Review Vol. 18, No. 4,
1988.
[FreeBSD] The FreeBSD Project, "http://www.freebsd.org".
[I-D.gont-tcpm-tcp-soft-errors]
Gont, F., "TCP's Reaction to Soft Errors",
draft-gont-tcpm-tcp-soft-errors-02 (work in progress),
September 2005.
[I-D.iab-link-indications]
Aboba, B., "Architectural Implications of Link
Indications", draft-iab-link-indications-03 (work in
progress), August 2005.
[I-D.ietf-pmtud-method]
Mathis, M., "Path MTU Discovery",
draft-ietf-pmtud-method-04 (work in progress),
February 2005.
[I-D.ietf-tcpm-tcpsecure]
Dalal, M., "Improving TCP's Robustness to Blind In-Window
Attacks", draft-ietf-tcpm-tcpsecure-03 (work in progress),
May 2005.
[I-D.larsen-tsvwg-port-randomisation]
Larsen, M., "Port Randomisation",
draft-larsen-tsvwg-port-randomisation-00 (work in
progress), October 2004.
[Linux] The Linux Project, "http://www.kernel.org".
[McKusick]
McKusick, M., Bostic, K., Karels, M., and J. Quarterman,
"The Design and Implementation of the 4.4BSD Operating
System", Addison-Wesley , 1996.
[NISCC] NISCC, "NISCC Vulnerability Advisory 532967/NISCC/ICMP:
Vulnerability Issues in ICMP packets with TCP payloads",
http://www.niscc.gov.uk/niscc/docs/
al-20050412-00308.html?lang=en, 2005.
[NetBSD] The NetBSD Project, "http://www.netbsd.org".
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[OpenBSD] The OpenBSD Project, "http://www.openbsd.org".
[RFC0816] Clark, D., "Fault isolation and recovery", RFC 816,
July 1982.
[RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
April 1992.
[RFC1323] Jacobson, V., Braden, B., and D. Borman, "TCP Extensions
for High Performance", RFC 1323, May 1992.
[RFC2385] Heffernan, A., "Protection of BGP Sessions via the TCP MD5
Signature Option", RFC 2385, August 1998.
[RFC2581] Allman, M., Paxson, V., and W. Stevens, "TCP Congestion
Control", RFC 2581, April 1999.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[RFC2821] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
April 2001.
[RFC2923] Lahey, K., "TCP Problems with Path MTU Discovery",
RFC 2923, September 2000.
[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP",
RFC 3168, September 2001.
[RFC3390] Allman, M., Floyd, S., and C. Partridge, "Increasing TCP's
Initial Window", RFC 3390, October 2002.
[TCPv2] Wright, G. and W. Stevens, "TCP/IP Illustrated, Volume 2:
The Implementation", Addison-Wesley , 1994.
[US-CERT] US-CERT, "US-CERT Vulnerability Note VU#222750: TCP/IP
Implementations do not adequately validate ICMP error
messages", http://www.kb.cert.org/vuls/id/222750, 2005.
[Watson] Watson, P., "Slipping in the Window: TCP Reset Attacks",
2004 CanSecWest Conference , 2004.
Appendix A. The counter-measure for the PMTUD attack in action
This appendix shows the proposed counter-measure for the ICMP attack
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against the PMTUD mechanism in action. It shows both how the fix
protects TCP from being attacked and how the counter-measure works in
normal scenarios. As discussed in Section 7.2, this Appendix assumes
the PMTUD-specific counter-measure is implemented in addition to the
TCP sequence number checking described in Section 4.1.
Figure 1 illustrates an hypothetical scenario in which two hosts are
connected by means of three intermediate routers. It also shows the
MTU of each hypothetical hop. All the following subsections assume
the network setup of this figure.
Also, for simplicity sake, all subsections assume an IP header of 20
octets and a TCP header of 20 octets. Thus, for example, when the
PMTU is assumed to be 1500 octets, TCP will send segments that
contain, at most, 1460 octets of data.
For simplicity sake, all the following subsections assume the TCP
implementation at Host 1 has chosen a a MAXSEGRTO of 1.
+----+ +----+ +----+ +----+ +----+
| H1 |--------| R1 |--------| R2 |--------| R3 |--------| H2 |
+----+ +----+ +----+ +----+ +----+
MTU=4464 MTU=2048 MTU=1500 MTU=4464
Figure 1: Hypothetical scenario
A.1. Normal operation for bulk transfers
This subsection shows the proposed counter-measure in normal
operation, when a TCP connection is used for bulk transfers. That
is, it shows how the proposed counter-measure works when there is no
attack taking place, and a TCP connection is used for transferring
large amounts of data. This section assumes that just after the
connection is established, one of the TCP endpoints begins to
transfer data in packets that are as large as possible.
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Host 1 Host 2
1. --> <SEQ=100><CTL=SYN> -->
2. <-- <SEQ=X><ACK=101><CTL=SYN,ACK> <--
3. --> <SEQ=101><ACK=X+1><CTL=ACK> -->
4. --> <SEQ=101><ACK=X+1><CTL=ACK><DATA=4424> -->
5. <--- ICMP "Packet Too Big" MTU=2048, TCPseq#=101 <--- R1
6. --> <SEQ=101><ACK=X+1><CTL=ACK><DATA=2008> -->
7. <--- ICMP "Packet Too Big" MTU=1500, TCPseq#=101 <--- R2
8. --> <SEQ=101><ACK=X+1><CTL=ACK><DATA=1460> -->
9. <-- <SEQ=X+1><ACK=1561><CTL=ACK> <--
Figure 2: Normal operation for bulk transfers
nsegrto is initialized to zero. Both maxsizeacked and maxsizesent
are initialized to the minimum MTU for the internet protocol being
used (68 for IPv4, and 1280 for IPv6).
In lines 1 to 3 the three-way handshake takes place, and the
connection is established. In line 4, H1 tries to send a full-sized
TCP segment. As described by [RFC1191] and [RFC1981], in this case
TCP will try to send a segment with 4424 bytes of data, which will
result in an IP packet of 4464 octets. Therefore, maxsizesent is set
to 4464. When the packet reaches R1, it elicits an ICMP "Packet Too
Big" error message.
In line 5, H1 receives the ICMP error message, which reports a Next-
Hop MTU of 2048 octets. After performing the TCP sequence number
check described in Section 4.1, the Next-Hop MTU reported by the ICMP
error message (claimedmtu) is compared with maxsizesent. As it is
smaller than maxsizesent, it passes the check, and thus is then
compared with maxsizeacked. As claimedmtu is larger than
maxsizeacked, TCP assumes that the corresponding TCP segment was
performing the Initial PMTU Discovery. Therefore, the TCP at H1
honors the ICMP message by updating the assumed Path-MTU. maxsizesent
is reset to the minimum MTU of the internet protocol in use (68 for
IPv4, and 1280 for IPv6).
In line 6, the TCP at H1 sends a segment with 2008 bytes of data,
which results in an IP packet of 2048 octets. maxsizesent is thus set
to 2008 bytes. When the packet reaches R2, it elicits an ICMP
"Packet Too Big" error message.
In line 7, H1 receives the ICMP error message, which reports a Next-
Hop MTU of 1500 octets. After performing the TCP sequence number
check, the Next-Hop MTU reported by the ICMP error message
(claimedmtu) is compared with maxsizesent. As it is smaller than
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maxsizesent, it passes the check, and thus is then compared with
maxsizeacked. As claimedmtu is larger than maxsizeacked, TCP assumes
that the corresponding TCP segment was performing the Initial PMTU
Discovery. Therefore, the TCP at H1 honors the ICMP message by
updating the assumed Path-MTU. maxsizesent is reset to the minimum
MTU of the internet protocol in use.
In line 8, the TCP at H1 sends a segment with 1460 bytes of data,
which results in an IP packet of 1500 octets. maxsizesent is thus set
to 1500. This packet reaches H2, where it elicits an acknowledgement
(ACK) segment.
In line 9, H1 finally gets the acknowledgement for the data segment.
As the corresponding packet was larger than maxsizeacked, TCP updates
maxsizeacked, setting it to 1500. At this point TCP has discovered
the Path-MTU for this TCP connection.
A.2. Operation during Path-MTU changes
Let us suppose a TCP connection between H1 and H2 has already been
established, and that the PMTU for the connection has already been
discovered to be 1500. At this point, both maxsizesent and
maxsizeacked are equal to 1500, and nsegrto is equal to 0. Suppose
some time later the PMTU decreases to 1492. For simplicity, let us
suppose that the Path-MTU has decreased because the MTU of the link
between R2 and R3 has decreased from 1500 to 1492. Figure 3
illustrates how the proposed counter-measure would work in this
scenario.
Host 1 Host 2
1. (Path-MTU decreases)
2. --> <SEQ=100><ACK=X><CTL=ACK><DATA=1500> -->
3. <--- ICMP "Packet Too Big" MTU=1492, TCPseq#=100 <--- R2
4. (Segment times out)
5. --> <SEQ=100><ACK=X><CTL=ACK><DATA=1452> -->
6. <-- <SEQ=X><ACK=1552><CTL=ACK> <--
Figure 3: Operation during Path-MTU changes
In line 1, the Path-MTU for this connection decreases from 1500 to
1492. In line 2, the TCP at H1, without being aware of the Path-MTU
change, sends a 1500-byte packet to H2. When the packet reaches R2,
it elicits an ICMP "Packet Too Big" error message.
In line 3, H1 receives the ICMP error message, which reports a Next-
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Hop MTU of 1492 octets. After performing the TCP sequence number
check, the Next-Hop MTU reported by the ICMP error message
(claimedmtu) is compared with maxsizesent. As claimedmtu is smaller
than maxsizesent, it is then compared with maxsizeacked. As
claimedmtu is smaller than maxsizeacked (full-sized packets were
getting to the remote end-point), this packet is assumed to be
performing Path-MTU Update. And a "pending error" condition is
recorded.
In line 4, the segment times out. Thus, nsegrto is incremented by 1.
As nsegrto is greater than or equal to MAXSEGRTO, the assumed Path-
MTU is updated. nsegrto is reset to 0, and maxsizeacked is set to
claimedmtu, and maxsizesent is set to the minimum MTU of the internet
protocol in use.
In line 5, H1 retransmits the data using the updated PMTU, and thus
maxsizesent is set to 1492. The resulting packet reaches H2, where
it elicits an acknowledgement (ACK) segment.
In line 6, H1 finally gets the acknowledgement for the data segment.
At this point TCP has discovered the new Path-MTU for this TCP
connection.
A.3. Idle connection being attacked
Let us suppose a TCP connection between H1 and H2 has already been
established, and the PMTU for the connection has already been
discovered to be 1500. Figure 4 shows a sample time-line diagram
that illustrates an idle connection being attacked.
Host 1 Host 2
1. --> <SEQ=100><ACK=X><CTL=ACK><DATA=50> -->
2. <-- <SEQ=X><ACK=150><CTL=ACK> <--
3. <--- ICMP "Packet Too Big" MTU=68, TCPseq#=100 <---
4. <--- ICMP "Packet Too Big" MTU=68, TCPseq#=100 <---
5. <--- ICMP "Packet Too Big" MTU=68, TCPseq#=100 <---
Figure 4: Idle connection being attacked
In line 1, H1 sends its last bunch of data. At line 2, H2
acknowledges the receipt of these data. Then the connection becomes
idle. In lines 3, 4, and 5, an attacker sends forged ICMP "Packet
Too Big" error messages to H1. Regardless of how many packets it
sends and the TCP sequence number each ICMP packet includes, none of
these ICMP error messages will pass the TCP sequence number check
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described in Section 4.1, as H1 has no unacknowledged data in flight
to H2. Therefore, the attack does not succeed.
A.4. Active connection being attacked after discovery of the Path-MTU
Let us suppose an attacker attacks a TCP connection for which the
PMTU has already been discovered. In this case, as illustrated in
Figure 1, the PMTU would be found to be 1500 bytes. Figure 5 shows a
possible packet exchange.
Host 1 Host 2
1. --> <SEQ=100><ACK=X><CTL=ACK><DATA=1460> -->
2. --> <SEQ=1560><ACK=X><CTL=ACK><DATA=1460> -->
3. --> <SEQ=3020><ACK=X><CTL=ACK><DATA=1460> -->
4. --> <SEQ=4480><ACK=X><CTL=ACK><DATA=1460> -->
5. <--- ICMP "Packet Too Big" MTU=68, TCPseq#=100 <---
6. <-- <SEQ=X><CTL=ACK><ACK=1560> <--
Figure 5: Active connection being attacked after discovery of PMTU
As we assume the PMTU has already been discovered, we also assume
both maxsizesent and maxsizeacked are equal to 1500. We assume
nsegrto is equal to zero, as there have been no segment timeouts.
In lines 1, 2, 3, and 4, H1 sends four data segments to H2. In line
5, an attacker sends a forged ICMP packet to H1. We assume the
attacker is lucky enough to guess both the four-tuple that identifies
the connection and a valid TCP sequence number. As the Next-Hop MTU
claimed in the ICMP "Packet Too Big" message (claimedmtu) is smaller
than maxsizeacked, this packet is assumed to be performing Path-MTU
Update. Thus, the error message is recorded.
In line 6, H1 receives an acknowledgement for the segment sent in
line 1, before it times out. At this point, the "pending error"
condition is cleared, and the recorded ICMP "Packet Too Big" error
message is ignored. Therefore, the attack does not succeed.
A.5. TCP peer attacked when sending small packets just after the three-
way handshake
This section analyzes an scenario in which a TCP peer that is sending
small segments just after the connection has been established, is
attacked. The connection could be being used by protocols such as
SMTP [RFC2821] and HTTP [RFC2616], for example, which usually behave
like this.
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Figure 6 shows a possible packet exchange for such scenario.
Host 1 Host 2
1. --> <SEQ=100><CTL=SYN> -->
2. <-- <SEQ=X><ACK=101><CTL=SYN,ACK> <--
3. --> <SEQ=101><ACK=X+1><CTL=ACK> -->
4. --> <SEQ=101><ACK=X+1><CTL=ACK><DATA=100> -->
5. <-- <SEQ=X+1><ACK=201><CTL=ACK> <--
6. --> <SEQ=201><ACK=X+1><CTL=ACK><DATA=100> -->
7. --> <SEQ=301><ACK=X+1><CTL=ACK><DATA=100> -->
8. <--- ICMP "Packet Too Big" MTU=150, TCPseq#=101 <---
Figure 6: TCP peer attacked when sending small packets just after the
three-way handshake
nsegrto is initialized to zero. Both maxsizesent and maxsizeacked
are initialized to the minimum MTU for the internet protocol being
used (68 for IPv4, and 1280 for IPv6).
In lines 1 to 3 the three-way handshake takes place, and the
connection is established. At this point, the assumed Path-MTU for
this connection is 4464. In line 4, H1 sends a small segment (which
results in a 140-byte packet) to H2. maxsizesent is thus set to 140.
In line 5 this segment is acknowledged, and thus maxsizeacked is set
to 140.
In lines 6 and 7, H1 sends two small segments to H2. In line 8,
while the segments from lines 6 and 7 are still in flight to H2, an
attacker sends a forged ICMP "Packet Too Big" error message to H1.
Assuming the attacker is lucky enough to guess a valid TCP sequence
number, this ICMP message will pass the TCP sequence number check.
The Next-Hop MTU reported by the ICMP error message (claimedmtu) is
then compared with maxsizesent. As claimedmtu is larger than
maxsizesent, the ICMP error message is silently discarded.
Therefore, the attack does not succeed.
Appendix B. Pseudo-code for the counter-measure for the blind
performance-degrading attack
This section contains a pseudo-code version of the counter-measure
described in Section 7.2 for the blind performance-degrading attack
described in Section 7. It is meant as guidance for developers on
how to implement this counter-measure.
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The pseudo-code makes use of the following variables, constants, and
functions:
ack
Variable holding the acknowledgement number contained in the TCP
segment that has just been received.
acked_packet_size
Variable holding the packet size (data, plus headers) the ACK that
has just been received is acknowledging.
adjust_mtu()
Function that adjusts the MTU for this connection, according to
the ICMP "Packet Too Big" that was last received.
claimedmtu
Variable holding the Next-Hop MTU advertised by the ICMP "Packet
Too Big" error message.
claimedtcpseq
Variable holding the TCP sequence number contained in the payload
of the ICMP "Packet Too Big" message that has just been received
or was last recorded.
current_mtu
Variable holding the assumed Path-MTU for this connection.
drop_message()
Function that performs the necessary actions to drop the ICMP
message being processed.
initial_mtu
Variable holding the MTU for new connections, as explained in
[RFC1191] and [RFC1981].
maxsizeacked
Variable holding the largest packet size (data, plus headers) that
has so for been acked for this connection, as explained in
Section 7.2
maxsizesent
Variable holding the largest packet size (data, plus headers) that
has so for been sent for this connection, as explained in
Section 7.2
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nsegrto
Variable holding the number of times this segment has timed out,
as explained in Section 7.2
packet_size
Variable holding the size of the IP datagram being sent
pending_message
Variable (flag) that indicates whether there is a pending ICMP
"Packet Too Big" message to be processed.
save_message()
Function that records the ICMP "Packet Too Big" message that has
just been received.
MINIMUM_MTU
Constant holding the minimum MTU for the internet protocol in use
(68 for IPv4, and 1280 for IPv6.
MAXSEGRTO
Constant holding the number of times a given segment must timeout
before an ICMP "Packet Too Big" error message can be honored.
EVENT: New TCP connection
current_mtu = initial_mtu;
maxsizesent = MINIMUM_MTU;
maxsizeacked = MINIMUM_MTU;
nsegrto = 0;
pending_message = 0;
EVENT: Segment is sent
if (packet_size > maxsizesent)
maxsizesent = packet_size;
EVENT: Segment is received
if (acked_packet_size > maxsizeacked)
maxsizeacked = acked_packet_size;
if (pending_mesage)
if (ack > claimedtcpseq){
pending_message = 0;
nsegrto = 0;
}
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EVENT: ICMP "Packet Too Big" message is received
if (claimedtcpseq < SND.UNA || claimed_TCP_SEQ >= SND.NXT){
drop_message();
}
else {
if (claimedmtu >= maxsizesent || claimedmtu >= current_mtu)
drop_message();
else {
if (claimedmtu > maxsizeacked){
adjust_mtu();
current_mtu = claimedmtu;
maxsizesent = MINIMUM_MTU;
}
else {
pending_message = 1;
save_message();
}
}
}
EVENT: Segment times out
nsegrto++;
if (pending_message && nsegrto >= MAXSEGRTO){
adjust_mtu();
nsegrto = 0;
pending_message = 0;
maxsizeacked = claimedmtu;
maxsizesent = MINIMUM_MTU;
current_mtu = claimedmtu;
}
Notes:
All comparisions between sequence numbers must be performed using
sequence number arithmethic.
The pseudo-code implements the mechanism described in Section 7.2,
the TCP sequence number checking described in Section 4.1, and the
validation check on the advertised Next-Hop MTU described in
[RFC1191] and [RFC1981].
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Appendix C. Additional considerations for the validation of ICMP error
messages
If a full TCP segment is contained in the payload of the ICMP error
message, then the first check that must be performed is that the TCP
checksum is valid. Then, if a TCP MD5 option is present, the MD5
signature should be recalculated for the encapsulated packet, and if
doesn't match the one contained in the TCP MD5 option, the packet
should be dropped.
Regardless of whether the received ICMP error message contains a full
packet or not, if a TCP timestamp option is present, it should be
used to validate the error message according to the rules specified
in [RFC1323].
It must be noted that all the checks discussed in this appendix imply
that the ICMP error message contains more data than just the full IP
header and the first 64 bits of the payload of the original datagram
that elicited the error message. As discussed in Section 3, for
obvious reasons one should not expect an attacker to include in the
packets it sends more information than that required to by the
current specifications.
Appendix D. Advice and guidance to vendors
Vendors are urged to contact NISCC (vulteam@niscc.gov.uk) if they
think they may be affected by the issues described in this document.
As the lead coordination center for these issues, NISCC is well
placed to give advice and guidance as required.
NISCC works extensively with government departments and agencies,
commercial organizations and the academic community to research
vulnerabilities and potential threats to IT systems especially where
they may have an impact on Critical National Infrastructure's (CNI).
Other ways to contact NISCC, plus NISCC's PGP public key, are
available at http://www.uniras.gov.uk/vuls/ .
Appendix E. Changes from previous versions of the draft
E.1. Changes from draft-gont-tcpm-icmp-attacks-04
o Added Appendix C
o Added reference to [I-D.iab-link-indications]
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o Added stress on the fact that ICMP error messages are unreliable
o Miscellaneous editorial changes
E.2. Changes from draft-gont-tcpm-icmp-attacks-03
o Added references to existing implementations of the proposed
counter-measures
o The discussion in Section 4 was improved
o The discussion in Section 5.2.1 was expanded and improved
o The proposed counter-measure for the attack against the PMTUD was
improved and simplified
o Appendix B was added
o Miscellaneous editorial changes
E.3. Changes from draft-gont-tcpm-icmp-attacks-02
o Fixed errors in Section 5.2.1
o The proposed counter-measure for the attack against the PMTUD
mechanism was refined to allow quick discovery of the Path-MTU
o Appendix A was added so as to clarify the operation of the
counter-measure for the attack against the PMTUD mechanism
o Added Appendix D
o Miscellaneous editorial changes
E.4. Changes from draft-gont-tcpm-icmp-attacks-01
o The document was restructured for easier reading
o A discussion of ICMPv6 was added in several sections of the
document
o Added Section on Acknowledgement number checking"/>
o Added Section 4.3
o Added Section 7
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o Fixed typo in the ICMP types, in several places
o Fixed typo in the TCP sequence number check formula
o Miscellaneous editorial changes
E.5. Changes from draft-gont-tcpm-icmp-attacks-00
o Added a proposal to change the handling of the so-called ICMP hard
errors during the synchronized states
o Added a summary of the relevant RFCs in several sections
o Miscellaneous editorial changes
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Author's Address
Fernando Gont
Universidad Tecnologica Nacional
Evaristo Carriego 2644
Haedo, Provincia de Buenos Aires 1706
Argentina
Phone: +54 11 4650 8472
Email: fernando@gont.com.ar
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Gont Expires April 27, 2006 [Page 36]
