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DESCRIPTION

An operator affects the source-address selection using sysctl(8) and ifconfig(8). Operators set policies with sysctl(8). Some policies consider 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 destination 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 policy 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” policy 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