sockmapd is a socketmap server daemon for Mail Transfer Agents (MTA) that uses configurable database repositories for lookup tables.
sockmapd is written in Go Lang
Mail Transfer Agents like Sendmail and Postfix can use either hash files, database queries, or socketmaps to lookup for valid recipients' eMail addresses, hosts or eMail addresses in blacklists or whitelists, or even header policies, just to mention a few examples.
Often we have this information stored in database servers that are not directly accessible from our DMZ where the MTAs are usually deployed. Although some MTAs (like Postifx) do support direct database access and queries, we want to keep access to our internal database well protected.
This is the case, for example, if we use dbmail, an IMAP, POP server. dbmail maintains mailboxes and their eMail aliases on the protected database server, but the MTA in the DMZ is prevented direct access to them. If we want our MTA to validate a recipient and reject, or accept the eMail for further delivery at the time it is contacted by the sender's MTA, then we need a service like sockmapd, that takes queries from the MTA and verifies whether the recipient is valid. The same applies when we want our MTA to check if a sender's eMail address or the sender MTA's host name is in our blacklist.
sockmapd is a configurable service daemon that accepts socketmap protocol queries (TCP protocol) on a specified TCP Port number and returns a response to the calling service client. Sockmapd queries database tables that are mapped to a service map via a configuration file that can be specified in the command line.
Socketmap request queries have the format of a netstring as follows:
"[len]:[query],"
Example:
if my request is to lookup for someone@somewhere.com in the recipient map, the request string will be:
"32:recipient someone@somewhere.com,"
where 32 is the length of "recipient somebody@somewhere.com". In front of this request, sockmapd queries the database table mapped to the socketmap in the configuration file and returns a response as follows:
"9:NOTFOUND ,"
or
"24:OK someone@somewhere.com,"
The response strings are documented in the socketmap table man page.
Database mapping is achieved through a json format configuration file. The configuration file includes 3 sections:
sysconfig includes the tcp port number and the path for an alternative log file.
database includes host, port, username, and password, these are common configuration parameters for the connection to a database server. Note that host is a string array that can contain multiple nodes of a database cluster (or replication nodes). sockmapd initializes a database connection for each database trying to connect to each node until a successful connection is achieved. In case the database connection fails when a request is being processed, sockmapd tries to re-initialize the database connections.
postmaps is an array of an object including service, database, table, key, value and reason. sockmapd maintains a database connection for each database specified (as we can have different databases holding map data), and creates the query using the key column and taking the result from the value column (if specified, otherwise the query checks only that key is present). If specified, the content of reason is added to the response.