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.
bafb48eca640a455dbb85cd6293af2853c07b0c0e758cd9e2820797a6f2459ae
TCP Maintenance and Minor F. Gont
Extensions (tcpm) UTN/FRH
Internet-Draft December 22, 2004
Expires: June 22, 2005
ICMP attacks against TCP
draft-gont-tcpm-icmp-attacks-03.txt
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Copyright Notice
Copyright (C) The Internet Society (2004).
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 . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Background . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1 The Internet Control Message Protocol (ICMP) . . . . . . . 3
2.1.1 ICMP for IP version 4 (ICMP) . . . . . . . . . . . . . 4
2.1.2 ICMP for IP version 6 (ICMPv6) . . . . . . . . . . . . 5
2.2 Handling of ICMP errors . . . . . . . . . . . . . . . . . 5
3. ICMP attacks against TCP . . . . . . . . . . . . . . . . . . . 6
4. Constraints in the possible solutions . . . . . . . . . . . . 6
5. General counter-measures against ICMP attacks . . . . . . . . 7
5.1 TCP sequence number checking . . . . . . . . . . . . . . . 7
5.2 TCP acknowledgement number checking . . . . . . . . . . . 8
5.3 Port randomization . . . . . . . . . . . . . . . . . . . . 8
5.4 Authentication . . . . . . . . . . . . . . . . . . . . . . 8
5.5 Filtering ICMP errors based on the ICMP payload . . . . . 9
6. Blind connection-reset attacks . . . . . . . . . . . . . . . . 9
6.1 Description . . . . . . . . . . . . . . . . . . . . . . . 9
6.2 Attack-specific counter-measures . . . . . . . . . . . . . 10
6.2.1 Changing the reaction to hard errors . . . . . . . . . 10
6.2.2 Delaying the connection-reset . . . . . . . . . . . . 11
7. Blind throughput-reduction attacks . . . . . . . . . . . . . . 12
7.1 ICMP Source Quench attack . . . . . . . . . . . . . . . . 12
7.1.1 Description . . . . . . . . . . . . . . . . . . . . . 12
7.1.2 Attack-specific counter-measures . . . . . . . . . . . 12
7.2 ICMP attack against the PMTU Discovery mechanism . . . . . 12
7.2.1 Description . . . . . . . . . . . . . . . . . . . . . 13
7.2.2 Attack-specific counter-measures . . . . . . . . . . . 14
8. Future work . . . . . . . . . . . . . . . . . . . . . . . . . 17
9. Security Considerations . . . . . . . . . . . . . . . . . . . 17
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 17
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 17
11.1 Normative References . . . . . . . . . . . . . . . . . . . . 17
11.2 Informative References . . . . . . . . . . . . . . . . . . . 18
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 19
A. The counter-measure for the PMTUD attack in action . . . . . . 19
A.1 Normal operation for bulk transfers . . . . . . . . . . . 20
A.2 Operation during Path-MTU changes . . . . . . . . . . . . 22
A.3 Idle connection being attacked . . . . . . . . . . . . . . 23
A.4 Active connection being attacked after discovery of
the Path-MTU . . . . . . . . . . . . . . . . . . . . . . . 23
B. An attack that could still succeed . . . . . . . . . . . . . . 24
C. Advice and guidance to vendors . . . . . . . . . . . . . . . . 26
D. Changes from previous versions of the draft . . . . . . . . . 26
D.1 Changes from draft-gont-tcpm-icmp-attacks-02 . . . . . . . 26
D.2 Changes from draft-gont-tcpm-icmp-attacks-01 . . . . . . . 27
D.3 Changes from draft-gont-tcpm-icmp-attacks-00 . . . . . . . 27
Intellectual Property and Copyright Statements . . . . . . . . 28
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1. Introduction
Recently, awareness has been raised about several threats against the
TCP [1] protocol, which include blind connection-reset attacks [12].
These attacks are based on sending forged TCP segments to any of the
TCP endpoints, requiring the attacker to be able to guess the
four-tuple that identifies the connection to be attacked.
While these attacks were known by the research community, they were
considered to be unfeasible. However, increases in bandwidth
availability, and the use of larger TCP windows [13] have made these
attacks feasible. Several general solutions have been proposed to
either eliminate or minimize the impact of these attacks
[14][15][16]. For protecting BGP sessions, specifically, a
counter-measure had already been documented in [17], which defines a
new TCP option that allows a sending TCP to include a MD5 [18]
signature in each transmitted segment.
All these counter-measures address attacks that require an attacker
to send forged TCP segments to the attacked host. However, there is
still a possibility for performing a number of attacks against the
TCP protocol, by means of ICMP [2]. These attacks include, among
others, blind connection-reset attacks.
This document aims to raise awareness of the use of ICMP to perform a
number of attacks against TCP, and proposes several counter-measures
that can eliminate or minimize the impact of these attacks.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [3].
2. Background
2.1 The Internet Control Message Protocol (ICMP)
The Internet Control Message Protocol (ICMP) is used in the Internet
Architecture 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 [19].
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. In the
same way, there are a number of cases in which an end-system may
generate an ICMP error message when it finds a problem while
processing a datagram. These error messages are notified to the
corresponding transport-protocol instance.
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When the transport protocol is notified of the error condition, it
will perform a fault recovery function. That is, it will try to
survive the network failure.
In the case of TCP, the typical fault recovery policy is as follows:
o If the network problem being reported is a hard error, abort the
corresponding connection.
o If the network problem being reported is a soft error, just record
this information, and repeatedly retransmit the segment until
either it gets acknowledged, or the connection times out.
Some stacks honor hard errors only for connections in any of the
synchronized states (ESTABLISHED, FIN-WAIT-1, FIN-WAIT-2, CLOSE-WAIT,
CLOSING, LAST-ACK or TIME-WAIT).
2.1.1 ICMP for IP version 4 (ICMP)
[2] 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 [4] 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 hard errors. Thus, any of these ICMP messages could
elicit a connection abort.
The ICMP specification also defines the ICMP Source Quench message
(type 4, code 0), which is meant to provide a mechanism for flow
control and congestion control. The Requirements for IP Version 4
Routers RFC [5], however, states that experience has shown this ICMP
message is ineffective for handling these issues.
[6] 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. For obvious
reasons, those systems implementing the Path MTU Discovery (PMTUD)
mechanism do not treat ICMP error messages of type 3 code 4 as hard
errors.
Appendix D of [7] 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)
[8] specifies the Internet Control Message Protocol (ICMPv6) to be
used with the Internet Protocol version 6 (IPv6) [9].
Even though ICMPv6 didn't exist when [4] was written, one could
extrapolate the concept of "hard errors" to ICMPv6 Type 1
(Destination Unreachable) codes 1 (communication with destination
administratively prohibited) and 4 (port unreachable). Thus, any of
these messages could elicit a connection abort.
ICMPv6 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. For IPv6, intermediate systems do
not fragment IP packets. Thus, there is an implicit "don't fragment"
bit set in every IPv6 datagram sent on a network. Therefore, hosts
do not treat ICMPv6 "Packet Too Big" messages as a hard errors, but
use them to discover the MTU of the corresponding internet path, as
part of the Path MTU Discovery mechanism for IP Version 6 [10].
Appendix D of [7] provides information about which ICMPv6 error
messages are produced by hosts, intermediate routers, or both.
2.2 Handling of ICMP errors
The Host Requirements RFC [4] states that a TCP MUST act on an ICMP
error message passed up from the IP layer, directing it to the
connection that created 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 instance of the transport protocol that elicited it.
Neither the Host Requirements RFC [4] nor the original TCP
specification [1] recommend any security checks on the received ICMP
messages. Thus, as long as the ICMP payload contains the correct
four-tuple that identifies the communication instance, it will be
processed by the corresponding transport-protocol instance, and the
corresponding action will be performed.
Therefore, 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 can use
ICMP to perform a variety of attacks.
As discussed in [12], there are a number of scenarios in which an
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attacker may be able to know or guess this four-tuple. Furthermore,
it must be noted that most Internet services use the so-called
"well-known" ports, so that only the client port would need to be
guessed. In the event that an attacker had no knowledge about the
range of port numbers used by clients, this would mean that an
attacker would need to send, at most, 65536 packets to perform any of
the attacks described in this document.
It is clear that security checks should be performed on the received
ICMP error messages, to mitigate the impact of the attacks described
in this document.
3. ICMP attacks against TCP
ICMP messages can be used to perform a variety of attacks. These
attacks have been discussed by the research community to a large
extent.
Some TCP/IP implementations have added security checks on the
received ICMP error messages to minimize the impact of these attacks.
However, as there has not been any official proposal about what would
be the best way to deal with these attacks, these security checks
have not been widely implemented.
Section 4 of this document discusses the constraints in the general
counter-measures that can be implemented against the attacks
described in this document. Section 5 proposes several general
counter-measures that apply to all the ICMP attacks described in this
document. Finally, 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 mitigate their
impact. These attack-specific counter-measures are meant to be
additional counter-measures to the ones proposed in Section 5. In
particular, all TCP implementations SHOULD perform the TCP sequence
number checking described in Section 5.1.
4. Constraints in the possible solutions
For ICMPv4, [2] 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. [4] 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 suggesting that
implementations may include more data from the original packet than
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that required by the original ICMP specification. The Requirements
for IP Version 4 Routers RFC [5] 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, that means that the only fields that will
be included are: the source port number, the destination port number,
and the 32-bit TCP sequence number. This clearly imposes a
constraint on the possible security checks that can be performed, as
there is not much information avalable on which to perform them.
While there exists a proposal to recommend hosts to include more data
from the original datagram in the payload of ICMP error messages
[20], and some TCP/IP implementations already do this, we cannot yet
propose any work-around based on checks performed on any data past
the first 64 bits of the payload of the original IP datagram that
elicited the ICMP error message. Thus, the only check that can be
performed on the ICMP error message is that of the TCP sequence
number contained in the payload.
As discussed above, for those ICMP error messages generated by
routers, we can expect to receive much more octets from the original
packet than just the entire IP header and the first 64 bits of the
transport protocol header. Therefore, not only can hosts check the
TCP sequence number contained in the payload of the ICMP error
message, but they can also perform further checks such as checking
the TCP acknowledgement number, as discussed in Section 5.2.
For ICMPv6, 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) [8]. Thus, further checks (as those
described above) can be performed on the received ICMP error
messages.
5. 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 5.1.
5.1 TCP sequence number checking
TCP SHOULD check that the TCP sequence number contained in the
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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 [21]). For
connections in the SYN-SENT or SYN-RECEIVED states, this would reduce
the possibility of considering a spoofed ICMP packet as valid to
1/2^^32. For a TCP endpoint with no data "in flight", this would
completely eliminate the possibility of success of these attacks.
5.2 TCP acknowledgement number checking
As discussed in Section 4, for those ICMP error messages that are
generated by intermediate routers, additional checks can be
performed. TCP SHOULD check that the TCP Acknowledgement number
contained in the payload of the ICMP error message is withing the
range SEG.ACK <= RCV.NXT. This means that the TCP Acknowledgement
number should correspond to data that have already been acknowledged.
This would reduce the possibility of considering a spoofed ICMP
packet as valid by a factor of two.
5.3 Port randomization
As discussed in the previous sections, in order to perform any of the
attacks described in this document, an attacker needs to guess (or
know) the four-tuple that identifies the connection to be attacked.
Randomizing the ephemeral ports used by the clients would make it
harder for an attacker to perform any of the attacks discussed in
this document.
[22] discusses a number of algorithms to randomize the ephemeral
ports used by clients.
Also, a proposal exists to enable TCP to reassign a well-known port
number to a random value [23].
5.4 Authentication
Hosts could require ICMP error messages to be authenticated [7], 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
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feasible for those ICMP error messages generated by intermediate
routers.
[7] contains a discussion on the authentication of ICMP messages.
5.5 Filtering ICMP errors 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. Thus, 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 used as a counter-measure.
Systems performing 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.
6. Blind connection-reset attacks
6.1 Description
The Host Requirements RFC [4] states that a host SHOULD abort the
corresponding connection when receiving an ICMP error message that
indicates a hard error.
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 [12].
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.
There are some points to be considered about this type of attack:
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o The source address of the ICMP error message need not be forged.
Thus, simple filtering based on the source address of ICMP packets
would not serve as a counter-measure against this type of attack.
o Even if TCP itself were protected against the blind
connection-reset attack described in [12] and [14], the type of
attack described in this document could still succeed.
6.2 Attack-specific counter-measures
6.2.1 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 minimize (or even
eliminate) the impact of 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 one would 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 [4] 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
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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
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 completely ignore this ICMP
error message. On the other hand, and for obvious reasons, those
systems implementing the Path-MTU Discovery (PMTUD) mechanism [6]
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.
Finally, it is important to note that, as discussed in Section 6.1,
hosts MUST NOT extrapolate ICMP errors across TCP connections.
6.2.2 Delaying the connection-reset
An alternative counter-measure could be to delay the connection
reset. Rather than immediately aborting a connection, a TCP could
abort a connection only after an ICMP error message indicating a hard
error has been received a specified number of times, and the
corresponding data have already been retransmitted more than some
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specified number of times.
For example, hosts could abort connections only after a fourth ICMP
error message indicating a hard error is received, and the
corresponding data have already been retransmitted more than six
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 6.2.1 is more simple to
implement and provides increased protection against this type of
attack.
7. Blind throughput-reduction attacks
The following subsections discuss a number of attacks that can be
performed against TCP to reduce the throughput of a TCP connection.
While these attacks do not reset the attacked TCP connections, they
may reduce their throughput to such an extent that they may become
practically unusable.
7.1 ICMP Source Quench attack
7.1.1 Description
The Host requirements RFC states 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 party. While this would not reset the connection, it would
certainly degrade the performance of the data transfer taking place
over it.
7.1.2 Attack-specific counter-measures
The Host Requirements RFC [4] 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 [5], research seems to suggest ICMP Source Quench is an
ineffective (and unfair) antidote for congestion. Thus, we recommend
hosts to completely ignore ICMP Source Quench messages.
7.2 ICMP attack against the PMTU Discovery mechanism
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7.2.1 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 [6].
A technique called "Path MTU Discovery mechanism" (PMTUD) lets IP
hosts determine the Path MTU of an arbitrary internet path. [6] and
[10] 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
reduces the assumed Path-MTU.
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 [6] and [10], the PMTUD can be used to attack
TCP. An attacker could reduce the throughput of a TCP connection by
sending forged ICMP "Destination Unreachable, fragmentation needed
and DF set" packets (or their IPv6 counterpart) to the sending host,
and making these packets report a low MTU.
For IPv4, this reported Next-Hop MTU could be as low as 68 octets, as
[11] 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)
[9].
Thus, this attack could considerably readuce the throughput that can
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be achieved with the attacked TCP connection.
7.2.2 Attack-specific counter-measures
An analogous counter-measure to that described in Section 6.2.2 could
be implemented to greatly minimize the impact of this attack.
For IPv4, this would mean that upon receipt of an ICMP "fragmentation
needed and DF bit set" error message, TCP would just record this
information, and would honor it only when it had received a specified
number of ICMP "fragmentation needed and DF bit set" messages, and
provided the corresponding data had already been retransmitted a
specified number of times.
For IPv6, the same mechanism would be implemented for handling ICMPv6
"Packet Too Big" error messages.
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) until 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.
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 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.
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In order to allow TCP to distinguish segments performing Initial
Path-MTU Discovery from those performing Path-MTU Update, a new
variable should be introduced to TCP: maxsizeacked.
This variable would hold the size (in octets) of the largest packet
that has so far been sent and 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.
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.
Upon receipt of an ICMP "Packet Too Big" error message, the Next-Hop
MTU claimed by the ICMP message (henceforth "claimedmtu") would be
compared with maxsizeacked. If claimedmtu is equal to or larger than
maxsizeacked, then TCP is supposed to be at the Initial Path-MTU
Discovery stage, and thus the ICMP "Packet Too Big" error message
should be honored. That is, the assumed Path-MTU should be updated
according to the Next-Hop MTU claimed in the ICMP error message.
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 will therefore delay the update of the assumed Path-MTU.
To perform this delay, two new parameters should be introduced to
TCP: MAXPKTTOOBIG, and MAXSEGRTO. MAXPKTTOOBIG would specify the
number of times an ICMP "Packet Too Big" must be received before it
can be honored to change the Path-MTU. MAXSEGRRTO would specify the
number of times a given segment must timeout before an ICMP "Packet
Too Big" error message can be honored.
Two variables would be needed to implement the proposed fix:
npkttoobig, and nsegrto. npkttoobig would be initialized to zero,
and would be incremented by one everytime a valid ICMP "Packet Too
Big" error message is received. It would be reset to zero everytime
an ICMP "Packet Too Big" error message is honored to change the
assumed Path-MTU for given internet path. nsegrto would be
initialized to zero, and would be incremented by one everytime the
corresponding segment times out.
Thus, the assumed Path-MTU for a given internet path would be changed
when an ICMP "Packet Too Big" is received, provided npkttoobig >=
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MAXPKTTOOBIG and nsegrto >= MAXSEGRTO. When the assumed Path-MTU is
updated, maxsizeacked should be set to claimedmtu, so as to allow the
Path-MTU to be discovered quickly in the event the Path-MTU for the
connection increases some time later.
The rationale behind this proposed delay is that if there is progress
on the connection, the ICMP "Packet Too Big" errors must be a false
claim.
MAXPKTTOOBIG can be set to any value greater than or equal to 1.
MAXSEGRTO can be set, in principle, to any value greater than or
equal to 0.
Setting MAXPKTTOOBIG to 1 and MAXSEGRTO to 0 would make TCP perform
the traditional PMTUD mechanism defined in [6] and [10].
When the values chosen for MAXSEGRTO and MAXPKTTOOBIG are such that
(MAXSEGRTO - MAXPKTTOOBIG) > 0, it somehow means the implementation
is acknowledging that segments may be lost and routers may be
rate-limiting their ICMP traffic.
MAXPKTTOOBIG and MAXSEGRTO might be a function of the Next-Hop MTU
claimed in the received ICMP "Packet Too Big" message. That is,
higher values for MAXPKTTOOBIG and MAXSEGRTO could be imposed when
the received ICMP "Packet Too Big" message claims a Next-Hop MTU that
is smaller some specified value.
A MAXPKTTOOBIG of 1 and a MAXSEGRTO of 1 should provide enough
protection for most cases. In any case, implementations are free to
choose higher values for any of these two constants.
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 describes an attack against the PMTUD mechanism that could
still succeed, along with a counter-measure against it. However,
this attack is unfeasible, and in most cases, non-sensical.
A mechanism that allows hosts to determine the Path-MTU of an
arbitrary internet path without the use of ICMP has been is described
in [24].
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8. Future work
The same considerations discussed in this document for TCP should be
applied to other similar protocols.
9. 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.
10. Acknowledgements
This document was inspired by Mika Liljeberg, while discussing some
issues related to [25] by private e-mail. The author would like to
thank James Carlson, Alan Cox, Juan Fraschini, Markus Friedl,
Guillermo Gont, Vivek Kakkar, Michael Kerrisk, Mika Liljeberg, David
Miller, Miles Nordin, Eloy Paris, Kacheong Poon, Andrew Powell, and
Pekka Savola for contributing many valuable comments.
The author wishes to express deep and heartfelt gratitude to Jorge
Oscar Gont and Nelida Garcia, for their precious motivation and
guidance.
11. References
11.1 Normative References
[1] Postel, J., "Transmission Control Protocol", STD 7, RFC 793,
September 1981.
[2] Postel, J., "Internet Control Message Protocol", STD 5, RFC
792, September 1981.
[3] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[4] Braden, R., "Requirements for Internet Hosts - Communication
Layers", STD 3, RFC 1122, October 1989.
[5] Baker, F., "Requirements for IP Version 4 Routers", RFC 1812,
June 1995.
[6] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
November 1990.
[7] Kent, S. and R. Atkinson, "Security Architecture for the
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Internet Protocol", RFC 2401, November 1998.
[8] Conta, A. and S. Deering, "Internet Control Message Protocol
(ICMPv6) for the Internet Protocol Version 6 (IPv6)
Specification", RFC 2463, December 1998.
[9] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6)
Specification", RFC 2460, December 1998.
[10] McCann, J., Deering, S. and J. Mogul, "Path MTU Discovery for
IP version 6", RFC 1981, August 1996.
[11] Postel, J., "Internet Protocol", STD 5, RFC 791, September
1981.
11.2 Informative References
[12] Watson, P., "Slipping in the Window: TCP Reset Attacks", 2004
CanSecWest Conference , 2004.
[13] Jacobson, V., Braden, B. and D. Borman, "TCP Extensions for
High Performance", RFC 1323, May 1992.
[14] Stewart, R., "Transmission Control Protocol security
considerations", draft-ietf-tcpm-tcpsecure-02 (work in
progress), November 2004.
[15] Touch, J., "ANONsec: Anonymous IPsec to Defend Against Spoofing
Attacks", draft-touch-anonsec-00 (work in progress), May 2004.
[16] Poon, K., "Use of TCP timestamp option to defend against blind
spoofing attack", draft-poon-tcp-tstamp-mod-01 (work in
progress), October 2004.
[17] Heffernan, A., "Protection of BGP Sessions via the TCP MD5
Signature Option", RFC 2385, August 1998.
[18] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, April
1992.
[19] Clark, D., "Fault isolation and recovery", RFC 816, July 1982.
[20] Gont, F., "Increasing the payload of ICMP error messages",
(work in progress) draft-gont-icmp-payload-00.txt, 2004.
[21] Allman, M., Paxson, V. and W. Stevens, "TCP Congestion
Control", RFC 2581, April 1999.
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[22] Larsen, M., "Port Randomisation",
draft-larsen-tsvwg-port-randomisation-00 (work in progress),
October 2004.
[23] Shepard, T., "Reassign Port Number option for TCP",
draft-shepard-tcp-reassign-port-number-00 (work in progress),
July 2004.
[24] Mathis, M., "Path MTU Discovery", draft-ietf-pmtud-method-03
(work in progress), October 2004.
[25] Gont, F., "TCP's Reaction to Soft Errors",
draft-gont-tcpm-tcp-soft-errors-01 (work in progress), October
2004.
[26] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821, April
2001.
[27] 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.
[28] Nagle, J., "Congestion control in IP/TCP internetworks", RFC
896, January 1984.
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
Appendix A. The counter-measure for the PMTUD attack in action
This appendix shows the proposed counter-measure for the ICMP attack
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 5, this Appendix assumes
the PMTUD-specific counter-measure is implemented in addition to the
TCP sequence number checking described in Section 5.1.
Figure 1 illustrates an hypothetical scenario in which two hosts are
connected by means of three intermediate routers. It also shows the
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MTU of each hypothetical hop. All the following subsections assume
the network setup of this figure.
Also, for simplicity, 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, all the following subsections assume the TCP
implementation at Host 1 has chosen a MAXPKTTOOBIG of 1, and 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
Both npkttoobig and nsegrto are initialized to zero. maxsizeacked is
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 [6] and [10], 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. 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, the Next-Hop MTU reported by the ICMP error message
(claimedmtu) is 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.
In line 6, the TCP at H1 sends a segment with 2008 bytes of data,
which results in an IP packet of 2048 octets. 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 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.
In line 8, the TCP at H1 sends a segment with 1460 bytes of data,
which results in an IP packet of 1500 octets. This packet reaches
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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, maxsizeacked is equal to 1500,
maxsegrto is equal to 0, and maxpkttoobig 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 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-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 maxsizeacked. As claimedmtu is smaller
than maxsizeacked, this packet is assumed to be performing Path-MTU
Update. Thus, npkttoobig is incremented by one. While npkttoobig is
greater than or equal to MAXPKTTOOBIG, nsegrto is still smaller than
MAXSEGRTO, and thus the assumed PMTU will not yet be updated.
In line 4, the segment times out. Thus, nsegrto is incremented by 1.
As npkttoobig is greater than or equal to MAXPKTTOOBIG and nsegrto is
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greater than or equal to MAXSEGRTO, the assumed Path-MTU is updated.
npkttoobig and nsegrto are reset to 0, and maxsizeacked is set to
claimedmtu.
In line 5, H1 retransmits the data using the updated PMTU. 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. We assume the
attacker has guessed the four-tuple that identifies the connection.
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
described in Section 5.1, 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.
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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
maxsizeacked is equal to 1500. Also, npkttoobig must be equal to
zero, as no ICMP "Packet Too Big" error messages have been received
for the outstanding segments. In the same way, 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, npkttoobig is incremented by one. While npkttoobig is
greater than or equal to MAXPKTTOOBIG, nsegrto is still smaller than
MAXSEGRTO, and thus the assumed PMTU will not yet be updated.
In line 6, H1 receives an acknowledgement for the segment from line
1, before it times out. At this point, npkttoobig is set back to
zero, and the pending ICMP "Packet Too Big" error message is ignored.
Therefore, the attack does not succeed.
Appendix B. An attack that could still succeed
This Appendix is for completeness-sake only. The author believes
this Appendix will be removed in future revisions of this document.
In any case, input on this issue is welcome.
As mentioned in Section 7.2.2, even if the proposed counter-measure
for the PMTUD attack were implemented, there is an attack that could
still succeed.
Suppose a TCP connection is used by an application which involves the
exchange of small amounts of data before large transfers take place.
Applications using protocols such as SMTP [26] and HTTP [27], for
example, usually behave like this.
Figure 6 shows a possible packet exchange for such scenario.
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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=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: An attack against the PMTUD that could still succeed
Both npkttoobig and nsegrto are initialized to zero. maxsizeacked is
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 sends a small segment to
H2. 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 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 will
incorrectly honor the ICMP message by updating the assumed MTU.
Thus, the connection will assume a PMTU of 150 octets until the next
PMTU-increase "probe" is sent. Depending on the implementation, this
"probe" could be sent several minutes later [6][10].
It must be noted that while this attack is theoretically possible, we
have assumed the attacker is lucky enough not only to guess the
four-tuple that identifies the connection, but also to guess the
sequence number of unacknowledged data that are in flight to H2.
Given that we assume that only a few small segments are in flight to
H2, this is very unlikely. Furthermore, in order to produce any
performance impact, the forged ICMP error message should report a
Next-Hop MTU that is small enough, but that is larger than
maxsizeacked, as TCP would otherwise assume this segment is
performing Path-MTU Update, and therefore would delay the update of
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the assumed Path-MTU.
Also, it must be noted that algorithms such as that described in [28]
will help to avoid sending small segments, and therefore will also
help to make maxsizeacked increase quickly, making this attack
non-sensical.
In any case, this attack could be completely eliminated by
introducing an additional variable: maxsizeinflight. For new
connections, this variable would be initialized to the minimum MTU of
the internet protocol being used (68 for IPv4, and 1280 for IPv6).
Whenever a packet is to be transmitted, the size of the packet is
compared with maxsizeinflight. If the size of the packet to be
transmitted is larger than maxsizeinflight, maxsizeinflight is set to
that packet size.
Whenever the assumed Path-MTU is updated (either as the result of a
segment performing Initial Path-MTU Discovery, or as the result of a
segment performing Path-MTU Update), maxsizeinflight is set to the
new assumed Path-MTU value.
This way, TCP has a record of the largest packet size it has in
flight, and thus can ignore ICMP "Packet Too Big" messages that claim
errors that could never have happened.
Appendix C. Advice and guidance to vendors
Vendors are urged to contact NISCC (vulteam@niscc.gov.uk) if they
think they will be effected 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 D. Changes from previous versions of the draft
D.1 Changes from draft-gont-tcpm-icmp-attacks-02
o Fixed errors in Section 6.2.1
o The proposed counter-measure for the attack against the PMTUD
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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 C
o Miscellaneous editorial changes
D.2 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 5.2
o Added Section 5.5
o Added Section 7.2
o Fixed typo in the ICMP types, in several places
o Fixed typo in the TCP sequence number check formula
o Miscellaneous editorial changes
D.3 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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