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RFC6056(7)             Miscellaneous Information Manual             RFC6056(7)

NAME
     rfc6056 - port randomization algorithms

DESCRIPTION
     The rfc6056 algorithms are used in order to randomize the port allocation
     of outgoing UDP packets, in order to provide protection from a series of
     "blind" attacks based on the attacker's ability to guess the sequence of
     ephemeral ports associated with outgoing packets.  For more information
     consult RFC 6056.

     The individual algorithms are described below.

   The RFC 6056 algorithms
     The following algorithms are available:

     bsd           This is the default NetBSD port selection algorithm, which
                   starts from anonportmax and proceeds decreasingly through
                   the available ephemeral ports.

     random_start  Select ports randomly from the available ephemeral ports.
                   In case a collision with a local port is detected, the
                   algorithm proceeds decreasingly through the sequence of
                   ephemeral ports until a free port is found.  Note that the
                   random port selection algorithms are not guaranteed to find
                   a free port.

     random_pick   Select ports randomly from the available ephemeral ports.
                   In case a collision with a local port is detected the
                   algorithm tries selecting a new port randomly until a free
                   port is found.

     hash          Select ports using a md5(3) hash of the local address, the
                   foreign address, and the foreign port.  Note that in the
                   case of a bind(2) call some of this information might be
                   unavailable and the port selection is delayed until the
                   time of a connect(2) call, performed either explicitly or
                   up calling sendto(2).

     doublehash    Select ports using a md5(3) hash of the local address,
                   foreign address, and foreign port coupled with a md5(3)
                   hash of the same components obtained using a separate table
                   that is associated with a subset of all outgoing
                   connections.  The same considerations regarding late
                   connection as in the case of hash apply.

     randinc       Use random increments in order to select the next port.

SYSCTL CONTROLS
     The following sysctl controls are available for selecting the default
     port randomization algorithm:

     sysctl name                              Type      Changeable
     net.inet.ip.anonportalgo.available       string    no
     net.inet.ip.anonportalgo.selected        string    yes
     net.inet6.ip6.anonportalgo.available     string    no
     net.inet6.ip6.anonportalgo.selected      string    yes

SOCKET OPTIONS
     The IP_PORTSEL socket option at the IPPROTO_IP level and the IPV6_PORTSEL
     socket option at the IPPROTO_IPV6 level can be used with a string
     argument specifying the algorithm's name in order to select the port
     randomization algorithm for a specific socket.  For more info see
     setsockopt(2).

SEE ALSO
     setsockopt(2), sysctl(3), sysctl(7)

HISTORY
     The rfc6056 algorithms first appeared in NetBSD 6.0.

NetBSD 10.99                    August 25, 2011                   NetBSD 10.99