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+# AdGuard Home Technical Document
+
+The document describes technical details and internal algorithms of AdGuard Home.
+
+Contents:
+* First startup
+* Installation wizard
+	* "Get install settings" command
+	* "Check configuration" command
+	* Disable DNSStubListener
+	* "Apply configuration" command
+* Enable DHCP server
+	* "Check DHCP" command
+	* "Enable DHCP" command
+	* Static IP check/set
+
+
+## First startup
+
+The first application startup is detected when there's no .yaml configuration file.
+
+We check if the user is root, otherwise we fail with an error.
+
+Web server is started up on port 3000 and automatically redirects requests to `/` to Installation wizard.
+
+After Installation wizard steps are completed, we write configuration to a file and start normal operation.
+
+
+## Installation wizard
+
+This is the collection of UI screens that are shown to a user on first application startup.
+
+The screens are:
+
+1. Welcome
+2. Set up network interface and listening ports for Web and DNS servers
+3. Set up administrator username and password
+4. Configuration complete
+5. Done
+
+Algorithm:
+
+Screen 2:
+* UI asks server for initial information and shows it
+* User edits the default settings, clicks on "Next" button
+* UI asks server to check new settings
+* Server searches for the known issues
+* UI shows information about the known issues and the means to fix them
+* Server applies automatic fixes of the known issues on command from UI
+
+Screen 3:
+* UI asks server to apply the configuration
+* Server restarts DNS server
+
+
+### "Get install settings" command
+
+Request:
+
+	GET /control/install/get_addresses
+
+Response:
+
+	200 OK
+
+	{
+	"web_port":80,
+	"dns_port":53,
+	"interfaces":{
+		"enp2s0":{"name":"enp2s0","mtu":1500,"hardware_address":"","ip_addresses":["",""],"flags":"up|broadcast|multicast"},
+		"lo":{"name":"lo","mtu":65536,"hardware_address":"","ip_addresses":["127.0.0.1","::1"],"flags":"up|loopback"},
+	}
+	}
+
+If `interfaces.flags` doesn't contain `up` flag, UI must show `(Down)` status next to its IP address in interfaces selector.
+
+
+### "Check configuration" command
+
+Request:
+
+	POST /control/install/check_config
+
+	{
+	"web":{"port":80,"ip":"192.168.11.33"},
+	"dns":{"port":53,"ip":"127.0.0.1","autofix":false},
+	}
+
+Server should check whether a port is available only in case it itself isn't already listening on that port.
+
+Server replies on success:
+
+	200 OK
+
+	{
+	"web":{"status":""},
+	"dns":{"status":""},
+	}
+
+Server replies on error:
+
+	200 OK
+
+	{
+	"web":{"status":"ERROR MESSAGE"},
+	"dns":{"status":"ERROR MESSAGE", "can_autofix": true|false},
+	}
+
+
+### Disable DNSStubListener
+
+On Linux, if 53 port is not available, server performs several additional checks to determine if the issue can be fixed automatically.
+
+#### Phase 1
+
+Request:
+
+	POST /control/install/check_config
+
+	{
+	"dns":{"port":53,"ip":"127.0.0.1","autofix":false}
+	}
+
+Check if DNSStubListener is enabled:
+
+	systemctl is-enabled systemd-resolved
+
+Check if DNSStubListener is active:
+
+	grep -E '#?DNSStubListener=yes' /etc/systemd/resolved.conf
+
+If the issue can be fixed automatically, server replies with `"can_autofix":true`
+
+	200 OK
+
+	{
+	"dns":{"status":"ERROR MESSAGE", "can_autofix":true},
+	}
+
+In this case UI shows "Fix" button next to error message.
+
+#### Phase 2
+
+If user clicks on "Fix" button, UI sends request to perform an automatic fix
+
+	POST /control/install/check_config
+
+	{
+	"dns":{"port":53,"ip":"127.0.0.1","autofix":true},
+	}
+
+Deactivate (save backup as `resolved.conf.orig`) and stop DNSStubListener:
+
+	sed -r -i.orig 's/#?DNSStubListener=yes/DNSStubListener=no/g' /etc/systemd/resolved.conf
+	systemctl reload-or-restart systemd-resolved
+
+Server replies:
+
+	200 OK
+
+	{
+	"dns":{"status":""},
+	}
+
+
+### "Apply configuration" command
+
+Request:
+
+	POST /control/install/configure
+
+	{
+	"web":{"port":80,"ip":"192.168.11.33"},
+	"dns":{"port":53,"ip":"127.0.0.1"},
+	"username":"u",
+	"password":"p",
+	}
+
+Server checks the parameters once again, restarts DNS server, replies:
+
+	200 OK
+
+On error, server responds with code 400 or 500.  In this case UI should show error message and reset to the beginning.
+
+	400 Bad Request
+
+	ERROR MESSAGE
+
+
+## Enable DHCP server
+
+Algorithm:
+
+* UI shows DHCP configuration screen with "Enabled DHCP" button disabled, and "Check DHCP" button enabled
+* User clicks on "Check DHCP"; UI sends request to server
+* Server may fail to detect whether there is another DHCP server working in the network.  In this case UI shows a warning.
+* Server may detect that a dynamic IP configuration is used for this interface.  In this case UI shows a warning.
+* UI enables "Enable DHCP" button
+* User clicks on "Enable DHCP"; UI sends request to server
+* Server sets a static IP (if necessary), enables DHCP server, sends the status back to UI
+* UI shows the status
+
+
+### "Check DHCP" command
+
+Request:
+
+	POST /control/dhcp/find_active_dhcp
+
+	vboxnet0
+
+Response:
+
+	200 OK
+
+	{
+		"other_server": {
+			"found": "yes|no|error",
+			"error": "Error message", // set if found=error
+		},
+		"static_ip": {
+			"static": "yes|no|error",
+			"ip": "<Current dynamic IP address>", // set if static=no
+		}
+	}
+
+If `other_server.found` is:
+* `no`: everything is fine - there is no other DHCP server
+* `yes`: we found another DHCP server.  UI shows a warning.
+* `error`: we failed to determine whether there's another DHCP server.  `other_server.error` contains error details.  UI shows a warning.
+
+If `static_ip.static` is:
+* `yes`: everything is fine - server uses static IP address.
+
+* `no`: `static_ip.ip` contains the current dynamic IP address which we may set as static.  In this case UI shows a warning:
+
+		Your system uses dynamic IP address configuration for interface <CURRENT INTERFACE NAME>.  In order to use DHCP server a static IP address must be set.  Your current IP address is <static_ip.ip>.  We will automatically set this IP address as static if you press Enable DHCP button.
+
+* `error`: this means that the server failed to check for a static IP.  In this case UI shows a warning:
+
+		In order to use DHCP server a static IP address must be set.  We failed to determine if this network interface is configured using static IP address.  Please set a static IP address manually.
+
+
+### "Enable DHCP" command
+
+Request:
+
+	POST /control/dhcp/set_config
+
+	{
+		"enabled":true,
+		"interface_name":"vboxnet0",
+		"gateway_ip":"192.169.56.1",
+		"subnet_mask":"255.255.255.0",
+		"range_start":"192.169.56.3",
+		"range_end":"192.169.56.3",
+		"lease_duration":60,
+		"icmp_timeout_msec":0
+	}
+
+Response:
+
+	200 OK
+
+	OK
+
+
+### Static IP check/set
+
+Before enabling DHCP server we have to make sure the network interface we use has a static IP configured.
+
+#### Phase 1
+
+On Debian systems DHCP is configured by `/etc/dhcpcd.conf`.
+
+To detect if a static IP is used currently we search for line
+
+	interface eth0
+
+and then look for line
+
+	static ip_address=...
+
+If the interface already has a static IP, everything is set up, we don't have to change anything.
+
+To get the current IP address along with netmask we execute
+
+	ip -oneline -family inet address show eth0
+
+which will print:
+
+	2: eth0    inet 192.168.0.1/24 brd 192.168.0.255 scope global eth0\       valid_lft forever preferred_lft forever
+
+To get the current gateway address:
+
+	ip route show dev enp2s0
+
+which will print:
+
+	default via 192.168.0.1 proto dhcp metric 100
+
+
+#### Phase 2
+
+This method only works on Raspbian.
+
+On Ubuntu DHCP for a network interface can't be disabled via `dhcpcd.conf`.  This must be configured in `/etc/netplan/01-netcfg.yaml`.
+
+Fedora doesn't use `dhcpcd.conf` configuration at all.
+
+Step 1.
+
+To set a static IP address we add these lines to `dhcpcd.conf`:
+
+	interface eth0
+	static ip_address=192.168.0.1/24
+	static routers=192.168.0.1
+	static domain_name_servers=192.168.0.1
+
+* Don't set 'routers' if we couldn't find gateway IP
+* Set 'domain_name_servers' equal to our IP
+
+Step 2.
+
+If we would set a different IP address, we'd need to replace the IP address for the current network configuration.  But currently this step isn't necessary.
+
+	ip addr replace dev eth0 192.168.0.1/24