add admin guide
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Ariadne Conill
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doc/ADMIN-GUIDE.md
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doc/ADMIN-GUIDE.md
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# ifupdown-ng for system administrators
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ifupdown-ng is a network device manager which is backwards
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compatible with traditional ifup and ifdown as used on Debian
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and Alpine systems, while solving many design deficits with
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the original approach through robust error handling and the
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use of a dependency-solver to determine interface bring-up
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order.
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This guide is intended to walk users through using the
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ifupdown-ng system without any assumption of familiarity
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with the legacy ifupdown system.
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## Important Filesystem Paths
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The ifupdown-ng system uses the following paths, ranked
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in order of importance:
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* `/etc/network/interfaces`: the interface configuration
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database, which contains information about what
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interfaces should be configured.
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* `/run/ifstate`: the interface state file, which denotes
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what physical interfaces are configured, and what
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interface definition they are configured as.
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* `/usr/libexec/ifupdown-ng`: this directory contains the
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native ifupdown-ng executors, which are run as necessary
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to configure an interface. See the ifupdown-executor(7)
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manual page for more information on how these programs
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are written.
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* `/etc/network/if-{up|down|pre-up|post-down}.d`:
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these directories contain scripts that are run when an
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interface is brought up or down. In general, they follow
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the same contract described in ifupdown-executor(7).
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All configuration examples in this guide concern the
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`/etc/network/interfaces` file.
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## Basic Configuration
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To begin with, lets look at a basic configuration for a
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desktop computer. This scenario involves using the DHCP
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helper to learn an IPv4 address dynamically.
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In this case, the `/etc/network/interfaces` file would
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look like:
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```
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auto eth0
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iface eth0
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use dhcp
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```
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These configuration statements do two things: designate
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that `eth0` should be started automatically with the `auto`
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keyword, and designate that the `dhcp` executor should be
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used to configure the interface.
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As a more detailed explanation, here is a commented version:
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```
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# Start eth0 automatically.
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auto eth0
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# Begin an interface definition for eth0.
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iface eth0
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# Use the dhcp executor to configure eth0.
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use dhcp
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```
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## IPv6 RA Configuration
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With IPv6, stateless auto-configuration is typically used to
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configure network interfaces. If you are not interested in
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using IPv4 at all, you can simply use the `ipv6-ra` executor
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to ensure that an interface is configured to accept IPv6 RA
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advertisements:
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```
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auto eth0
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iface eth0
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use ipv6-ra
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```
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## Static Configuration
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We can use the `static` executor to configure static IPv4 and
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IPv6 addresses. If you use the `address` keyword, the `static`
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executor will automatically be used to configure the interface:
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```
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auto eth0
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iface eth0
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address 203.0.113.2/24
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gateway 203.0.113.1
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```
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### Multi-homing
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A typical scenario on servers is multi-homing, where a server
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has multiple IP addresses on a single interface. In this case
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you simply add additional `address` lines like this:
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```
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auto eth0
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iface eth0
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address 203.0.113.2/24
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address 203.0.113.3/24
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address 203.0.113.4/24
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gateway 203.0.113.1
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```
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### Dual-stack configurations
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Another typical scenario for servers is to run a dual-stack
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configuration, where interfaces have both an IPv4 and an IPv6
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address. This is accomplished in a similar way as multi-homing.
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You specify the IPv4 and IPv6 addresses you want, followed by
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gateways for each:
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```
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auto eth0
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iface eth0
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address 203.0.113.2/24
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address 203.0.113.3/24
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address 203.0.113.4/24
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gateway 203.0.113.1
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address 2001:db8:1000:2::2/64
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address 2001:db8:1000:2::3/64
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address 2001:db8:1000:2::4/64
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gateway 2001:db8:1000:2::1
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```
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## Questions
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If you have further questions about how to use ifupdown-ng to
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configure a specific scenario, drop by the [ifupdown-ng IRC channel][irc].
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[irc]: irc://irc.as7007.net/#ifupdown-ng
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