Transport Area Working Group M. Larsen
(tsvwg) TietoEnator
Internet-Draft F. Gont
Intended status: BCP UTN/FRH
Expires: June 3, 2010 November 30, 2009
Port Randomization
draft-ietf-tsvwg-port-randomization-05
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 five-
tuple (Protocol, Source Address, Destination Address, Source Port,
Destination Port) that identifies the transport protocol instance to
be attacked. This document describes a number of simple and
efficient methods for the 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
for protecting the connection, the described port number obfuscation
algorithms provide improved security/obfuscation with very little
effort and without any key management overhead. The algorithms
described in this document are local policies that may be
incrementally deployed, and that do not violate the specifications of
any of the transport protocols that may benefit from them, such as
TCP, UDP, UDP-lite, SCTP, DCCP, and RTP.
Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79.
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The list of current Internet-Drafts can be accessed at
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The list of Internet-Draft Shadow Directories can be accessed at
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This Internet-Draft will expire on June 3, 2010.
Copyright Notice
Copyright (c) 2009 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the BSD License.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5
2. Ephemeral Ports . . . . . . . . . . . . . . . . . . . . . . . 7
2.1. Traditional Ephemeral Port Range . . . . . . . . . . . . . 7
2.2. Ephemeral port selection . . . . . . . . . . . . . . . . . 7
2.3. Collision of connection-id's . . . . . . . . . . . . . . . 8
3. Obfuscating the Ephemeral Ports . . . . . . . . . . . . . . . 10
3.1. Characteristics of a good ephemeral port obfuscation
algorithm . . . . . . . . . . . . . . . . . . . . . . . . 10
3.2. Ephemeral port number range . . . . . . . . . . . . . . . 11
3.3. Ephemeral Port Obfuscation Algorithms . . . . . . . . . . 12
3.3.1. Algorithm 1: Simple port randomization algorithm . . . 12
3.3.2. Algorithm 2: Another simple port randomization
algorithm . . . . . . . . . . . . . . . . . . . . . . 14
3.3.3. Algorithm 3: Simple hash-based algorithm . . . . . . . 14
3.3.4. Algorithm 4: Double-hash obfuscation algorithm . . . . 17
3.3.5. Algorithm 5: Random-increments port selection
algorithm . . . . . . . . . . . . . . . . . . . . . . 18
3.4. Secret-key considerations for hash-based port
obfuscation algorithms . . . . . . . . . . . . . . . . . . 20
3.5. Choosing an ephemeral port obfuscation algorithm . . . . . 21
4. Port obfuscation and Network Address Port Translation
(NAPT) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
5. Security Considerations . . . . . . . . . . . . . . . . . . . 24
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 25
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 26
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 27
8.1. Normative References . . . . . . . . . . . . . . . . . . . 27
8.2. Informative References . . . . . . . . . . . . . . . . . . 27
Appendix A. Survey of the algorithms in use by some popular
implementations . . . . . . . . . . . . . . . . . . . 30
A.1. FreeBSD . . . . . . . . . . . . . . . . . . . . . . . . . 30
A.2. Linux . . . . . . . . . . . . . . . . . . . . . . . . . . 30
A.3. NetBSD . . . . . . . . . . . . . . . . . . . . . . . . . . 30
A.4. OpenBSD . . . . . . . . . . . . . . . . . . . . . . . . . 30
A.5. OpenSolaris . . . . . . . . . . . . . . . . . . . . . . . 30
Appendix B. Changes from previous versions of the draft (to
be removed by the RFC Editor before publication
of this document as a RFC . . . . . . . . . . . . . . 31
B.1. Changes from draft-ietf-tsvwg-port-randomization-04 . . . 31
B.2. Changes from draft-ietf-tsvwg-port-randomization-03 . . . 31
B.3. Changes from draft-ietf-tsvwg-port-randomization-02 . . . 31
B.4. Changes from draft-ietf-tsvwg-port-randomization-01 . . . 31
B.5. Changes from draft-ietf-tsvwg-port-randomization-00 . . . 31
B.6. Changes from draft-larsen-tsvwg-port-randomization-02 . . 31
B.7. Changes from draft-larsen-tsvwg-port-randomization-01 . . 32
B.8. Changes from draft-larsen-tsvwg-port-randomization-00 . . 32
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B.9. Changes from draft-larsen-tsvwg-port-randomisation-00 . . 32
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 33
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1. Introduction
Recently, awareness has been raised about a number of "blind" attacks
(i.e., attacks that can be performed without the need to sniff the
packets that correspond to the transport protocol instance to be
attacked) 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] [RFC4953] [Watson].
All these attacks rely on the attacker's ability to guess or know the
five-tuple (Protocol, 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 accurately guessed by an attacker, the ephemeral port
of the client is usually unknown and must be guessed.
This document describes a number of algorithms for the selection of
the ephemeral ports, such that the possibility of an off-path
attacker guessing the exact value is reduced. They are not a
replacement for cryptographic methods of protecting a connection such
as IPsec [RFC4301], the TCP MD5 signature option [RFC2385], or the
TCP Authentication Option [I-D.ietf-tcpm-tcp-auth-opt]. For example,
they do not provide any mitigation in those scenarios in which the
attacker is able to sniff the packets that correspond to the
transport protocol connection to be attacked. However, the proposed
algorithms provide improved obfuscation with very little effort and
without any key management overhead.
The mechanisms described in this document are local modifications
that may be incrementally deployed, and that does not violate the
specifications of any of the transport protocols that may benefit
from it, such as TCP [RFC0793], UDP [RFC0768], SCTP [RFC4960], DCCP
[RFC4340], UDP-lite [RFC3828], and RTP [RFC3550].
Since these mechanisms are obfuscation techniques, focus has been on
a reasonable compromise between the level of obfuscation and the ease
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of implementation. Thus the algorithms must be computationally
efficient, and not require substantial state.
We note that while the technique of mitigating "blind" attacks by
obfuscating the ephemeral port election is well-known as "port
randomization", the goal of the algorithms described in this document
is to reduce the chances of an attacker guessing the ephemeral ports
selected for new connections, rather than to actually produce
mathematically random sequences of ephemeral ports.
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 [RFC2119].
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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 defined by IANA has traditionally consisted
of the 49152-65535 range.
2.2. Ephemeral port selection
As each communication instance is identified by the five-tuple
{protocol, local IP address, local port, remote IP address, remote
port}, the selection of ephemeral port numbers must result in a
unique five-tuple.
Selection of ephemeral ports such that they result in unique five-
tuples is handled by some implementations by having a per-protocol
global 'next_ephemeral' variable that is equal to the previously
chosen ephemeral port + 1, i.e. the selection process is:
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/* Initialization at system boot time. Could be random */
next_ephemeral = min_ephemeral;
/* Ephemeral port selection function */
count = max_ephemeral - min_ephemeral + 1;
do {
port = next_ephemeral;
if (next_ephemeral == max_ephemeral) {
next_ephemeral = min_ephemeral;
} else {
next_ephemeral++;
}
if (five-tuple is unique)
return port;
count--;
} while (count > 0);
return ERROR;
Figure 1
This algorithm works well provided that the number of connections for
a each transport protocol that have a life-time longer than it takes
to exhaust the total ephemeral port range is small, so that five-
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. Additionally, if an
attacker could force a client to periodically establish a new TCP
connection to an attacker controlled machine (or through an attacker
observable routing path), the attacker could subtract consecutive
source port values to obtain the number of outoing TCP connections
established globally by the target host within that time period (up
to wrap-around issues and 5-tuple collisions, of course).
2.3. Collision of connection-id's
While it is possible for the ephemeral port selection algorithm to
verify that the selected port number results in connection-id that is
not currently in use at that system, the resulting connection-id may
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still be in use at a remote system. For example, consider a scenario
in which a client establishes a TCP connection with a remote web
server, and the web server performs the active close on the
connection. While the state information for this connection will
disappear at the client side (that is, the connection will be moved
to the fictional CLOSED state), the connection-id will remain in the
TIME-WAIT state at the web server for 2*MSL (Maximum Segment
Lifetime). If the same client tried to create a new incarnation of
the previous connection (that is, a connection with the same
connection-id as the one in the TIME_WAIT state at the server), a
connection-id "collision" would occur. The effect of these
collisions range from connection-establishment failures to TIME-WAIT
state assassination (with the potential of data corruption)
[RFC1337]. In scenarios in which a specific client establishes TCP
connections with a specific service at a server, these problems
become evident. Therefore, an ephemeral port selection algorithm
should ideally minimize the rate of connection-id collisions.
A simple approach to minimize the rate of these collisions would be
to choose port numbers incrementally, so that a given port number
would not be reused until the rest of the port numbers in ephemeral
port range have been used for a transport protocol instance.
However, if a single global variable were used to keep track of the
last ephemeral port selected, ephemeral port numbers would be
trivially predictable, thus making it easier for an off-path attacker
to "guess" the connection-id in use by a target connection.
Section 3.3.3 and Section 3.3.4 describe algorithms that select port
numbers incrementally, while still making it difficult for an off-
path attacker to predict the ephemeral ports used for future
connections.
Another possible approach to minimize the rate of collisions of
connection-id's would be for both end-points of a TCP connection to
keep state about recent connections (e.g., have both end-points end
up in the TIME-WAIT state).
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3. Obfuscating the Ephemeral Ports
3.1. Characteristics of a good ephemeral port obfuscation algorithm
There are a number of factors to consider when designing a policy of
selection of ephemeral ports, which include:
o Minimizing the predictability of the ephemeral port numbers used
for future connections.
o Minimizing collisions of connection-id's
o Avoiding conflict with applications that depend on the use of
specific port numbers.
Given the goal of improving the transport protocol's resistance to
attack by obfuscation of the five-tuple that identifies a transport-
protocol instance, it is key to minimize the predictability of the
ephemeral ports that will be selected for new connections. While the
obvious approach to address this requirement would be to select the
ephemeral ports by simply picking a random value within the chosen
port number range, this straightforward policy may lead to collisions
of connection-id's, which could lead to the interoperability problems
(namely delays in the establishment of new connections, failures in
connection-establishment, or data curruption) discussed in
Section 2.3. As discussed in Section 1, it is worth noting that
while the technique of mitigating "blind" attacks by obfuscating the
ephemeral port election is well-known as "port randomization", the
goal of the algorithms described in this document is to reduce the
chances of an attacker guessing the ephemeral ports selected for new
connections, rather than to actually produce sequences of
mathematically random ephemeral port numbers.
It is also worth noting that, provided adequate algorithms are in
use, the larger the range from which ephemeral pots are selected, the
smaller the chances of an attacker are to guess the selected port
number.
In scenarios in which a specific client establishes connections with
a specific service at a server, the problems described in Section 2.3
become evident. A good algorithm to minimize the collisions of
connection-id's would consider the time a given five-tuple was last
used, and would avoid reusing the last recently used five-tuples. A
simple approach to minimize the rate of collisions would be to choose
port numbers incrementally, so that a given port number would not be
reused until the rest of the port numbers in the ephemeral port range
have been used for a transport protocol instance. However, if a
single global variable were used to keep track of the last ephemeral
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port selected, ephemeral port numbers would be trivially predictable.
It is important to note that a number of applications rely on binding
specific port numbers that may be within the ephemeral ports range.
If such an application was run while the corresponding port number
was in use, the application would fail. Therefore, transport
protocols should avoid using those port numbers as ephemeral ports.
Port numbers that are currently in use by a TCP in the LISTEN state
should not be allowed for use as ephemeral ports. If this rule is
not complied, an attacker could potentially "steal" an incoming
connection to a local server application by issuing a connection
request to the victim client at roughly the same time the client
tries to connect to the victim server application [CPNI-TCP]
[I-D.gont-tcp-security]. If the SYN segment corresponding to the
attacker's connection request and the SYN segment corresponding to
the victim client "cross each other in the network", and provided the
attacker is able to know or guess the ephemeral port used by the
client, a TCP simultaneous open scenario would take place, and the
incoming connection request sent by the client would be matched with
the attacker's socket rather than with the victim server
application's socket.
It should be noted that most applications based on popular
implementations of TCP API (such as the Sockets API) perform "passive
opens" in three steps. Firstly, the application obtains a file
descriptor to be used for inter-process communication (e.g., by
issuing a socket() call). Secondly, the application binds the file
descriptor to a local TCP port number (e.g., by issuing a bind()
call), thus creating a TCP in the fictional CLOSED state. Thirdly,
the aforementioned TCP is put in the LISTEN state (e.g., by issuing a
listen() call). As a result, with such an implementation of the TCP
API, even if port numbers in use for TCPs in the LISTEN state were
not allowed for use as ephemeral ports, there is a window of time
between the second and the third steps in which an attacker could be
allowed to select a port number that would be later used for
listening to incoming connections. Therefore, these implementations
of the TCP API should enforce a stricter requirement for the
allocation of port numbers: port numbers that are in use by a TCP in
the LISTEN or CLOSED states should not be allowed for allocation as
ephemeral ports [CPNI-TCP] [I-D.gont-tcp-security].
3.2. 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.
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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 a local list of the port numbers that
should not be allocated as ephemeral ports. Thus, before allocating
a port number, the ephemeral port selection function would check this
list, 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 the ephemeral port
selection algorithms.
3.3. Ephemeral Port Obfuscation Algorithms
Transport protocols SHOULD obfuscate the allocatation of their
ephemeral ports, since this help to mitigate a number of attacks that
depend on the attacker's ability to guess or know the five-tuple that
identifies the transport protocol instance to be attacked.
The following subsections describe a number of algorithms that could
be implemented in order to obfuscate the selection of ephemeral port
numbers.
3.3.1. Algorithm 1: Simple port randomization algorithm
In order to address the security issues discussed in Section 1 and
Section 2.2, a number of systems have implemented simple ephemeral
port number randomization, as follows:
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/* Ephemeral port selection function */
num_ephemeral = max_ephemeral - min_ephemeral + 1;
next_ephemeral = min_ephemeral + (random() % num_ephemeral);
count = num_ephemeral;
do {
if(five-tuple is unique)
return next_ephemeral;
if (next_ephemeral == max_ephemeral) {
next_ephemeral = min_ephemeral;
} else {
next_ephemeral++;
}
count--;
} while (count > 0);
return ERROR;
Figure 2
We will refer to this algorithm as 'Algorithm 1'.
Since the initially chosen port may already be in use with identical
IP addresses and server port, the resulting five-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.
Web proxy servers, NAPTs [RFC2663], and other middle-boxes aggregate
multiple peers into the same port space and thus increse the
population of used ephemeral ports, and hence the chances of
collisions of connection-id's. However, [Allman] has shown that at
least in the network scenarios used for measuring the collision
properties of the algorithms described in this document, the
collision rate resulting from the use of the aforementioned middle-
boxes is nevertheless very low.
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,
thus possibly leading to collisions of connection-id's. Even if a
given five-tuple is verified to be unique by the port selection
algorithm, the five-tuple might still be in use at the remote system.
In such a scenario, the connection request could possibly fail
([Silbersack] describes this problem for the TCP case).
This algorithm selects ephemeral port numbers randomly and thus
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reduces the chances of an attacker of guessing the ephemeral port
selected for a target connection. Additionally, it prevents
attackers from obtaining the number of outgoing connections
established by the client in some period of time.
3.3.2. Algorithm 2: Another simple port randomization algorithm
Another algorithm for selecting a random port number is shown in
Figure 3, in which in the event a local connection-id collision is
detected, another port number is selected randomly, as follows:
/* Ephemeral port selection function */
num_ephemeral = max_ephemeral - min_ephemeral + 1;
next_ephemeral = min_ephemeral + (random() % num_ephemeral);
count = num_ephemeral;
do {
if(five-tuple is unique)
return next_ephemeral;
next_ephemeral = min_ephemeral + (random() % num_ephemeral);
count--;
} while (count > 0);
return ERROR;
Figure 3
We will refer to this algorithm as 'Algorithm 2'. This algorithm
might be unable to select an ephemeral port (i.e., return "ERROR")
even if there are port numbers that would result in unique five-
tuples, when there are a large number of port numbers already in use.
However, the results in [Allman] have shown that in common scenarios,
one port choice is enough, and in most cases where more than one
choice is needed two choices suffice. Therefore, in those scenarios
this would not be problem.
3.3.3. Algorithm 3: Simple hash-based algorithm
We would like to achieve the port reuse properties of the traditional
BSD port selection algorithm (described in Section 2.2), while at the
same time achieve the obfuscation properties of Algorithm 1 and
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
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'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 at system boot time. Could be random. */
next_ephemeral = 0;
/* Ephemeral port selection function */
num_ephemeral = max_ephemeral - min_ephemeral + 1;
offset = F(local_IP, remote_IP, remote_port, secret_key);
count = num_ephemeral;
do {
port = min_ephemeral +
(next_ephemeral + offset) % num_ephemeral;
next_ephemeral++;
if(five-tuple is unique)
return port;
count--;
} while (count > 0);
return ERROR;
Figure 4
We will refer to this algorithm as 'Algorithm 3'.
In other words, the function F() provides a per-connection fixed
offset within the global ephemeral port range. 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 3. This allows us to simply increment the
'next_ephemeral' variable and 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
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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 Algorithm #3 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. In fact, any
identifier of the remote entity could be used, depending on
availability an the granularity requested. With SCTP both hostnames
and alternative IP addresses may be included in the association
negotiation and either of these could be used in the offset function
F().
When multiple unique identifiers are available, any of these can be
chosen as input to the offset function F() since they all uniquely
identify the remote entity. However, in cases like SCTP where the
ephemeral port must be unique across all IP address permutations, we
should ideally always use the same IP address to get a single
starting offset for each association negotiation from a given remote
entity to minimize the possibility of collisions. A simple numerical
sorting of the IP addresses and always using the numerically lowest
could achieve this. However, since most protocols most likely will
report the same IP addresses in the same order in each association
setup, this sorting is most likely not necessary and the 'first one'
can simply be used.
The ability of hostnames to uniquely define hosts can be discussed,
and since SCTP always includes at least one IP address, we recommend
to use this as input to the offset function F() and ignore hostnames
chunks when searching for ephemeral ports.
It should be note that, as this algorithm uses a global counter
("next_ephemeral") for selecting ephemeral ports, if an attacker
could force a client to periodically establish a new TCP connection
to an attacker controlled machine (or through an attacker observable
routing path), the attacker could subtract consecutive source port
values to obtain the number of outoing TCP connections established
globally by the target host within that time period (up to wrap-
around issues and 5-tuple collisions, of course).
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3.3.4. Algorithm 4: Double-hash obfuscation algorithm
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:
/* Initialization at system boot time */
for(i = 0; i < TABLE_LENGTH; i++)
table[i] = random() % 65536;
/* Ephemeral port selection function */
num_ephemeral = max_ephemeral - min_ephemeral + 1;
offset = F(local_IP, remote_IP, remote_port, secret_key1);
index = G(local_IP, remote_IP, remote_port, secret_key2);
count = num_ephemeral;
do {
port = min_ephemeral + (offset + table[index]) % num_ephemeral;
table[index]++;
if(five-tuple is unique)
return port;
count--;
} while (count > 0);
return ERROR;
Figure 5
We will refer to this algorithm as 'Algorithm 4'.
'table[]' could be initialized with mathematically random values, as
indicated by the initialization code in Figure 5. The function G()
should be a cryptographic hash function like MD5 [RFC1321]. It
should use both IP addresses, the remote port and a secret key value
to compute a value between 0 and (TABLE_LENGTH-1). Alternatively,
G() could take as "offset" as input, and perform the exclusive-or
(xor) operation between all the bytes in 'offset'.
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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 3. That is, a
connection established with some remote end-point will not
necessarily cause the 'next_ephemeral' variable corresponding to
other end-points 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 port sequence
will result from adding the corresponding entry of 'table[]' to the
variable 'offset', which selects the actual port sequence (as in
Algorithm 3). [Allman] has found that a TABLE_LENGTH of 10 can
result in an improvement over Algorithm 3. Further increasing the
TABLE_LENGTH will increase the obfuscation, and possibly further
decrease the collision rate.
An attacker can perform traffic analysis for any "increment space"
into which the attacker has "visibility", namely that the attacker
can force the client to establish a transport-protocol connection
whose G(offset) identifies the target "increment space". However,
the attacker's ability to perform traffic analysis is very reduced
when compared to the traditional BSD algorithm (described in
Section 2.2) and Algorithm 3. Additionally, an implementation can
further limit the attacker's ability to perform traffic analysis by
further separating the increment space (that is, using a larger value
for TABLE_LENGTH).
3.3.5. Algorithm 5: Random-increments port selection algorithm
[Allman] introduced another port obfuscation algorithm, which offers
a middle ground between the algorithms that select ephemeral ports
randomly (such as those described in Section 3.3.1 and
Section 3.3.2), and those that offer obfuscation but no randomization
(such as those described in Section 3.3.3 and Section 3.3.4). We
will refer to this algorithm as 'Algorithm 5'.
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/* Initialization code at system boot time. */
next_ephemeral = random() % 65536; /* Initialization value */
N = 500; /* Determines the tradeoff */
/* Ephemeral port selection function */
num_ephemeral = max_ephemeral - min_ephemeral + 1;
count = num_ephemeral;
do {
next_ephemeral = next_ephemeral + (random() % N) + 1;
port = min_ephemeral + (next_ephemeral % num_ephemeral);
if(five-tuple is unique)
return port;
count--;
} while (count > 0);
return ERROR;
Figure 6
This algorithm aims at at producing a monotonically-increasing
sequence to prevent the collision of connection-id's, while avoiding
the use of fixed increments, which would lead to trivially-
predictable sequences. The value "N" allows for direct control of
the tradeoff between the level of obfuscation and the port reuse
frequency. The smaller the value of "N", the more linear the more
similar this algorithm is to the traditioanl BSD port selection
algorithm (described in Section 2.2. The larger the value of "N",
the more similar this algorithm is to the algorithm described in
Section 3.3.1 of this document.
When the port numbers wrap, there's the risk of collisions of
connection-id's. Therefore, "N" should be selecting according to the
following criteria:
o It should maximize the wrapping time of the ephemeral port space
o It should minimize collisions of connection-id's
o It should maximize obfuscation
Clearly, these are competing goals, and the decision of which value
of "N" to use is a tradeoff. Therefore, the value of "N" should be
configurable so that system administrators can make the tradeoff for
themselves.
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3.4. Secret-key considerations for hash-based port obfuscation
algorithms
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
automatically.
Changing the secret will cause abrupt shifts in the chosen ephemeral
ports, and consequently collisions may occur. That is, upon changing
the secret, the "offset" value (see Figure 4 and Figure 5) used for
each destination end-point will be different from that computed with
the previous secret, ths leading to the selection of a port number
recently used for connecting to the same end-point.
Thus the change in secret key should be done with consideration and
could be performed whenever one of the following events occur:
o The system is being bootstrapped.
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).
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3.5. Choosing an ephemeral port obfuscation algorithm
[Allman] is an empyrical study of the properties of the algorithms
described in this document, which has found that all the algorithms
described in this document offer low collision rates -- at most 0.3%.
That is, in those network scenarios asessed by [Allman] all of the
algorithms described in this document perform good in terms of
collisions of connection-id's. However, these results may vary
depending on the characteristics of network traffic and the specific
network setup.
The algorithm sketched in Figure 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. However, it is very simple, and
leads to a low port resuse frequency.
Algorithm 1 and Algorithm 2 have the advantage that they provide
complete randomization. However, they may increase the chances of
port number collisions, which could lead to the failure of the
connection establishment attempt. [Allman] found that these two
algorithms show the largest collision rates (among all the algorithms
described in this document).
Algorithm 3 provides complete separation in local and remote IP
addresses and remote port space, and only limited separation in other
dimensions (see Section 3.4). 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[]'.
Algorithm 5 offers middle ground between the simple randomization
algorithms (Algorithm 1 and Algorthm 2) and the hash-based algorithms
(Algorithm 3 and Algorithm 4). The upper limit on the random
increments (the value "N" in Figure 6 controls the trade-off between
randomization and port-reuse frequency.
Finally, a special case that may preclude the utilization of
Algorithm 3 and Algorithm 4 should be analyzed. There exist some
applications that contain the following code sequence:
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s = socket();
bind(s, IP_address, port = *);
Figure 7
In some BSD-derived systems, the call to bind() will result in the
selection of an ephemeral port number. However, as neither the
remote IP address nor the remote 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 implementating Algorithm 3 or Algorithm
4 should consider using Algorithm 2 when facing the scenario just
described.
An alternative to this behavior would be to implement "lazy binding"
in response to the bind() call. That is, selection of an epphemeral
port would be delayed until, e.g., connect() or send() are called.
Thus, at that point the ephemeral port is actually selected, all the
necessary arguments for the hash function F() would be available, and
thus Algorithm 3 and Algorithm 4 could still be used in this
scenario. This policy has been implemented by Linux [Linux].
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4. Port obfuscation and Network Address Port Translation (NAPT)
Network Address Port Translation (NAPT) translate both the network
address and transport-protocol port number, thus allowing the
transport identifiers of a number of private hosts to be multiplexed
into the transport identifiers of a single external address.
[RFC2663]
In those scenarios in which a NAPT is present between the two end-
points of transport-protocol connection, the obfuscation of the
ephemeral ports (from the point of view of the external network) will
depend on the ephemeral port selection function at the NAPT.
Therefore, NAPTs should consider obfuscating the ephemeral ports by
means of any of the algorithms discussed in this document. It should
be noted that in some network scenarios, a NAPT may naturally obscure
ephemeral port selections simply due to the vast range of services
with which it establishes connections and to the overall rate of the
traffic [Allman].
Section 3.5 provides guidance in choosing a port obfuscation
algorithm.
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5. Security Considerations
Obfuscating ephemeral ports is no replacement for cryptographic
mechanisms, such as IPsec [RFC4301], in terms of protecting transport
protocol instances against blind attacks.
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. Ephemeral port obfuscation 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 an attacker uses dynamically assigned IP addresses, the current
ephemeral port offset (Algorithm 3 and Algorithm 4) for a given five-
tuple can be sampled and subsequently 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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6. IANA Considerations
There are no IANA registries within this document. The RFC-Editor
can remove this section before publication of this document as an
RFC.
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7. 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 (in alphabetical order) Mark Allman,
Matthias Bethke, Stephane Bortzmeyer, Brian Carpenter, Vincent
Deffontaines, Lars Eggert, Gorry Fairhurst, Guillermo Gont, Alfred
Hoenes, Amit Klein, Carlos Pignataro, Kacheong Poon, Joe Touch, and
Dan Wing 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.
Fernando Gont would like to thank Carolina Suarez for her love and
support.
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8. References
8.1. Normative References
[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
August 1980.
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, September 1981.
[RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
April 1992.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2385] Heffernan, A., "Protection of BGP Sessions via the TCP MD5
Signature Option", RFC 2385, August 1998.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
[RFC3828] Larzon, L-A., Degermark, M., Pink, S., Jonsson, L-E., and
G. Fairhurst, "The Lightweight User Datagram Protocol
(UDP-Lite)", RFC 3828, July 2004.
[RFC4086] Eastlake, D., Schiller, J., and S. Crocker, "Randomness
Requirements for Security", BCP 106, RFC 4086, June 2005.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
[RFC4340] Kohler, E., Handley, M., and S. Floyd, "Datagram
Congestion Control Protocol (DCCP)", RFC 4340, March 2006.
[RFC4960] Stewart, R., "Stream Control Transmission Protocol",
RFC 4960, September 2007.
8.2. Informative References
[FreeBSD] The FreeBSD Project, "http://www.freebsd.org".
[IANA] "IANA Port Numbers",
<http://www.iana.org/assignments/port-numbers>.
[I-D.ietf-tcpm-icmp-attacks]
Gont, F., "ICMP attacks against TCP",
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draft-ietf-tcpm-icmp-attacks-06 (work in progress),
August 2009.
[RFC1337] Braden, B., "TIME-WAIT Assassination Hazards in TCP",
RFC 1337, May 1992.
[RFC1948] Bellovin, S., "Defending Against Sequence Number Attacks",
RFC 1948, May 1996.
[RFC2663] Srisuresh, P. and M. Holdrege, "IP Network Address
Translator (NAT) Terminology and Considerations",
RFC 2663, August 1999.
[RFC4953] Touch, J., "Defending TCP Against Spoofing Attacks",
RFC 4953, July 2007.
[Allman] Allman, M., "Comments On Selecting Ephemeral Ports", ACM
Computer Communicatiion Review, 39(2), 2009.
[CPNI-TCP]
Gont, F., "CPNI Technical Note 3/2009: Security Assessment
of the Transmission Control Protocol (TCP)", UK Centre
for the Protection of National Infrastructure, 2009.
[I-D.gont-tcp-security]
Gont, F., "Security Assessment of the Transmission Control
Protocol (TCP)", draft-gont-tcp-security-00 (work in
progress), February 2009.
[Linux] The Linux Project, "http://www.kernel.org".
[NetBSD] The NetBSD Project, "http://www.netbsd.org".
[OpenBSD] The OpenBSD Project, "http://www.openbsd.org".
[OpenSolaris]
OpenSolaris, "http://www.opensolaris.org".
[Silbersack]
Silbersack, M., "Improving TCP/IP security through
randomization without sacrificing interoperability.",
EuroBSDCon 2005 Conference .
[Stevens] Stevens, W., "Unix Network Programming, Volume 1:
Networking APIs: Socket and XTI", Prentice Hall , 1998.
[I-D.ietf-tcpm-tcp-auth-opt]
Touch, J., Mankin, A., and R. Bonica, "The TCP
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Authentication Option", draft-ietf-tcpm-tcp-auth-opt-08
(work in progress), October 2009.
[Watson] Watson, P., "Slipping in the Window: TCP Reset Attacks",
CanSecWest 2004 Conference .
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Appendix A. Survey of the algorithms in use by some popular
implementations
A.1. FreeBSD
FreeBSD implements Algorithm 1, and in response to this document now
uses a 'min_port' of 10000 and a 'max_port' of 65535. [FreeBSD]
A.2. Linux
Linux implements Algorithm 3. If the algorithm is faced with the
corner-case scenario described in Section 3.5, Algorithm 1 is used
instead [Linux].
A.3. NetBSD
NetBSD does not obfuscate its ephemeral 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 1, with a 'min_port' of 1024 and a
'max_port' of 49151. [OpenBSD]
A.5. OpenSolaris
OpenSolaris implements Algorithm 1, with a 'min_port' of 32768 and a
'max_port' of 65535. [OpenSolaris]
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Appendix B. Changes from previous versions of the draft (to be removed
by the RFC Editor before publication of this document as a
RFC
B.1. Changes from draft-ietf-tsvwg-port-randomization-04
o Fixes nits.
B.2. Changes from draft-ietf-tsvwg-port-randomization-03
o Addresses WGLC comments from Mark Allman. See:
http://www.ietf.org/mail-archive/web/tsvwg/current/msg09149.html
B.3. Changes from draft-ietf-tsvwg-port-randomization-02
o Added clarification of what we mean by "port randomization".
o Addresses feedback sent on-list and off-list by Mark Allman.
o Added references to [Allman] and [CPNI-TCP].
B.4. Changes from draft-ietf-tsvwg-port-randomization-01
o Added Section 2.3.
o Added discussion of "lazy binding in Section 3.5.
o Added discussion of obtaining the number of outgoing connections.
o Miscellaneous editorial changes
B.5. Changes from draft-ietf-tsvwg-port-randomization-00
o Added Section 3.1.
o Changed Intended Status from "Standards Track" to "BCP".
o Miscellaneous editorial changes.
B.6. Changes from draft-larsen-tsvwg-port-randomization-02
o Draft resubmitted as draft-ietf.
o Included references and text on protocols other than TCP.
o Added the second variant of the simple port randomization
algorithm
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o Reorganized the algorithms into different sections
o Miscellaneous editorial changes.
B.7. Changes from draft-larsen-tsvwg-port-randomization-01
o No changes. Draft resubmitted after expiration.
B.8. 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.9. 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
TietoEnator
Skanderborgvej 232
Aarhus DK-8260
Denmark
Phone: +45 8938 5100
Email: michael.larsen@tietoenator.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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