ETRN Polling
Mail hosts which have intermittent connectivity to the Internet will need a connected host to store mail for them when they are not connected and then transmit it to them when they are.
The SMTP protocol has the necessary facilities to accomplish this. First, SMTP uses DNS MX records to determine what mail exchange server(s) to connect to in order to send mail to a particular domain.
These MX records have priority values associated with them. The lower the numerical value of the MX priority. If a sending site looks up a domain name and finds that there are two MX servers, one has a value of 10, the other 100, it will first attempt to connect to the server with the value of 10. If it can not, it will next try the server with the MX priority value of 100.
Eskimo North uses this to implement ETRN polling. A DNS record for your domain with an MX record for your server is added with a MX priority value that is low, like '10', and an MX record for a server at the ISP is added with a high MX priority value, say '100'.
Eskimo North also must configure it's MTA to relay for your domain so that it can accept mail for your domain.
Now what happens is a remote site attempts to send mail, it will first attempt to send directly to your server because it has the lower value priority MX record. If your server is available, it will send the mail to your server. If your server is not available, it will then try the higher priority value MX record for the server at Eskimo North, and our server will accept the mail. It will attempt to forward it but it will be unable to do so because your site is off line so it will queue the incoming mail.
Now when your site connects to the net, it will connect to the mail server at Eskimo North and it will issue an ETRN command that asks the remote server to initiate a connection and session towards your server.
In the early days a command called TURN was used to reverse the session direction. However, this had a security hole in that a site could connect, issue an EHLO command informing the MTA that it was a different site, then issue an ETRN command and steal that sites mail. This was not a good thing.
The ETRN command works differently. Since it initiates the connection, it can be more confident that it's talking to the right system, as opposed to trusting an existing connection.
On this end then it basically requires DNS and sendmail (the MTA we use) configuration. On your end, it requires software capable of issuing an ETRN command.
In order for this to work, a static IP address is not required on your end, but a static host name is. This can be accomplished on a dynamic IP address by using dynamic DNS that dynamically maps a static host name to your dynamic IP. There are a number of dynamic DNS services that do this, many are free.
The SMTP protocol has the necessary facilities to accomplish this. First, SMTP uses DNS MX records to determine what mail exchange server(s) to connect to in order to send mail to a particular domain.
These MX records have priority values associated with them. The lower the numerical value of the MX priority. If a sending site looks up a domain name and finds that there are two MX servers, one has a value of 10, the other 100, it will first attempt to connect to the server with the value of 10. If it can not, it will next try the server with the MX priority value of 100.
Eskimo North uses this to implement ETRN polling. A DNS record for your domain with an MX record for your server is added with a MX priority value that is low, like '10', and an MX record for a server at the ISP is added with a high MX priority value, say '100'.
Eskimo North also must configure it's MTA to relay for your domain so that it can accept mail for your domain.
Now what happens is a remote site attempts to send mail, it will first attempt to send directly to your server because it has the lower value priority MX record. If your server is available, it will send the mail to your server. If your server is not available, it will then try the higher priority value MX record for the server at Eskimo North, and our server will accept the mail. It will attempt to forward it but it will be unable to do so because your site is off line so it will queue the incoming mail.
Now when your site connects to the net, it will connect to the mail server at Eskimo North and it will issue an ETRN command that asks the remote server to initiate a connection and session towards your server.
In the early days a command called TURN was used to reverse the session direction. However, this had a security hole in that a site could connect, issue an EHLO command informing the MTA that it was a different site, then issue an ETRN command and steal that sites mail. This was not a good thing.
The ETRN command works differently. Since it initiates the connection, it can be more confident that it's talking to the right system, as opposed to trusting an existing connection.
On this end then it basically requires DNS and sendmail (the MTA we use) configuration. On your end, it requires software capable of issuing an ETRN command.
In order for this to work, a static IP address is not required on your end, but a static host name is. This can be accomplished on a dynamic IP address by using dynamic DNS that dynamically maps a static host name to your dynamic IP. There are a number of dynamic DNS services that do this, many are free.
posted by Nanook at 9:58 PM
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