Strange networking behavior when sending from PHP via Exim to the world

eitanc

• November 09, 2023 10:36

Hello, I use the latest WordPress with the latest version of a WP email SMTP delivery plugin. It works fine, sending emails using STARTTLS to port 25 of a remote server on the Internet. I wanted to understand how it works so I did a test delivery using this plugin and in parallel did a network packet capture using tcpdump, catching from all interfaces, without any filtering or limitations. The background: The server is Ubuntu using the latest version 114 CP. The server has a local private IP, which access the Internet via a local FW, using 1:1 NAT. The FW is connected to the Internet and use a public IP. The session goes like this: 1. Internal-Private-IP at some source port (say 55661) > 127.0.0.1 port 25 - SYN 2. Target-External-IP at source port of 25 > Internal-Private-IP at port 55661 - SYN, ACK 3. Internal-Private-IP at port 55661 > 127.0.0.1 port 25 - ACK What looks strange to me is it looks like a step is missing between action #1 and #2, I don't see the Internal-Private-IP connecting to the Target-External-IP at destination port 25, I gust get the Target-External-IP reply, to a request I don't have evidence to. When the session continues with SMTP commands - the same happens, there are no steps that show connection to the Target-External-IP server, only replies from it. (I realize that probably also the sending of the "SYN, ACK" back from Internal-Private-IP to 127.0.0.1 is missing, to have full tidy session, but I guess this is Exim saving time of obvious sub-session steps) I guess the Internal-Private-IP is being sent using a PHP function of WP and that 127.0.0.1 is probably Exim. Any idea what is going on here?

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  cPRex Jurassic Moderator

  • November 09, 2023 21:17

  Hey hey! This doesn't sound like it would be related to any cPanel tools, but I'll leave this thread marked as new for a bit for visibility to see if other users have ideas on what may be happening here.

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  ffeingol

  • November 09, 2023 21:23

  I'll let other more Exim experts chime in, but from WordPress/SMTP plugin the email is not going to immediately send. The SMTP plugin will put the email into Exim's queue and then Exim will give a status code to the STMP plugin that the email was accepted. Exim will then attempt to deliver the email that has been placed in the queue.

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