diff --git a/app.go b/app.go
index 91f4fbdd..ba8aee3c 100644
--- a/app.go
+++ b/app.go
@@ -12,7 +12,6 @@ import (
 	"strconv"
 	"sync"
 	"syscall"
-	"time"
 
 	"github.com/AdguardTeam/golibs/log"
 	"github.com/gobuffalo/packr"
@@ -101,24 +100,7 @@ func run(args options) {
 		config.BindPort = args.bindPort
 	}
 
-	// Load filters from the disk
-	// And if any filter has zero ID, assign a new one
-	for i := range config.Filters {
-		filter := &config.Filters[i] // otherwise we're operating on a copy
-		if filter.ID == 0 {
-			filter.ID = assignUniqueFilterID()
-		}
-		err = filter.load()
-		if err != nil {
-			// This is okay for the first start, the filter will be loaded later
-			log.Debug("Couldn't load filter %d contents due to %s", filter.ID, err)
-			// clear LastUpdated so it gets fetched right away
-		}
-
-		if len(filter.Rules) == 0 {
-			filter.LastUpdated = time.Time{}
-		}
-	}
+	loadFilters()
 
 	// Save the updated config
 	err = config.write()
@@ -145,11 +127,6 @@ func run(args options) {
 	// Update filters we've just loaded right away, don't wait for periodic update timer
 	go func() {
 		refreshFiltersIfNecessary(false)
-		// Save the updated config
-		err := config.write()
-		if err != nil {
-			log.Fatal(err)
-		}
 	}()
 	// Schedule automatic filters updates
 	go periodicallyRefreshFilters()
diff --git a/control.go b/control.go
index ad2da0ca..be0d61b2 100644
--- a/control.go
+++ b/control.go
@@ -576,11 +576,9 @@ func handleFilteringAddURL(w http.ResponseWriter, r *http.Request) {
 	}
 
 	// Check for duplicates
-	for i := range config.Filters {
-		if config.Filters[i].URL == f.URL {
-			httpError(w, http.StatusBadRequest, "Filter URL already added -- %s", f.URL)
-			return
-		}
+	if filterExists(f.URL) {
+		httpError(w, http.StatusBadRequest, "Filter URL already added -- %s", f.URL)
+		return
 	}
 
 	// Set necessary properties
@@ -588,7 +586,7 @@ func handleFilteringAddURL(w http.ResponseWriter, r *http.Request) {
 	f.Enabled = true
 
 	// Download the filter contents
-	ok, err := f.update(true)
+	ok, err := f.update()
 	if err != nil {
 		httpError(w, http.StatusBadRequest, "Couldn't fetch filter from url %s: %s", f.URL, err)
 		return
@@ -611,7 +609,11 @@ func handleFilteringAddURL(w http.ResponseWriter, r *http.Request) {
 
 	// URL is deemed valid, append it to filters, update config, write new filter file and tell dns to reload it
 	// TODO: since we directly feed filters in-memory, revisit if writing configs is always necessary
-	config.Filters = append(config.Filters, f)
+	if !filterAdd(f) {
+		httpError(w, http.StatusBadRequest, "Filter URL already added -- %s", f.URL)
+		return
+	}
+
 	err = writeAllConfigs()
 	if err != nil {
 		httpError(w, http.StatusInternalServerError, "Couldn't write config file: %s", err)
@@ -621,6 +623,7 @@ func handleFilteringAddURL(w http.ResponseWriter, r *http.Request) {
 	err = reconfigureDNSServer()
 	if err != nil {
 		httpError(w, http.StatusInternalServerError, "Couldn't reconfigure the DNS server: %s", err)
+		return
 	}
 
 	_, err = fmt.Fprintf(w, "OK %d rules\n", f.RulesCount)
@@ -649,6 +652,7 @@ func handleFilteringRemoveURL(w http.ResponseWriter, r *http.Request) {
 	}
 
 	// go through each element and delete if url matches
+	config.Lock()
 	newFilters := config.Filters[:0]
 	for _, filter := range config.Filters {
 		if filter.URL != url {
@@ -664,6 +668,7 @@ func handleFilteringRemoveURL(w http.ResponseWriter, r *http.Request) {
 	}
 	// Update the configuration after removing filter files
 	config.Filters = newFilters
+	config.Unlock()
 	httpUpdateConfigReloadDNSReturnOK(w, r)
 }
 
@@ -686,22 +691,12 @@ func handleFilteringEnableURL(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	found := false
-	for i := range config.Filters {
-		filter := &config.Filters[i] // otherwise we will be operating on a copy
-		if filter.URL == url {
-			filter.Enabled = true
-			found = true
-		}
-	}
-
+	found := filterEnable(url, true)
 	if !found {
 		http.Error(w, "URL parameter was not previously added", http.StatusBadRequest)
 		return
 	}
 
-	// kick off refresh of rules from new URLs
-	refreshFiltersIfNecessary(false)
 	httpUpdateConfigReloadDNSReturnOK(w, r)
 }
 
@@ -724,15 +719,7 @@ func handleFilteringDisableURL(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	found := false
-	for i := range config.Filters {
-		filter := &config.Filters[i] // otherwise we will be operating on a copy
-		if filter.URL == url {
-			filter.Enabled = false
-			found = true
-		}
-	}
-
+	found := filterEnable(url, false)
 	if !found {
 		http.Error(w, "URL parameter was not previously added", http.StatusBadRequest)
 		return
diff --git a/filter.go b/filter.go
index 9625b207..819e683e 100644
--- a/filter.go
+++ b/filter.go
@@ -2,10 +2,10 @@ package main
 
 import (
 	"fmt"
+	"hash/crc32"
 	"io/ioutil"
 	"os"
 	"path/filepath"
-	"reflect"
 	"regexp"
 	"strconv"
 	"strings"
@@ -28,6 +28,7 @@ type filter struct {
 	Name        string    `json:"name" yaml:"name"`
 	RulesCount  int       `json:"rulesCount" yaml:"-"`
 	LastUpdated time.Time `json:"lastUpdated,omitempty" yaml:"-"`
+	checksum    uint32    // checksum of the file data
 
 	dnsfilter.Filter `yaml:",inline"`
 }
@@ -43,6 +44,88 @@ func userFilter() filter {
 	}
 }
 
+// Enable or disable a filter
+func filterEnable(url string, enable bool) bool {
+	r := false
+	config.Lock()
+	for i := range config.Filters {
+		filter := &config.Filters[i] // otherwise we will be operating on a copy
+		if filter.URL == url {
+			filter.Enabled = enable
+			if enable {
+				e := filter.load()
+				if e != nil {
+					// This isn't a fatal error,
+					//  because it may occur when someone removes the file from disk.
+					// In this case the periodic update task will try to download the file.
+					filter.LastUpdated = time.Time{}
+					log.Tracef("%s filter load: %v", url, e)
+				}
+			} else {
+				filter.unload()
+			}
+			r = true
+			break
+		}
+	}
+	config.Unlock()
+	return r
+}
+
+// Return TRUE if a filter with this URL exists
+func filterExists(url string) bool {
+	r := false
+	config.RLock()
+	for i := range config.Filters {
+		if config.Filters[i].URL == url {
+			r = true
+			break
+		}
+	}
+	config.RUnlock()
+	return r
+}
+
+// Add a filter
+// Return FALSE if a filter with this URL exists
+func filterAdd(f filter) bool {
+	config.Lock()
+
+	// Check for duplicates
+	for i := range config.Filters {
+		if config.Filters[i].URL == f.URL {
+			config.Unlock()
+			return false
+		}
+	}
+
+	config.Filters = append(config.Filters, f)
+	config.Unlock()
+	return true
+}
+
+// Load filters from the disk
+// And if any filter has zero ID, assign a new one
+func loadFilters() {
+	for i := range config.Filters {
+		filter := &config.Filters[i] // otherwise we're operating on a copy
+		if filter.ID == 0 {
+			filter.ID = assignUniqueFilterID()
+		}
+
+		if !filter.Enabled {
+			// No need to load a filter that is not enabled
+			continue
+		}
+
+		err := filter.load()
+		if err != nil {
+			// This is okay for the first start, the filter will be loaded later
+			log.Debug("Couldn't load filter %d contents due to %s", filter.ID, err)
+		}
+	}
+}
+
 func deduplicateFilters() {
 	// Deduplicate filters
 	i := 0 // output index, used for deletion later
@@ -84,41 +167,83 @@ func periodicallyRefreshFilters() {
 
 // Checks filters updates if necessary
 // If force is true, it ignores the filter.LastUpdated field value
+//
+// Algorithm:
+// . Get the list of filters to be updated
+// . For each filter run the download and checksum check operation
+//  . If filter data hasn't changed, set new update time
+//  . If filter data has changed, parse it, save it on disk, set new update time
+//  . Apply changes to the current configuration
+// . Restart server
 func refreshFiltersIfNecessary(force bool) int {
-	config.Lock()
+	var updateFilters []filter
 
-	// fetch URLs
-	updateCount := 0
+	config.RLock()
 	for i := range config.Filters {
-		filter := &config.Filters[i] // otherwise we will be operating on a copy
+		f := &config.Filters[i] // otherwise we will be operating on a copy
 
-		if filter.ID == 0 { // protect against users modifying the yaml and removing the ID
-			filter.ID = assignUniqueFilterID()
+		if !f.Enabled {
+			continue
 		}
 
-		if len(filter.Rules) == 0 {
-			// Try reloading filter from the disk before updating
-			// This is useful for the case when we simply enable a previously downloaded filter
-			_ = filter.load()
+		if !force && time.Since(f.LastUpdated) <= updatePeriod {
+			continue
 		}
 
-		updated, err := filter.update(force)
+		var uf filter
+		uf.ID = f.ID
+		uf.URL = f.URL
+		uf.checksum = f.checksum
+		updateFilters = append(updateFilters, uf)
+	}
+	config.RUnlock()
+
+	updateCount := 0
+	for i := range updateFilters {
+		uf := &updateFilters[i]
+		updated, err := uf.update()
 		if err != nil {
-			log.Printf("Failed to update filter %s: %s\n", filter.URL, err)
+			log.Printf("Failed to update filter %s: %s\n", uf.URL, err)
 			continue
 		}
 		if updated {
 			// Saving it to the filters dir now
-			err = filter.save()
+			err = uf.save()
 			if err != nil {
-				log.Printf("Failed to save the updated filter %d: %s", filter.ID, err)
+				log.Printf("Failed to save the updated filter %d: %s", uf.ID, err)
 				continue
 			}
 
+		} else {
+			mtime := time.Now()
+			e := os.Chtimes(uf.Path(), mtime, mtime)
+			if e != nil {
+				log.Error("os.Chtimes(): %v", e)
+			}
+			uf.LastUpdated = mtime
+		}
+
+		config.Lock()
+		for k := range config.Filters {
+			f := &config.Filters[k]
+			if f.ID != uf.ID || f.URL != uf.URL {
+				continue
+			}
+			f.LastUpdated = uf.LastUpdated
+			if !updated {
+				continue
+			}
+
+			log.Info("Updated filter #%d.  Rules: %d -> %d",
+				f.ID, f.RulesCount, uf.RulesCount)
+			f.Name = uf.Name
+			f.Rules = uf.Rules
+			f.RulesCount = uf.RulesCount
+			f.checksum = uf.checksum
 			updateCount++
 		}
+		config.Unlock()
 	}
-	config.Unlock()
 
 	if updateCount > 0 && isRunning() {
 		err := reconfigureDNSServer()
@@ -139,13 +264,19 @@ func parseFilterContents(contents []byte) (int, string, []string) {
 
 	// Count lines in the filter
 	for _, line := range lines {
+
 		line = strings.TrimSpace(line)
-		if len(line) > 0 && line[0] == '!' {
-			if m := filterTitleRegexp.FindAllStringSubmatch(line, -1); len(m) > 0 && len(m[0]) >= 2 && !seenTitle {
+		if len(line) == 0 {
+			continue
+		}
+
+		if line[0] == '!' {
+			m := filterTitleRegexp.FindAllStringSubmatch(line, -1)
+			if len(m) > 0 && len(m[0]) >= 2 && !seenTitle {
 				name = m[0][1]
 				seenTitle = true
 			}
-		} else if len(line) != 0 {
+		} else {
 			rulesCount++
 		}
 	}
@@ -153,20 +284,8 @@ func parseFilterContents(contents []byte) (int, string, []string) {
 	return rulesCount, name, lines
 }
 
-// Checks for filters updates
-// If "force" is true -- does not check the filter's LastUpdated field
-// Call "save" to persist the filter contents
-func (filter *filter) update(force bool) (bool, error) {
-	if filter.ID == 0 { // protect against users deleting the ID
-		filter.ID = assignUniqueFilterID()
-	}
-	if !filter.Enabled {
-		return false, nil
-	}
-	if !force && time.Since(filter.LastUpdated) <= updatePeriod {
-		return false, nil
-	}
-
+// Perform upgrade on a filter
+func (filter *filter) update() (bool, error) {
 	log.Tracef("Downloading update for filter %d from %s", filter.ID, filter.URL)
 
 	resp, err := client.Get(filter.URL)
@@ -195,22 +314,22 @@ func (filter *filter) update(force bool) (bool, error) {
 		return false, err
 	}
 
-	// Extract filter name and count number of rules
-	rulesCount, filterName, rules := parseFilterContents(body)
-
-	if filterName != "" {
-		filter.Name = filterName
-	}
-
 	// Check if the filter has been really changed
-	if reflect.DeepEqual(filter.Rules, rules) {
+	checksum := crc32.ChecksumIEEE(body)
+	if filter.checksum == checksum {
 		log.Tracef("Filter #%d at URL %s hasn't changed, not updating it", filter.ID, filter.URL)
 		return false, nil
 	}
 
+	// Extract filter name and count number of rules
+	rulesCount, filterName, rules := parseFilterContents(body)
 	log.Printf("Filter %d has been updated: %d bytes, %d rules", filter.ID, len(body), rulesCount)
+	if filterName != "" {
+		filter.Name = filterName
+	}
 	filter.RulesCount = rulesCount
 	filter.Rules = rules
+	filter.checksum = checksum
 
 	return true, nil
 }
@@ -230,11 +349,6 @@ func (filter *filter) save() error {
 
 // loads filter contents from the file in dataDir
 func (filter *filter) load() error {
-	if !filter.Enabled {
-		// No need to load a filter that is not enabled
-		return nil
-	}
-
 	filterFilePath := filter.Path()
 	log.Tracef("Loading filter %d contents to: %s", filter.ID, filterFilePath)
 
@@ -253,11 +367,18 @@ func (filter *filter) load() error {
 
 	filter.RulesCount = rulesCount
 	filter.Rules = rules
+	filter.checksum = crc32.ChecksumIEEE(filterFileContents)
 	filter.LastUpdated = filter.LastTimeUpdated()
 
 	return nil
 }
 
+// Clear filter rules
+func (filter *filter) unload() {
+	filter.Rules = []string{}
+	filter.RulesCount = 0
+}
+
 // Path to the filter contents
 func (filter *filter) Path() string {
 	return filepath.Join(config.ourWorkingDir, dataDir, filterDir, strconv.FormatInt(filter.ID, 10)+".txt")
@@ -266,12 +387,12 @@ func (filter *filter) Path() string {
 // LastTimeUpdated returns the time when the filter was last time updated
 func (filter *filter) LastTimeUpdated() time.Time {
 	filterFilePath := filter.Path()
-	if _, err := os.Stat(filterFilePath); os.IsNotExist(err) {
+	s, err := os.Stat(filterFilePath)
+	if os.IsNotExist(err) {
 		// if the filter file does not exist, return 0001-01-01
 		return time.Time{}
 	}
 
-	s, err := os.Stat(filterFilePath)
 	if err != nil {
 		// if the filter file does not exist, return 0001-01-01
 		return time.Time{}