tsvwg                                                          M. Larsen
Internet-Draft                                               TietoEnator
Intended status: Standards Track                                 F. Gont
Expires: June 6, 2008                                            UTN/FRH
                                                        December 4, 2007


                           Port Randomization
                 draft-ietf-tsvwg-port-randomization-00

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Copyright Notice

   Copyright (C) The IETF Trust (2007).













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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 simple and efficient method
   for random selection of the client port number, such that the
   possibility of an attacker guessing the exact value is reduced.
   While this is not a replacement for cryptographic methods, the
   described port number randomization algorithms provide improved
   security/obfuscation with very little effort and without any key
   management overhead.  The mechanisms described in this document are a
   local modification 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, UDP, SCTP, DCCP, and RTP.
































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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Ephemeral Ports  . . . . . . . . . . . . . . . . . . . . . . .  5
     2.1.  Traditional Ephemeral Port Range . . . . . . . . . . . . .  5
     2.2.  Ephemeral port selection . . . . . . . . . . . . . . . . .  5
   3.  Randomizing the Ephemeral Ports  . . . . . . . . . . . . . . .  7
     3.1.  Ephemeral port number range  . . . . . . . . . . . . . . .  7
     3.2.  Ephemeral Port Randomization Algorithms  . . . . . . . . .  7
       3.2.1.  Algorithm 1: Simple port randomization algorithm . . .  7
       3.2.2.  Algorithm 2: Another simple port randomization
               algorithm  . . . . . . . . . . . . . . . . . . . . . .  9
       3.2.3.  Algorithm 3: Simple hash-based algorithm . . . . . . .  9
       3.2.4.  Algorithm 4: Double-hash randomization algorithm . . . 11
     3.3.  Secret-key considerations for hash-based port
           randomization algorithms . . . . . . . . . . . . . . . . . 13
     3.4.  Choosing an ephemeral port randomization algorithm . . . . 14
   4.  Security Considerations  . . . . . . . . . . . . . . . . . . . 15
   5.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 16
   6.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
     6.1.  Normative References . . . . . . . . . . . . . . . . . . . 17
     6.2.  Informative References . . . . . . . . . . . . . . . . . . 17
   Appendix A.  Survey of the algorithms in use by some popular
                implementations . . . . . . . . . . . . . . . . . . . 19
     A.1.  FreeBSD  . . . . . . . . . . . . . . . . . . . . . . . . . 19
     A.2.  Linux  . . . . . . . . . . . . . . . . . . . . . . . . . . 19
     A.3.  NetBSD . . . . . . . . . . . . . . . . . . . . . . . . . . 19
     A.4.  OpenBSD  . . . . . . . . . . . . . . . . . . . . . . . . . 19
   Appendix B.  Changes from previous versions of the draft . . . . . 20
     B.1.  Changes from draft-larsen-tsvwg-port-randomisation-02  . . 20
     B.2.  Changes from draft-larsen-tsvwg-port-randomisation-01  . . 20
     B.3.  Changes from draft-larsen-tsvwg-port-randomization-00  . . 20
     B.4.  Changes from draft-larsen-tsvwg-port-randomisation-00  . . 20
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 21
   Intellectual Property and Copyright Statements . . . . . . . . . . 22
















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1.  Introduction

   Recently, awareness has been raised about a number of "blind" attacks
   that can be performed against the Transmission Control Protocol (TCP)
   [RFC0793] and similar protocols.  The consequences of these attacks
   range from throughput-reduction to broken connections or data
   corruption [I-D.ietf-tcpm-icmp-attacks] [I-D.ietf-tcpm-tcp-antispoof]
   [Watson].

   All these attacks rely on the attacker's ability to guess or know the
   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] or the TCP MD5 signature option [RFC2385].  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 level of obfuscation and ease of
   implementation.  Thus the algorithms must be computationally
   efficient, and not require substantial data structures.






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2.  Ephemeral Ports

2.1.  Traditional Ephemeral Port Range

   The Internet Assigned Numbers Authority (IANA) assigns the unique
   parameters and values used in protocols developed by the Internet
   Engineering Task Force (IETF), including well-known ports [IANA].
   IANA has traditionally reserved the following use of the 16-bit port
   range of TCP and UDP:

   o  The Well Known Ports, 0 through 1023.

   o  The Registered Ports, 1024 through 49151

   o  The Dynamic and/or Private Ports, 49152 through 65535

   The range for assigned ports managed by the IANA is 0-1023, with the
   remainder being registered by IANA but not assigned.

   The ephemeral port range has traditionally consisted of the 49152-
   65535 range.

2.2.  Ephemeral port selection

   As each communication instance is identified by the 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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       next_ephemeral = min_ephemeral;  /*initialization, could be random */

       /* Ephemeral port selection */
       count = max_ephemeral - min_ephemeral + 1;

       do {
           port = next_ephemeral;
           if (next_ephemeral == max_ephemeral) {
               next_ephemeral = min_ephemeral;
           } else {
               next_ephemeral++;
           }

           if (five-tuple is unique)
               return port;

       } 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.


















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3.  Randomizing the Ephemeral Ports

3.1.  Ephemeral port number range

   As mentioned in Section 2.1, the ephemeral port range has
   traditionally consisted of the 49152-65535 range.  However, it should
   also include the range 1024-49151 range.

   Since this range includes user-specific server ports, this may not
   always be possible, though.  A possible workaround for this potential
   problem would be to maintain an array of bits, in which each bit
   would correspond to each of the port numbers in the range 1024-65535.
   A bit set to 0 would indicate that the corresponding port is
   available for allocation, while a bit set to one would indicate that
   the port is reserved and therefore cannot be allocated.  Thus, before
   allocating a port number, the ephemeral port selection function would
   check this array of bits, avoiding the allocation of ports that may
   be needed for specific applications.

   Transport protocols SHOULD use the largest possible port range, since
   this improves the obfuscation provided by randomizing the ephemeral
   ports.

3.2.  Ephemeral Port Randomization Algorithms

3.2.1.  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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       next_ephemeral = min_ephemeral + random()
                           % (max_ephemeral - min_ephemeral + 1);

       count = max_ephemeral - min_ephemeral + 1;

       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).  Gateways such as proxy
   servers are an example of such a system.

   Since this algorithm performs a completely random port selection
   (i.e., without taking into account the port numbers previously
   chosen), it has the potential of reusing port numbers too quickly.
   Even if a given 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



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   possible.

3.2.2.  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:


       next_ephemeral = min_ephemeral + random()
                           % (max_ephemeral - min_ephemeral + 1);

       count = max_ephemeral - min_ephemeral + 1;

       do {
           if(five-tuple is unique)
                   return next_ephemeral;

           next_ephemeral = min_ephemeral + random()
                               % (max_ephemeral - min_ephemeral + 1);
           count--;
       } while (count > 0);

       return ERROR;

                                 Figure 3

   We will refer to this algorithm as 'Algorithm 2'.  The only
   difference between this algorithm and Algorithm 1 is that the search
   time for this variant may be longer than for the later, particularly
   when there are a large number of port numbers already in use.

3.2.3.  Algorithm 3: Simple hash-based algorithm

   We would like to achieve the port reuse properties of 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
   '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.:



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       /* Initialization code */
       next_ephemeral = 0;  /* could be random */

       /* Ephemeral port selection */
       offset = F(local_IP, remote_IP, remote_port, secret_key);
       count = max_ephemeral - min_ephemeral + 1;

       do {
           port = min_ephemeral + (next_ephemeral + offset)
                      % (max_ephemeral - min_ephemeral + 1);
           next_ephemeral++;
           count--;

           if(five-tuple is unique)
               return port;

       } 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
   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



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   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 discusses,
   and since SCTP always include 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.

3.2.4.  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
   system would keep an array of, TABLE_LENGTH short integers, which
   would provide a separation of the increment of the 'next_ephemeral'
   variable.  This improvement could be incorporated into Algorithm 3 as
   follows:


















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       /* Initialization code */
       for(i = 0; i < TABLE_LENGTH; i++)   /* Initialization code */
           table[i] = random % 65536;


       /* Ephemeral port selection */
       offset = F(local_IP, remote_IP, remote_port, secret_key);
       index = G(offset);
       count = max_ephemeral - min_ephemeral + 1;

       do {
           port = min_ephemeral + (offset + table[index])
                      % (max_ephemeral - min_ephemeral + 1);

           table[index]++;
           count--;

             if(five-tuple is unique)
           return port;

       } 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 another cryptographic hash
   function such as that used in F().

   The array 'table[]' assures that succesive connections to the same
   end-point will use increasing ephemeral port numbers.  However,
   incrementation of the port numbers is separated into TABLE_LENGTH
   different spaces, and thus the port reuse frequency will be
   (probabilistically) lower than that of Algorithm 2.  That is, a
   connection established 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 actual port
   sequence will result from adding the corresponding entry of 'table[]'



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   to the variable 'offset', which actually selects the actual port
   sequence (as in Algorithm 3).

3.3.  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:

   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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3.4.  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 failure of the connection
   establishment attempts.

   Algorithm 3 provides complete separation in local and remote IP
   addresses and remote port space, and only limited separation in other
   dimensions (See Section Section 3.3), and thus 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 precludes the utilization of Algorithm 3
   and Algorithm 4 should be analyzed.  There exist some applications
   that contain the following code sequence:


       s = socket();
       bind(s, IP_address, port = *);


                                 Figure 6

   This code sequence results 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 implementing Algorithm 3 or Algorithm 4 should
   consider using Algorithm 2 when facing the scenario just described.
   This policy has been implemented by Linux [Linux].





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4.  Security Considerations

   Randomizing ports is no replacement for cryptographic mechanisms,
   such as IPsec [RFC4301], 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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5.  Acknowledgements

   The offset function was inspired by the mechanism proposed by Steven
   Bellovin in [RFC1948] for defending against TCP sequence number
   attacks.

   The authors would like to thank Mark Allman, Lars Eggert, Gorry
   Fairhurst, Alfred Hoenes, Carlos Pignataro, 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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6.  References

6.1.  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.

   [RFC2385]  Heffernan, A., "Protection of BGP Sessions via the TCP MD5
              Signature Option", RFC 2385, August 1998.

   [RFC2960]  Stewart, R., Xie, Q., Morneault, K., Sharp, C.,
              Schwarzbauer, H., Taylor, T., Rytina, I., Kalla, M.,
              Zhang, L., and V. Paxson, "Stream Control Transmission
              Protocol", RFC 2960, October 2000.

   [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.

6.2.  Informative References

   [FreeBSD]  The FreeBSD Project, "http://www.freebsd.org".




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   [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-02 (work in progress),
              May 2007.

   [I-D.ietf-tcpm-tcp-antispoof]
              Touch, J., "Defending TCP Against Spoofing Attacks",
              draft-ietf-tcpm-tcp-antispoof-06 (work in progress),
              February 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 , 2005.

   [Stevens]  Stevens, W., "Unix Network Programming, Volume 1:
              Networking APIs: Socket and XTI,  Prentice Hall", 1998.

   [Watson]   Watson, P., "Slipping in the Window: TCP Reset attacks",
              december 2003.






















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Appendix A.  Survey of the algorithms in use by some popular
             implementations

A.1.  FreeBSD

   FreeBSD implements Algorithm 2. with a 'min_port' of 49152 and a
   'max_port' of 65535.  If the selected port number is in use, the next
   available port number is tried next [FreeBSD].

A.2.  Linux

   Linux implements Algorithm 3.  If the algorithm is faced with the
   corner-case scenario described in Section 3.4, Algorithm 2 is used
   instead [Linux].

A.3.  NetBSD

   NetBSD does not randomize ehemeral port numbers.  It selects
   ephemeral port numbers from the range 49152-65535, starting from port
   65535, and decreasing the port number for each ephemeral port number
   selected [NetBSD].

A.4.  OpenBSD

   OpenBSD implements Algorithm 2. with a 'min_port' of 1024 and a
   'max_port' of 49151.  If the selected port number is in use, the next
   available port number is tried next [OpenBSD].
























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Appendix B.  Changes from previous versions of the draft

B.1.  Changes from draft-larsen-tsvwg-port-randomisation-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.2.  Changes from draft-larsen-tsvwg-port-randomisation-01

   o  No changes.  Draft resubmitted after expiration.

B.3.  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.4.  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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