IN_GETIFA(9) | 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 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

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`.

February 22, 2007 | NetBSD 8.2