- NetBSD Manual Pages
RFC6056(7) NetBSD Miscellaneous Information Manual RFC6056(7)
Powered by man-cgi (2021-06-01).
Maintained for NetBSD
by Kimmo Suominen.
Based on man-cgi by Panagiotis Christias.
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 packets. For more informa-
tion 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 algo-
rithm tries selecting a new port randomly until a free port
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, for-
eign 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.
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
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 argu-
ment specifying the algorithm's name in order to select the port random-
ization algorithm for a specific socket. For more info see
setsockopt(2), sysctl(3), sysctl(7)
The rfc6056 algorithms first appeared in NetBSD 6.0.
NetBSD 10.99 August 25, 2011 NetBSD 10.99