Fix ACME IPv6 HTTP-01 challenges

Fixes #890

acme/challenge.go

@@ -79,7 +79,7 @@ func (ch *Challenge) Validate(ctx context.Context, db DB, jwk *jose.JSONWebKey,
 }
 
 func http01Validate(ctx context.Context, ch *Challenge, db DB, jwk *jose.JSONWebKey, vo *ValidateChallengeOptions) error {
-	u := &url.URL{Scheme: "http", Host: ch.Value, Path: fmt.Sprintf("/.well-known/acme-challenge/%s", ch.Token)}
+	u := &url.URL{Scheme: "http", Host: http01ChallengeHost(ch.Value), Path: fmt.Sprintf("/.well-known/acme-challenge/%s", ch.Token)}
 
 	resp, err := vo.HTTPGet(u.String())
 	if err != nil {
@@ -119,6 +119,17 @@ func http01Validate(ctx context.Context, ch *Challenge, db DB, jwk *jose.JSONWeb
 	return nil
 }
 
+// http01ChallengeHost checks if a Challenge value is an IPv6 address
+// and adds square brackets if that's the case, so that it can be used
+// as a hostname. Returns the original Challenge value as the host to
+// use in other cases.
+func http01ChallengeHost(value string) string {
+	if ip := net.ParseIP(value); ip != nil && ip.To4() == nil {
+		value = "[" + value + "]"
+	}
+	return value
+}
+
 func tlsAlert(err error) uint8 {
 	var opErr *net.OpError
 	if errors.As(err, &opErr) {

acme/challenge_test.go

@@ -13,6 +13,7 @@ import (
 	"encoding/asn1"
 	"encoding/base64"
 	"encoding/hex"
+	"errors"
 	"fmt"
 	"io"
 	"math/big"
@@ -23,9 +24,9 @@ import (
 	"testing"
 	"time"
 
-	"github.com/pkg/errors"
-	"github.com/smallstep/assert"
 	"go.step.sm/crypto/jose"
+
+	"github.com/smallstep/assert"
 )
 
 func Test_storeError(t *testing.T) {
@@ -2350,3 +2351,34 @@ func Test_serverName(t *testing.T) {
 		})
 	}
 }
+
+func Test_http01ChallengeHost(t *testing.T) {
+	tests := []struct {
+		name  string
+		value string
+		want  string
+	}{
+		{
+			name:  "dns",
+			value: "www.example.com",
+			want:  "www.example.com",
+		},
+		{
+			name:  "ipv4",
+			value: "127.0.0.1",
+			want:  "127.0.0.1",
+		},
+		{
+			name:  "ipv6",
+			value: "::1",
+			want:  "[::1]",
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := http01ChallengeHost(tt.value); got != tt.want {
+				t.Errorf("http01ChallengeHost() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}