diff --git a/internal/agherr/agherr.go b/internal/agherr/agherr.go
index aedf2a8ba06..4cef546925f 100644
--- a/internal/agherr/agherr.go
+++ b/internal/agherr/agherr.go
@@ -1,5 +1,4 @@
-// Package agherr contains the extended error type, and the function for
-// wrapping several errors.
+// Package agherr contains AdGuard Home's error handling helpers.
 package agherr
 
 import (
@@ -23,8 +22,8 @@ type manyError struct {
 }
 
 // Many wraps several errors and returns a single error.
-func Many(message string, underlying ...error) error {
-	err := &manyError{
+func Many(message string, underlying ...error) (err error) {
+	err = &manyError{
 		message:    message,
 		underlying: underlying,
 	}
@@ -33,7 +32,7 @@ func Many(message string, underlying ...error) error {
 }
 
 // Error implements the error interface for *manyError.
-func (e *manyError) Error() string {
+func (e *manyError) Error() (msg string) {
 	switch len(e.underlying) {
 	case 0:
 		return e.message
@@ -58,7 +57,7 @@ func (e *manyError) Error() string {
 }
 
 // Unwrap implements the hidden errors.wrapper interface for *manyError.
-func (e *manyError) Unwrap() error {
+func (e *manyError) Unwrap() (err error) {
 	if len(e.underlying) == 0 {
 		return nil
 	}
@@ -71,3 +70,37 @@ func (e *manyError) Unwrap() error {
 type wrapper interface {
 	Unwrap() error
 }
+
+// Annotate annotates the error with the message, unless the error is nil.  This
+// is a helper function to simplify code like this:
+//
+//   func (f *foo) doStuff(s string) (err error) {
+//           defer func() {
+//                   if err != nil {
+//                           err = fmt.Errorf("bad foo string %q: %w", s, err)
+//                   }
+//           }()
+//
+//           // …
+//   }
+//
+// Instead, write:
+//
+//   func (f *foo) doStuff(s string) (err error) {
+//           defer agherr.Annotate("bad foo string %q: %w", &err, s)
+//
+//           // …
+//   }
+//
+func Annotate(msg string, errPtr *error, args ...interface{}) {
+	if errPtr == nil {
+		return
+	}
+
+	err := *errPtr
+	if err != nil {
+		args = append(args, err)
+
+		*errPtr = fmt.Errorf(msg, args...)
+	}
+}
diff --git a/internal/agherr/agherr_test.go b/internal/agherr/agherr_test.go
index 3ac5aeab426..b9a1ff900c0 100644
--- a/internal/agherr/agherr_test.go
+++ b/internal/agherr/agherr_test.go
@@ -6,30 +6,32 @@ import (
 	"testing"
 
 	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
 )
 
 func TestError_Error(t *testing.T) {
 	testCases := []struct {
+		err  error
 		name string
 		want string
-		err  error
 	}{{
+		err:  Many("a"),
 		name: "simple",
 		want: "a",
-		err:  Many("a"),
 	}, {
+		err:  Many("a", errors.New("b")),
 		name: "wrapping",
 		want: "a: b",
-		err:  Many("a", errors.New("b")),
 	}, {
+		err:  Many("a", errors.New("b"), errors.New("c"), errors.New("d")),
 		name: "wrapping several",
 		want: "a: b (hidden: c, d)",
-		err:  Many("a", errors.New("b"), errors.New("c"), errors.New("d")),
 	}, {
+		err:  Many("a", Many("b", errors.New("c"), errors.New("d"))),
 		name: "wrapping wrapper",
 		want: "a: b: c (hidden: d)",
-		err:  Many("a", Many("b", errors.New("c"), errors.New("d"))),
 	}}
+
 	for _, tc := range testCases {
 		assert.Equal(t, tc.want, tc.err.Error(), tc.name)
 	}
@@ -43,33 +45,78 @@ func TestError_Unwrap(t *testing.T) {
 		errWrapped
 		errNil
 	)
+
 	errs := []error{
 		errSimple:  errors.New("a"),
 		errWrapped: fmt.Errorf("err: %w", errors.New("nested")),
 		errNil:     nil,
 	}
+
 	testCases := []struct {
-		name    string
 		want    error
 		wrapped error
+		name    string
 	}{{
-		name:    "simple",
 		want:    errs[errSimple],
 		wrapped: Many("a", errs[errSimple]),
+		name:    "simple",
 	}, {
-		name:    "nested",
 		want:    errs[errWrapped],
 		wrapped: Many("b", errs[errWrapped]),
+		name:    "nested",
 	}, {
-		name:    "nil passed",
 		want:    errs[errNil],
 		wrapped: Many("c", errs[errNil]),
+		name:    "nil passed",
 	}, {
-		name:    "nil not passed",
 		want:    nil,
 		wrapped: Many("d"),
+		name:    "nil not passed",
 	}}
+
 	for _, tc := range testCases {
 		assert.Equal(t, tc.want, errors.Unwrap(tc.wrapped), tc.name)
 	}
 }
+
+func TestAnnotate(t *testing.T) {
+	const s = "1234"
+	const wantMsg = `bad string "1234": test`
+
+	// Don't use const, because we can't take a pointer of a contant.
+	var errTest error = Error("test")
+
+	t.Run("nil", func(t *testing.T) {
+		var errPtr *error
+		assert.NotPanics(t, func() {
+			Annotate("bad string %q: %w", errPtr, s)
+		})
+	})
+
+	t.Run("non_nil", func(t *testing.T) {
+		errPtr := &errTest
+		assert.NotPanics(t, func() {
+			Annotate("bad string %q: %w", errPtr, s)
+		})
+
+		require.NotNil(t, errPtr)
+
+		err := *errPtr
+		require.NotNil(t, err)
+
+		assert.Equal(t, wantMsg, err.Error())
+	})
+
+	t.Run("defer", func(t *testing.T) {
+		f := func() (err error) {
+			defer Annotate("bad string %q: %w", &errTest, s)
+
+			return errTest
+		}
+
+		err := f()
+		require.NotNil(t, err)
+
+		assert.Equal(t, wantMsg, err.Error())
+	})
+}
diff --git a/internal/dhcpd/iprange.go b/internal/dhcpd/iprange.go
index fa70c8f6ab8..50242255dff 100644
--- a/internal/dhcpd/iprange.go
+++ b/internal/dhcpd/iprange.go
@@ -5,6 +5,8 @@ import (
 	"math"
 	"math/big"
 	"net"
+
+	"github.com/AdguardTeam/AdGuardHome/internal/agherr"
 )
 
 // ipRange is an inclusive range of IP addresses.
@@ -18,17 +20,14 @@ type ipRange struct {
 	end   *big.Int
 }
 
-// maxRangeLen is the maximum IP range length.
+// maxRangeLen is the maximum IP range length.  The bitsets used in servers only
+// accept uints, which can have the size of 32 bit.
 const maxRangeLen = math.MaxUint32
 
 // newIPRange creates a new IP address range.  start must be less than end.  The
 // resulting range must not be greater than maxRangeLen.
 func newIPRange(start, end net.IP) (r *ipRange, err error) {
-	defer func() {
-		if err != nil {
-			err = fmt.Errorf("invalid ip range: %w", err)
-		}
-	}()
+	defer agherr.Annotate("invalid ip range: %w", &err)
 
 	// Make sure that both are 16 bytes long to simplify handling in
 	// methods.
@@ -85,7 +84,7 @@ func (r *ipRange) find(p ipPredicate) (ip net.IP) {
 
 // offset returns the offset of ip from the beginning of r.  It returns 0 and
 // false if ip is not in r.
-func (r *ipRange) offset(ip net.IP) (offset uint64, ok bool) {
+func (r *ipRange) offset(ip net.IP) (offset uint, ok bool) {
 	ip = ip.To16()
 	ipInt := (&big.Int{}).SetBytes(ip)
 	if !r.containsInt(ipInt) {
@@ -96,5 +95,5 @@ func (r *ipRange) offset(ip net.IP) (offset uint64, ok bool) {
 
 	// Assume that the range was checked against maxRangeLen during
 	// construction.
-	return offsetInt.Uint64(), true
+	return uint(offsetInt.Uint64()), true
 }
diff --git a/internal/dhcpd/iprange_test.go b/internal/dhcpd/iprange_test.go
index 2a95e691836..01991532a39 100644
--- a/internal/dhcpd/iprange_test.go
+++ b/internal/dhcpd/iprange_test.go
@@ -115,7 +115,7 @@ func TestIPRange_Offset(t *testing.T) {
 	testCases := []struct {
 		name       string
 		in         net.IP
-		wantOffset uint64
+		wantOffset uint
 		wantOK     bool
 	}{{
 		name:       "in",
diff --git a/internal/dhcpd/options.go b/internal/dhcpd/options.go
index 9992764efc0..780eeeab132 100644
--- a/internal/dhcpd/options.go
+++ b/internal/dhcpd/options.go
@@ -100,11 +100,7 @@ func newDHCPOptionParser() (p *dhcpOptionParser) {
 
 // parse parses an option.  See the handlers' documentation for more info.
 func (p *dhcpOptionParser) parse(s string) (code uint8, data []byte, err error) {
-	defer func() {
-		if err != nil {
-			err = fmt.Errorf("invalid option string %q: %w", s, err)
-		}
-	}()
+	defer agherr.Annotate("invalid option string %q: %w", &err, s)
 
 	s = strings.TrimSpace(s)
 	parts := strings.SplitN(s, " ", 3)
diff --git a/internal/dhcpd/v4.go b/internal/dhcpd/v4.go
index c243157a061..a4d7c5c82e9 100644
--- a/internal/dhcpd/v4.go
+++ b/internal/dhcpd/v4.go
@@ -20,15 +20,18 @@ import (
 //
 // TODO(a.garipov): Think about unifying this and v6Server.
 type v4Server struct {
-	srv        *server4.Server
-	leasesLock sync.Mutex
-	leases     []*Lease
+	conf V4ServerConf
+	srv  *server4.Server
 
 	// leasedOffsets contains offsets from conf.ipRange.start that have been
 	// leased.
 	leasedOffsets *bitset.BitSet
 
-	conf V4ServerConf
+	// leases contains all dynamic and static leases.
+	leases []*Lease
+
+	// leasesLock protects leases and leasedOffsets.
+	leasesLock sync.Mutex
 }
 
 // WriteDiskConfig4 - write configuration
@@ -116,11 +119,16 @@ func (s *v4Server) blacklistLease(lease *Lease) {
 	lease.Expiry = time.Now().Add(s.conf.leaseTime)
 }
 
-// removeLease removes a lease by its offset from the beginning of the IP range.
-func (s *v4Server) removeLease(offset uint) {
-	l := s.leases[offset]
-	s.leases = append(s.leases[:offset], s.leases[offset+1:]...)
-	s.leasedOffsets.Clear(offset)
+// rmLeaseByIndex removes a lease by its index in the leases slice.
+func (s *v4Server) rmLeaseByIndex(i int) {
+	l := s.leases[i]
+	s.leases = append(s.leases[:i], s.leases[i+1:]...)
+
+	r := s.conf.ipRange
+	offset, ok := r.offset(l.IP)
+	if ok {
+		s.leasedOffsets.Clear(offset)
+	}
 
 	log.Debug("dhcpv4: removed lease %s (%s)", l.IP, l.HWAddr)
 }
@@ -136,7 +144,7 @@ func (s *v4Server) rmDynamicLease(lease Lease) error {
 				return fmt.Errorf("static lease already exists")
 			}
 
-			s.removeLease(uint(i))
+			s.rmLeaseByIndex(i)
 			if i == len(s.leases) {
 				break
 			}
@@ -149,7 +157,7 @@ func (s *v4Server) rmDynamicLease(lease Lease) error {
 				return fmt.Errorf("static lease already exists")
 			}
 
-			s.removeLease(uint(i))
+			s.rmLeaseByIndex(i)
 		}
 	}
 	return nil
@@ -175,14 +183,13 @@ func (s *v4Server) addLease(l *Lease) {
 // Remove a lease with the same properties
 func (s *v4Server) rmLease(lease Lease) error {
 	for i, l := range s.leases {
-		if net.IP.Equal(l.IP, lease.IP) {
-
+		if l.IP.Equal(lease.IP) {
 			if !bytes.Equal(l.HWAddr, lease.HWAddr) ||
 				l.Hostname != lease.Hostname {
 				return fmt.Errorf("lease not found")
 			}
 
-			s.removeLease(uint(i))
+			s.rmLeaseByIndex(i)
 
 			return nil
 		}
@@ -255,7 +262,7 @@ func (s *v4Server) addrAvailable(target net.IP) bool {
 	pinger.Timeout = time.Duration(s.conf.ICMPTimeout) * time.Millisecond
 	pinger.Count = 1
 	reply := false
-	pinger.OnRecv = func(pkt *ping.Packet) {
+	pinger.OnRecv = func(_ *ping.Packet) {
 		reply = true
 	}
 	log.Debug("dhcpv4: Sending ICMP Echo to %v", target)
diff --git a/staticcheck.conf b/staticcheck.conf
index 4dd931764d5..b997f6a9477 100644
--- a/staticcheck.conf
+++ b/staticcheck.conf
@@ -1,6 +1,8 @@
 checks = ["all"]
 initialisms = [
   # See https://github.com/dominikh/go-tools/blob/master/config/config.go.
+  #
+  # Do not add "PTR" since we use "Ptr" as a suffix.
   "inherit"
 , "DHCP"
 , "DOH"
@@ -8,7 +10,6 @@ initialisms = [
 , "DOT"
 , "EDNS"
 , "MX"
-, "PTR"
 , "QUIC"
 , "RA"
 , "SDNS"