Pull request: aghnet: imp host validation for system resolvers

Updates #3022.

Squashed commit of the following:

commit 2f63b4e1765d9c9bfeadafcfa42c9d8741b628e1
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Wed Apr 28 21:29:28 2021 +0300

    aghnet: fix doc

commit efdc1bb2c8959a9f888d558c32c415e6f3678b0c
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Wed Apr 28 21:19:54 2021 +0300

    all: doc changes

commit 8154797095874771bcf04d109644e6ae33fcb470
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Wed Apr 28 21:15:42 2021 +0300

    aghnet: imp host validation for system resolvers

CHANGELOG.md

@@ -17,6 +17,14 @@ and this project adheres to
 ## [v0.106.1] - 2021-05-17 (APPROX.)
 -->
 
+### Fixed
+
+- Validation of IPv6 addresses with zones in system resolvers ([#3022]).
+
+[#3022]: https://github.com/AdguardTeam/AdGuardHome/issues/3022
+
+
+
 ## [v0.106.0] - 2021-04-28
 
 ### Added

internal/aghnet/systemresolvers.go

@@ -26,11 +26,15 @@ type SystemResolvers interface {
 }
 
 const (
-	// fakeDialErr is an error which dialFunc is expected to return.
+	// errBadAddrPassed is returned when dialFunc can't parse an IP address.
-	fakeDialErr agherr.Error = "this error signals the successful dialFunc work"
+	errBadAddrPassed agherr.Error = "the passed string is not a valid IP address"
 
-	// badAddrPassedErr is returned when dialFunc can't parse an IP address.
+	// errFakeDial is an error which dialFunc is expected to return.
-	badAddrPassedErr agherr.Error = "the passed string is not a valid IP address"
+	errFakeDial agherr.Error = "this error signals the successful dialFunc work"
+
+	// errUnexpectedHostFormat is returned by validateDialedHost when the host has
+	// more than one percent sign.
+	errUnexpectedHostFormat agherr.Error = "unexpected host format"
 )
 
 // refreshWithTicker refreshes the cache of sr after each tick form tickCh.

internal/aghnet/systemresolvers_others.go

@@ -7,6 +7,7 @@ import (
 	"errors"
 	"fmt"
 	"net"
+	"strings"
 	"sync"
 	"time"
 
@@ -35,7 +36,7 @@ func (sr *systemResolvers) refresh() (err error) {
 
 	_, err = sr.resolver.LookupHost(context.Background(), sr.hostGenFunc())
 	dnserr := &net.DNSError{}
-	if errors.As(err, &dnserr) && dnserr.Err == fakeDialErr.Error() {
+	if errors.As(err, &dnserr) && dnserr.Err == errFakeDial.Error() {
 		return nil
 	}
 
@@ -58,19 +59,43 @@ func newSystemResolvers(refreshIvl time.Duration, hostGenFunc HostGenFunc) (sr S
 	return s
 }
 
+// validateDialedHost validated the host used by resolvers in dialFunc.
+func validateDialedHost(host string) (err error) {
+	defer agherr.Annotate("parsing %q: %w", &err, host)
+
+	var ipStr string
+	parts := strings.Split(host, "%")
+	switch len(parts) {
+	case 1:
+		ipStr = host
+	case 2:
+		// Remove the zone and check the IP address part.
+		ipStr = parts[0]
+	default:
+		return errUnexpectedHostFormat
+	}
+
+	if net.ParseIP(ipStr) == nil {
+		return errBadAddrPassed
+	}
+
+	return nil
+}
+
 // dialFunc gets the resolver's address and puts it into internal cache.
 func (sr *systemResolvers) dialFunc(_ context.Context, _, address string) (_ net.Conn, err error) {
 	// Just validate the passed address is a valid IP.
 	var host string
 	host, err = SplitHost(address)
 	if err != nil {
-		// TODO(e.burkov): Maybe use a structured badAddrPassedErr to
+		// TODO(e.burkov): Maybe use a structured errBadAddrPassed to
 		// allow unwrapping of the real error.
-		return nil, fmt.Errorf("%s: %w", err, badAddrPassedErr)
+		return nil, fmt.Errorf("%s: %w", err, errBadAddrPassed)
 	}
 
-	if net.ParseIP(host) == nil {
+	err = validateDialedHost(host)
-		return nil, fmt.Errorf("parsing %q: %w", host, badAddrPassedErr)
+	if err != nil {
+		return nil, fmt.Errorf("validating dialed host: %w", err)
 	}
 
 	sr.addrsLock.Lock()
@@ -78,7 +103,7 @@ func (sr *systemResolvers) dialFunc(_ context.Context, _, address string) (_ net
 
 	sr.addrs.Add(host)
 
-	return nil, fakeDialErr
+	return nil, errFakeDial
 }
 
 func (sr *systemResolvers) Get() (rs []string) {

internal/aghnet/systemresolvers_others_test.go

@@ -46,21 +46,33 @@ func TestSystemResolvers_DialFunc(t *testing.T) {
 	imp := createTestSystemResolversImp(t, 0, nil)
 
 	testCases := []struct {
+		want    error
 		name    string
 		address string
-		want    error
 	}{{
+		want:    errFakeDial,
 		name:    "valid",
 		address: "127.0.0.1",
-		want:    fakeDialErr,
 	}, {
+		want:    errFakeDial,
+		name:    "valid_ipv6_port",
+		address: "[::1]:53",
+	}, {
+		want:    errFakeDial,
+		name:    "valid_ipv6_zone_port",
+		address: "[::1%lo0]:53",
+	}, {
+		want:    errBadAddrPassed,
 		name:    "invalid_split_host",
 		address: "127.0.0.1::123",
-		want:    badAddrPassedErr,
 	}, {
+		want:    errUnexpectedHostFormat,
+		name:    "invalid_ipv6_zone_port",
+		address: "[::1%%lo0]:53",
+	}, {
+		want:    errBadAddrPassed,
 		name:    "invalid_parse_ip",
 		address: "not-ip",
-		want:    badAddrPassedErr,
 	}}
 
 	for _, tc := range testCases {