mod_smtp_recipient_monitor: Add recipient-based greylisting for submi…

…ssion.

Add a new module which can be leveraged by users to automatically help
prevent them from sending emails from the wrong identities or to the
wrong recipients.

README.rst

@@ -58,6 +58,7 @@ Key features and capabilities include:
 
     * Sieve filtering scripts and ManageSieve service
     * `MailScript filtering engine <configs/.rules>`_ for flexible, custom, dynamic mail filtering rules (Sieve alternative)
+	* Intelligent sender/recipient analysis - prevent yourself from ever sending an email to the wrong people by mistake!
 
   * Webmail client backend
 

bbs/stringlist.c

@@ -107,7 +107,7 @@ void stringlist_empty(struct stringlist *list)
 	RWLIST_UNLOCK(list);
 }
 
-const char *stringlist_next(struct stringlist *list, struct stringitem **i)
+const char *stringlist_next(const struct stringlist *list, struct stringitem **i)
 {
 	struct stringitem *inext;
 	if (!*i) {
@@ -155,6 +155,25 @@ int stringlist_push(struct stringlist *list, const char *s)
 	return 0;
 }
 
+int stringlist_push_sorted(struct stringlist *list, const char *s)
+{
+	struct stringitem *i;
+	char *sdup = strdup(s);
+
+	if (ALLOC_FAILURE(sdup)) {
+		return -1;
+	}
+
+	i = calloc(1, sizeof(*i));
+	if (ALLOC_FAILURE(i)) {
+		free(sdup);
+		return -1;
+	}
+	i->s = sdup;
+	RWLIST_INSERT_SORTALPHA(list, i, entry, s);
+	return 0;
+}
+
 int stringlist_push_tail(struct stringlist *list, const char *s)
 {
 	struct stringitem *i;

configs/modules.conf

@@ -77,8 +77,9 @@ load = mod_smtp_filter.so
 load = mod_smtp_filter_arc.so
 load = mod_smtp_filter_dkim.so
 load = mod_smtp_filter_dmarc.so
-load = mod_smtp_mailing_lists.so
 load = mod_spamassassin.so
+load = mod_smtp_mailing_lists.so
+load = mod_smtp_recipient_monitor.so
 load = mod_webmail.so
 load = net_imap.so
 load = net_nntp.so

include/net_smtp.h

@@ -22,6 +22,18 @@
 /* Mainly for message submission agents, not encrypted by default, but may use STARTTLS */
 #define DEFAULT_SMTP_MSA_PORT 587
 
+#define _smtp_reply(smtp, fmt, ...) \
+	bbs_debug(6, "%p <= " fmt, smtp, ## __VA_ARGS__); \
+	bbs_auto_fd_writef(smtp_node(smtp), smtp_node(smtp) ? smtp_node(smtp)->wfd : -1, fmt, ## __VA_ARGS__); \
+
+/*! \brief Final SMTP response with this code */
+#define smtp_resp_reply(smtp, code, subcode, reply) _smtp_reply(smtp, "%d %s %s\r\n", code, subcode, reply)
+#define smtp_reply(smtp, code, status, fmt, ...) _smtp_reply(smtp, "%d %s " fmt "\r\n", code, #status, ## __VA_ARGS__)
+#define smtp_reply_nostatus(smtp, code, fmt, ...) _smtp_reply(smtp, "%d " fmt "\r\n", code, ## __VA_ARGS__)
+
+/*! \brief Non-final SMTP response (subsequent responses with the same code follow) */
+#define smtp_reply0_nostatus(smtp, code, fmt, ...) _smtp_reply(smtp, "%d-" fmt "\r\n", code, ## __VA_ARGS__)
+
 struct smtp_session;
 
 void __attribute__ ((format (gnu_printf, 3, 4))) bbs_smtp_log(int level, struct smtp_session *smtp, const char *fmt, ...);
@@ -72,13 +84,33 @@ enum smtp_direction {
 	SMTP_DIRECTION_OUT = (1 << 2),		/*!< Outgoing mail to another MTA */
 };
 
+/*!
+ * \note There are two different "filtering" APIs available,
+ * based on the smtp_filter_data (filters) and smtp_msg_process (message processors) structures.
+ * They are very similar and they probably could theoretically be combined.
+ * That said, there are a few major differences between them as they exist now,
+ * that should be considered when deciding which one to use.
+ *
+ * - The filtering API can modify/rewrite messages. The message processors do not.
+ *   Thus, filtering here is probably also more akin to "milters" in standard POSIX MTAs.
+ *   This is probably the most important difference. Message processors don't (and can't)
+ *   rewrite messages, so they are probably more efficient to run if you don't need to modify the input.
+ * - Both can reject messages; message processors have more control over how they are rejected, though filters can quarantine the message.
+ * - Both can operation on incoming and outgoing messages to varying degrees.
+ * - In practice, message processors are used for "filtering engines" (ironically) - Sieve and MailScript.
+ *   "Filters" are used for more traditional milter applications, such as SPF, DKIM, etc. - things that prepend headers to the message.
+ * - Other differences exist. Compare the structs and APIs to see what information is available.
+ */
+
+/* == SMTP filter callbacks - these receive a message and potentially modify it == */
+
 struct smtp_filter_data {
 	struct smtp_session *smtp;		/*!< SMTP session */
 	int inputfd;					/*!< File descriptor from which message can be read */
 	const char *recipient;			/*!< Recipient (RCPT TO). Only available for SMTP_DIRECTION_IN and SMTP_DIRECTION_SUBMIT, and if the scope is SMTP_SCOPE_INDIVIDUAL */
 	size_t size;					/*!< Message length */
 	enum smtp_direction dir;		/*!< Direction */
-	int received;					/*!< Time that message was received */
+	time_t received;				/*!< Time that message was received */
 	/* Duplicated fields: these are simply duplicated from smtp: */
 	struct bbs_node *node;			/*!< Node */
 	const char *from;				/*!< Envelope from */
@@ -200,16 +232,19 @@ int smtp_message_quarantinable(struct smtp_session *smtp);
 
 struct smtp_msg_process {
 	/* Inputs */
+	struct smtp_session *smtp;	/*!< SMTP session. Not originally included, so try to avoid using this! */
 	int fd;						/*!< File descriptor of SMTP session */
 	struct mailbox *mbox;		/*!< Mailbox (incoming only) */
 	struct bbs_user *user;		/*!< BBS user (outgoing only) */
 	struct bbs_node *node;		/*!< BBS node */
 	const char *datafile;		/*!< Name of email data file */
 	FILE *fp;					/*!< Email data file (used internally only) */
 	const char *from;			/*!< Envelope from */
+	const struct stringlist *recipients; /*!< All envelope recipients (RCPT TO). Only available for SMTP_SCOPE_COMBINED/SMTP_DIRECTION_SUBMIT, not SMTP_DIRECTION_IN or SMTP_DIRECTION_OUT */
 	const char *recipient;		/*!< Envelope to - only for INCOMING messages */
 	int size;					/*!< Size of email */
-	int userid;					/*!< User ID (outgoing only) */
+	unsigned int userid;		/*!< User ID (outgoing only) */
+	enum smtp_direction dir;	/*!< Full direction (IN, OUT, or SUBMIT) */
 	unsigned int direction:1;	/*!< 0 = incoming, 1 = outgoing */
 	/* Outputs */
 	unsigned int bounce:1;		/*!< Whether to send a bounce */

include/stringlist.h

@@ -82,7 +82,7 @@ void stringlist_empty(struct stringlist *list);
  * \returns Next string, or NULL if end of list reached.
  * \note The returned string must not be modified, since it remains in the list.
  */
-const char *stringlist_next(struct stringlist *list, struct stringitem **i);
+const char *stringlist_next(const struct stringlist *list, struct stringitem **i);
 
 /*!
  * \brief Pop the most recently added item to a string list
@@ -101,6 +101,15 @@ char *stringlist_pop(struct stringlist *list);
  */
 int stringlist_push(struct stringlist *list, const char *s);
 
+/*!
+ * \brief Add an item to a stringlist, such that the stringlist remains sorted alphabetically
+ * \param list
+ * \param s String to add
+ * \note Assumes list is WRLOCKed
+ * \retval 0 on success, -1 on failure
+ */
+int stringlist_push_sorted(struct stringlist *list, const char *s);
+
 /*!
  * \brief Add an item to the end of a stringlist
  * \param list

modules/mod_smtp_delivery_local.c

@@ -252,6 +252,7 @@ static int do_local_delivery(struct smtp_session *smtp, struct smtp_response *re
 	smtp_mproc_init(smtp, &mproc);
 	mproc.size = (int) datalen;
 	mproc.recipient = recip_buf + 1; /* Without <> */
+	mproc.dir = SMTP_DIRECTION_IN;
 	mproc.direction = SMTP_MSG_DIRECTION_IN;
 	mproc.mbox = mbox;
 	mproc.userid = 0;