* DNS: resolve host names from DHCP: improve

#1956

Squashed commit of the following:

commit efeacd92b8b82a9a0a0cce8c5648f2d024b4bc9e
Author: Simon Zolin <s.zolin@adguard.com>
Date:   Tue Aug 18 13:54:15 2020 +0300

    * DNS: resolve host names from DHCP: improve

    . Require a valid host name from DHCP lease
    . Use lower-case names

AGHTechDoc.md

@@ -22,7 +22,8 @@ Contents:
 	* Update client
 	* Delete client
 	* API: Find clients by IP
-* Enable DHCP server
+* DHCP server
+	* DHCP server in DNS
 	* "Show DHCP status" command
 	* "Check DHCP" command
 	* "Enable DHCP" command
@@ -375,9 +376,9 @@ Error response:
 UI shows error message "Auto-update has failed"
 
 
-## Enable DHCP server
+## DHCP server
 
-Algorithm:
+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
@@ -389,6 +390,21 @@ Algorithm:
 * UI shows the status
 
 
+### DHCP server in DNS
+
+DHCP leases are used in several ways by DNS module.
+
+* For "A" DNS reqeust we reply with an IP address leased by our DHCP server.
+
+		< A bills-notebook.lan.
+		> A bills-notebook.lan. = 192.168.1.100
+
+* For "PTR" DNS request we reply with a hostname from an active DHCP lease.
+
+		< PTR 100.1.168.192.in-addr.arpa.
+		> PTR 100.1.168.192.in-addr.arpa. = bills-notebook.
+
+
 ### "Show DHCP status" command
 
 Request: