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.Dd August 25, 2011
.Dt RFC6056 7
.Os
.Sh NAME
.Nm rfc6056
.Nd port randomization algorithms
.Sh DESCRIPTION
The
.Nm
algorithms are used in order to randomize the port allocation of outgoing UDP
packets, in order to provide protection from a series of
.Dq 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.
.Pp
The individual algorithms are described below:
.Ss The RFC 6056 algorithms
.Li The following algorithms are available:
.Bl -tag -width "random_start"
.It Sy bsd
This is the default
.Nx
port selection algorithm, which starts from
.Dv anonportmax
and proceeds decreasingly through the available ephemeral ports.
.It Sy 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.
.It Sy 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.
.It Sy hash
Select ports using a
.Xr md5 3
hash of the local address, the foreign address, and the foreign port.
Note that in the case of a
.Xr bind 2
call some of this information might be unavailable and the
port selection is delayed until the time of a
.Xr connect 2
call, performed either explicitly or up calling
.Xr sendto 2 .
.It Sy doublehash
Select ports using a
.Xr md5 3
hash of the local address, foreign address, and foreign port coupled with a
.Xr 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.
.It Sy randinc
Use random increments in order to select the next port.
.El
.Sh SYSCTL CONTROLS
The following sysctl controls are available for selecting the default
port randomization algorithm:
.Bl -column "net.inet6.udp6.rfc6056.available" "string" "Changeable"
.It Sy sysctl name                   Ta Sy Type Ta Sy Changeable
.It net.inet.udp.rfc6056.available   Ta string  Ta no
.It net.inet.udp.rfc6056.selected    Ta string  Ta yes
.It net.inet6.udp6.rfc6056.available Ta string  Ta no
.It net.inet6.udp6.rfc6056.selected  Ta string  Ta yes
.El
.Sh SOCKET OPTIONS
The socket option
.Dv UDP_RFC6056ALGO
at the
.Dv IPPROTO_UDP
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
.Xr setsockopt 2 .
.Sh SEE ALSO
.Xr setsockopt 2 ,
.Xr sysctl 3 ,
.Xr sysctl 7
.Sh HISTORY
The
.Nm
algorithms first appeared in
.Nx 6.0 .