userdb: Add support for receive-only users

Some use cases, like receive-only MTAs, need domain users for receiving
emails, but have no real need for passwords since they will never use
submission.

Today, that is not supported, and those use-cases require the
administrator to come up with a password unnecessarily, adding
complexity and possibly risk.

This patch implements "receive-only users", which don't have a valid
password, thus exist for the purposes of delivering mail, but always
fail authentication.

See https://github.com/albertito/chasquid/issues/44 for more details and
rationale.

Thanks to xavierg who suggested this feature on IRC.

cmd/chasquid-util/chasquid-util.go

@@ -29,7 +29,7 @@ import (
 // Usage to show users on --help or invocation errors.
 const usage = `
 Usage:
-  chasquid-util [options] user-add <user@domain> [--password=<password>]
+  chasquid-util [options] user-add <user@domain> [--password=<password>] [--receive_only]
   chasquid-util [options] user-remove <user@domain>
   chasquid-util [options] authenticate <user@domain> [--password=<password>]
   chasquid-util [options] check-userdb <domain>
@@ -140,12 +140,25 @@ func checkUserDB() {
 	fmt.Println("Database loaded")
 }
 
-// chasquid-util user-add <user@domain> [--password=<password>]
+// chasquid-util user-add <user@domain> [--password=<password>] [--receive_only]
 func userAdd() {
 	user, _, db := userDBFromArgs(true)
-	password := getPassword()
 
-	err := db.AddUser(user, password)
+	_, recvOnly := args["--receive_only"]
+	_, hasPassword := args["--password"]
+
+	if recvOnly && hasPassword {
+		Fatalf("Cannot specify both --receive_only and --password")
+	}
+
+	var err error
+	if recvOnly {
+		err = db.AddDeniedUser(user)
+	} else {
+		password := getPassword()
+		err = db.AddUser(user, password)
+	}
+
 	if err != nil {
 		Fatalf("Error adding user: %v", err)
 	}

cmd/chasquid-util/test.sh

@@ -34,6 +34,17 @@ if ! r user-add user@domain --password=passwd > /dev/null; then
 fi
 check_userdb
 
+if ! r user-add denied@domain --receive_only > /dev/null; then
+	echo user-add --receive_only failed
+	exit 1
+fi
+check_userdb
+
+if r user-add xxx@domain --password=passwd --receive_only > /dev/null; then
+	echo user-add --password --receive_only worked
+	exit 1
+fi
+
 if ! r authenticate user@domain --password=passwd > /dev/null; then
 	echo authenticate failed
 	exit 1
@@ -44,6 +55,11 @@ if r authenticate user@domain --password=abcd > /dev/null; then
 	exit 1
 fi
 
+if r authenticate denied@domain --password=abcd > /dev/null; then
+	echo authenticate on a no-submission user worked
+	exit 1
+fi
+
 # Interactive authentication.
 # Need to wrap the execution under "script" since the interaction requires an
 # actual TTY, and that's a fairly portable way to do that.

docs/man/chasquid-util.1

@@ -133,7 +133,7 @@
 .\" ========================================================================
 .\"
 .IX Title "chasquid-util 1"
-.TH chasquid-util 1 "2023-07-29" "" ""
+.TH chasquid-util 1 "2023-12-03" "" ""
 .\" For nroff, turn off justification.  Always turn off hyphenation; it makes
 .\" way too many mistakes in technical documents.
 .if n .ad l
@@ -142,7 +142,7 @@
 chasquid\-util \- chasquid management tool
 .SH "SYNOPSIS"
 .IX Header "SYNOPSIS"
-\&\fBchasquid-util\fR [\fIoptions\fR] user-add \fIuser@domain\fR [\-\-password=\fIpassword\fR]
+\&\fBchasquid-util\fR [\fIoptions\fR] user-add \fIuser@domain\fR [\-\-password=\fIpassword\fR] [\-\-receive_only]
 .PP
 \&\fBchasquid-util\fR [\fIoptions\fR] user-remove \fIuser@domain\fR
 .PP
@@ -160,9 +160,12 @@ chasquid\-util \- chasquid management tool
 chasquid-util is a command-line utility for \fBchasquid\fR\|(1) operations.
 .SH "OPTIONS"
 .IX Header "OPTIONS"
-.IP "\fBuser-add\fR \fIuser@domain\fR [\-\-password=\fIpassword\fR]" 8
+.IP "\fBuser-add\fR \fIuser@domain\fR [\-\-password=\fIpassword\fR] [\-\-receive_only]" 8
-.IX Item "user-add user@domain [--password=password]"
+.IX Item "user-add user@domain [--password=password] [--receive_only]"
 Add a new user to the domain.
+.Sp
+If \fI\-\-receive_only\fR is given, then the user will never successfully
+authenticate. This is useful when creating receive-only users.
 .IP "\fBuser-remove\fR \fIuser@domain\fR" 8
 .IX Item "user-remove user@domain"
 Remove the user from the domain.

docs/man/chasquid-util.1.md

@@ -4,7 +4,7 @@ chasquid-util - chasquid management tool
 
 # SYNOPSIS
 
-**chasquid-util** \[_options_\] user-add _user@domain_ \[--password=_password_\]
+**chasquid-util** \[_options_\] user-add _user@domain_ \[--password=_password_\] \[--receive\_only\]
 
 **chasquid-util** \[_options_\] user-remove _user@domain_
 
@@ -24,10 +24,13 @@ chasquid-util is a command-line utility for [chasquid(1)](chasquid.1.md) operati
 
 # OPTIONS
 
-- **user-add** _user@domain_ \[--password=_password_\]
+- **user-add** _user@domain_ \[--password=_password_\] \[--receive\_only\]
 
     Add a new user to the domain.
 
+    If _--receive\_only_ is given, then the user will never successfully
+    authenticate. This is useful when creating receive-only users.
+
 - **user-remove** _user@domain_
 
     Remove the user from the domain.

docs/man/chasquid-util.1.pod

@@ -5,7 +5,7 @@ chasquid-util - chasquid management tool
 
 =head1 SYNOPSIS
 
-B<chasquid-util> [I<options>] user-add I<user@domain> [--password=I<password>]
+B<chasquid-util> [I<options>] user-add I<user@domain> [--password=I<password>] [--receive_only]
 
 B<chasquid-util> [I<options>] user-remove I<user@domain>
 
@@ -28,10 +28,13 @@ chasquid-util is a command-line utility for chasquid(1) operations.
 
 =over 8
 
-=item B<user-add> I<user@domain> [--password=I<password>]
+=item B<user-add> I<user@domain> [--password=I<password>] [--receive_only]
 
 Add a new user to the domain.
 
+If I<--receive_only> is given, then the user will never successfully
+authenticate. This is useful when creating receive-only users.
+
 =item B<user-remove> I<user@domain>
 
 Remove the user from the domain.

internal/userdb/userdb.go

@@ -123,6 +123,8 @@ func (p *Password) PasswordMatches(plain string) bool {
 		return s.Scrypt.PasswordMatches(plain)
 	case *Password_Plain:
 		return s.Plain.PasswordMatches(plain)
+	case *Password_Denied:
+		return false
 	default:
 		return false
 	}
@@ -164,6 +166,22 @@ func (db *DB) AddUser(name, plainPassword string) error {
 	return nil
 }
 
+// AddDenied to the database. If the user is already present, override it.
+// Note we enforce that the name has been normalized previously.
+func (db *DB) AddDeniedUser(name string) error {
+	if norm, err := normalize.User(name); err != nil || name != norm {
+		return errors.New("invalid username")
+	}
+
+	db.mu.Lock()
+	db.db.Users[name] = &Password{
+		Scheme: &Password_Denied{&Denied{}},
+	}
+	db.mu.Unlock()
+
+	return nil
+}
+
 // RemoveUser from the database. Returns True if the user was there, False
 // otherwise.
 func (db *DB) RemoveUser(name string) bool {

internal/userdb/userdb.pb.go

@@ -1,6 +1,6 @@
 // Code generated by protoc-gen-go. DO NOT EDIT.
 // versions:
-// 	protoc-gen-go v1.28.1
+// 	protoc-gen-go v1.30.0
 // 	protoc        v3.21.12
 // source: userdb.proto
 
@@ -73,8 +73,10 @@ type Password struct {
 	unknownFields protoimpl.UnknownFields
 
 	// Types that are assignable to Scheme:
+	//
 	//	*Password_Scrypt
 	//	*Password_Plain
+	//	*Password_Denied
 	Scheme isPassword_Scheme `protobuf_oneof:"scheme"`
 }
 
@@ -131,6 +133,13 @@ func (x *Password) GetPlain() *Plain {
 	return nil
 }
 
+func (x *Password) GetDenied() *Denied {
+	if x, ok := x.GetScheme().(*Password_Denied); ok {
+		return x.Denied
+	}
+	return nil
+}
+
 type isPassword_Scheme interface {
 	isPassword_Scheme()
 }
@@ -143,10 +152,16 @@ type Password_Plain struct {
 	Plain *Plain `protobuf:"bytes,3,opt,name=plain,proto3,oneof"`
 }
 
+type Password_Denied struct {
+	Denied *Denied `protobuf:"bytes,4,opt,name=denied,proto3,oneof"`
+}
+
 func (*Password_Scrypt) isPassword_Scheme() {}
 
 func (*Password_Plain) isPassword_Scheme() {}
 
+func (*Password_Denied) isPassword_Scheme() {}
+
 type Scrypt struct {
 	state         protoimpl.MessageState
 	sizeCache     protoimpl.SizeCache
@@ -281,6 +296,44 @@ func (x *Plain) GetPassword() []byte {
 	return nil
 }
 
+type Denied struct {
+	state         protoimpl.MessageState
+	sizeCache     protoimpl.SizeCache
+	unknownFields protoimpl.UnknownFields
+}
+
+func (x *Denied) Reset() {
+	*x = Denied{}
+	if protoimpl.UnsafeEnabled {
+		mi := &file_userdb_proto_msgTypes[4]
+		ms := protoimpl.X.MessageStateOf(protoimpl.Pointer(x))
+		ms.StoreMessageInfo(mi)
+	}
+}
+
+func (x *Denied) String() string {
+	return protoimpl.X.MessageStringOf(x)
+}
+
+func (*Denied) ProtoMessage() {}
+
+func (x *Denied) ProtoReflect() protoreflect.Message {
+	mi := &file_userdb_proto_msgTypes[4]
+	if protoimpl.UnsafeEnabled && x != nil {
+		ms := protoimpl.X.MessageStateOf(protoimpl.Pointer(x))
+		if ms.LoadMessageInfo() == nil {
+			ms.StoreMessageInfo(mi)
+		}
+		return ms
+	}
+	return mi.MessageOf(x)
+}
+
+// Deprecated: Use Denied.ProtoReflect.Descriptor instead.
+func (*Denied) Descriptor() ([]byte, []int) {
+	return file_userdb_proto_rawDescGZIP(), []int{4}
+}
+
 var File_userdb_proto protoreflect.FileDescriptor
 
 var file_userdb_proto_rawDesc = []byte{
@@ -294,27 +347,30 @@ var file_userdb_proto_rawDesc = []byte{
 	0x03, 0x6b, 0x65, 0x79, 0x12, 0x26, 0x0a, 0x05, 0x76, 0x61, 0x6c, 0x75, 0x65, 0x18, 0x02, 0x20,
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 }
 
 var (
@@ -329,24 +385,26 @@ func file_userdb_proto_rawDescGZIP() []byte {
 	return file_userdb_proto_rawDescData
 }
 
-var file_userdb_proto_msgTypes = make([]protoimpl.MessageInfo, 5)
+var file_userdb_proto_msgTypes = make([]protoimpl.MessageInfo, 6)
 var file_userdb_proto_goTypes = []interface{}{
 	(*ProtoDB)(nil),  // 0: userdb.ProtoDB
 	(*Password)(nil), // 1: userdb.Password
 	(*Scrypt)(nil),   // 2: userdb.Scrypt
 	(*Plain)(nil),    // 3: userdb.Plain
-	nil,              // 4: userdb.ProtoDB.UsersEntry
+	(*Denied)(nil),   // 4: userdb.Denied
+	nil,              // 5: userdb.ProtoDB.UsersEntry
 }
 var file_userdb_proto_depIdxs = []int32{
-	4, // 0: userdb.ProtoDB.users:type_name -> userdb.ProtoDB.UsersEntry
+	5, // 0: userdb.ProtoDB.users:type_name -> userdb.ProtoDB.UsersEntry
 	2, // 1: userdb.Password.scrypt:type_name -> userdb.Scrypt
 	3, // 2: userdb.Password.plain:type_name -> userdb.Plain
-	1, // 3: userdb.ProtoDB.UsersEntry.value:type_name -> userdb.Password
+	4, // 3: userdb.Password.denied:type_name -> userdb.Denied
-	4, // [4:4] is the sub-list for method output_type
+	1, // 4: userdb.ProtoDB.UsersEntry.value:type_name -> userdb.Password
-	4, // [4:4] is the sub-list for method input_type
+	5, // [5:5] is the sub-list for method output_type
-	4, // [4:4] is the sub-list for extension type_name
+	5, // [5:5] is the sub-list for method input_type
-	4, // [4:4] is the sub-list for extension extendee
+	5, // [5:5] is the sub-list for extension type_name
-	0, // [0:4] is the sub-list for field type_name
+	5, // [5:5] is the sub-list for extension extendee
+	0, // [0:5] is the sub-list for field type_name
 }
 
 func init() { file_userdb_proto_init() }
@@ -403,10 +461,23 @@ func file_userdb_proto_init() {
 				return nil
 			}
 		}
+		file_userdb_proto_msgTypes[4].Exporter = func(v interface{}, i int) interface{} {
+			switch v := v.(*Denied); i {
+			case 0:
+				return &v.state
+			case 1:
+				return &v.sizeCache
+			case 2:
+				return &v.unknownFields
+			default:
+				return nil
+			}
+		}
 	}
 	file_userdb_proto_msgTypes[1].OneofWrappers = []interface{}{
 		(*Password_Scrypt)(nil),
 		(*Password_Plain)(nil),
+		(*Password_Denied)(nil),
 	}
 	type x struct{}
 	out := protoimpl.TypeBuilder{
@@ -414,7 +485,7 @@ func file_userdb_proto_init() {
 			GoPackagePath: reflect.TypeOf(x{}).PkgPath(),
 			RawDescriptor: file_userdb_proto_rawDesc,
 			NumEnums:      0,
-			NumMessages:   5,
+			NumMessages:   6,
 			NumExtensions: 0,
 			NumServices:   0,
 		},

internal/userdb/userdb.proto

@@ -12,6 +12,7 @@ message Password {
 	oneof scheme {
 		Scrypt scrypt = 2;
 		Plain plain = 3;
+		Denied denied = 4;
 	}
 }
 
@@ -27,3 +28,5 @@ message Scrypt {
 message Plain {
 	bytes password = 1;
 }
+
+message Denied { }

internal/userdb/userdb_test.go

@@ -122,24 +122,27 @@ func TestWrite(t *testing.T) {
 		t.Fatalf("expected %v, got %v", emptyDB, db)
 	}
 
-	// Add two users, write, and load again.
+	// Add users, write, and load again.
 	if err := db.AddUser("user1", "passwd1"); err != nil {
 		t.Fatalf("failed to add user1: %v", err)
 	}
 	if err := db.AddUser("ñoño", "añicos"); err != nil {
 		t.Fatalf("failed to add ñoño: %v", err)
 	}
+	if err := db.AddDeniedUser("ñaca"); err != nil {
+		t.Fatalf("failed to add ñaca: %v", err)
+	}
 	if err := db.Write(); err != nil {
 		t.Fatalf("error writing database: %v", err)
 	}
 
 	db = mustLoad(t, fname)
-	for _, name := range []string{"user1", "ñoño"} {
+	for _, name := range []string{"user1", "ñoño", "ñaca"} {
 		if !db.Exists(name) {
 			t.Errorf("user %q not in database", name)
 		}
 		if db.db.Users[name].GetScheme() == nil {
-			t.Errorf("user %q not using scrypt: %#v", name, db.db.Users[name])
+			t.Errorf("user %q missing scheme: %#v", name, db.db.Users[name])
 		}
 	}
 
@@ -153,6 +156,8 @@ func TestWrite(t *testing.T) {
 		{"user1", "passwd12", false},
 		{"ñoño", "añicos", true},
 		{"ñoño", "anicos", false},
+		{"ñaca", "", false},
+		{"ñaca", "lalala", false},
 		{"notindb", "something", false},
 		{"", "", false},
 		{" ", "  ", false},
@@ -202,6 +207,11 @@ func TestInvalidUsername(t *testing.T) {
 		if err == nil {
 			t.Errorf("AddUser(%q) worked, expected it to fail", name)
 		}
+
+		err = db.AddDeniedUser(name)
+		if err == nil {
+			t.Errorf("AddDeniedUser(%q) worked, expected it to fail", name)
+		}
 	}
 }
 
@@ -235,6 +245,24 @@ func TestPlainScheme(t *testing.T) {
 	}
 }
 
+// Test the denied scheme.
+func TestDeniedScheme(t *testing.T) {
+	fname := mustCreateDB(t, "")
+	defer removeIfSuccessful(t, fname)
+	db := mustLoad(t, fname)
+
+	db.db.Users["user"] = &Password{Scheme: &Password_Denied{}}
+	err := db.Write()
+	if err != nil {
+		t.Errorf("Write failed: %v", err)
+	}
+
+	db = mustLoad(t, fname)
+	if db.Authenticate("user", "anything") {
+		t.Errorf("denied authentication worked but it shouldn't")
+	}
+}
+
 func TestReload(t *testing.T) {
 	content := "users:< key: 'u1' value:< plain:< password: 'pass' >>>"
 	fname := mustCreateDB(t, content)
@@ -326,4 +354,12 @@ func TestExists(t *testing.T) {
 	if !db.Exists("user") {
 		t.Errorf("known user does not exist")
 	}
+
+	if err := db.AddDeniedUser("denieduser"); err != nil {
+		t.Fatalf("error adding user: %v", err)
+	}
+
+	if !db.Exists("denieduser") {
+		t.Errorf("known (denied) user does not exist")
+	}
 }