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doc/ADMIN-GUIDE.md

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