I run Postfix on my VPS’s exclusively to send mail. They’re configured to route mail to Mailgun, and I’ve made sure that they’re not an open relay. I also receive a daily logwatch email and it doesn’t help to have a bunch of this:
Feb 4 18:31:47 alpha postfix/smtpd: connect from unknown[18.104.22.168] Feb 4 18:31:47 alpha postfix/smtpd: NOQUEUE: reject: RCPT from unknown[22.214.171.124]: 554 5.7.1 <[email protected] yahoo.com>: Relay access denied; from=<[email protected]> to=<[email protected]> proto=ESMTP helo=<[192.168.2.33]> Feb 4 18:46:31 alpha postfix/smtpd: connect from client002.c039228.customers.cinergycom.net[126.96.36.199] Feb 4 18:46:31 alpha postfix/smtpd: NOQUEUE: reject: RCPT from client002.c039228.customers.cinergycom.net[69.196 .196.138]: 554 5.7.1 <[email protected]>: Relay access denied; from=<[email protected]> to=<[email protected]> proto=ESMTP helo=<[192.168.2.33]>
I had a couple of options. This first is to modify master.cf to deny SMTP access from “inet”. I tried it and it did work, but I didn’t really care for the solution since it didn’t gel that well with my Ansible configuration for the host. The other option was to just lock down port 25, period. I was down the road of re-re-re-acquainting myself with iptables–something I do seldom and immediate forget–and then I came across ufw. The cryptic iptables syntax is replaced with:
ufw allow http/tcp ufw enable
(by default everything is rejected)
This worked just fine and I was able to easily whitelist a few key ports in my base Ansible file. We’ll see tomorrow whether the logwatch email thins out.
** Interesting note: testing this stuff from my laptop didn’t work. By spinning up another VPS and successfully connect to port 25 (so I could then verify that I couldn’t connect after the changes), I confirmed that it was my ISP (Comcast). I’ve now learned that blocking port 25 except to the ISP’s mail server is a thing.