IN_GETIFA(9)						   BSD Kernel Developer's Manual					      IN_GETIFA(9)

NAME

     in_getifa -- Look up the IPv4 source address best matching an IPv4 destination

SYNOPSIS

     options IPSELSRC
     #include <netinet/in_selsrc.h>

     struct ifaddr *
     in_getifa(struct ifaddr *ifa, const struct sockaddr *dst0);

DESCRIPTION

     in_getifa enforces the IPv4 source-address selection policy.  Add the source-address selection policy mechanism to your kernel with options
     IPSELSRC.	options IPSELSRC lets the operator set the policy for choosing the source address of any socket bound to the ``wildcard'' address,
     INADDR_ANY.  Note that the policy is applied after the kernel makes its forwarding decision, thereby choosing the output interface; in other
     words, this mechanism does not affect whether or not NetBSD is a ``strong ES''.

     An operator affects the source-address selection using sysctl(8) and ifconfig(8).	Operators set policies with sysctl(8).	Some policies con-
     sider the ``preference number'' of an address.  An operator may set preference numbers for each address with ifconfig(8).

     A source-address policy is a priority-ordered list of source-address ranking functions.  A ranking function maps its arguments, (source
     address, source index, source preference, destination address), to integers.  The source index is the position of source address in the
     interface address list; the index of the first address is 0.  The source preference is the preference number the operator assigned to source
     address.  The destination address is the socket peer / packet destination.

     Presently, there are four ranking functions to choose from:

     index		ranks by source index; lower indices are ranked more highly.

     preference 	ranks by source preference; higher preference numbers are ranked more highly.

     common-prefix-len	ranks each source address by the length of the longest prefix it has in common with destination address; longer common
			prefixes rank more highly.

     same-category	determines the "categories" of source and destination address.	A category is one of private, link-local, or other.  If
			the categories exactly match, same-category assigns a rank of 2.  Some sources are ranked 1 by category: a link-local
			source with a private destination, a private source with a link-local destination, and a private source with an other des-
			tination rank 1.  All other sources rank 0.

			Categories are defined as follows.

			private     RFC1918 networks, 192.168/16, 172.16/12, and 10/8

			link-local  169.254/16, 224/24

			other	    all other networks---i.e., not private, not link-local

     To apply a policy, the kernel applies all ranking functions in the policy to every source address, producing a vector of ranks for each
     source.  The kernel sorts the sources in descending, lexicographical order by their rank-vector, and chooses the highest-ranking (first)
     source.  The kernel breaks ties by choosing the source with the least source index.

     The operator may set a policy on individual interfaces.  The operator may also set a global policy that applies to all interfaces whose pol-
     icy he does not set individually.

     Here is the sysctl tree for the policy at system startup:

	   net.inet.ip.selectsrc.default = index
	   net.inet.ip.interfaces.ath0.selectsrc =
	   net.inet.ip.interfaces.sip0.selectsrc =
	   net.inet.ip.interfaces.sip1.selectsrc =
	   net.inet.ip.interfaces.lo0.selectsrc =
	   net.inet.ip.interfaces.pflog0.selectsrc =

     The policy on every interface is the ``empty'' policy, so the default policy applies.  The default policy, index, is the ``historical'' pol-
     icy in NetBSD.

     The operator may override the default policy on ath0,

		   # sysctl -w net.inet.ip.interfaces.ath0.selectsrc=same-category,common-prefix-len,preference

     yielding this policy:

	   net.inet.ip.selectsrc.default = index
	   net.inet.ip.interfaces.ath0.selectsrc = same-category,common-prefix-len,preference

     The operator may set a new default,

	   # sysctl -w net.inet.ip.selectsrc.debug=> same-category,common-prefix-len,preference
	   # sysctl -w net.inet.ip.interfaces.ath0.selectsrc=

     yielding this policy:

	   net.inet.ip.selectsrc.default = same-category,common-prefix-len,preference
	   net.inet.ip.interfaces.ath0.selectsrc =

     In a number of applications, the policy above will usually pick suitable source addresses if ath0 is configured in this way:

	   # ifconfig ath0 inet 64.198.255.1/24
	   # ifconfig ath0 inet 10.0.0.1/24
	   # ifconfig ath0 inet 169.254.1.1/24
	   # ifconfig ath0 inet 192.168.49.1/24 preference 5
	   # ifconfig ath0 inet 192.168.37.1/24 preference 9
     A sysctl, net.inet.ip.selectsrc.debug, turns on and off debug messages concerned with source selection.  You may set it to 0 (no messages) or
     1.

SEE ALSO

     ifconfig(8), sysctl(8)

STANDARDS

     The family of IPv6 source-address selection policies defined by RFC3484 resembles the family of IPv4 policies that in_getifa enforces.

AUTHORS

     David Young <dyoung@NetBSD.org>

BUGS

     With options IPSELSRC, a new interface ioctl(2), SIOCSIFADDRPREF, was introduced.	It ought to be documented in inet(4).  Also, options(4)
     ought to cross-reference this manual page.

     This work should be used to set IPv6 source-address selection policies, especially the family of policies defined by RFC3484.

BSD
								 February 22, 2007							       BSD