Add a flag to enable strict DNS resolution

This commit adds a flag to enable strict DNS resolution on ACME
challenges.

acme/challenge.go

@@ -63,6 +63,11 @@ var (
 	//
 	// This variable can be used for testing purposes.
 	InsecurePortTLSALPN01 int
+
+	// StrictFQDN allows to enforce a fully qualified domain name in the DNS
+	// resolution. By default it allows domain resolution using a search list
+	// defined in the resolv.conf or similar configuration.
+	StrictFQDN bool
 )
 
 // Challenge represents an ACME response Challenge type.
@@ -163,9 +168,11 @@ func http01Validate(ctx context.Context, ch *Challenge, db DB, jwk *jose.JSONWeb
 
 // rootedName adds a trailing "." to a given domain name.
 func rootedName(name string) string {
+	if StrictFQDN {
 		if name == "" || name[len(name)-1] != '.' {
 			return name + "."
 		}
+	}
 	return name
 }
 

acme/challenge_test.go

@@ -2768,16 +2768,31 @@ func Test_serverName(t *testing.T) {
 func Test_http01ChallengeHost(t *testing.T) {
 	tests := []struct {
 		name       string
+		strictFQDN bool
 		value      string
 		want       string
 	}{
 		{
 			name:       "dns",
+			strictFQDN: false,
+			value:      "www.example.com",
+			want:       "www.example.com",
+		},
+		{
+			name:       "dns strict",
+			strictFQDN: true,
 			value:      "www.example.com",
 			want:       "www.example.com.",
 		},
 		{
 			name:       "rooted dns",
+			strictFQDN: false,
+			value:      "www.example.com.",
+			want:       "www.example.com.",
+		},
+		{
+			name:       "rooted dns strict",
+			strictFQDN: true,
 			value:      "www.example.com.",
 			want:       "www.example.com.",
 		},
@@ -2794,6 +2809,11 @@ func Test_http01ChallengeHost(t *testing.T) {
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
+			tmp := StrictFQDN
+			t.Cleanup(func() {
+				StrictFQDN = tmp
+			})
+			StrictFQDN = tt.strictFQDN
 			if got := http01ChallengeHost(tt.value); got != tt.want {
 				t.Errorf("http01ChallengeHost() = %v, want %v", got, tt.want)
 			}
@@ -4501,16 +4521,24 @@ func Test_tlsAlpn01ChallengeHost(t *testing.T) {
 	}
 	tests := []struct {
 		name       string
+		strictFQDN bool
 		args       args
 		want       string
 	}{
-		{"dns", args{"smallstep.com"}, "smallstep.com."},
-		{"rooted dns", args{"smallstep.com."}, "smallstep.com."},
-		{"ipv4", args{"1.2.3.4"}, "1.2.3.4"},
-		{"ipv6", args{"2607:f8b0:4023:1009::71"}, "2607:f8b0:4023:1009::71"},
+		{"dns", false, args{"smallstep.com"}, "smallstep.com"},
+		{"dns strict", true, args{"smallstep.com"}, "smallstep.com."},
+		{"rooted dns", false, args{"smallstep.com."}, "smallstep.com."},
+		{"rooted dns strict", true, args{"smallstep.com."}, "smallstep.com."},
+		{"ipv4", true, args{"1.2.3.4"}, "1.2.3.4"},
+		{"ipv6", true, args{"2607:f8b0:4023:1009::71"}, "2607:f8b0:4023:1009::71"},
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
+			tmp := StrictFQDN
+			t.Cleanup(func() {
+				StrictFQDN = tmp
+			})
+			StrictFQDN = tt.strictFQDN
 			assert.Equal(t, tt.want, tlsAlpn01ChallengeHost(tt.args.name))
 		})
 	}
@@ -4522,14 +4550,22 @@ func Test_dns01ChallengeHost(t *testing.T) {
 	}
 	tests := []struct {
 		name       string
+		strictFQDN bool
 		args       args
 		want       string
 	}{
-		{"dns", args{"smallstep.com"}, "_acme-challenge.smallstep.com."},
-		{"rooted dns", args{"smallstep.com."}, "_acme-challenge.smallstep.com."},
+		{"dns", false, args{"smallstep.com"}, "_acme-challenge.smallstep.com"},
+		{"dns strict", true, args{"smallstep.com"}, "_acme-challenge.smallstep.com."},
+		{"rooted dns", false, args{"smallstep.com."}, "_acme-challenge.smallstep.com."},
+		{"rooted dns strict", true, args{"smallstep.com."}, "_acme-challenge.smallstep.com."},
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
+			tmp := StrictFQDN
+			t.Cleanup(func() {
+				StrictFQDN = tmp
+			})
+			StrictFQDN = tt.strictFQDN
 			assert.Equal(t, tt.want, dns01ChallengeHost(tt.args.domain))
 		})
 	}

commands/app.go

@@ -83,6 +83,10 @@ Requires **--insecure** flag.`,
 			Usage: `the <port> used on tls-alpn-01 challenges. It can be changed for testing purposes.
 Requires **--insecure** flag.`,
 		},
+		cli.BoolFlag{
+			Name:  "acme-strict-fqdn",
+			Usage: `enable strict DNS resolution using a fully qualified domain name.`,
+		},
 		cli.StringFlag{
 			Name:  "pidfile",
 			Usage: "the path to the <file> to write the process ID.",
@@ -126,6 +130,9 @@ func appAction(ctx *cli.Context) error {
 		}
 	}
 
+	// Set the strict DNS resolution on ACME challenges. Defaults to false.
+	acme.StrictFQDN = ctx.Bool("acme-strict-fqdn")
+
 	// Allow custom contexts.
 	if caCtx := ctx.String("context"); caCtx != "" {
 		if _, ok := step.Contexts().Get(caCtx); ok {