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 #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
-.IX Item "user-add user@domain [--password=password]"
+.IP "\fBuser-add\fR \fIuser@domain\fR [\-\-password=\fIpassword\fR] [\-\-receive_only]" 8
+.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 {