.\" $NetBSD: in_getifa.9,v 1.11 2020/01/21 07:53:51 wiz Exp $
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.\" Copyright (c) 2006 David Young.  All rights reserved.
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.Dd February 22, 2007
.Dt IN_GETIFA 9
.Os
.Sh NAME
.Nm in_getifa
.Nd Look up the IPv4 source address best matching an IPv4 destination
.Sh SYNOPSIS
.Cd options IPSELSRC
.In netinet/in_selsrc.h
.Ft struct ifaddr *
.Fn in_getifa "struct ifaddr *ifa" "const struct sockaddr *dst0"
.Sh DESCRIPTION
.Nm
enforces the IPv4 source-address selection policy.
Add the source-address selection policy mechanism to your kernel with
.Cd options IPSELSRC .
.Cd options IPSELSRC
lets the operator set the policy for choosing the source address
of any socket bound to the
.Dq wildcard
address,
.Dv INADDR_ANY .
Note that the policy is applied
.Em after
the kernel makes its forwarding decision, thereby choosing the
output interface;
in other words, this mechanism does not affect whether or not
.Nx
is a
.Dq strong ES .
.Pp
An operator affects the source-address selection using
.Xr sysctl 8
and
.Xr ifconfig 8 .
Operators set policies with
.Xr sysctl 8 .
Some policies consider the
.Dq preference number
of an address.
An operator may set preference numbers for each address with
.Xr ifconfig 8 .
.Pp
A source-address policy is a priority-ordered list of source-address
ranking functions.
A ranking function maps its arguments,
.Po
.Em source address ,
.Em source index ,
.Em source preference ,
.Em destination address
.Pc ,
to integers.
The
.Em source index
is the position of
.Em source address
in the interface address list; the index of the first address is 0.
The
.Em source preference
is the preference number the operator assigned
to
.Em source address .
The
.Em destination address
is the socket peer / packet destination.
.Pp
Presently, there are four ranking functions to choose from:
.Bl -tag -width "common-prefix-len"
.It index
ranks by
.Em source index ;
lower indices are ranked more highly.
.It preference
ranks by
.Em source preference ;
higher preference numbers are ranked more highly.
.It common-prefix-len
ranks each
.Em source address
by the length of the longest prefix it has in common with
.Em destination address ;
longer common prefixes rank more highly.
.It same-category
determines the "categories" of
.Em source
and
.Em destination address .
A category is one of
.Em private ,
.Em link-local ,
or
.Em other .
If the categories exactly match, same-category assigns a rank of 2.
Some sources are ranked 1 by category:
a
.Em link-local
source with a
.Em private
destination, a
.Em private
source with a
.Em link-local
destination, and a
.Em private
source with an
.Em other
destination rank 1.
All other sources rank 0.
.Pp
Categories are defined as follows.
.Bl -tag -width "link-local"
.It private
RFC1918 networks, 192.168/16, 172.16/12, and 10/8
.It link-local
169.254/16, 224/24
.It other
all other networks---i.e., not private, not link-local
.El
.El
.Pp
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
.Em source index .
.Pp
The operator may set a policy on individual interfaces.
The operator may also set a global policy that applies to all
interfaces whose policy they do not set individually.
.Pp
Here is the sysctl tree for the policy at system startup:
.Bd -literal -offset indent
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 =
.Ed
.Pp
The policy on every interface is the
.Dq empty
policy, so the default policy applies.
The default policy,
.Em index ,
is the
.Dq historical
policy in
.Nx .
.Pp
The operator may override the default policy on ath0,
.Bd -literal -offset indent
	# sysctl -w net.inet.ip.interfaces.ath0.selectsrc=same-category,common-prefix-len,preference
.Ed
.Pp
yielding this policy:
.Bd -literal -offset indent
net.inet.ip.selectsrc.default = index
net.inet.ip.interfaces.ath0.selectsrc = same-category,common-prefix-len,preference
.Ed
.Pp
The operator may set a new default,
.Bd -literal -offset indent
# sysctl -w net.inet.ip.selectsrc.debug=\
> same-category,common-prefix-len,preference
# sysctl -w net.inet.ip.interfaces.ath0.selectsrc=
.Ed
.Pp
yielding this policy:
.Bd -literal -offset indent
net.inet.ip.selectsrc.default = same-category,common-prefix-len,preference
net.inet.ip.interfaces.ath0.selectsrc =
.Ed
.Pp
In a number of applications, the policy above will usually pick
suitable source addresses if ath0 is configured in this way:
.Bd -literal -offset indent
# 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
.Ed
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.
.Sh SEE ALSO
.Xr ifconfig 8 ,
.Xr sysctl 8
.Sh STANDARDS
The family of IPv6 source-address selection policies defined by
.Li RFC3484
resembles the family of IPv4 policies that
.Nm
enforces.
.Sh AUTHORS
.An David Young Aq Mt dyoung@NetBSD.org
.Sh BUGS
With
.Cd options IPSELSRC ,
a new interface
.Xr ioctl 2 ,
.Dv SIOCSIFADDRPREF ,
was introduced.
It ought to be documented in
.Xr inet 4 .
Also,
.Xr options 4
ought to cross-reference this manual page.
.Pp
This work should be used to set IPv6 source-address selection
policies, especially the family of policies defined by
.Li RFC3484 .