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Versions: (draft-larsen-tsvwg-port-randomization) 00 01 02 03 04 05 06 07 08 09 RFC 6056

Transport Area Working Group                                   M. Larsen
(tsvwg)                                                      TietoEnator
Internet-Draft                                                   F. Gont
Intended status: BCP                                             UTN/FRH
Expires: March 4, 2009                                   August 31, 2008


                           Port Randomization
                 draft-ietf-tsvwg-port-randomization-02

Status of this Memo

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   This Internet-Draft will expire on March 4, 2009.

















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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 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 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.  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.
































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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Ephemeral Ports  . . . . . . . . . . . . . . . . . . . . . . .  6
     2.1.  Traditional Ephemeral Port Range . . . . . . . . . . . . .  6
     2.2.  Ephemeral port selection . . . . . . . . . . . . . . . . .  6
     2.3.  Collision of connection-id's . . . . . . . . . . . . . . .  7
   3.  Randomizing the Ephemeral Ports  . . . . . . . . . . . . . . .  9
     3.1.  Characteristics of a good ephemeral port randomization
           algorithm  . . . . . . . . . . . . . . . . . . . . . . . .  9
     3.2.  Ephemeral port number range  . . . . . . . . . . . . . . . 10
     3.3.  Ephemeral Port Randomization Algorithms  . . . . . . . . . 10
       3.3.1.  Algorithm 1: Simple port randomization algorithm . . . 10
       3.3.2.  Algorithm 2: Another simple port randomization
               algorithm  . . . . . . . . . . . . . . . . . . . . . . 12
       3.3.3.  Algorithm 3: Simple hash-based algorithm . . . . . . . 12
       3.3.4.  Algorithm 4: Double-hash randomization algorithm . . . 14
     3.4.  Secret-key considerations for hash-based port
           randomization algorithms . . . . . . . . . . . . . . . . . 16
     3.5.  Choosing an ephemeral port randomization algorithm . . . . 17
   4.  Port randomization and Network Address Port Translation
       (NAPT) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 20
   6.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 21
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 22
     7.1.  Normative References . . . . . . . . . . . . . . . . . . . 22
     7.2.  Informative References . . . . . . . . . . . . . . . . . . 23
   Appendix A.  Survey of the algorithms in use by some popular
                implementations . . . . . . . . . . . . . . . . . . . 24
     A.1.  FreeBSD  . . . . . . . . . . . . . . . . . . . . . . . . . 24
     A.2.  Linux  . . . . . . . . . . . . . . . . . . . . . . . . . . 24
     A.3.  NetBSD . . . . . . . . . . . . . . . . . . . . . . . . . . 24
     A.4.  OpenBSD  . . . . . . . . . . . . . . . . . . . . . . . . . 24
   Appendix B.  Changes from previous versions of the draft . . . . . 25
     B.1.  Changes from draft-ietf-tsvwg-port-randomization-01  . . . 25
     B.2.  Changes from draft-ietf-tsvwg-port-randomization-00  . . . 25
     B.3.  Changes from draft-larsen-tsvwg-port-randomization-02  . . 25
     B.4.  Changes from draft-larsen-tsvwg-port-randomization-01  . . 25
     B.5.  Changes from draft-larsen-tsvwg-port-randomization-00  . . 25
     B.6.  Changes from draft-larsen-tsvwg-port-randomisation-00  . . 26
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 27
   Intellectual Property and Copyright Statements . . . . . . . . . . 28









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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 random selection
   of the client ephemeral port, that reduce the possibility of an off-
   path attacker guessing the exact value.  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 data structures.

   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 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 operating systems 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. Initialization value 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
   port number "collision" would occur.  The effect of these port number
   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 lead to a low port reuse frequency, to reduce the
   chances of port number collisions.

   A simple approach to maximize the five-tuple reuse cycle 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.
























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3.  Randomizing the Ephemeral Ports

3.1.  Characteristics of a good ephemeral port randomization 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  Maximizing the port reuse cycle.

   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 a short
   reuse cycle of port numbers, which could lead to the interoperability
   problems discussed in Section 2.3.  It is also worth noting that,
   provided adequate randomization 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.  Therefore, an ephemeral port selection algorithm
   should ideally lead to a low port reuse frequency, to reduce the
   chances of port number collisions.  A good algorithm to maximize the
   port reuse cycle 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 maximize the five-tuple reuse cycle 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.

   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.



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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.

   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.

3.3.  Ephemeral Port Randomization Algorithms

   Transport protocols SHOULD allocate their ephemeral ports randomly,
   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.
   Although carefully chosen random sources and optimized five-tuple
   lookup mechanisms (e.g., optimized through hashing) will mitigate the
   cost of this verification, some systems may still not want to incur
   this search time.

   Systems that may be specially susceptible to this kind of repeated
   five-tuple collisions are those that create many connections from a
   single local IP address to a single service (i.e. both of the IP
   addresses and the server port are fixed).  Web proxy servers and
   NAPTs [RFC2663] are an examples of such systems.

   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 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).
   Therefore, it is desirable to keep the port reuse frequency as low as
   possible.



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   This algorithm selects ephemeral port numbers randomly and thus
   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'.  The difference
   between this algorithm and Algorithm 1 is that the search time for
   this variant may be longer than for the latter, particularly when
   there is a large number of port numbers already in use.  Also, this
   algorithm may be unable to select an ephemeral port (i.e., return
   "ERROR") even if there are port numbers that would result in unique
   five-tuples, particularly when there are a large number of port
   numbers already in use.

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, 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 code at system boot time. Initialization value 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
   calculation.  The local IP address and remote port may in some cases
   be constant and not improve the connection separation, however, they



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   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.  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).

3.3.4.  Algorithm 4: Double-hash randomization 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



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   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_key);
     index = G(offset);
     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 random values, as indicated by
   the initialization code in Figure 5.  G() would return a value
   between 0 and (TABLE_LENGTH-1) taking 'offset' as its input.  G()
   could, for example, perform the exclusive-or (xor) operation between
   all the bytes in 'offset', or could be some 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 3.  That is, a
   connection established with some remote end-point will not



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   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 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).

   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 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).

3.4.  Secret-key considerations for hash-based port randomization
      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.  Thus the change in
   secret key should be done with consideration and could be performed
   whenever one of the following events occur:




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   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).

3.5.  Choosing an ephemeral port randomization algorithm

   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.

   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.

   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 3.4), and thus may scale better than
   Algorithm 1 and 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 may preclude 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 6



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   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 randomization 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 randomizing the ephemeral ports by
   means of any of the algorithms discussed in this document.

   Section 3.5 provides guidance in choosing a port randomization
   algorithm.


































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5.  Security Considerations

   Randomizing 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.  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 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.  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, 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.





































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7.  References

7.1.  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.

   [RFC1948]  Bellovin, S., "Defending Against Sequence Number Attacks",
              RFC 1948, May 1996.

   [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.

   [RFC2663]  Srisuresh, P. and M. Holdrege, "IP Network Address
              Translator (NAT) Terminology and Considerations",
              RFC 2663, August 1999.

   [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.






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7.2.  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",
              draft-ietf-tcpm-icmp-attacks-03 (work in progress),
              March 2008.

   [RFC1337]  Braden, B., "TIME-WAIT Assassination Hazards in TCP",
              RFC 1337, May 1992.

   [RFC4953]  Touch, J., "Defending TCP Against Spoofing Attacks",
              RFC 4953, July 2007.

   [Linux]    The Linux Project, "http://www.kernel.org".

   [NetBSD]   The NetBSD Project, "http://www.netbsd.org".

   [OpenBSD]  The OpenBSD Project, "http://www.openbsd.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
              Authentication Option", draft-ietf-tcpm-tcp-auth-opt-01
              (work in progress), July 2008.

   [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 randomize 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]


























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Appendix B.  Changes from previous versions of the draft

B.1.  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.2.  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.3.  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

   o  Reorganized the algorithms into different sections

   o  Miscellaneous editorial changes.

B.4.  Changes from draft-larsen-tsvwg-port-randomization-01

   o  No changes.  Draft resubmitted after expiration.

B.5.  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





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   o  Miscellaneous editorial changes

B.6.  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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Full Copyright Statement

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