Handle DHCP edge case [Fix #9]

proxy/main.go

@@ -315,9 +315,15 @@ func handleDHCP(content []byte, dstMAC net.HardwareAddr, srcMAC net.HardwareAddr
 		switch reqType {
 		case dhcp4.Discover:
 			if OldIP == nil {
-				log.Fatal(prefix, "DHCPDISCOVER but not previous address is known")
+				if NewIP == nil || !NewIP.IsGlobalUnicast() {
+					log.Warning(prefix, "DHCP request, but neither OldIP nor NewIP known")
+					sendDHCPRequest(dhcp4.Discover, net.IPv4zero)
+					return
 				}
+				sendDHCPReply(req, dhcp4.Offer, NewIP, options, dstMAC)
+			} else {
 				sendDHCPReply(req, dhcp4.Offer, OldIP, options, dstMAC)
+			}
 		case dhcp4.Inform:
 			fallthrough
 		case dhcp4.Request: