refactor(client): Clean up client dns resolver

client/config.go

@@ -4,10 +4,11 @@ import (
 	"context"
 	"crypto/tls"
 	"errors"
-	"fmt"
+	"log"
 	"net"
 	"net/http"
 	"regexp"
+	"strconv"
 	"time"
 
 	"golang.org/x/oauth2"
@@ -20,6 +21,7 @@ const (
 
 var (
 	ErrInvalidDNSResolver        = errors.New("invalid DNS resolver specified. Required format is {proto}://{ip}:{port}")
+	ErrInvalidDNSResolverPort    = errors.New("invalid DNS resolver port")
 	ErrInvalidClientOAuth2Config = errors.New("invalid OAuth2 configuration, all fields are required")
 
 	defaultConfig = Config{
@@ -46,8 +48,8 @@ type Config struct {
 	// Timeout for the client
 	Timeout time.Duration `yaml:"timeout"`
 
-	// DNSResolver override for the HTTPClient
-	// Expected format is {protocol}://{host}:{port}
+	// DNSResolver override for the HTTP client
+	// Expected format is {protocol}://{host}:{port}, e.g. tcp://1.1.1.1:53
 	DNSResolver string `yaml:"dns-resolver,omitempty"`
 
 	// OAuth2Config is the OAuth2 configuration used for the client.
@@ -80,9 +82,9 @@ func (c *Config) ValidateAndSetDefaults() error {
 		c.Timeout = 10 * time.Second
 	}
 	if c.HasCustomDNSResolver() {
-		_, err := c.ParseDNSResolver()
-		if err != nil {
-			return ErrInvalidDNSResolver
+		// Validate the DNS resolver now to make sure it will not return an error later.
+		if _, err := c.parseDNSResolver(); err != nil {
+			return err
 		}
 	}
 	if c.HasOAuth2Config() && !c.OAuth2Config.isValid() {
@@ -91,17 +93,17 @@ func (c *Config) ValidateAndSetDefaults() error {
 	return nil
 }
 
-// Returns true if the DNSResolver is set in the configuration
+// HasCustomDNSResolver returns whether a custom DNSResolver is configured
 func (c *Config) HasCustomDNSResolver() bool {
 	return len(c.DNSResolver) > 0
 }
 
-// Parses the DNSResolver configuration string into the DNSResolverConfig struct
-func (c *Config) ParseDNSResolver() (DNSResolverConfig, error) {
+// parseDNSResolver parses the DNS resolver into the DNSResolverConfig struct
+func (c *Config) parseDNSResolver() (*DNSResolverConfig, error) {
 	re := regexp.MustCompile(`^(?P<proto>(.*))://(?P<host>[A-Za-z0-9\-\.]+):(?P<port>[0-9]+)?(.*)$`)
 	matches := re.FindStringSubmatch(c.DNSResolver)
 	if len(matches) == 0 {
-		return DNSResolverConfig{}, errors.New("ParseError")
+		return nil, ErrInvalidDNSResolver
 	}
 	r := make(map[string]string)
 	for i, k := range re.SubexpNames() {
@@ -109,8 +111,14 @@ func (c *Config) ParseDNSResolver() (DNSResolverConfig, error) {
 			r[k] = matches[i]
 		}
 	}
-
-	return DNSResolverConfig{
+	port, err := strconv.Atoi(r["port"])
+	if err != nil {
+		return nil, err
+	}
+	if port < 1 || port > 65535 {
+		return nil, ErrInvalidDNSResolverPort
+	}
+	return &DNSResolverConfig{
 		Protocol: r["proto"],
 		Host:     r["host"],
 		Port:     r["port"],
@@ -150,20 +158,25 @@ func (c *Config) getHTTPClient() *http.Client {
 			},
 		}
 		if c.HasCustomDNSResolver() {
-			dnsResolver, _ := c.ParseDNSResolver()
-			dialer := &net.Dialer{
-				Resolver: &net.Resolver{
-					PreferGo: true,
-					Dial: func(ctx context.Context, network, address string) (net.Conn, error) {
-						d := net.Dialer{}
-						return d.DialContext(ctx, dnsResolver.Protocol, fmt.Sprintf("%s:%s", dnsResolver.Host, dnsResolver.Port))
+			dnsResolver, err := c.parseDNSResolver()
+			if err != nil {
+				// We're ignoring the error, because it should have been validated on startup ValidateAndSetDefaults.
+				// It shouldn't happen, but if it does, we'll log it... Better safe than sorry ;)
+				log.Println("[client][getHTTPClient] THIS SHOULD NOT HAPPEN. Silently ignoring invalid DNS resolver due to error:", err.Error())
+			} else {
+				dialer := &net.Dialer{
+					Resolver: &net.Resolver{
+						PreferGo: true,
+						Dial: func(ctx context.Context, network, address string) (net.Conn, error) {
+							d := net.Dialer{}
+							return d.DialContext(ctx, dnsResolver.Protocol, dnsResolver.Host+":"+dnsResolver.Port)
+						},
 					},
-				},
+				}
+				c.httpClient.Transport.(*http.Transport).DialContext = func(ctx context.Context, network, addr string) (net.Conn, error) {
+					return dialer.DialContext(ctx, network, addr)
+				}
 			}
-			dialCtx := func(ctx context.Context, network, addr string) (net.Conn, error) {
-				return dialer.DialContext(ctx, network, addr)
-			}
-			c.httpClient.Transport.(*http.Transport).DialContext = dialCtx
 		}
 		if c.HasOAuth2Config() {
 			c.httpClient = configureOAuth2(c.httpClient, *c.OAuth2Config)