SMTP Proxy

HI F5_Jeff -

So SMTP is a tricky protocol when it comes to intercepting it within the F5 iRules.

In the example `mailx` output below, each exchange is a separate set of rules firing in the LTM irule. So the "connecting..." line will fire your CLIENT_ACCEPTED event. Then each >>> will fire your CLIENT_DATA event, which needs to respond with a response code and status. Basically being a middleman between your client and the smtp server. So if you want to do the load balancing up front, you need to intercept the commands up until the RCPT TO command, make the determination, and THEN route.

There is an example here, https://devcentral.f5.com/s/question/0D51T00006j2svT/irule-based-on-smtp-receiver-domain where its "queuing" or saving up the HELO and FROM commands in variables and responding to the client, and then once a RCPT comes through and the destination domain is known, it "replays" the saved commands to the back end SMTP server (after picking a pool).

I will admit, I have not been able to get it working perfectly (yet) but hopefully this helps explain the way SMTP communications are handled in the F5 LTM iRules, where its a series of events that you need to orchestrate (and its not trivial).

echo "Testing" | mailx -v -r "testing-sender@example.com" -s "New test email through LTM" -S smtp="smtp-vip.example.com:25" testing-recipient@example.com
 
Resolving host smtp-vip.example.com . . . done.
Connecting to 10.1.2.3:25 . . . connected.
220 EXGHOST03.example.com Microsoft ESMTP MAIL Service ready at Thu, 16 Jan 2020 08:59:52 -0500
>>> HELO mydesktop.example.com
250 EXGHOST03.example.com Hello [10.4.5.6]
>>> MAIL FROM:<testing-sender@example.com>
250 2.1.0 Sender OK
>>> RCPT TO:<testing-recipient@example.com>
250 2.1.5 Recipient OK
>>> DATA
354 Start mail input; end with <CRLF>.<CRLF>
>>> .
250 2.6.0 <5c216d54./ukd0CI0g7ZM5ydM%testing-sender@example.com> [InternalId=106902174639121, Hostname=EXGHOST03.example.com] Queued mail for delivery
>>> QUIT
221 2.0.0 Service closing transmission channel