IN_GETIFA(9) BSD Kernel Developer's Manual IN_GETIFA(9)NAMEin_getifa — Look up the IPv4 source address best matching an IPv4 desti‐
nation
SYNOPSIS
options IPSELSRC
#include <netinet/in_selsrc.h>
struct ifaddr *
in_getifa(struct ifaddr *ifa, const struct sockaddr *dst0);
DESCRIPTIONin_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 prefer‐
ence 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 inte‐
gers. The source index is the position of source address in the inter‐
face 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 destina‐
tion.
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 destina‐
tion, 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 pol‐
icy 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 ALSOifconfig(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
