diff --git a/internal/dhcpd/db.go b/internal/dhcpd/db.go
index 2618f12e..7408c9e0 100644
--- a/internal/dhcpd/db.go
+++ b/internal/dhcpd/db.go
@@ -4,6 +4,7 @@ package dhcpd
 
 import (
 	"encoding/json"
+	"errors"
 	"io/ioutil"
 	"net"
 	"os"
@@ -39,9 +40,10 @@ func (s *Server) dbLoad() {
 
 	data, err := ioutil.ReadFile(s.conf.DBFilePath)
 	if err != nil {
-		if !os.IsNotExist(err) {
-			log.Error("DHCP: can't read file %s: %v", s.conf.DBFilePath, err)
+		if !errors.Is(err, os.ErrNotExist) {
+			log.Error("dhcp: can't read file %q: %v", s.conf.DBFilePath, err)
 		}
+
 		return
 	}
 
diff --git a/internal/dhcpd/http.go b/internal/dhcpd/http.go
index b0817ab6..9fea7bf6 100644
--- a/internal/dhcpd/http.go
+++ b/internal/dhcpd/http.go
@@ -508,8 +508,8 @@ func (s *Server) handleReset(w http.ResponseWriter, r *http.Request) {
 	s.Stop()
 
 	err := os.Remove(s.conf.DBFilePath)
-	if err != nil && !os.IsNotExist(err) {
-		log.Error("DHCP: os.Remove: %s: %s", s.conf.DBFilePath, err)
+	if err != nil && !errors.Is(err, os.ErrNotExist) {
+		log.Error("dhcp: removing %q: %s", s.conf.DBFilePath, err)
 	}
 
 	oldconf := s.conf
diff --git a/internal/home/control.go b/internal/home/control.go
index 19876b12..e7724794 100644
--- a/internal/home/control.go
+++ b/internal/home/control.go
@@ -239,7 +239,7 @@ func handleHTTPSRedirect(w http.ResponseWriter, r *http.Request) (ok bool) {
 		}
 
 		httpsURL := &url.URL{
-			Scheme:   "https",
+			Scheme:   schemeHTTPS,
 			Host:     hostPort,
 			Path:     r.URL.Path,
 			RawQuery: r.URL.RawQuery,
@@ -255,7 +255,7 @@ func handleHTTPSRedirect(w http.ResponseWriter, r *http.Request) (ok bool) {
 	//
 	// See https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Access-Control-Allow-Origin.
 	originURL := &url.URL{
-		Scheme: "http",
+		Scheme: schemeHTTP,
 		Host:   r.Host,
 	}
 	w.Header().Set("Access-Control-Allow-Origin", originURL.String())
diff --git a/internal/home/controlfiltering.go b/internal/home/controlfiltering.go
index b64423d1..2de312e4 100644
--- a/internal/home/controlfiltering.go
+++ b/internal/home/controlfiltering.go
@@ -12,26 +12,31 @@ import (
 	"strings"
 	"time"
 
-	"github.com/AdguardTeam/AdGuardHome/internal/util"
 	"github.com/AdguardTeam/golibs/log"
 	"github.com/miekg/dns"
 )
 
-// isValidURL - return TRUE if URL or file path is valid
-func isValidURL(rawurl string) bool {
-	if filepath.IsAbs(rawurl) {
-		// this is a file path
-		return util.FileExists(rawurl)
+// validateFilterURL validates the filter list URL or file name.
+func validateFilterURL(urlStr string) (err error) {
+	if filepath.IsAbs(urlStr) {
+		_, err = os.Stat(urlStr)
+		if err != nil {
+			return fmt.Errorf("checking filter file: %w", err)
+		}
+
+		return nil
 	}
 
-	url, err := url.ParseRequestURI(rawurl)
+	url, err := url.ParseRequestURI(urlStr)
 	if err != nil {
-		return false // Couldn't even parse the rawurl
+		return fmt.Errorf("checking filter url: %w", err)
 	}
-	if len(url.Scheme) == 0 {
-		return false // No Scheme found
+
+	if s := url.Scheme; s != schemeHTTP && s != schemeHTTPS {
+		return fmt.Errorf("checking filter url: invalid scheme %q", s)
 	}
-	return true
+
+	return nil
 }
 
 type filterAddJSON struct {
@@ -48,8 +53,11 @@ func (f *Filtering) handleFilteringAddURL(w http.ResponseWriter, r *http.Request
 		return
 	}
 
-	if !isValidURL(fj.URL) {
-		http.Error(w, "Invalid URL or file path", http.StatusBadRequest)
+	err = validateFilterURL(fj.URL)
+	if err != nil {
+		msg := fmt.Sprintf("invalid url: %s", err)
+		http.Error(w, msg, http.StatusBadRequest)
+
 		return
 	}
 
@@ -168,8 +176,11 @@ func (f *Filtering) handleFilteringSetURL(w http.ResponseWriter, r *http.Request
 		return
 	}
 
-	if !isValidURL(fj.Data.URL) {
-		http.Error(w, "invalid URL or file path", http.StatusBadRequest)
+	err = validateFilterURL(fj.Data.URL)
+	if err != nil {
+		msg := fmt.Sprintf("invalid url: %s", err)
+		http.Error(w, msg, http.StatusBadRequest)
+
 		return
 	}
 
diff --git a/internal/home/home.go b/internal/home/home.go
index 54bae585..21307c05 100644
--- a/internal/home/home.go
+++ b/internal/home/home.go
@@ -570,8 +570,8 @@ func loadOptions() options {
 	return o
 }
 
-// prints IP addresses which user can use to open the admin interface
-// proto is either "http" or "https"
+// printHTTPAddresses prints the IP addresses which user can use to open the
+// admin interface.  proto is either schemeHTTP or schemeHTTPS.
 func printHTTPAddresses(proto string) {
 	tlsConf := tlsConfigSettings{}
 	if Context.tls != nil {
@@ -579,12 +579,12 @@ func printHTTPAddresses(proto string) {
 	}
 
 	port := strconv.Itoa(config.BindPort)
-	if proto == "https" {
+	if proto == schemeHTTPS {
 		port = strconv.Itoa(tlsConf.PortHTTPS)
 	}
 
 	var hostStr string
-	if proto == "https" && tlsConf.ServerName != "" {
+	if proto == schemeHTTPS && tlsConf.ServerName != "" {
 		if tlsConf.PortHTTPS == 443 {
 			log.Printf("Go to https://%s", tlsConf.ServerName)
 		} else {
diff --git a/internal/home/mobileconfig.go b/internal/home/mobileconfig.go
index 3953e2e6..9ea3e7f5 100644
--- a/internal/home/mobileconfig.go
+++ b/internal/home/mobileconfig.go
@@ -58,7 +58,7 @@ func getMobileConfig(d dnsSettings) ([]byte, error) {
 		dspName = fmt.Sprintf("%s DoH", d.ServerName)
 
 		u := &url.URL{
-			Scheme: "https",
+			Scheme: schemeHTTPS,
 			Host:   d.ServerName,
 			Path:   "/dns-query",
 		}
diff --git a/internal/home/service.go b/internal/home/service.go
index 94364cbb..caebb582 100644
--- a/internal/home/service.go
+++ b/internal/home/service.go
@@ -236,7 +236,7 @@ func handleServiceInstallCommand(s service.Service) {
 AdGuard Home is successfully installed and will automatically start on boot.
 There are a few more things that must be configured before you can use it.
 Click on the link below and follow the Installation Wizard steps to finish setup.`)
-		printHTTPAddresses("http")
+		printHTTPAddresses(schemeHTTP)
 	}
 }
 
diff --git a/internal/home/web.go b/internal/home/web.go
index 38ba6dcb..6b650576 100644
--- a/internal/home/web.go
+++ b/internal/home/web.go
@@ -15,6 +15,12 @@ import (
 	"github.com/gobuffalo/packr"
 )
 
+// HTTP scheme constants.
+const (
+	schemeHTTP  = "http"
+	schemeHTTPS = "https"
+)
+
 const (
 	// readTimeout is the maximum duration for reading the entire request,
 	// including the body.
@@ -159,7 +165,7 @@ func (web *Web) Start() {
 
 	// this loop is used as an ability to change listening host and/or port
 	for !web.httpsServer.shutdown {
-		printHTTPAddresses("http")
+		printHTTPAddresses(schemeHTTP)
 		errs := make(chan error, 2)
 
 		hostStr := web.conf.BindHost.String()
@@ -256,7 +262,7 @@ func (web *Web) tlsServerLoop() {
 			WriteTimeout:      web.conf.WriteTimeout,
 		}
 
-		printHTTPAddresses("https")
+		printHTTPAddresses(schemeHTTPS)
 		err := web.httpsServer.server.ListenAndServeTLS("", "")
 		if err != http.ErrServerClosed {
 			cleanupAlways()
diff --git a/internal/querylog/http.go b/internal/querylog/http.go
index b235c2fe..4f71c21d 100644
--- a/internal/querylog/http.go
+++ b/internal/querylog/http.go
@@ -8,8 +8,6 @@ import (
 	"strconv"
 	"time"
 
-	"github.com/AdguardTeam/AdGuardHome/internal/util"
-
 	"github.com/AdguardTeam/golibs/jsonutil"
 	"github.com/AdguardTeam/golibs/log"
 )
@@ -127,6 +125,17 @@ func getDoubleQuotesEnclosedValue(s *string) bool {
 	return false
 }
 
+// inStr checks if string is in the slice of strings.
+func inStr(strs []string, str string) (ok bool) {
+	for _, s := range strs {
+		if s == str {
+			return true
+		}
+	}
+
+	return false
+}
+
 // parseSearchCriteria - parses "searchCriteria" from the specified query parameter
 func (l *queryLog) parseSearchCriteria(q url.Values, name string, ct criteriaType) (bool, searchCriteria, error) {
 	val := q.Get(name)
@@ -142,7 +151,7 @@ func (l *queryLog) parseSearchCriteria(q url.Values, name string, ct criteriaTyp
 		c.strict = true
 	}
 
-	if ct == ctFilteringStatus && !util.ContainsString(filteringStatusValues, c.value) {
+	if ct == ctFilteringStatus && !inStr(filteringStatusValues, c.value) {
 		return false, c, fmt.Errorf("invalid value %s", c.value)
 	}
 
diff --git a/internal/querylog/qlogreader.go b/internal/querylog/qlogreader.go
index 19909110..03a93a0f 100644
--- a/internal/querylog/qlogreader.go
+++ b/internal/querylog/qlogreader.go
@@ -4,8 +4,10 @@ import (
 	"errors"
 	"fmt"
 	"io"
+	"os"
 
 	"github.com/AdguardTeam/AdGuardHome/internal/agherr"
+	"github.com/AdguardTeam/golibs/log"
 )
 
 // QLogReader allows reading from multiple query log files in the reverse order.
@@ -30,8 +32,16 @@ func NewQLogReader(files []string) (*QLogReader, error) {
 	for _, f := range files {
 		q, err := NewQLogFile(f)
 		if err != nil {
-			// Close what we've already opened
-			_ = closeQFiles(qFiles)
+			if errors.Is(err, os.ErrNotExist) {
+				continue
+			}
+
+			// Close what we've already opened.
+			cerr := closeQFiles(qFiles)
+			if cerr != nil {
+				log.Debug("querylog: closing files: %s", cerr)
+			}
+
 			return nil, err
 		}
 
diff --git a/internal/querylog/search.go b/internal/querylog/search.go
index b114c4e9..216f9167 100644
--- a/internal/querylog/search.go
+++ b/internal/querylog/search.go
@@ -5,7 +5,6 @@ import (
 	"sort"
 	"time"
 
-	"github.com/AdguardTeam/AdGuardHome/internal/util"
 	"github.com/AdguardTeam/golibs/log"
 )
 
@@ -87,9 +86,15 @@ func (l *queryLog) searchFiles(params *searchParams) ([]*logEntry, time.Time, in
 	entries := make([]*logEntry, 0)
 	oldest := time.Time{}
 
-	r, err := l.openReader()
+	files := []string{
+		l.logFile + ".1",
+		l.logFile,
+	}
+
+	r, err := NewQLogReader(files)
 	if err != nil {
-		log.Error("Failed to open qlog reader: %v", err)
+		log.Error("querylog: failed to open qlog reader: %s", err)
+
 		return entries, oldest, 0
 	}
 	defer r.Close()
@@ -174,17 +179,3 @@ func (l *queryLog) readNextEntry(r *QLogReader, params *searchParams) (*logEntry
 
 	return &entry, timestamp, nil
 }
-
-// openReader - opens QLogReader instance
-func (l *queryLog) openReader() (*QLogReader, error) {
-	files := make([]string, 0)
-
-	if util.FileExists(l.logFile + ".1") {
-		files = append(files, l.logFile+".1")
-	}
-	if util.FileExists(l.logFile) {
-		files = append(files, l.logFile)
-	}
-
-	return NewQLogReader(files)
-}
diff --git a/internal/updater/updater.go b/internal/updater/updater.go
index 4bc96fc3..d6a09a04 100644
--- a/internal/updater/updater.go
+++ b/internal/updater/updater.go
@@ -5,6 +5,7 @@ import (
 	"archive/tar"
 	"archive/zip"
 	"compress/gzip"
+	"errors"
 	"fmt"
 	"io"
 	"io/ioutil"
@@ -19,7 +20,6 @@ import (
 	"time"
 
 	"github.com/AdguardTeam/AdGuardHome/internal/aghio"
-	"github.com/AdguardTeam/AdGuardHome/internal/util"
 	"github.com/AdguardTeam/AdGuardHome/internal/version"
 	"github.com/AdguardTeam/golibs/log"
 )
@@ -160,7 +160,7 @@ func (u *Updater) VersionCheckURL() (vcu string) {
 	return u.versionCheckURL
 }
 
-func (u *Updater) prepare() error {
+func (u *Updater) prepare() (err error) {
 	u.updateDir = filepath.Join(u.workDir, fmt.Sprintf("agh-update-%s", u.newVersion))
 
 	_, pkgNameOnly := filepath.Split(u.packageURL)
@@ -181,12 +181,11 @@ func (u *Updater) prepare() error {
 
 	log.Info("Updating from %s to %s.  URL:%s", version.Version(), u.newVersion, u.packageURL)
 
-	// If the binary file isn't found in working directory, we won't be able
-	// to auto-update.  Getting the full path to the current binary file on
-	// Unix and checking write permissions is more difficult.
+	// TODO(a.garipov): Use os.Args[0] instead?
 	u.currentExeName = filepath.Join(u.workDir, exeName)
-	if !util.FileExists(u.currentExeName) {
-		return fmt.Errorf("executable file %s doesn't exist", u.currentExeName)
+	_, err = os.Stat(u.currentExeName)
+	if err != nil {
+		return fmt.Errorf("checking %q: %w", u.currentExeName, err)
 	}
 
 	return nil
@@ -367,11 +366,14 @@ func tarGzFileUnpack(tarfile, outdir string) ([]string, error) {
 			}
 
 			err = os.Mkdir(outputName, os.FileMode(header.Mode&0o777))
-			if err != nil && !os.IsExist(err) {
-				err2 = fmt.Errorf("os.Mkdir(%s): %w", outputName, err)
+			if err != nil && !errors.Is(err, os.ErrExist) {
+				err2 = fmt.Errorf("os.Mkdir(%q): %w", outputName, err)
+
 				break
 			}
-			log.Debug("updater: created directory %s", outputName)
+
+			log.Debug("updater: created directory %q", outputName)
+
 			continue
 		} else if header.Typeflag != tar.TypeReg {
 			log.Debug("updater: %s: unknown file type %d, skipping", inputNameOnly, header.Typeflag)
@@ -443,11 +445,14 @@ func zipFileUnpack(zipfile, outdir string) ([]string, error) {
 			}
 
 			err = os.Mkdir(outputName, fi.Mode())
-			if err != nil && !os.IsExist(err) {
-				err2 = fmt.Errorf("os.Mkdir(): %w", err)
+			if err != nil && !errors.Is(err, os.ErrExist) {
+				err2 = fmt.Errorf("os.Mkdir(%q): %w", outputName, err)
+
 				break
 			}
-			log.Tracef("created directory %s", outputName)
+
+			log.Tracef("created directory %q", outputName)
+
 			continue
 		}
 
@@ -501,11 +506,12 @@ func copySupportingFiles(files []string, srcdir, dstdir string) error {
 		dst := filepath.Join(dstdir, name)
 
 		err := copyFile(src, dst)
-		if err != nil && !os.IsNotExist(err) {
+		if err != nil && !errors.Is(err, os.ErrNotExist) {
 			return err
 		}
 
-		log.Debug("updater: copied: %s -> %s", src, dst)
+		log.Debug("updater: copied: %q -> %q", src, dst)
 	}
+
 	return nil
 }
diff --git a/internal/util/autohosts.go b/internal/util/autohosts.go
index ad157171..c3156920 100644
--- a/internal/util/autohosts.go
+++ b/internal/util/autohosts.go
@@ -2,10 +2,12 @@ package util
 
 import (
 	"bufio"
+	"errors"
 	"io"
 	"io/ioutil"
 	"net"
 	"os"
+	"path/filepath"
 	"runtime"
 	"strings"
 	"sync"
@@ -76,7 +78,7 @@ func (a *AutoHosts) Init(hostsFn string) {
 	var err error
 	a.watcher, err = fsnotify.NewWatcher()
 	if err != nil {
-		log.Error("AutoHosts: %s", err)
+		log.Error("autohosts: %s", err)
 	}
 }
 
@@ -127,7 +129,7 @@ func (a *AutoHosts) Process(host string, qtype uint16) []net.IP {
 		copy(ipsCopy, ips)
 	}
 
-	log.Debug("AutoHosts: answer: %s -> %v", host, ipsCopy)
+	log.Debug("autohosts: answer: %s -> %v", host, ipsCopy)
 	return ipsCopy
 }
 
@@ -153,7 +155,7 @@ func (a *AutoHosts) ProcessReverse(addr string, qtype uint16) (hosts []string) {
 		return nil // not found
 	}
 
-	log.Debug("AutoHosts: reverse-lookup: %s -> %s", addr, hosts)
+	log.Debug("autohosts: reverse-lookup: %s -> %s", addr, hosts)
 
 	return hosts
 }
@@ -191,7 +193,7 @@ func (a *AutoHosts) updateTable(table map[string][]net.IP, host string, ipAddr n
 		ok = true
 	}
 	if ok {
-		log.Debug("AutoHosts: added %s -> %s", ipAddr, host)
+		log.Debug("autohosts: added %s -> %s", ipAddr, host)
 	}
 }
 
@@ -201,7 +203,7 @@ func (a *AutoHosts) updateTableRev(tableRev map[string][]string, newHost string,
 	hosts, ok := tableRev[ipStr]
 	if !ok {
 		tableRev[ipStr] = []string{newHost}
-		log.Debug("AutoHosts: added reverse-address %s -> %s", ipStr, newHost)
+		log.Debug("autohosts: added reverse-address %s -> %s", ipStr, newHost)
 
 		return
 	}
@@ -213,7 +215,7 @@ func (a *AutoHosts) updateTableRev(tableRev map[string][]string, newHost string,
 	}
 
 	tableRev[ipStr] = append(tableRev[ipStr], newHost)
-	log.Debug("AutoHosts: added reverse-address %s -> %s", ipStr, newHost)
+	log.Debug("autohosts: added reverse-address %s -> %s", ipStr, newHost)
 }
 
 // Read IP-hostname pairs from file
@@ -221,12 +223,12 @@ func (a *AutoHosts) updateTableRev(tableRev map[string][]string, newHost string,
 func (a *AutoHosts) load(table map[string][]net.IP, tableRev map[string][]string, fn string) {
 	f, err := os.Open(fn)
 	if err != nil {
-		log.Error("AutoHosts: %s", err)
+		log.Error("autohosts: %s", err)
 		return
 	}
 	defer f.Close()
 	r := bufio.NewReader(f)
-	log.Debug("AutoHosts: loading hosts from file %s", fn)
+	log.Debug("autohosts: loading hosts from file %s", fn)
 
 	for done := false; !done; {
 		var line string
@@ -234,7 +236,7 @@ func (a *AutoHosts) load(table map[string][]net.IP, tableRev map[string][]string
 		if err == io.EOF {
 			done = true
 		} else if err != nil {
-			log.Error("AutoHosts: %s", err)
+			log.Error("autohosts: %s", err)
 
 			return
 		}
@@ -310,7 +312,7 @@ func (a *AutoHosts) watcherLoop() {
 			}
 
 			if event.Op&fsnotify.Write == fsnotify.Write {
-				log.Debug("AutoHosts: modified: %s", event.Name)
+				log.Debug("autohosts: modified: %s", event.Name)
 				a.updateHosts()
 			}
 
@@ -318,7 +320,7 @@ func (a *AutoHosts) watcherLoop() {
 			if !ok {
 				return
 			}
-			log.Error("AutoHosts: %s", err)
+			log.Error("autohosts: %s", err)
 		}
 	}
 }
@@ -333,14 +335,15 @@ func (a *AutoHosts) updateHosts() {
 	for _, dir := range a.hostsDirs {
 		fis, err := ioutil.ReadDir(dir)
 		if err != nil {
-			if !os.IsNotExist(err) {
-				log.Error("AutoHosts: Opening directory: %s: %s", dir, err)
+			if !errors.Is(err, os.ErrNotExist) {
+				log.Error("autohosts: Opening directory: %q: %s", dir, err)
 			}
+
 			continue
 		}
 
 		for _, fi := range fis {
-			a.load(table, tableRev, dir+"/"+fi.Name())
+			a.load(table, tableRev, filepath.Join(dir, fi.Name()))
 		}
 	}
 
diff --git a/internal/util/helpers.go b/internal/util/helpers.go
index 0dad08b9..2ccc9bbe 100644
--- a/internal/util/helpers.go
+++ b/internal/util/helpers.go
@@ -8,29 +8,12 @@ import (
 	"bytes"
 	"fmt"
 	"io/ioutil"
-	"os"
 	"os/exec"
 	"path/filepath"
 	"runtime"
 	"strings"
 )
 
-// ContainsString checks if string is in the slice of strings.
-func ContainsString(strs []string, str string) bool {
-	for _, s := range strs {
-		if s == str {
-			return true
-		}
-	}
-	return false
-}
-
-// FileExists returns true if file exists.
-func FileExists(fn string) bool {
-	_, err := os.Stat(fn)
-	return err == nil || !os.IsNotExist(err)
-}
-
 // RunCommand runs shell command.
 func RunCommand(command string, arguments ...string) (int, string, error) {
 	cmd := exec.Command(command, arguments...)
diff --git a/internal/util/tls.go b/internal/util/tls.go
index f1aa4976..ed70d24f 100644
--- a/internal/util/tls.go
+++ b/internal/util/tls.go
@@ -3,6 +3,7 @@ package util
 import (
 	"crypto/tls"
 	"crypto/x509"
+	"errors"
 	"io/ioutil"
 	"os"
 	"runtime"
@@ -29,8 +30,8 @@ func LoadSystemRootCAs() *x509.CertPool {
 	for _, dir := range dirs {
 		fis, err := ioutil.ReadDir(dir)
 		if err != nil {
-			if !os.IsNotExist(err) {
-				log.Error("Opening directory: %s: %s", dir, err)
+			if !errors.Is(err, os.ErrNotExist) {
+				log.Error("opening directory: %q: %s", dir, err)
 			}
 
 			continue