IPADDRSEL(8) Maintenance Procedures IPADDRSEL(8)


ipaddrsel - configure IPv6 default address selection



/usr/sbin/ipaddrsel -f file

/usr/sbin/ipaddrsel -d


Use the ipaddrsel utility to configure the IPv6 default address selection
policy table. The policy table is a longest-matching-prefix lookup table
that is used for IPv6 source address selection and for destination
address ordering when resolving names to AF_INET6 addresses. For a
description of how the policy table is used for source address selection,
see inet6(4P). For a description of how the policy table is used for
destination address ordering, see getaddrinfo(3SOCKET).

The unmodified policy table is valid for all typical IPv6 deployments.
Modify the table only if a circumstance exists for which the default
behavior of the IPv6 source address selection or destination address
ordering mechanism is unsatisfactory. See the section for examples of
such circumstances. You should carefully consider your addressing
strategy before you change the table from the provided default.

When the ipaddrsel command is issued without any arguments, the address
selection policy currently in use is printed. The format of the output is
compatible with the format of the configuration file that the -f option

Note -

If the usesrc subcommand to ifconfig(8) is applied to a particular
physical interface, the selection policy specified by usesrc overrides
the source address selection policies specified by ipaddrsel. This is
true for packets that are locally generated and for applications that
do not choose a non-zero source address using bind(3SOCKET).

The Configuration File

The configuration file that the -f option accepts can contain either
comment lines or policy entries. Comment lines have a '#' character as
the first non-blank character. and they are ignored by the ipaddrsel
utility. Policy entry lines have the following format:

prefix/prefix_length precedence label [# comment]

The prefix must be an IPv6 prefix in a format consistent with
inet(3SOCKET). The prefix_length is an integer ranging from 0 to 128. The
IPv6 source address selection and destination address ordering algorithms
determine the precedence or label of an address by doing a longest-
prefix-match lookup using the prefixes in this table, much like next-hop
determination for a destination is done by doing a longest-prefix-match
lookup using an IP routing table.

The precedence is a non-negative integer that represents how the
destination address ordering mechanism will sort addresses returned from
name lookups. In general, addresses with a higher precedence will be in
front of addresses with a lower precedence. Other factors, such as
destinations with undesirable source addresses can, however, override
these precedence values.

The label is a string of at most fifteen characters, not including the
NULL terminator. The label allows particular source address prefixes to
be used with destination prefixes of the same label. Specifically, for a
particular destination address, the IPv6 source address selection
algorithm prefers source addresses whose label is equal that of the

The label may be followed by an optional comment.

The file must contain a default policy entry, which is an entry with
::0/0 as its prefix and prefix_length. This is to ensure that all
possible addresses match a policy.


The ippadrsel utility supports the following options:

-f file
Replace the address selection policy table with the policy
specified in the file.

Revert the kernel's address selection policy table back to the
default table. Invoking ipaddrsel in this way only changes
the currently running kernel's policy table, and does not
alter the configuration file /etc/inet/ipaddrsel.conf. To
revert the configuration file back to its default settings,
use ipaddrsel -d, then dump the contents of the table to the
configuration file by redirecting the output of ipaddrsel to

example# ipaddrsel -d
example# ipaddrsel > /etc/inet/ipaddrsel.conf


Example 1: The Default Policy in /etc/inet/ipaddrsel.conf

The following example is the default policy that is located in

# Prefix Precedence Label
::1/128 50 Loopback
::/0 40 Default
::ffff: 35 IPv4
2002::/16 30 6to4
2001::/32 5 Teredo
fc00::/7 3 ULA
::/96 1 IPv4_Compatible
fec0::/10 1 Site_Local
3ffe::/16 1 6bone

Example 2: Assigning a Lower Precedence to Link-local and Site-local


By default, the destination address ordering rules sort addresses of
smaller scope before those of larger scope. For example, if a name
resolves to a global and a site-local address, the site local address
would be ordered before the global address. An administrator can override
this ordering rule by assigning a lower precedence to addresses of
smaller scope, as the following table demonstrates.

# Prefix Precedence Label
::1/128 50 Loopback
::/0 40 Default
2002::/16 30 6to4
fec0::/10 27 Site-Local
fe80::/10 23 Link-Local
::/96 20 IPv4_Compatible
::ffff: 10 IPv4

Example 3: Assigning Higher Precedence to IPv4 Destinations

By default, IPv6 addresses are ordered in front of IPv4 addresses in name
lookups. ::ffff: has the precedence of 35 in the default table.
In the following example, IPv4 addresses are assigned higher precedence
and are ordered in front of IPv6 destinations:

# Prefix Precedence Label
::1/128 50 Loopback
::/0 40 Default
::ffff: 60 IPv4
2002::/16 30 6to4
2001::/32 5 Teredo
fc00::/7 3 ULA
::/96 1 IPv4_Compatible
fec0::/10 1 Site_Local
3ffe::/16 1 6bone

Example 4: Ensuring that a Particular Source Address is Used

This example ensures that a particular source address is used only when
communicating with destinations in a particular network.

The following policy table assigns the label "ClientNet" to a particular
source address on the local system, 2001:1111:1111::1. The table assigns
the same label to a network, 2001:2222:2222::/48. The result of this
policy is that the 2001:1111:1111::1 source address will only be used
when communicating with destinations contained in the 2001:2222:2222::/48
network. For this example, this network is the ClientNet, which could
represent a particular client's network.

# Prefix Precedence Label
::1/128 50 Loopback
2001:1111:1111::1/128 40 ClientNet
2001:2222:2222::/48 40 ClientNet
::/0 40 Default
2002::/16 30 6to4
::/96 20 IPv4_Compatible
::ffff: 10 IPv4

This example assumes that the local system has one physical interface,
and that all global prefixes are assigned to that physical interface.


ipaddrsel returns the following exit values:

ipaddrsel successfully completed.

An error occurred. If a failure is encountered, the kernel's
current policy table is unchanged.


The file that contains the IPv6 default
address selection policy to be installed at
boot time. This file is loaded before any
Internet services are started.


See attributes(7) for descriptions of the following attributes:

|Interface Stability | Evolving |


getaddrinfo(3SOCKET), inet(3SOCKET), inet6(4P), ipaddrsel.conf(5),
attributes(7), nscd(8)


The ipnodes cache kept by nscd(8) contains addresses that are ordered
using the destination address ordering algorithm, which is one of the
reasons why ipaddrsel is called before nscd in the boot sequence. If
ipaddrsel is used to change the address selection policy after nscd has
started, you should invalidate the nscd ipnodes cache by invoking the
following command:

example# /usr/sbin/nscd -i ipnodes

November 4, 2017 IPADDRSEL(8)