tsvwg M. Larsen
Internet-Draft Ericsson
Intended status: Standards Track F. Gont
Expires: August 15, 2007 UTN/FRH
February 11, 2007
Port Randomization
draft-larsen-tsvwg-port-randomization-01
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Abstract
Recently, awareness has been raised about a number of "blind" attacks
that can be performed against the Transmission Control Protocol (TCP)
and similar protocols. The consequences of these attacks range from
throughput-reduction to broken connections or data corruption. These
attacks rely on the attacker's ability to guess or know the four-
tuple (Source Address, Destination Address, Source port, Destination
Port) that identifies the transport protocol instance to be attacked.
This document describes a simple and efficient method for random
selection of the client port number, such that the possibility of an
attacker guessing the exact value is reduced. While this is not a
replacement for cryptographic methods, the described port number
randomization algorithms provide improved security/obfuscation with
very little effort and without any key management overhead.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Ephemeral Ports . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Traditional Ephemeral Port Range . . . . . . . . . . . . . 4
2.2. Ephemeral port selection . . . . . . . . . . . . . . . . . 4
3. Randomizing the Ephemeral Ports . . . . . . . . . . . . . . . 6
3.1. Ephemeral port number range . . . . . . . . . . . . . . . 6
3.2. Ephemeral Port Randomization Algorithms . . . . . . . . . 6
3.3. Secret Key . . . . . . . . . . . . . . . . . . . . . . . . 11
3.4. Choosing Algorithm . . . . . . . . . . . . . . . . . . . . 11
4. Security Considerations . . . . . . . . . . . . . . . . . . . 13
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 14
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
6.1. Normative References . . . . . . . . . . . . . . . . . . . 15
6.2. Informative References . . . . . . . . . . . . . . . . . . 15
Appendix A. Survey of the algorithms in use by some popular
implementations . . . . . . . . . . . . . . . . . . . 17
A.1. FreeBSD . . . . . . . . . . . . . . . . . . . . . . . . . 17
A.2. Linux . . . . . . . . . . . . . . . . . . . . . . . . . . 17
A.3. NetBSD . . . . . . . . . . . . . . . . . . . . . . . . . . 17
A.4. OpenBSD . . . . . . . . . . . . . . . . . . . . . . . . . 17
Appendix B. Changes from previous versions of the draft . . . . . 18
B.1. Changes from draft-larsen-tsvwg-port-randomization-00 . . 18
B.2. Changes from draft-larsen-tsvwg-port-randomisation-00 . . 18
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 19
Intellectual Property and Copyright Statements . . . . . . . . . . 20
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1. Introduction
Recently, awareness has been raised about a number of "blind" attacks
that can be performed against the Transmission Control Protocol (TCP)
[RFC0793] and similar protocols. The consequences of these attacks
range from throughput-reduction to broken connections or data
corruption [I-D.ietf-tcpm-icmp-attacks] [I-D.ietf-tcpm-tcp-antispoof]
[Watson].
All these attacks rely on the attacker's ability to guess or know the
four-tuple (Source Address, Source port, Destination Address,
Destination Port) that identifies the transport protocol instance to
be attacked.
Services are usually located at fixed, 'well-known' ports [IANA] at
the host supplying the service (the server). Client applications
connecting to any such service will contact the server by specifying
the server IP address and service port number. The IP address and
port number of the client are normally left unspecified by the client
application and thus chosen automatically by the client networking
stack. Ports chosen automatically by the networking stack are known
as ephemeral ports [Stevens].
While the server IP address and well-known port and the client IP
address may be available to the attacker, the ephemeral port of the
client is usually unknown and must be guessed.
This document describes a method for random selection of the client
ephemeral port, thereby reducing the possibility of an off-path
attacker guessing the exact value. This is not a replacement for
cryptographic methods such as IPsec [RFC4301] or the TCP MD5
signature option [RFC2385]. However, the proposed algorithm provides
improved obfuscation with very little effort and without any key
management overhead.
The mechanism described is a local modification that may be
incrementally deployed, and does not violate the specifications of
any of the transport protocols that may benefit from it [RFC0793]
[RFC0768] [RFC2960] [RFC4340].
Since the mechanism is an obfuscation technique, focus has been on a
reasonable compromise between level of obfuscation and ease of
implementation. Thus the algorithm must be computationally
efficient, and not require substantial data structures.
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2. Ephemeral Ports
2.1. Traditional Ephemeral Port Range
The Internet Assigned Numbers Authority (IANA) assigns the unique
parameters and values used in protocols developed by the Internet
Engineering Task Force (IETF), including well-known ports [IANA].
IANA has traditionally reserved the following use of the 16-bit port
range of TCP and UDP:
o The Well Known Ports, 0 through 1023.
o The Registered Ports, 1024 through 49151
o The Dynamic and/or Private Ports, 49152 through 65535
The range for assigned ports managed by the IANA is 0-1023, with the
remainder being registered by IANA but not assigned.
The ephemeral port range has traditionally consisted of the 49152-
65535 range.
2.2. Ephemeral port selection
As each communication instance is identified by the four-tuple {local
IP address, local port, remote IP address, remote port}, selection
ephemeral port numbers must result in a unique four-tuple.
Selection of ephemeral ports such that they result in unique four-
tuples is handled by some operating systems by having a global 'next
ephemeral port' variable that is equal to the previously chosen
ephemeral port + 1, i.e. the selection process is:
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next_ephemeral_port = 1024; /*initialization, could be random */
/* Ephemeral port selection */
count = max_ephemeral - min_ephemeral + 1;
do {
port = next_ephemeral;
if (next_ephemeral == max_ephemeral) {
next_ephemeral = min_ephemeral;
} else {
next_ephemeral++;
}
if (four-tuple is unique)
return port;
} while (count > 0);
return ERROR;
Figure 1
We will refer to this as 'Algorithm 1'.
This algorithm works well provided that the number of connections
(globally, across all four-tuples) that has a life-time longer than
it takes to exhaust the total ephemeral port range is small, so that
four-tuple collisions are rare.
However, this method has the drawback that the 'next_ephemeral'
variable and thus the ephemeral port range is shared between all
connections and the next ports chosen by the client are easy to
predict. If an attacker operates an "innocent" server to which the
client connects, it is easy to obtain a reference point for the
current value of the 'next_ephemeral' variable.
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3. Randomizing the Ephemeral Ports
3.1. Ephemeral port number range
As mentioned in Section 2.1, the ephemeral port range has
traditionally consisted of the 49152-65535 range. However, it should
also include the range 1024-49151 range.
Since this range includes user-specific server ports, this may not
always be possible, though. A possible workaround for this potential
problem would be to maintain an array of bits, in which each bit
would correspond to each of the port numbers in the range 1024-65535.
A bit set to 0 would indicate that the corresponding port is
available for allocation, while a bit set to one would indicate that
the port is reserved and therefore cannot be allocated. Thus, before
allocating a port number, the ephemeral port selection function would
check this array of bits, avoiding the allocation of ports that may
be needed for specific applications.
Transport protocols SHOULD use the largest possible port range, since
this improves the obfuscation provided by randomizing the ephemeral
ports.
3.2. Ephemeral Port Randomization Algorithms
In order to address the security issues discussed in Section 2.2, a
number of systems have implemented simple ephemeral port number
randomization, as follows:
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next_ephemeral = min_ephemeral + random()
% (max_ephemeral - min_ephemeral + 1);
count = max_ephemeral - min_ephemeral + 1;
do {
if(four-tuple is unique)
return next_ephemeral;
if (next_ephemeral == max_ephemeral) {
next_ephemeral = min_ephemeral;
} else {
next_ephemeral_port++;
}
count--;
} while (count > 0);
return ERROR;
Figure 2
We will refer to this algorithm as 'Algorithm 2'.
Since the the chosen port may already be in use with identical IP
addresses and server port, the resulting four-tuple might not be
unique. Therefore, multiple ports may have to be tried and verified
against all existing connections before a port can be chosen.
Although carefully chosen random sources and optimized four-tuple
lookup mechanisms (e.g., optimized through hashing), will mitigate
the cost of this verification, some systems may still not want to
incur this unknown search time.
Systems that may be specially susceptible to this kind of repeated
four-tuple collisions are those that create many connections from a
single local IP address to a single service (i.e. both IP addresses
and server port are fixed). Gateways such as proxy servers are an
example of such a system.
Since this algorithm performs a completely random port selection
(i.e., without taking into account the port numbers previously
chosen), it has the potential of reusing port numbers too quickly.
Even if a given four-tuple is verified to be unique by the port
selection algorithm, there four-tuple might still be in use at the
remote system. In such a scenario, the connection request would
possible fail ([Silbersack] describes this problem in detail).
Therefore, it is desirable to keep the port reuse frequency as low as
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possible.
We would like to achieve the port reuse properties of Algorithm 1,
while at the same time achieve the obfuscation properties of
Algorithm 2.
Ideally, we would like a 'next_ephemeral' value for each set of
(local IP address, remote IP addresses, remote port), so that the
port reuse frequency is the lowest possible. Each of these
'next_ephemeral' variables should be initialized with random values
within the ephemeral port range and would thus separate the ephemeral
port ranges of the connections entirely. Since we do not want to
maintain in memory all these 'next_ephemeral' values, we propose an
offset function F(), that can be computed from the local IP address,
remote IP address, remote port and a secret key. F() will yield
(practically) different values for each set of arguments, i.e.:
/* Initialization code */
next_ephemeral = 0; /* could be random */
/* Ephemeral port selection */
offset = F(local_IP, remote_IP, remote_port, secret_key);
count = max_ephemeral - min_ephemeral + 1;
do {
port = min_ephemeral + (next_ephemeral + offset)
% (max_ephemeral - min_ephemeral + 1);
next_ephemeral++;
count--;
if(four-tuple is unique)
return port;
} while (count > 0);
return ERROR;
Figure 3
We will refer to this algorithm as 'Algorithm 3'.
In other words, the function F() provides a per-connection fixed
offset of the global ephemeral port range controlled by
'next_ephemeral'. Both the 'offset' and 'next_ephemeral' variables
may take any value within the storage type range since we are
restricting the resulting port similar to that shown in Figure 2.
This allows us to simply increment the 'next_ephemeral' variable and
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rely on the unsigned integer to simply wrap-around.
The function F() should be a cryptographic hash function like MD5
[RFC1321]. The function should use both IP addresses, the remote
port and a secret key value to compute the offset. The remote IP
address is the primary separator and must be included in the offset
calculation. The local IP address and remote port may in some cases
be constant and not improve the connection separation, however, they
should also be included in the offset calculation.
Cryptographic algorithms stronger than e.g. MD5 should not be
necessary, given that port randomization is simply an obfuscation
technique. The secret should be chosen as random as possible, see
[RFC4086] for recommendations on choosing secrets.
Note that on multiuser systems, the function F() could include user
specific information, thereby providing protection not only on a host
to host basis, but on a user to service basis.
A tradeoff between maintaining a single global 'next_ephemeral'
variable and maintaining 2**N 'next_ephemeral' variables (where N is
the width of of the result of F()) could be achieved as follows. The
system would keep an array of, TABLE_LENGTH short integers, which
would provide a separation of the increment of the 'next_ephemeral'
variable. This improvement could be incorporated into Algorithm 3 as
follows:
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/* Initialization code */
for(i = 0; i < TABLE_LENGTH; i++) /* Initialization code */
table[i] = random % 65536;
/* Ephemeral port selection */
offset = F(local_IP, remote_IP, remote_port, secret_key);
index = G(offset);
count = max_ephemeral - min_ephemeral + 1;
do {
port = min_ephemeral + (offset + table[index])
% (max_ephemeral - min_ephemeral + 1);
table[index]++;
count--;
if(four-tuple is unique)
return port;
} while (count > 0);
return ERROR;
Figure 4
'table[]' could be initialized with random values, as indicated by
the initialization code in Figure 4. G() would return a value
between 0 and (TABLE_LENGTH-1) taking 'offset' as its input. G()
could, for example, perform exclusive-or (xor) operation between all
the bytes in 'offset', or could be another cryptographic hash
function such as that used in F().
The array 'table[]' assures that succesive connections to the same
end-point will use increasing ephemeral port numbers. However,
incrementation of the port numbers is separated into TABLE_LENGTH
different spaces, and thus the port reuse frequency will be
(probabilistically) lower than that of Algorithm 2. That is, a
connection established for a given for-tuple will not necessarily
cause the 'next_ephemeral' variable corresponding to other four-
tuples to be incremented.
It is interesting to note that the size of 'table[]' does not limit
the number of different port sequences, but rather separates the
*increments* into TABLE_LENGTH different spaces. The actual port
sequence will result from adding the corresponding entry of 'table[]'
to the variable 'offset', which actually selects the actual port
sequence (as in Algorithm 3).
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3.3. Secret Key
Every complex manipulation (like MD5) is no more secure than the
input values, and in the case of ephemeral ports, the secret key. If
an attacker is aware of which cryptographic hash function is being
used by the victim (which we should expect), and the attacker can
obtain enough material (e.g. ephemeral ports chosen by the victim),
the attacker may simply search the entire secret key space to find
matches.
To protect against this, the secret key should be of a reasonable
length. Key-lengths of 32-bits should be adequate, since a 32-bit
secret would result in approximately 65k possible secrets if the
attacker is able to obtain a single ephemeral port (assuming a good
hash function). If the attacker is able to obtain more ephemeral
ports, key-lengths of 64-bits or more should be used.
Another possible mechanism for protecting the secret key is to change
it after some time. If the host platform is capable of producing
reasonable good random data, the secret key can be changed.
Changing the secret will cause abrupt shifts in the chosen ephemeral
ports, and consequently collisions may occur. Thus the change in
secret key should be done with consideration and could be performed
whenever one of the following events occur:
o Some predefined/random time has expired.
o The secret has been used N times (i.e. we consider it insecure).
o There are few active connections (i.e., possibility of collision
is low).
o There is little traffic (the performance overhead of collisions is
tolerated).
o There is enough random data available to change the secret key
(pseudo-random changes should not be done).
3.4. Choosing Algorithm
Algorithm 1 is the traditional ephemeral port selection algorithm
implemented in BSD-derived systems. It generates a global sequence
of ephemeral port numbers, which makes it trivial for an attacker to
predict the port number that will be used for a future transport
protocol instance.
Algorithm 2 has the advantage that it provides complete
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randomization. However, it may increase the chances of port number
collisions, which could lead to failure of the connection
establishment attempts.
Algorithm 3 provides complete separation in local and remote IP
addresses and remote port space, and only limited separation in other
dimensions (See Section Section 3.3), and thus scales better than
Algorithm 2. However, implementations should consider the
performance impact of computing the cryptographic hash used for the
offset.
Algorithm 4 improves Algorithm 3, usually leading to a lower port
reuse frequency, at the expense of more processor cycles used for
computing G(), and additional kernel memory for storing the array
'table[]'.
Finally, a special case that precludes the utilization of Algorithm 3
and Algorithm 4 should be analyzed. There exist some applications
that contain the following code sequence:
s = socket();
bind(s, IP_address, port = *);
Figure 5
This code sequence results in the selection of an ephemeral port
number. However, as neither the remote IP address nor the remote TCP
port will be available to the ephemeral port selection function, the
hash function F() used in Algorithm 3 and Algorithm 4 will not have
all the required arguments, and thus the result of the hash function
will be impossible to compute.
Transport protocols implementing Algorithm 3 or Algorithm 4 should
consider using Algorithm 2 when facing the scenario just described.
This policy has been implemented by Linux [Linux].
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4. Security Considerations
Randomizing ports is no replacement for cryptographic mechanisms,
such as IPsec [RFC4301].
An eavesdropper, which can monitor the packets that correspond to the
connection to be attacked could learn the IP addresses and port
numbers in use (and also sequence numbers etc.) and easily attack the
connection. Randomizing ports does not provide any additional
protection against this kind of attacks. In such situations, proper
authentication mechanisms such as those described in [RFC4301] should
be used.
If the local offset function F() results in identical offsets for
different inputs, the port-offset mechanism proposed in this document
has no or reduced effect.
If random numbers are used as the only source of the secret key, they
must be chosen in accordance with the recommendations given in
[RFC4086].
If all ports available in the ephemeral port range are in use,
randomization provides no obfuscation.
If an attacker uses dynamically assigned IP addresses, the current
ephemeral port offset (Algorithm 3 and Algorithm 4) for a given four-
tuple can be sampled and subsequently be used to attack an innocent
peer reusing this address. However, this is only possible until a
re-keying happens as described above. Also, since ephemeral ports
are only used on the client side (e.g. the one initiating the
connection), both the attacker and the new peer need to act as
servers in the scenario just described. While servers using dynamic
IP addresses exist, they are not very common and with an appropriate
re-keying mechanism the effect of this attack is limited.
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5. Acknowledgements
The offset function was inspired by the mechanism proposed by Steven
Bellovin in [RFC1948] for defending against TCP sequence number
attacks.
The authors would like to thank Alfred Hoenes and Carlos Pignataro
for their valuable feedback on earlier versions of this document.
The authors would like to thank FreeBSD's Mike Silbersack for a very
fruitful discussion about ephemeral port selection techniques.
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6. References
6.1. Normative References
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, September 1981.
[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
August 1980.
[RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
April 1992.
[RFC4086] Eastlake, D., Schiller, J., and S. Crocker, "Randomness
Requirements for Security", BCP 106, RFC 4086, June 2005.
[RFC1948] Bellovin, S., "Defending Against Sequence Number Attacks",
RFC 1948, May 1996.
[RFC2385] Heffernan, A., "Protection of BGP Sessions via the TCP MD5
Signature Option", RFC 2385, August 1998.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
[RFC2960] Stewart, R., Xie, Q., Morneault, K., Sharp, C.,
Schwarzbauer, H., Taylor, T., Rytina, I., Kalla, M.,
Zhang, L., and V. Paxson, "Stream Control Transmission
Protocol", RFC 2960, October 2000.
[RFC4340] Kohler, E., Handley, M., and S. Floyd, "Datagram
Congestion Control Protocol (DCCP)", RFC 4340, March 2006.
6.2. Informative References
[Watson] Watson, P., "Slipping in the Window: TCP Reset attacks",
december 2003.
[IANA] "IANA Port Numbers",
<http://www.iana.org/assignments/port-numbers>.
[Stevens] Stevens, W., "Unix Network Programming, Volume 1:
Networking APIs: Socket and XTI, Prentice Hall", 1998.
[Silbersack]
Silbersack, M., "Improving TCP/IP security through
randomization without sacrificing interoperability.",
EuroBSDCon 2005 Conference , 2005.
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[I-D.ietf-tcpm-tcp-antispoof]
Touch, J., "Defending TCP Against Spoofing Attacks",
draft-ietf-tcpm-tcp-antispoof-05 (work in progress),
October 2006.
[I-D.ietf-tcpm-icmp-attacks]
Gont, F., "ICMP attacks against TCP",
draft-ietf-tcpm-icmp-attacks-01 (work in progress),
October 2006.
[Linux] The Linux Project, "http://www.kernel.org".
[FreeBSD] The FreeBSD Project, "http://www.freebsd.org".
[NetBSD] The NetBSD Project, "http://www.netbsd.org".
[OpenBSD] The OpenBSD Project, "http://www.openbsd.org".
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Appendix A. Survey of the algorithms in use by some popular
implementations
A.1. FreeBSD
FreeBSD implements Algorithm 2. with a 'min_port' of 49152 and a
'max_port' of 65535. If the selected port number is in use, the next
available port number is tried next [FreeBSD].
A.2. Linux
Linux implements Algorithm 3. If the algorithm is faced with the
corner-case scenario described in Section 3.4, Algorithm 2 is used
instead [Linux].
A.3. NetBSD
NetBSD does not randomize ehemeral port numbers. It selects
ephemeral port numbers from the range 49152-65535, starting from port
65535, and decreasing the port number for each ephemeral port number
selected [NetBSD].
A.4. OpenBSD
OpenBSD implements Algorithm 2. with a 'min_port' of 1024 and a
'max_port' of 49151. If the selected port number is in use, the next
available port number is tried next [OpenBSD].
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Appendix B. Changes from previous versions of the draft
B.1. Changes from draft-larsen-tsvwg-port-randomization-00
o Fixed a bug in expressions used to calculate number of ephemeral
ports
o Added a survey of the algorithms in use by popular TCP
implementations
o The whole document was reorganizaed
o Miscellaneous editorial changes
B.2. Changes from draft-larsen-tsvwg-port-randomisation-00
o Document resubmitted after original document by M. Larsen expired
in 2004
o References were included to current WG documents of the TCPM WG
o The document was made more general, to apply to all transport
protocols
o Miscellaneous editorial changes
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Authors' Addresses
Michael Vittrup Larsen
Ericsson
Skanderborgvej 232
Aarhus DK-8260
Denmark
Phone: +45 8938 5100
Email: michael.vittrup.larsen@ericsson.com
Fernando Gont
Universidad Tecnologica Nacional / Facultad Regional Haedo
Evaristo Carriego 2644
Haedo, Provincia de Buenos Aires 1706
Argentina
Phone: +54 11 4650 8472
Email: fernando@gont.com.ar
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