Feature: more robust updater DNS resolution

- Parallel resolver to resolve multiple hosts
- Repeat resolver to repeat resolution for a single host
- Additional parameters for fault toleration
- Do not update servers if e.g. > 10% DNS resolutions failed
- resolver package in updater package

internal/updater/alias.go

@@ -1,10 +0,0 @@
-package updater
-
-import (
-	"context"
-	"net"
-)
-
-type (
-	lookupIPFunc func(ctx context.Context, host string) (ips []net.IP, err error)
-)

internal/updater/cyberghost.go

@@ -2,16 +2,16 @@ package updater
 
 import (
 	"context"
-	"fmt"
 	"sort"
 	"time"
 
 	"github.com/qdm12/gluetun/internal/constants"
 	"github.com/qdm12/gluetun/internal/models"
+	"github.com/qdm12/gluetun/internal/updater/resolver"
 )
 
 func (u *updater) updateCyberghost(ctx context.Context) (err error) {
-	servers, err := findCyberghostServers(ctx, u.lookupIP)
+	servers, err := findCyberghostServers(ctx, u.presolver)
 	if err != nil {
 		return err
 	}
@@ -23,62 +23,82 @@ func (u *updater) updateCyberghost(ctx context.Context) (err error) {
 	return nil
 }
 
-func findCyberghostServers(ctx context.Context, lookupIP lookupIPFunc) (servers []models.CyberghostServer, err error) {
+func findCyberghostServers(ctx context.Context, presolver resolver.Parallel) (
+	servers []models.CyberghostServer, err error) {
 	groups := getCyberghostGroups()
 	allCountryCodes := constants.CountryCodes()
 	cyberghostCountryCodes := getCyberghostSubdomainToRegion()
 	possibleCountryCodes := mergeCountryCodes(cyberghostCountryCodes, allCountryCodes)
 
-	results := make(chan models.CyberghostServer)
-	const maxGoroutines = 10
-	guard := make(chan struct{}, maxGoroutines)
-	defer close(guard)
+	// key is the host
+	possibleServers := make(map[string]models.CyberghostServer, len(groups)*len(possibleCountryCodes))
+	possibleHosts := make([]string, 0, len(groups)*len(possibleCountryCodes))
 	for groupID, groupName := range groups {
 		for countryCode, region := range possibleCountryCodes {
-			if err := ctx.Err(); err != nil {
-				return nil, err
-			}
 			const domain = "cg-dialup.net"
-			host := fmt.Sprintf("%s-%s.%s", groupID, countryCode, domain)
-			go tryCyberghostHostname(ctx, lookupIP, host, groupName, region, results, guard)
+			possibleHost := groupID + "-" + countryCode + "." + domain
+			possibleHosts = append(possibleHosts, possibleHost)
+			possibleServer := models.CyberghostServer{
+				Region: region,
+				Group:  groupName,
+			}
+			possibleServers[possibleHost] = possibleServer
 		}
 	}
-	for i := 0; i < len(groups)*len(possibleCountryCodes); i++ {
-		server := <-results
-		if server.IPs == nil {
-			continue
+
+	const (
+		maxFailRatio = 1
+		minFound     = 100
+		maxDuration  = 10 * time.Second
+		maxNoNew     = 2
+		maxFails     = 1
+	)
+	settings := resolver.ParallelSettings{
+		MaxFailRatio: maxFailRatio,
+		MinFound:     minFound,
+		Repeat: resolver.RepeatSettings{
+			MaxDuration:     maxDuration,
+			BetweenDuration: time.Second,
+			MaxNoNew:        maxNoNew,
+			MaxFails:        maxFails,
+			SortIPs:         true,
+		},
+	}
+	hostToIPs, _, err := presolver.Resolve(ctx, possibleHosts, settings)
+	if err != nil {
+		return nil, err
+	}
+
+	if err := ctx.Err(); err != nil {
+		return nil, err
+	}
+
+	// Set IPs for servers found
+	for host, IPs := range hostToIPs {
+		server := possibleServers[host]
+		server.IPs = IPs
+		possibleServers[host] = server
+	}
+
+	// Remove servers with no IPs (aka not found)
+	for host, server := range possibleServers {
+		if len(server.IPs) == 0 {
+			delete(possibleServers, host)
 		}
+	}
+
+	// Flatten possibleServers to a slice
+	servers = make([]models.CyberghostServer, 0, len(possibleServers))
+	for _, server := range possibleServers {
 		servers = append(servers, server)
 	}
-	if err := ctx.Err(); err != nil {
-		return servers, err
-	}
+
 	sort.Slice(servers, func(i, j int) bool {
 		return servers[i].Region < servers[j].Region
 	})
 	return servers, nil
 }
 
-func tryCyberghostHostname(ctx context.Context, lookupIP lookupIPFunc,
-	host, groupName, region string,
-	results chan<- models.CyberghostServer, guard chan struct{}) {
-	guard <- struct{}{}
-	defer func() {
-		<-guard
-	}()
-	const repetition = 10
-	IPs, err := resolveRepeat(ctx, lookupIP, host, repetition, time.Second)
-	if err != nil || len(IPs) == 0 {
-		results <- models.CyberghostServer{}
-		return
-	}
-	results <- models.CyberghostServer{
-		Region: region,
-		Group:  groupName,
-		IPs:    IPs,
-	}
-}
-
 //nolint:goconst
 func stringifyCyberghostServers(servers []models.CyberghostServer) (s string) {
 	s = "func CyberghostServers() []models.CyberghostServer {\n"

internal/updater/fastestvpn.go

@@ -7,12 +7,14 @@ import (
 	"regexp"
 	"sort"
 	"strings"
+	"time"
 
 	"github.com/qdm12/gluetun/internal/models"
+	"github.com/qdm12/gluetun/internal/updater/resolver"
 )
 
 func (u *updater) updateFastestvpn(ctx context.Context) (err error) {
-	servers, warnings, err := findFastestvpnServersFromZip(ctx, u.client, u.lookupIP)
+	servers, warnings, err := findFastestvpnServersFromZip(ctx, u.client, u.presolver)
 	if u.options.CLI {
 		for _, warning := range warnings {
 			u.logger.Warn("FastestVPN: %s", warning)
@@ -29,7 +31,7 @@ func (u *updater) updateFastestvpn(ctx context.Context) (err error) {
 	return nil
 }
 
-func findFastestvpnServersFromZip(ctx context.Context, client *http.Client, lookupIP lookupIPFunc) (
+func findFastestvpnServersFromZip(ctx context.Context, client *http.Client, presolver resolver.Parallel) (
 	servers []models.FastestvpnServer, warnings []string, err error) {
 	const zipURL = "https://support.fastestvpn.com/download/openvpn-tcp-udp-config-files"
 	contents, err := fetchAndExtractFiles(ctx, client, zipURL)
@@ -98,10 +100,22 @@ func findFastestvpnServersFromZip(ctx context.Context, client *http.Client, look
 		i++
 	}
 
-	const repetition = 1
-	const timeBetween = 0
-	const failOnErr = true
-	hostToIPs, _, err := parallelResolve(ctx, lookupIP, hosts, repetition, timeBetween, failOnErr)
+	const (
+		maxFailRatio = 0.1
+		maxNoNew     = 1
+		maxFails     = 2
+	)
+	settings := resolver.ParallelSettings{
+		MaxFailRatio: maxFailRatio,
+		Repeat: resolver.RepeatSettings{
+			MaxDuration: time.Second,
+			MaxNoNew:    maxNoNew,
+			MaxFails:    maxFails,
+			SortIPs:     true,
+		},
+	}
+	hostToIPs, newWarnings, err := presolver.Resolve(ctx, hosts, settings)
+	warnings = append(warnings, newWarnings...)
 	if err != nil {
 		return nil, warnings, err
 	}
@@ -120,7 +134,7 @@ func findFastestvpnServersFromZip(ctx context.Context, client *http.Client, look
 			TCP:      data.TCP,
 			UDP:      data.UDP,
 			Country:  data.Country,
-			IPs:      uniqueSortedIPs(IPs),
+			IPs:      IPs,
 		}
 		servers = append(servers, server)
 	}

internal/updater/hma.go

@@ -12,10 +12,11 @@ import (
 	"unicode"
 
 	"github.com/qdm12/gluetun/internal/models"
+	"github.com/qdm12/gluetun/internal/updater/resolver"
 )
 
 func (u *updater) updateHideMyAss(ctx context.Context) (err error) {
-	servers, warnings, err := findHideMyAssServers(ctx, u.client, u.lookupIP)
+	servers, warnings, err := findHideMyAssServers(ctx, u.client, u.presolver)
 	if u.options.CLI {
 		for _, warning := range warnings {
 			u.logger.Warn("HideMyAss: %s", warning)
@@ -32,7 +33,7 @@ func (u *updater) updateHideMyAss(ctx context.Context) (err error) {
 	return nil
 }
 
-func findHideMyAssServers(ctx context.Context, client *http.Client, lookupIP lookupIPFunc) (
+func findHideMyAssServers(ctx context.Context, client *http.Client, presolver resolver.Parallel) (
 	servers []models.HideMyAssServer, warnings []string, err error) {
 	TCPhostToURL, err := findHideMyAssHostToURLForProto(ctx, client, "TCP")
 	if err != nil {
@@ -59,10 +60,27 @@ func findHideMyAssServers(ctx context.Context, client *http.Client, lookupIP loo
 		i++
 	}
 
-	const failOnErr = false
-	const resolveRepetition = 5
-	const timeBetween = 2 * time.Second
-	hostToIPs, warnings, _ := parallelResolve(ctx, lookupIP, hosts, resolveRepetition, timeBetween, failOnErr)
+	const (
+		maxFailRatio    = 0.1
+		maxDuration     = 15 * time.Second
+		betweenDuration = 2 * time.Second
+		maxNoNew        = 2
+		maxFails        = 2
+	)
+	settings := resolver.ParallelSettings{
+		MaxFailRatio: maxFailRatio,
+		Repeat: resolver.RepeatSettings{
+			MaxDuration:     maxDuration,
+			BetweenDuration: betweenDuration,
+			MaxNoNew:        maxNoNew,
+			MaxFails:        maxFails,
+			SortIPs:         true,
+		},
+	}
+	hostToIPs, warnings, err := presolver.Resolve(ctx, hosts, settings)
+	if err != nil {
+		return nil, warnings, err
+	}
 
 	servers = make([]models.HideMyAssServer, 0, len(hostToIPs))
 	for host, IPs := range hostToIPs {

internal/updater/privado.go

@@ -5,12 +5,14 @@ import (
 	"fmt"
 	"net/http"
 	"sort"
+	"time"
 
 	"github.com/qdm12/gluetun/internal/models"
+	"github.com/qdm12/gluetun/internal/updater/resolver"
 )
 
 func (u *updater) updatePrivado(ctx context.Context) (err error) {
-	servers, warnings, err := findPrivadoServersFromZip(ctx, u.client, u.lookupIP)
+	servers, warnings, err := findPrivadoServersFromZip(ctx, u.client, u.presolver)
 	if u.options.CLI {
 		for _, warning := range warnings {
 			u.logger.Warn("Privado: %s", warning)
@@ -27,7 +29,7 @@ func (u *updater) updatePrivado(ctx context.Context) (err error) {
 	return nil
 }
 
-func findPrivadoServersFromZip(ctx context.Context, client *http.Client, lookupIP lookupIPFunc) (
+func findPrivadoServersFromZip(ctx context.Context, client *http.Client, presolver resolver.Parallel) (
 	servers []models.PrivadoServer, warnings []string, err error) {
 	const zipURL = "https://privado.io/apps/ovpn_configs.zip"
 	contents, err := fetchAndExtractFiles(ctx, client, zipURL)
@@ -47,11 +49,26 @@ func findPrivadoServersFromZip(ctx context.Context, client *http.Client, lookupI
 		hosts = append(hosts, hostname)
 	}
 
-	const repetition = 1
-	const timeBetween = 1
-	const failOnErr = false
-	hostToIPs, newWarnings, _ := parallelResolve(ctx, lookupIP, hosts, repetition, timeBetween, failOnErr)
+	const (
+		maxFailRatio = 0.1
+		maxDuration  = 3 * time.Second
+		maxNoNew     = 1
+		maxFails     = 2
+	)
+	settings := resolver.ParallelSettings{
+		MaxFailRatio: maxFailRatio,
+		Repeat: resolver.RepeatSettings{
+			MaxDuration: maxDuration,
+			MaxNoNew:    maxNoNew,
+			MaxFails:    maxFails,
+			SortIPs:     true,
+		},
+	}
+	hostToIPs, newWarnings, err := presolver.Resolve(ctx, hosts, settings)
 	warnings = append(warnings, newWarnings...)
+	if err != nil {
+		return nil, warnings, err
+	}
 
 	for hostname, IPs := range hostToIPs {
 		switch len(IPs) {

internal/updater/privatevpn.go

@@ -11,10 +11,11 @@ import (
 
 	"github.com/qdm12/gluetun/internal/constants"
 	"github.com/qdm12/gluetun/internal/models"
+	"github.com/qdm12/gluetun/internal/updater/resolver"
 )
 
 func (u *updater) updatePrivatevpn(ctx context.Context) (err error) {
-	servers, warnings, err := findPrivatevpnServersFromZip(ctx, u.client, u.lookupIP)
+	servers, warnings, err := findPrivatevpnServersFromZip(ctx, u.client, u.presolver)
 	if u.options.CLI {
 		for _, warning := range warnings {
 			u.logger.Warn("Privatevpn: %s", warning)
@@ -31,7 +32,7 @@ func (u *updater) updatePrivatevpn(ctx context.Context) (err error) {
 	return nil
 }
 
-func findPrivatevpnServersFromZip(ctx context.Context, client *http.Client, lookupIP lookupIPFunc) (
+func findPrivatevpnServersFromZip(ctx context.Context, client *http.Client, presolver resolver.Parallel) (
 	servers []models.PrivatevpnServer, warnings []string, err error) {
 	// Note: all servers do both TCP and UDP
 	const zipURL = "https://privatevpn.com/client/PrivateVPN-TUN.zip"
@@ -93,10 +94,27 @@ func findPrivatevpnServersFromZip(ctx context.Context, client *http.Client, look
 		i++
 	}
 
-	const failOnError = false
-	hostToIPs, newWarnings, _ := parallelResolve(ctx, lookupIP, hostnames, 5, time.Second, failOnError)
-	if len(newWarnings) > 0 {
-		warnings = append(warnings, newWarnings...)
+	const (
+		maxFailRatio    = 0.1
+		maxDuration     = 6 * time.Second
+		betweenDuration = time.Second
+		maxNoNew        = 2
+		maxFails        = 2
+	)
+	settings := resolver.ParallelSettings{
+		MaxFailRatio: maxFailRatio,
+		Repeat: resolver.RepeatSettings{
+			MaxDuration:     maxDuration,
+			BetweenDuration: betweenDuration,
+			MaxNoNew:        maxNoNew,
+			MaxFails:        maxFails,
+			SortIPs:         true,
+		},
+	}
+	hostToIPs, newWarnings, err := presolver.Resolve(ctx, hostnames, settings)
+	warnings = append(warnings, newWarnings...)
+	if err != nil {
+		return nil, warnings, err
 	}
 
 	for hostname, server := range uniqueServers {

internal/updater/purevpn.go

@@ -10,10 +10,11 @@ import (
 
 	"github.com/qdm12/gluetun/internal/models"
 	"github.com/qdm12/gluetun/internal/publicip"
+	"github.com/qdm12/gluetun/internal/updater/resolver"
 )
 
 func (u *updater) updatePurevpn(ctx context.Context) (err error) {
-	servers, warnings, err := findPurevpnServers(ctx, u.client, u.lookupIP)
+	servers, warnings, err := findPurevpnServers(ctx, u.client, u.presolver)
 	if u.options.CLI {
 		for _, warning := range warnings {
 			u.logger.Warn("PureVPN: %s", warning)
@@ -30,7 +31,7 @@ func (u *updater) updatePurevpn(ctx context.Context) (err error) {
 	return nil
 }
 
-func findPurevpnServers(ctx context.Context, client *http.Client, lookupIP lookupIPFunc) (
+func findPurevpnServers(ctx context.Context, client *http.Client, presolver resolver.Parallel) (
 	servers []models.PurevpnServer, warnings []string, err error) {
 	const zipURL = "https://s3-us-west-1.amazonaws.com/heartbleed/windows/New+OVPN+Files.zip"
 	contents, err := fetchAndExtractFiles(ctx, client, zipURL)
@@ -53,10 +54,25 @@ func findPurevpnServers(ctx context.Context, client *http.Client, lookupIP looku
 		hosts = append(hosts, host)
 	}
 
-	const repetition = 20
-	const timeBetween = time.Second
-	const failOnErr = true
-	hostToIPs, _, err := parallelResolve(ctx, lookupIP, hosts, repetition, timeBetween, failOnErr)
+	const (
+		maxFailRatio    = 0.1
+		maxDuration     = 20 * time.Second
+		betweenDuration = time.Second
+		maxNoNew        = 2
+		maxFails        = 2
+	)
+	settings := resolver.ParallelSettings{
+		MaxFailRatio: maxFailRatio,
+		Repeat: resolver.RepeatSettings{
+			MaxDuration:     maxDuration,
+			BetweenDuration: betweenDuration,
+			MaxNoNew:        maxNoNew,
+			MaxFails:        maxFails,
+			SortIPs:         true,
+		},
+	}
+	hostToIPs, newWarnings, err := presolver.Resolve(ctx, hosts, settings)
+	warnings = append(warnings, newWarnings...)
 	if err != nil {
 		return nil, warnings, err
 	}

internal/updater/resolver.go

@@ -1,134 +0,0 @@
-package updater
-
-import (
-	"bytes"
-	"context"
-	"net"
-	"sort"
-	"time"
-)
-
-func newResolver(resolverAddress string) *net.Resolver {
-	d := net.Dialer{}
-	resolverAddress = net.JoinHostPort(resolverAddress, "53")
-	return &net.Resolver{
-		PreferGo: true,
-		Dial: func(ctx context.Context, network, address string) (net.Conn, error) {
-			return d.DialContext(ctx, "udp", resolverAddress)
-		},
-	}
-}
-
-func newLookupIP(r *net.Resolver) lookupIPFunc {
-	return func(ctx context.Context, host string) (ips []net.IP, err error) {
-		addresses, err := r.LookupIPAddr(ctx, host)
-		if err != nil {
-			return nil, err
-		}
-		ips = make([]net.IP, len(addresses))
-		for i := range addresses {
-			ips[i] = addresses[i].IP
-		}
-		return ips, nil
-	}
-}
-
-func parallelResolve(ctx context.Context, lookupIP lookupIPFunc, hosts []string,
-	repetition int, timeBetween time.Duration, failOnErr bool) (
-	hostToIPs map[string][]net.IP, warnings []string, err error) {
-	ctx, cancel := context.WithCancel(ctx)
-	defer cancel()
-
-	type result struct {
-		host string
-		ips  []net.IP
-	}
-
-	results := make(chan result)
-	defer close(results)
-	errors := make(chan error)
-	defer close(errors)
-
-	for _, host := range hosts {
-		go func(host string) {
-			ips, err := resolveRepeat(ctx, lookupIP, host, repetition, timeBetween)
-			if err != nil {
-				errors <- err
-				return
-			}
-			results <- result{
-				host: host,
-				ips:  ips,
-			}
-		}(host)
-	}
-
-	hostToIPs = make(map[string][]net.IP, len(hosts))
-
-	for range hosts {
-		select {
-		case newErr := <-errors:
-			if !failOnErr {
-				warnings = append(warnings, newErr.Error())
-			} else if err == nil {
-				err = newErr
-				cancel()
-			}
-		case r := <-results:
-			hostToIPs[r.host] = r.ips
-		}
-	}
-
-	return hostToIPs, warnings, err
-}
-
-func resolveRepeat(ctx context.Context, lookupIP lookupIPFunc, host string,
-	repetition int, timeBetween time.Duration) (ips []net.IP, err error) {
-	uniqueIPs := make(map[string]struct{})
-
-	i := 0
-	for {
-		newIPs, newErr := lookupIP(ctx, host)
-		if err == nil {
-			err = newErr // it's fine to fail some of the resolutions
-		}
-		for _, ip := range newIPs {
-			key := ip.String()
-			uniqueIPs[key] = struct{}{}
-		}
-
-		i++
-		if i == repetition {
-			break
-		}
-
-		timer := time.NewTimer(timeBetween)
-		select {
-		case <-timer.C:
-		case <-ctx.Done():
-			if !timer.Stop() {
-				<-timer.C
-			}
-			return nil, ctx.Err()
-		}
-	}
-
-	if len(uniqueIPs) == 0 {
-		return nil, err
-	}
-
-	ips = make([]net.IP, 0, len(uniqueIPs))
-	for key := range uniqueIPs {
-		ip := net.ParseIP(key)
-		if ipv4 := ip.To4(); ipv4 != nil {
-			ip = ipv4
-		}
-		ips = append(ips, ip)
-	}
-
-	sort.Slice(ips, func(i, j int) bool {
-		return bytes.Compare(ips[i], ips[j]) < 1
-	})
-
-	return ips, err
-}

internal/updater/resolver/ips.go

@@ -0,0 +1,15 @@
+package resolver
+
+import "net"
+
+func uniqueIPsToSlice(uniqueIPs map[string]struct{}) (ips []net.IP) {
+	ips = make([]net.IP, 0, len(uniqueIPs))
+	for key := range uniqueIPs {
+		IP := net.ParseIP(key)
+		if IPv4 := IP.To4(); IPv4 != nil {
+			IP = IPv4
+		}
+		ips = append(ips, IP)
+	}
+	return ips
+}

internal/updater/resolver/ips_test.go

@@ -0,0 +1,41 @@
+package resolver
+
+import (
+	"net"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func Test_uniqueIPsToSlice(t *testing.T) {
+	t.Parallel()
+	testCases := map[string]struct {
+		inputIPs  map[string]struct{}
+		outputIPs []net.IP
+	}{
+		"nil": {
+			inputIPs:  nil,
+			outputIPs: []net.IP{},
+		},
+		"empty": {
+			inputIPs:  map[string]struct{}{},
+			outputIPs: []net.IP{},
+		},
+		"single IPv4": {
+			inputIPs:  map[string]struct{}{"1.1.1.1": {}},
+			outputIPs: []net.IP{{1, 1, 1, 1}},
+		},
+		"two IPv4s": {
+			inputIPs:  map[string]struct{}{"1.1.1.1": {}, "1.1.2.1": {}},
+			outputIPs: []net.IP{{1, 1, 1, 1}, {1, 1, 2, 1}},
+		},
+	}
+	for name, testCase := range testCases {
+		testCase := testCase
+		t.Run(name, func(t *testing.T) {
+			t.Parallel()
+			outputIPs := uniqueIPsToSlice(testCase.inputIPs)
+			assert.Equal(t, testCase.outputIPs, outputIPs)
+		})
+	}
+}

internal/updater/resolver/net.go

@@ -0,0 +1,17 @@
+package resolver
+
+import (
+	"context"
+	"net"
+)
+
+func newResolver(resolverAddress string) *net.Resolver {
+	d := net.Dialer{}
+	resolverAddress = net.JoinHostPort(resolverAddress, "53")
+	return &net.Resolver{
+		PreferGo: true,
+		Dial: func(ctx context.Context, network, address string) (net.Conn, error) {
+			return d.DialContext(ctx, "udp", resolverAddress)
+		},
+	}
+}

internal/updater/resolver/parallel.go

@@ -0,0 +1,127 @@
+package resolver
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"net"
+)
+
+type Parallel interface {
+	Resolve(ctx context.Context, hosts []string, settings ParallelSettings) (
+		hostToIPs map[string][]net.IP, warnings []string, err error)
+}
+
+type parallel struct {
+	repeatResolver Repeat
+}
+
+func NewParallelResolver(address string) Parallel {
+	return &parallel{
+		repeatResolver: NewRepeat(address),
+	}
+}
+
+type ParallelSettings struct {
+	Repeat    RepeatSettings
+	FailEarly bool
+	// Maximum ratio of the hosts failing DNS resolution
+	// divided by the total number of hosts requested.
+	// This value is between 0 and 1. Note this is only
+	// applicable if FailEarly is not set to true.
+	MaxFailRatio float64
+	// MinFound is the minimum number of hosts to be found.
+	// If it is bigger than the number of hosts given, it
+	// is set to the number of hosts given.
+	MinFound int
+}
+
+type parallelResult struct {
+	host string
+	IPs  []net.IP
+}
+
+var (
+	ErrMinFound     = errors.New("not enough hosts found")
+	ErrMaxFailRatio = errors.New("maximum failure ratio reached")
+)
+
+func (pr *parallel) Resolve(ctx context.Context, hosts []string,
+	settings ParallelSettings) (hostToIPs map[string][]net.IP, warnings []string, err error) {
+	minFound := settings.MinFound
+	if minFound > len(hosts) {
+		minFound = len(hosts)
+	}
+
+	ctx, cancel := context.WithCancel(ctx)
+	defer cancel()
+
+	results := make(chan parallelResult)
+	defer close(results)
+	errors := make(chan error)
+	defer close(errors)
+
+	for _, host := range hosts {
+		go pr.resolveAsync(ctx, host, settings.Repeat, results, errors)
+	}
+
+	hostToIPs = make(map[string][]net.IP, len(hosts))
+	maxFails := int(settings.MaxFailRatio * float64(len(hosts)))
+
+	for range hosts {
+		select {
+		case newErr := <-errors:
+			if settings.FailEarly {
+				if err == nil {
+					// only set the error to the first error encountered
+					// and not the context canceled errors coming after.
+					err = newErr
+					cancel()
+				}
+				break
+			}
+
+			// do not add warnings coming from the call to cancel()
+			if len(warnings) < maxFails {
+				warnings = append(warnings, newErr.Error())
+			}
+
+			if len(warnings) == maxFails {
+				cancel() // cancel only once when we reach maxFails
+			}
+		case result := <-results:
+			hostToIPs[result.host] = result.IPs
+		}
+	}
+
+	if err != nil { // fail early
+		return nil, warnings, err
+	}
+
+	if len(hostToIPs) < minFound {
+		return nil, warnings,
+			fmt.Errorf("%w: found %d hosts but expected at least %d",
+				ErrMinFound, len(hostToIPs), minFound)
+	}
+
+	failureRatio := float64(len(warnings)) / float64(len(hosts))
+	if failureRatio > settings.MaxFailRatio {
+		return hostToIPs, warnings,
+			fmt.Errorf("%w: %.2f failure ratio reached", ErrMaxFailRatio, failureRatio)
+	}
+
+	return hostToIPs, warnings, nil
+}
+
+func (pr *parallel) resolveAsync(ctx context.Context, host string,
+	settings RepeatSettings, results chan<- parallelResult, errors chan<- error) {
+	IPs, err := pr.repeatResolver.Resolve(ctx, host, settings)
+	if err != nil {
+		errors <- err
+		return
+	}
+	results <- parallelResult{
+		host: host,
+		IPs:  IPs,
+	}
+}

internal/updater/resolver/repeat.go

@@ -0,0 +1,141 @@
+package resolver
+
+import (
+	"bytes"
+	"context"
+	"errors"
+	"fmt"
+	"net"
+	"sort"
+	"time"
+)
+
+type Repeat interface {
+	Resolve(ctx context.Context, host string, settings RepeatSettings) (IPs []net.IP, err error)
+}
+
+type repeat struct {
+	resolver *net.Resolver
+}
+
+func NewRepeat(address string) Repeat {
+	return &repeat{
+		resolver: newResolver(address),
+	}
+}
+
+type RepeatSettings struct {
+	MaxDuration     time.Duration
+	BetweenDuration time.Duration
+	MaxNoNew        int
+	// Maximum consecutive DNS resolution failures
+	MaxFails int
+	SortIPs  bool
+}
+
+func (r *repeat) Resolve(ctx context.Context, host string, settings RepeatSettings) (ips []net.IP, err error) {
+	timedCtx, cancel := context.WithTimeout(ctx, settings.MaxDuration)
+	defer cancel()
+
+	noNewCounter := 0
+	failCounter := 0
+	uniqueIPs := make(map[string]struct{})
+
+	for err == nil {
+		// TODO
+		// - one resolving every 100ms for round robin DNS responses
+		// - one every second for time based DNS cycling responses
+		noNewCounter, failCounter, err = r.resolveOnce(ctx, timedCtx, host, settings, uniqueIPs, noNewCounter, failCounter)
+	}
+
+	if len(uniqueIPs) == 0 {
+		return nil, err
+	}
+
+	ips = uniqueIPsToSlice(uniqueIPs)
+
+	if settings.SortIPs {
+		sort.Slice(ips, func(i, j int) bool {
+			return bytes.Compare(ips[i], ips[j]) < 1
+		})
+	}
+
+	return ips, nil
+}
+
+var (
+	ErrMaxNoNew = errors.New("reached the maximum number of no new update")
+	ErrMaxFails = errors.New("reached the maximum number of consecutive failures")
+	ErrTimeout  = errors.New("reached the timeout")
+)
+
+func (r *repeat) resolveOnce(ctx, timedCtx context.Context, host string,
+	settings RepeatSettings, uniqueIPs map[string]struct{}, noNewCounter, failCounter int) (
+	newNoNewCounter, newFailCounter int, err error) {
+	IPs, err := r.lookupIPs(timedCtx, host)
+	if err != nil {
+		failCounter++
+		if settings.MaxFails > 0 && failCounter == settings.MaxFails {
+			return noNewCounter, failCounter, fmt.Errorf("%w: %d failed attempts resolving %s: %s",
+				ErrMaxFails, settings.MaxFails, host, err)
+		}
+		// it's fine to fail some of the resolutions
+		return noNewCounter, failCounter, nil
+	}
+	failCounter = 0 // reset the counter if we had no error
+
+	anyNew := false
+	for _, IP := range IPs {
+		key := IP.String()
+		if _, ok := uniqueIPs[key]; !ok {
+			anyNew = true
+			uniqueIPs[key] = struct{}{}
+		}
+	}
+
+	if !anyNew {
+		noNewCounter++
+	}
+
+	if settings.MaxNoNew > 0 && noNewCounter == settings.MaxNoNew {
+		// we reached the maximum number of resolutions without
+		// finding any new IP address to our unique IP addresses set.
+		return noNewCounter, failCounter,
+			fmt.Errorf("%w: %d times no updated for %d IP addresses found",
+				ErrMaxNoNew, noNewCounter, len(uniqueIPs))
+	}
+
+	timer := time.NewTimer(settings.BetweenDuration)
+	select {
+	case <-timer.C:
+		return noNewCounter, failCounter, nil
+	case <-ctx.Done():
+		if !timer.Stop() {
+			<-timer.C
+		}
+		return noNewCounter, failCounter, ctx.Err()
+	case <-timedCtx.Done():
+		if err := ctx.Err(); err != nil {
+			// timedCtx was canceled from its parent context
+			return noNewCounter, failCounter, err
+		}
+		return noNewCounter, failCounter,
+			fmt.Errorf("%w: %s", ErrTimeout, timedCtx.Err())
+	}
+}
+
+func (r *repeat) lookupIPs(ctx context.Context, host string) (ips []net.IP, err error) {
+	addresses, err := r.resolver.LookupIPAddr(ctx, host)
+	if err != nil {
+		return nil, err
+	}
+	ips = make([]net.IP, 0, len(addresses))
+	for i := range addresses {
+		ip := addresses[i].IP
+		if ip == nil {
+			continue
+		}
+		ips = append(ips, ip)
+	}
+	return ips, nil
+}

internal/updater/resolver/resolver.go

@@ -0,0 +1,3 @@
+// Package resolver defines custom resolvers to resolve
+// hosts multiple times with adjustable settings.
+package resolver

internal/updater/resolver_test.go

@@ -1,83 +0,0 @@
-package updater
-
-import (
-	"context"
-	"fmt"
-	"net"
-	"sync"
-	"testing"
-
-	"github.com/stretchr/testify/assert"
-	"github.com/stretchr/testify/require"
-)
-
-func Test_resolveRepeat(t *testing.T) {
-	t.Parallel()
-	testCases := map[string]struct {
-		lookupIPResult [][]net.IP
-		lookupIPErr    error
-		n              int
-		ips            []net.IP
-		err            error
-	}{
-		"failure twice": {
-			lookupIPResult: [][]net.IP{{}, {}},
-			lookupIPErr:    fmt.Errorf("feeling sick"),
-			n:              2,
-			err:            fmt.Errorf("feeling sick"),
-		},
-		"failure once": {
-			lookupIPResult: [][]net.IP{{}, {{1, 1, 1, 1}}},
-			lookupIPErr:    fmt.Errorf("feeling sick"),
-			n:              2,
-			ips:            []net.IP{{1, 1, 1, 1}},
-			err:            fmt.Errorf("feeling sick"),
-		},
-		"successful": {
-			lookupIPResult: [][]net.IP{
-				{{1, 1, 1, 1}, {1, 1, 1, 2}},
-				{{2, 1, 1, 1}, {2, 1, 1, 2}},
-				{{2, 1, 1, 3}, {2, 1, 1, 2}},
-			},
-			n: 3,
-			ips: []net.IP{
-				{1, 1, 1, 1},
-				{1, 1, 1, 2},
-				{2, 1, 1, 1},
-				{2, 1, 1, 2},
-				{2, 1, 1, 3},
-			},
-		},
-	}
-	for name, testCase := range testCases {
-		testCase := testCase
-		t.Run(name, func(t *testing.T) {
-			t.Parallel()
-			if testCase.lookupIPErr == nil {
-				require.Len(t, testCase.lookupIPResult, testCase.n)
-			}
-			const host = "blabla"
-			i := 0
-			mutex := &sync.Mutex{}
-			lookupIP := func(ctx context.Context, argHost string) (
-				ips []net.IP, err error) {
-				assert.Equal(t, host, argHost)
-				mutex.Lock()
-				result := testCase.lookupIPResult[i]
-				i++
-				mutex.Unlock()
-				return result, testCase.err
-			}
-
-			ips, err := resolveRepeat(
-				context.Background(), lookupIP, host, testCase.n, 0)
-			if testCase.err != nil {
-				require.Error(t, err)
-				assert.Equal(t, testCase.err.Error(), err.Error())
-			} else {
-				assert.NoError(t, err)
-			}
-			assert.Equal(t, testCase.ips, ips)
-		})
-	}
-}

internal/updater/surfshark.go

@@ -10,10 +10,11 @@ import (
 	"time"
 
 	"github.com/qdm12/gluetun/internal/models"
+	"github.com/qdm12/gluetun/internal/updater/resolver"
 )
 
 func (u *updater) updateSurfshark(ctx context.Context) (err error) {
-	servers, warnings, err := findSurfsharkServersFromZip(ctx, u.client, u.lookupIP)
+	servers, warnings, err := findSurfsharkServersFromZip(ctx, u.client, u.presolver)
 	if u.options.CLI {
 		for _, warning := range warnings {
 			u.logger.Warn("Surfshark: %s", warning)
@@ -31,7 +32,7 @@ func (u *updater) updateSurfshark(ctx context.Context) (err error) {
 }
 
 //nolint:deadcode,unused
-func findSurfsharkServersFromAPI(ctx context.Context, client *http.Client, lookupIP lookupIPFunc) (
+func findSurfsharkServersFromAPI(ctx context.Context, client *http.Client, presolver resolver.Parallel) (
 	servers []models.SurfsharkServer, warnings []string, err error) {
 	const url = "https://my.surfshark.com/vpn/api/v4/server/clusters"
 
@@ -69,12 +70,25 @@ func findSurfsharkServersFromAPI(ctx context.Context, client *http.Client, looku
 		hosts[i] = jsonServers[i].Host
 	}
 
-	const repetition = 20
-	const timeBetween = time.Second
-	const failOnErr = true
-	hostToIPs, _, err := parallelResolve(ctx, lookupIP, hosts, repetition, timeBetween, failOnErr)
+	const (
+		maxFailRatio    = 0.1
+		maxDuration     = 20 * time.Second
+		betweenDuration = time.Second
+		maxNoNew        = 2
+		maxFails        = 2
+	)
+	settings := resolver.ParallelSettings{
+		MaxFailRatio: maxFailRatio,
+		Repeat: resolver.RepeatSettings{
+			MaxDuration:     maxDuration,
+			BetweenDuration: betweenDuration,
+			MaxNoNew:        maxNoNew,
+			MaxFails:        maxFails,
+		},
+	}
+	hostToIPs, warnings, err := presolver.Resolve(ctx, hosts, settings)
 	if err != nil {
-		return nil, nil, err
+		return nil, warnings, err
 	}
 
 	for _, jsonServer := range jsonServers {
@@ -87,14 +101,14 @@ func findSurfsharkServersFromAPI(ctx context.Context, client *http.Client, looku
 		}
 		server := models.SurfsharkServer{
 			Region: jsonServer.Country + " " + jsonServer.Location,
-			IPs:    uniqueSortedIPs(IPs),
+			IPs:    IPs,
 		}
 		servers = append(servers, server)
 	}
 	return servers, warnings, nil
 }
 
-func findSurfsharkServersFromZip(ctx context.Context, client *http.Client, lookupIP lookupIPFunc) (
+func findSurfsharkServersFromZip(ctx context.Context, client *http.Client, presolver resolver.Parallel) (
 	servers []models.SurfsharkServer, warnings []string, err error) {
 	const zipURL = "https://my.surfshark.com/vpn/api/v1/server/configurations"
 	contents, err := fetchAndExtractFiles(ctx, client, zipURL)
@@ -119,10 +133,24 @@ func findSurfsharkServersFromZip(ctx context.Context, client *http.Client, looku
 		hosts = append(hosts, host)
 	}
 
-	const repetition = 20
-	const timeBetween = time.Second
-	const failOnErr = true
-	hostToIPs, _, err := parallelResolve(ctx, lookupIP, hosts, repetition, timeBetween, failOnErr)
+	const (
+		maxFailRatio    = 0.1
+		maxDuration     = 20 * time.Second
+		betweenDuration = time.Second
+		maxNoNew        = 2
+		maxFails        = 2
+	)
+	settings := resolver.ParallelSettings{
+		MaxFailRatio: maxFailRatio,
+		Repeat: resolver.RepeatSettings{
+			MaxDuration:     maxDuration,
+			BetweenDuration: betweenDuration,
+			MaxNoNew:        maxNoNew,
+			MaxFails:        maxFails,
+		},
+	}
+	hostToIPs, newWarnings, err := presolver.Resolve(ctx, hosts, settings)
+	warnings = append(warnings, newWarnings...)
 	if err != nil {
 		return nil, warnings, err
 	}
@@ -144,14 +172,18 @@ func findSurfsharkServersFromZip(ctx context.Context, client *http.Client, looku
 		}
 		server := models.SurfsharkServer{
 			Region: region,
-			IPs:    uniqueSortedIPs(IPs),
+			IPs:    IPs,
 		}
 		servers = append(servers, server)
 	}
 
 	// process entries in mapping that were not in zip file
-	remainingServers, newWarnings := getRemainingServers(ctx, mapping, lookupIP)
+	remainingServers, newWarnings, err := getRemainingServers(ctx, mapping, presolver)
 	warnings = append(warnings, newWarnings...)
+	if err != nil {
+		return nil, warnings, err
+	}
+
 	servers = append(servers, remainingServers...)
 
 	sort.Slice(servers, func(i, j int) bool {
@@ -160,28 +192,46 @@ func findSurfsharkServersFromZip(ctx context.Context, client *http.Client, looku
 	return servers, warnings, nil
 }
 
-func getRemainingServers(ctx context.Context, mapping map[string]string, lookupIP lookupIPFunc) (
-	servers []models.SurfsharkServer, warnings []string) {
+func getRemainingServers(ctx context.Context, mapping map[string]string, presolver resolver.Parallel) (
+	servers []models.SurfsharkServer, warnings []string, err error) {
 	hosts := make([]string, 0, len(mapping))
 	for subdomain := range mapping {
 		hosts = append(hosts, subdomain+".prod.surfshark.com")
 	}
 
-	const repetition = 20
-	const timeBetween = time.Second
-	const failOnErr = false
-	hostToIPs, warnings, _ := parallelResolve(ctx, lookupIP, hosts, repetition, timeBetween, failOnErr)
+	const (
+		maxFailRatio    = 0.3
+		maxDuration     = 20 * time.Second
+		betweenDuration = time.Second
+		maxNoNew        = 2
+		maxFails        = 2
+	)
+	settings := resolver.ParallelSettings{
+		MaxFailRatio: maxFailRatio,
+		Repeat: resolver.RepeatSettings{
+			MaxDuration:     maxDuration,
+			BetweenDuration: betweenDuration,
+			MaxNoNew:        maxNoNew,
+			MaxFails:        maxFails,
+			SortIPs:         true,
+		},
+	}
+	hostToIPs, warnings, err := presolver.Resolve(ctx, hosts, settings)
+	if err != nil {
+		return nil, warnings, err
+	}
 
+	servers = make([]models.SurfsharkServer, 0, len(hostToIPs))
 	for host, IPs := range hostToIPs {
 		subdomain := strings.TrimSuffix(host, ".prod.surfshark.com")
 		server := models.SurfsharkServer{
 			Region: mapping[subdomain],
-			IPs:    uniqueSortedIPs(IPs),
+			IPs:    IPs,
 		}
 		servers = append(servers, server)
 	}
 
-	return servers, warnings
+	return servers, warnings, nil
 }
 
 func stringifySurfsharkServers(servers []models.SurfsharkServer) (s string) {

internal/updater/updater.go

@@ -9,6 +9,7 @@ import (
 
 	"github.com/qdm12/gluetun/internal/configuration"
 	"github.com/qdm12/gluetun/internal/models"
+	"github.com/qdm12/gluetun/internal/updater/resolver"
 	"github.com/qdm12/golibs/logging"
 )
 
@@ -24,11 +25,11 @@ type updater struct {
 	servers models.AllServers
 
 	// Functions for tests
-	logger   logging.Logger
-	timeNow  func() time.Time
-	println  func(s string)
-	lookupIP lookupIPFunc
-	client   *http.Client
+	logger    logging.Logger
+	timeNow   func() time.Time
+	println   func(s string)
+	presolver resolver.Parallel
+	client    *http.Client
 }
 
 func New(settings configuration.Updater, httpClient *http.Client,
@@ -36,15 +37,14 @@ func New(settings configuration.Updater, httpClient *http.Client,
 	if len(settings.DNSAddress) == 0 {
 		settings.DNSAddress = "1.1.1.1"
 	}
-	resolver := newResolver(settings.DNSAddress)
 	return &updater{
-		logger:   logger,
-		timeNow:  time.Now,
-		println:  func(s string) { fmt.Println(s) },
-		lookupIP: newLookupIP(resolver),
-		client:   httpClient,
-		options:  settings,
-		servers:  currentServers,
+		logger:    logger,
+		timeNow:   time.Now,
+		println:   func(s string) { fmt.Println(s) },
+		presolver: resolver.NewParallelResolver(settings.DNSAddress),
+		client:    httpClient,
+		options:   settings,
+		servers:   currentServers,
 	}
 }
 

internal/updater/vyprvpn.go

@@ -6,12 +6,14 @@ import (
 	"net/http"
 	"sort"
 	"strings"
+	"time"
 
 	"github.com/qdm12/gluetun/internal/models"
+	"github.com/qdm12/gluetun/internal/updater/resolver"
 )
 
 func (u *updater) updateVyprvpn(ctx context.Context) (err error) {
-	servers, warnings, err := findVyprvpnServers(ctx, u.client, u.lookupIP)
+	servers, warnings, err := findVyprvpnServers(ctx, u.client, u.presolver)
 	if u.options.CLI {
 		for _, warning := range warnings {
 			u.logger.Warn("Vyprvpn: %s", warning)
@@ -28,7 +30,7 @@ func (u *updater) updateVyprvpn(ctx context.Context) (err error) {
 	return nil
 }
 
-func findVyprvpnServers(ctx context.Context, client *http.Client, lookupIP lookupIPFunc) (
+func findVyprvpnServers(ctx context.Context, client *http.Client, presolver resolver.Parallel) (
 	servers []models.VyprvpnServer, warnings []string, err error) {
 	const zipURL = "https://support.vyprvpn.com/hc/article_attachments/360052617332/Vypr_OpenVPN_20200320.zip"
 	contents, err := fetchAndExtractFiles(ctx, client, zipURL)
@@ -60,18 +62,31 @@ func findVyprvpnServers(ctx context.Context, client *http.Client, lookupIP looku
 		i++
 	}
 
-	const repetition = 1
-	const timeBetween = 1
-	const failOnErr = true
-	hostToIPs, _, err := parallelResolve(ctx, lookupIP, hosts, repetition, timeBetween, failOnErr)
+	const (
+		maxFailRatio = 0.1
+		maxNoNew     = 2
+		maxFails     = 2
+	)
+	settings := resolver.ParallelSettings{
+		MaxFailRatio: maxFailRatio,
+		Repeat: resolver.RepeatSettings{
+			MaxDuration: time.Second,
+			MaxNoNew:    maxNoNew,
+			MaxFails:    maxFails,
+			SortIPs:     true,
+		},
+	}
+	hostToIPs, newWarnings, err := presolver.Resolve(ctx, hosts, settings)
+	warnings = append(warnings, newWarnings...)
 	if err != nil {
 		return nil, warnings, err
 	}
 
+	servers = make([]models.VyprvpnServer, 0, len(hostToIPs))
 	for host, IPs := range hostToIPs {
 		server := models.VyprvpnServer{
 			Region: hostToRegion[host],
-			IPs:    uniqueSortedIPs(IPs),
+			IPs:    IPs,
 		}
 		servers = append(servers, server)
 	}