Roller Network Help - IP Address Helper
If you're using a mail domain with SMTP Redirection and a hostname destination, this tool will automatically update the destination as an IP address or switch to "Accept and Hold" based on the results of the DNS lookup. This is useful for Dynamic DNS hosts, hosts that may occasionally withdraw their type "A" record, or hosts with unreliable DNS.
When Postfix (which we use) receives a negative answer from a DNS server for a domain used in SMTP Redirection (a Postfix transport map), it will return a fatal error. This is expected behavior because the DNS lookup returned a negative response (opposed to a lookup failure where no answer was available), however, in practice this is undesirable for some sites. Our IP address helper tool solves the problem by taking the DNS lookup away from Postfix.
A mail domain with an active IP address helper will be automatically switched between two modes depending on the lookup results. If a lookup returns a valid public IP address, the mode is changed to "SMTP Redirection (Direct Delivery) using the configured port number and IP address from the lookup result. In the event that more than one IP address is returned one will be chosen at random. If no IP address is returned, the DNS lookup times out, or is otherwise invalid, the mode is changed to "Accept and Hold Messages". In this mode, mail will be held in the queue until a valid lookup is returned. The IP address helper will interpret lookups that return a RFC 1918 or loopback address as an invalid result.
Configuration
Only one IP address helper may be assigned per mail domain. To create a new helper, click on the Add New IP Helper button. To prevent accidental mode changes, only domains in "SMTP Redirection (Direct Delivery)" or "Accept and Hold Messages" mode are shown below. To add an IP address helper to a domain not shown in the selector, change its mode to one of the two supported modes. An IP address helper is not applicable to any of the other mail domain modes.
The Destination Host must be a valid fully qualified domain name. This should be the DNS name of your mail server or other DNS resource that will return the current IP address of your mail server. The lookup result of the destination host entered here will determine what action an IP address helper will take. A Destination Port is also required and will be applied to the IP address of the host when the mail domain is changed to SMTP Redirection (Direct Delivery) mode.
Alert on Changes
An optional Alert on Changes feature is available to notify you when a mode change by an IP address helper takes place. When enabled, a short alert will be sent to the alert email address as configured in your account preferences. If this preference is not set, no alerts will be sent even if they are enabled here.
A sample alert from an IP address helper:
Subject: [rollernet] IP Helper example.com SMTP Redirection to 1.2.3.4:2525 -- Helper alert: 'alert@example.net' account 'example'. This is an automated message. |
Alert are only sent on mode changes, not for every lookup result.
Scanning and Update Intervals
The IP address helper system checks individual helpers approximately every minute. The last check time for a helper is shown by the "Last" date in the status column. The next scheduled check will take place in the next interval after the date shown as the "Next" date.
The TTL returned with the DNS lookup result determines the time of the next DNS check. An early check can be requested by using the "Check Now" action. (Keep in mind that it won't actually perform an instant check, only signal that a check should be performed within the next interval.) Lookup failures are given a retry of 5 minutes and invalid lookup results after 60 minutes.
Unsupported Mail Domain Modes
In the event that a mail domain is changed to an unsupported mode the IP address helper associated with that domain will be automatically removed. The only mail domain modes supported by the IP address helper feature are "SMTP Redirection (Direct Delivery)" or "Accept and Hold Messages"