Write page on DHCP (#127)

source/manual.rst

@@ -19,6 +19,7 @@ User Manual
    manual/virtuals
    manual/gui
    manual/users
+   manual/dhcp
    manual/ipv6
    manual/two_factor
    manual/systemhealth

source/manual/dhcp.rst

@@ -0,0 +1,43 @@
+====
+DHCP
+====
+
+DHCP is used to automatically provide clients with an IP address (instead of clients having to set one themselves).
+DHCP is available for both IPv4 and IPv6 clients, referred to as DHCPv4 and DHCPv6, respectively.
+
+-----------------
+Settings overview
+-----------------
+
+DHCPv4 settings can be found at **Services -> DHCPv4**. DHCPv6 settings can be found at **Services -> DHCPv6**.
+
+The DHCPv4 submenu further consists of:
+
+* An entry per interface of general settings, like a toggle to enable/disable DHCPv4 for this interface, DHCP range, DNS servers…
+* **Relay**: DHCP requests can be "forwarded" to another server. This is called relaying.
+* **Leases**: Shows all IP addresses that are handed out to clients (can be filtered to only show active and static leases).
+* **Log File**: Shows the log file of the DHCPv4 server.
+
+The DHCPv6 submenu further consists of:
+
+* **Relay**: DHCP requests can be "forwarded" to another server. This is called relaying.
+* **Leases**: Shows all IP addresses that are handed out to clients (can be filtered to only show active and static leases).
+
+------------
+Using DHCPv4
+------------
+
+A typical DHCPv4 usage scenario is using it on your LAN with an IP range of 192.168.1.x, where x can be a number from 1
+through 254. This means a subnet mask of 255.255.255.0. The range can also be written as 192.168.1.0/24. (The “1” in
+the third group can also be another number, and there are also other ranges available for private use. These are
+described in `RFC 1918 <https://tools.ietf.org/html/rfc1918#section-3>`_.)
+
+The LAN IP of the OPNsense device that serves DHCP to the LAN should fall in the same DHCP IP range. Typically, it gets
+the address ending in .1 (so 192.168.1.1) in this example.
+
+To set the LAN IP, go to **Interfaces -> [LAN]**, set “IPv4 Configuration Type” to “Static”, and under
+“Static IPv4 configuration”, set “IPv4 address” to ``192.168.1.1`` and the subnet dropdown to “24”. Then click Save.
+
+To set the DHCP settings, go to **Services -> DHCPv4 -> [LAN]**. Under “Gateway”, put ``192.168.1.1``. Under range,
+put ``192.168.1.100`` as the start address and ``192.168.1.200`` as the end address. Then click Save. After saving,
+click the “Apply Settings” button.