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RFC6056(7) BSD Miscellaneous Information Manual RFC6056(7)
rfc6056 -- port randomization algorithms
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 pack-
ets. 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 ran-
domly 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
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
randinc Use random increments in order to select the next port.
The following sysctl controls are available for selecting the default port randomization
sysctl name Type Changeable
net.inet.udp.rfc6056.available string no
net.inet.udp.rfc6056.selected string yes
net.inet6.udp6.rfc6056.available string no
net.inet6.udp6.rfc6056.selected string yes
The socket option UDP_RFC6056ALGO at the IPPROTO_UDP level can be used with a string argu-
ment specifying the algorithm's name in order to select the port randomization algorithm for
a specific socket. For more info see setsockopt(2).
setsockopt(2), sysctl(3), sysctl(7)
The rfc6056 algorithms first appeared in NetBSD 6.0.
BSD August 25, 2011 BSD
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