TrustStore: Implement recurser

Adds:
- Recurser that decides whether recursion is allowed for a given client.
  - ASLocalRecurser allows all clients in the local AS.
  - LocalOnlyRecurser allows only the local application.

go/lib/infra/modules/trust/v2/BUILD.bazel

@@ -23,6 +23,7 @@ go_library(
         "//go/lib/scrypto:go_default_library",
         "//go/lib/scrypto/trc/v2:go_default_library",
         "//go/lib/serrors:go_default_library",
+        "//go/lib/snet:go_default_library",
         "@org_golang_x_xerrors//:go_default_library",
     ],
 )
@@ -33,6 +34,7 @@ go_test(
         "export_test.go",
         "main_test.go",
         "provider_test.go",
+        "recurser_test.go",
         "resolver_test.go",
     ],
     data = [
@@ -48,6 +50,7 @@ go_test(
         "//go/lib/scrypto:go_default_library",
         "//go/lib/scrypto/trc/v2:go_default_library",
         "//go/lib/serrors:go_default_library",
+        "//go/lib/snet:go_default_library",
         "//go/lib/util:go_default_library",
         "//go/lib/xtest:go_default_library",
         "@com_github_golang_mock//gomock:go_default_library",

go/lib/infra/modules/trust/v2/recurser.go

@@ -16,8 +16,15 @@ package trust
 
 import (
 	"net"
+
+	"github.com/scionproto/scion/go/lib/addr"
+	"github.com/scionproto/scion/go/lib/serrors"
+	"github.com/scionproto/scion/go/lib/snet"
 )
 
+// ErrRecursionNotAllowed indicates that recursion is not allowed.
+var ErrRecursionNotAllowed = serrors.New("recursion not allowed")
+
 // Recurser decides whether a recursive request is permitted for a given peer.
 // For infra services use either ASLocalRecurser or LocalOnlyRecurser.
 type Recurser interface {
@@ -31,10 +38,32 @@ type Recurser interface {
 
 // ASLocalRecurser allows AS local addresses to start recursive requests.
 type ASLocalRecurser struct {
-	// TODO(roosd): implement
+	IA addr.IA
 }
 
-// LocalOnlyRecurser only allows requests from the application to recurse.
-type LocalOnlyRecurser struct {
-	// TODO(roosd): implement
+// AllowRecursion returns an error if address is not part of the local AS (or if
+// the check cannot be made).
+func (r *ASLocalRecurser) AllowRecursion(peer net.Addr) error {
+	if peer != nil {
+		switch saddr, ok := peer.(*snet.Addr); {
+		case !ok:
+			return serrors.WrapStr("unable to determine AS of peer", ErrRecursionNotAllowed,
+				"addr", peer)
+		case !r.IA.Equal(saddr.IA):
+			return serrors.WrapStr("client outside local AS", ErrRecursionNotAllowed,
+				"addr", peer)
+		}
+	}
+	return nil
+}
+
+// LocalOnlyRecurser returns an error if the address is not nil.
+type LocalOnlyRecurser struct{}
+
+// AllowRecursion returns an error if the address is not nil.
+func (r LocalOnlyRecurser) AllowRecursion(peer net.Addr) error {
+	if peer != nil {
+		return serrors.WrapStr("client not host-local", ErrRecursionNotAllowed, "addr", peer)
+	}
+	return nil
 }

go/lib/infra/modules/trust/v2/recurser_test.go

@@ -0,0 +1,85 @@
+// Copyright 2019 Anapaya Systems
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package trust_test
+
+import (
+	"net"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+
+	"github.com/scionproto/scion/go/lib/infra/modules/trust/v2"
+	"github.com/scionproto/scion/go/lib/snet"
+)
+
+func TestASLocalRecurserAllowRecursion(t *testing.T) {
+	tests := map[string]struct {
+		Addr      net.Addr
+		Assertion assert.ErrorAssertionFunc
+	}{
+		"host local": {
+			Assertion: assert.NoError,
+		},
+		"AS local": {
+			Addr:      &snet.Addr{IA: ia110},
+			Assertion: assert.NoError,
+		},
+		"remote AS": {
+			Addr:      &snet.Addr{IA: ia120},
+			Assertion: assert.Error,
+		},
+		"invalid Address": {
+			Addr:      &net.IPAddr{IP: net.IP{127, 0, 0, 1}},
+			Assertion: assert.Error,
+		},
+	}
+	for name, test := range tests {
+		t.Run(name, func(t *testing.T) {
+			r := trust.ASLocalRecurser{IA: ia110}
+			err := r.AllowRecursion(test.Addr)
+			test.Assertion(t, err)
+		})
+	}
+}
+
+func TestLocalOnlyRecurserAllowRecursion(t *testing.T) {
+	tests := map[string]struct {
+		Addr      net.Addr
+		Assertion assert.ErrorAssertionFunc
+	}{
+		"host local": {
+			Assertion: assert.NoError,
+		},
+		"AS local": {
+			Addr:      &snet.Addr{IA: ia110},
+			Assertion: assert.Error,
+		},
+		"remote AS": {
+			Addr:      &snet.Addr{IA: ia120},
+			Assertion: assert.Error,
+		},
+		"invalid Address": {
+			Addr:      &net.IPAddr{IP: net.IP{127, 0, 0, 1}},
+			Assertion: assert.Error,
+		},
+	}
+	for name, test := range tests {
+		t.Run(name, func(t *testing.T) {
+			r := trust.LocalOnlyRecurser{}
+			err := r.AllowRecursion(test.Addr)
+			test.Assertion(t, err)
+		})
+	}
+}