I run my own email server, and a friend received a compromised laptop from work which resulted in a spam attack from Russia yesterday. Turtle settings saved the days with thousands of emails still in the queue when I saw the problem, however it made me realize that everyone with accounts on my server are local, do not travel, and have no requirement to send emails from outside the country.
I found how to use the smtpd_discard_ehlo_keyword_address_maps setting in postfix to block a CIDR list of IPs, then found a maintained list of IPs by country codes on github. Cool so far, and a script to keep my local list updated was easy enough.
Now the question is, what countries should I be blocking? There are plenty of lists of the top hacking sources, but it’s hard to block #2 (the US) when that’s where I am located. But otherwise, does anyone have a list of countries they outright block from logging on to their servers? From the above google searches I have 17 countries blocked so far, and in the first 30 minutes already stopped login attempts from three of those countries, so it appears to be working.
Of course I could write a script to parse my logs to see who has already made attempts, but that’s what services like fail2ban are for, and I’m just wondering if there are any countries in particular I should directly block? My list so far includes the following: ae bg br cn de hk id in ir iq il kp ng ru sa th vn
The question itself may not be that interesting, but I thought at the very least some folks might be interested in my experience and think about doing something similar themselves. I can post more details of what I did if there is any interest.
The advice I’ve read (and implemented myself) is to not so much run a block list, but an allow list. So first things first, have a rule to block all connections, then have overriding rules to allow connections using criteria you would deem safe. If you know someone needs to access the server from the UK, include the UK on the allow list. Everything else can remain locked down until you have a reason to open it up to another country.
That’s pretty much the way a firewall works, but I’m not sure it’s quite so practical for email. When you get into something like cell phone access, the IPs can be all over the place. I’ve certainly seen enough attempts from addresses of my own cell provider. I’ve even seen fail2ban block IPs from my local city ISP, so it’s really difficult just blacklist everything and not expect there to be nearly immediate problems for those of us who have legitimate access. This is one of the reasons I run multiple tools, between the standard blocklists to weed out spammers and public VPNs, to things like fail2ban providing more realtime protection. I look at the country blocklist as just another tool in the arsenal to try and find a balance between protecting my services but still allowing easy access where it is needed.
They said country-based, not location-based. Your cellphone provider will probably only be using a handful of countries at most to relay traffic.