mx-connect

Establish TCP connection to a MX server. This module takes a target domain or email address, resolves appropriate MX servers for this target and tries to get a connection, starting from higher priority servers.

Supports unicode hostnames and IPv6.

npm install mx-connect

Usage

const mxConnect = require('mx-connect');

// Using promises (recommended)
const connection = await mxConnect(options);

// Using callbacks
mxConnect(options, (err, connection) => { ... });

Where

• options is the target domain, address, or configuration object
• callback (optional) is the function to run once connection is established or it fails

Example using async/await

const mxConnect = require('mx-connect');

try {
    const connection = await mxConnect('[email protected]');
    console.log('Connection to %s:%s', connection.hostname, connection.port);
    // Connection to aspmx.l.google.com:25

    connection.socket.pipe(process.stdout);
    // 220 mx.google.com ESMTP k11-v6si869487ljk.7 - gsmtp
} catch (err) {
    console.error(err);
}

Example using callbacks

const mxConnect = require('mx-connect');

mxConnect('[email protected]', (err, connection) => {
    if (err) {
        console.error(err);
    } else {
        console.log('Connection to %s:%s', connection.hostname, connection.port);
        // Connection to aspmx.l.google.com:25

        connection.socket.pipe(process.stdout);
        // 220 mx.google.com ESMTP k11-v6si869487ljk.7 - gsmtp
    }
});

Configuration options

You can use a domain name or an email address as the target, for additional configuration you would need to use a configuration object with the following properties (most are optional)

• target is either a domain name or an email address or an IP address or IP address literal (basically anything you can have after the @-sign in an email address). Unicode is allowed.

• port is the port number to connect to. Defaults to 25.

• maxConnectTime is the timeout in milliseconds to wait for a connection to be established (per MX host). Defaults to 5 minutes.

• localAddress is the local IP address to use for the connection

• localHostname is the hostname of the local address

• localAddressIPv4 is the local IPv4 address to use for the connection if you want to specify an address both for IPv4 and IPv6

• localHostnameIPv4 is the local hostname to use for IPv4 connections

• localAddressIPv6 is the local IPv6 address to use for the connection if you want to specify an address both for IPv4 and IPv6

• localHostnameIPv6 is the local hostname to use for IPv6 connections

• dnsOptions is an object for IP address related options

  • ignoreIPv6 (boolean, defaults to false) If true then never use IPv6 addresses for sending
  • preferIPv6 (boolean, defaults to false) If true then use IPv6 address even if IPv4 address is also available
  • blockLocalAddresses (boolean, defaults to false) If true then refuses to connect to IP addresses that are either in loopback, private network or attached to the server. People put every kind of stuff in MX records, you do not want to flood your loopback interface because someone thought it is a great idea to set 127.0.0.1 as the MX server
  • resolve (function, defaults to native dns.promises, callback-style) Custom DNS resolver function with signature resolve(domain, type, callback) or resolve(domain, callback)

• mx is a resolved MX object or an array of MX objects to skip DNS resolving. Useful if you want to connect to a specific host.

  • exchange is the hostname of the MX
  • priority (defaults to 0) is the MX priority number that is used to sort available MX servers (servers with higher priority are tried first)
  • A is an array of IPv4 addresses. Optional, resolved from exchange hostname if not set
  • AAAA is an array of IPv6 addresses. Optional, resolved from exchange hostname if not set

• ignoreMXHosts is an array of IP addresses to skip when connecting

• mxLastError is an error object to use if all MX hosts are filtered out by ignoreMXHosts

• connectHook function (delivery, options, callback) is a function handler to run before establishing a tcp connection to current target (defined in options). If the options object has a socket property after the callback then connection is not established. Useful if you want to divert the connection in some cases, for example if the target domain is in the Onion network then you could create a socket against a SOCKS proxy yourself.

• connectError function (err, delivery, options) is a function handler to run when a connection to a MX fails.

• mtaSts is an object for MTA-STS configuration

  • enabled - if not true then does not run MTA-STS checks, disabled by default
  • logger(logObj) - method to log MTA-STS information, logging is disabled by default
  • cache - an object to manage MTA-STS policy cache
    • get(domain) -> returns cached policy object
    • set(domain, policyObj) -> caches a policy object

Connection object

Function callback or promise resolution provides a connection object with the following properties:

• socket is a socket object against target
• hostname is the hostname of the exchange
• host is the IP address of the exchange
• port is the port used to connect
• localAddress is the local IP address used for the connection
• localHostname is the local hostname used for the connection
• localPort is the local port used for the connection

License

EUPL v1.1 or newer