(network) added section on how to create a simple local network

network.md

@@ -1,22 +1,104 @@
-# network
+# Network
 
 This document holds network related notes.
 
 <!--toc:start-->
-- [network](#network)
-  - [enable routing](#enable-routing)
+- [Network](#network)
+  - [Enable routing](#enable-routing)
+  - [Create local network with dhcp/dns](#create-local-network-with-dhcpdns)
+    - [`dnsmasq` configuration](#dnsmasq-configuration)
+    - [`nftables` configuration](#nftables-configuration)
+    - [starting the routing system](#starting-the-routing-system)
 <!--toc:end-->
 
 
-## enable routing
+## Enable routing
 
-To enable routing permanently add a file `/etc/sysctl.d/` with the following contents:
+To enable routing **temporarily** use the following command:
+
+```sh
+echo 1 > /proc/sys/net/ipv4/ip_forward
+```
+
+To enable routing **permanently** add a file `/etc/sysctl.d/` with the following contents:
 
 ```ini
 net.ipv4.ip_forward=1
 ```
 
-## simple gateway with nftables
+
+## Create local network with dhcp/dns
 
 > **WARNING:** do not use this configuration for an edge router (directly connected to the internet/isp)!
+> for an edge router you *absolutely need* to set up a reasonable firewall!
+
+(On the device that acts as router) install `dnsmasq` and `nftables`.
+Also install a trusted network manager (e.g. `NetworkManager` or `dhcpcd`).
+
+Decide on an interface each for the local and the parent network.
+In this chapter the examples use `enp1s0` for the local network and is referred to as local interface.
+For the external or parent network the interface `enp0s0` is used and is referred to as external interface.
+
+Use the network manager to set a static ip for the local interface, in this example `10.1.0.1` is used.
+Make sure this address is in the same subnet as the dhcp-range configured later in the dnsmasq configuration.
+Set up the external interface to request an ip address via DHCP.
+
+
+### `dnsmasq` configuration
+
+Configure `dnsmasq` by editing `/etc/dnsmasq.conf`:
+
+```ini
+# required settings
+## set interface for local network
+interface=enp1s0
+## set address range for local network
+## exclude the first two and last address in your subnet (network-, router- and broadcast address)
+## the last value is the lease time
+dhcp-range=10.1.0.2,10.1.0.254,12h
+
+# optional settings
+domain-needed
+bogus-priv
+filterwin2k
+expand-hosts
+domain=hans
+```
+
+
+### `nftables` configuration
+
+Configure `nftables` by editing `/etc/nftables.conf`:
+
+```
+define local_network=10.1.0.0/24
+define lan_interface="enp1s0"
+define wan_interface="enp0s0"
+
+flush ruleset
+
+table inet filter {
+    chain routing {
+        type nat hook postrouting priority srcnat; policy accept;
+        ip saddr { $local_network } oifname { $wan_interface } masquerade
+    }
+}
+```
+
+> **NOTE:** This is where one would insert a few more "chains" to set up a firewall,
+> if the external network is not trusted.
+
+
+### starting the routing system
+
+Enable the services for `dnsmasq` and `nftables`.
+On a system with `systemd` use:
+
+```sh
+sudo systemd enable --now dnsmasq nftables
+```
+
+Enable routing in the kernel, see [Enable routing](#enable-routing).
+
+The system should now serve a DHCP/DNS server on the local interface and mask packets from the local to the external network with its own ip address.