dhcp: allow reserved leases outside scope pool range but within subnet

Reserved DHCP leases whose IP is outside the scope's [startingAddress,
endingAddress] pool range but still inside the scope's subnet were rejected on
add and, when added via the API back door, failed unicast renewal.

- Scope: validate reserved-lease addresses by subnet membership
  (IsAddressInNetwork) instead of pool range (IsAddressInRange), in both the
  ReservedLeases setter and TryAddReservedLease so the console form and API
  agree. Wrong-subnet addresses are still rejected.
- DhcpServer.FindScope: in the no-relay unicast (RENEW/REBIND) branch, match the
  scope by the client's reserved lease (keyed on MAC) before the pool-range
  check, mirroring the broadcast and relay branches. Previously an out-of-pool
  ciaddr matched no scope, so the REQUEST was silently dropped and the client
  lost its lease at T1.

Author: CoryCharlton

DnsServerCore/Dhcp/DhcpServer.cs

@@ -655,7 +655,14 @@ private Scope FindScope(DhcpMessage request, IPAddress remoteAddress, IPPacketIn
                     {
                         Scope scope = entry.Value;
 
-                        if (scope.Enabled && scope.IsAddressInRange(request.ClientIpAddress))
+                        if (!scope.Enabled)
+                            continue;
+
+                        Lease reservedLease = scope.GetReservedLease(request);
+                        if ((reservedLease is not null) && reservedLease.Address.Equals(request.ClientIpAddress))
+                            return scope; //ciaddr is this client's reserved address (may be out of pool range)
+
+                        if (scope.IsAddressInRange(request.ClientIpAddress))
                             return scope;
                     }
 

DnsServerCore/Dhcp/Scope.cs

@@ -1477,6 +1477,9 @@ public void ChangeNetwork(IPAddress startingAddress, IPAddress endingAddress, IP
 
         public bool TryAddReservedLease(Lease reservedLease)
         {
+            if (!IsAddressInNetwork(reservedLease.Address))
+                throw new ArgumentOutOfRangeException(nameof(reservedLease), "Reserved address must be within the scope's subnet.");
+
             if (_reservedLeases.TryAdd(reservedLease.ClientIdentifier, reservedLease))
             {
                 _dhcpServer.AddDnsEntries(this, reservedLease);
@@ -2175,8 +2178,8 @@ public IReadOnlyCollection<Lease> ReservedLeases
                 {
                     foreach (Lease reservedLease in value)
                     {
-                        if (!IsAddressInRange(reservedLease.Address))
-                            throw new ArgumentOutOfRangeException(nameof(ReservedLeases), "Reserved address must be in scope range.");
+                        if (!IsAddressInNetwork(reservedLease.Address))
+                            throw new ArgumentOutOfRangeException(nameof(ReservedLeases), "Reserved address must be within the scope's subnet.");
                     }
 
                     //remove DNS entries for reserved leases being removed or has updated domain name