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man pages section 8: System Administration Commands

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Updated: Thursday, June 13, 2019
 
 

ipaddrsel(8)

Name

ipaddrsel - configure IPv6 default address selection

Synopsis

/usr/sbin/ipaddrsel 
/usr/sbin/ipaddrsel -f file
/usr/sbin/ipaddrsel -d

Description

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(3C).

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 Examples 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 accepts.


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(3C).

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(3C). 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 destination.

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.

Options

The ipaddrsel utility supports the following options:

–f file

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

–d

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 /etc/inet/ipaddrsel.conf.

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

Examples

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

The following example is the default policy that is located in /etc/inet/ipaddrsel.conf:


# Prefix                            Precedence Label
::1/128                             50 Loopback
::/96                               20 IPv4_Compatible
::ffff:0.0.0.0/96                   10 IPv4
2002::/16                           30 6to4
::/0                                40 Default
Example 2 Assigning a Lower Precedence to Link-local and Site-local Addresses

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:0.0.0.0/96                    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:0.0.0.0/96 has the lowest precedence 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
2002::/16                            30 6to4
::/96                                20 IPv4_Compatible
::ffff:0.0.0.0/96                    60 IPv4
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 a label of 5 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:0.0.0.0/96                     10 IPv4

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

Exit Status

ipaddrsel returns the following exit values:

0

ipaddrsel successfully completed.

>0

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

Files

/etc/inet/ipaddrsel.conf

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.

Attributes

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

ATTRIBUTE TYPE
ATTRIBUTE VALUE
Availability
system/network
Interface Stability
Committed

See Also

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

Notes

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