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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.
p The individual algorithms are described below: .Ss The RFC 6056 algorithms .Li The following algorithms are available: l -tag -width "random_start" t Sy bsd This is the default .Nx port selection algorithm, which starts from .Dv anonportmax and proceeds decreasingly through the available ephemeral ports. t 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. t 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. t 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 . t 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. t 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: l -column "net.inet6.udp6.rfc6056.available" "string" "Changeable" t Sy sysctl name Ta Sy Type Ta Sy Changeable t net.inet.udp.rfc6056.available Ta string Ta no t net.inet.udp.rfc6056.selected Ta string Ta yes t net.inet6.udp6.rfc6056.available Ta string Ta no t 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 .