Often I hear that one should block spam email based on content match rather than IP match. Sometimes even that blocking Chinese ranges in particular is prejudiced and racist. Allow me to debunk that after I've been looking at traffic on port 25 with tcpdump for several weeks now, and got rid of most of my incoming spam too.

There are these spamhausen that communicate with my mail server as much as every minute.
- biz-smtp.com
- mailing-expert.com
- smtp-shop.com

All of them are Chinese. They make up - rough guess - around 90% of the traffic that hits my edge nodes, if not more.

The network ranges I've blocked are apparently as follows:
- 193.106.175.0/24 (Russia)
- 49.64.0.0/11 (China)
- 181.39.88.172 (Ecuador)
- 188.130.160.216 (Russia)
- 106.75.144.0/20 (China)
- 183.227.0.0/16 (China)
- 106.75.32.0/19 (China)
.. apparently I blocked that one twice, heh
- 116.16.0.0/12 (China)
- 123.58.160.0/19 (China)

It's not all China but holy hell, a lot of spam sure comes from there, given how Golden Shield supposedly blocks internet access to the Chinese citizens. A friend of mine who lives in China (how he got past the firewall is beyond me, and he won't tell me either) told me that while incoming information is "regulated", they don't give half a shit about outgoing traffic to foreign countries. Hence all those shitty filter bag suppliers and whatnot. The Chinese government doesn't care.

So what is the alternative like, that would block based on content? Well there are a few solutions out there, namely SpamAssassin, ClamAV and Amavis among others. The problem is that they're all very memory intensive (especially compared to e.g. Postfix and Dovecot themselves) and that they must scan every email, and keep up with evasion techniques (such as putting the content in an image, or using characters from different character sets t̾h̾a̾t̾ ̾l̾o̾o̾k̾ ̾s̾i̾m̾i̾l̾a̾r̾).

But the thing is, all of that traffic comes from a certain few offending IP ranges, and an iptables rule that covers a whole range is very cheap. China (or any country for that matter) has too many IP ranges to block all of them. But the certain few offending IP ranges? I'll take a cheap IP-based filter over expensive content-based filters any day. And I don't want to be shamed for that.

Okay so my brother in law has a laptop that is... To put it mildly, chockful of viruses of all sort, as it's an old machine still running w7 while still being online and an av about 7 years out of date.

So my bro in law (let's just call him my bro) asked me to install an adblock.

As I launched chrome and went to install it, how ever, the addon page said something like "Cannot install, chrome is managed by your company" - wtf?

Also, the out of date AV couldn't even be updated as its main service just wouldn't start.

Okay, something fishy going on... Uninstalled the old av, downloaded malware bytes and went to scan the whole pc.

Before I went to bed, it'd already found >150 detections. Though as the computer is so old, the progress was slow.

Thinking it would have enough time over night, I went to bed... Only to find out the next morning... It BSoD'd over night, and so none of the finds were removed.

Uuugh! Okay, so... Scanning out of a live booted linux it is I thought! Little did I know how much it'd infuriate me!

Looking through google, I found several live rescue images from popular AV brands. But:
1 - Kaspersky Sys Rescue -- Doesn't even support non-EFI systems

2 - Eset SysRescue -- Doesn't mount the system drive, terminal emulator is X64 while the CPU of the laptop is X86 meaning I cannot run that. Doesn't provide any info on username and passwords, had to dig around the image from the laptop I used to burn it to the USB drive to find the user was, in fact, called eset and had an empty password. Root had pass set but not in the image shadow file, so no idea really. Couldn't sudo as the eset user, except for the terminal emulator, which crashes thanks to the architecture mismatch.
3 - avast - live usb / cd cannot be downloaded from web, has to be installed through avast, which I really didn't want to install on my laptop just to make a rescue flash drive
4 - comodo - didn't even boot due to architecture mismatch

Fuck it! Sick and tired of this, I'm downloading Debian with XFCE. Switched to a tty1 after kernel loads, killed lightdm and Xserver to minimize usb drive reads, downloaded clamav (which got stuck on man-db update. After 20 minutes... I just killed it from a second tty, and the install finished successfully)

A definitions update, short manual skimover, and finally, got scanning!

Only... It's taking forever and not printing anything. Stracing the clamscan command showed it was... Loading the virus definitions lol... Okay, it's doing its thing, I can finally go have dinner

Man I didn't know x86 support got so weak in the couple years I haven't used Linux on a laptop lol.

It still feels weird to be installing an antivirus on a CentOS server :D

Is clamav good and reliable for Linux?

Bullshittery continues. This time around, absolutely innocent, clamav is root cause. For once not incompetent idiot, but piece of software. IDK if that makes me happy or upset.

So our email server that I configured and took care of died. RIP. Damn, better put it back together ASAP. So Im under pressure, while still pissed at everything that I ranted before (actually my last 2 rants were throttled, and in total all of that happened past 60 minutes but devrant rate limiting) I start auditing logs. You imagine, we kindda need it NOW, and it's second time last month clamav is pulling stunts and MTA refuses (properly) to work without antivirus. So pressurized, I look at logs, what the fuck went wrong.

clamav deamonize() failed - cannot allocate memory

Hmm. Intresting, but sounds like bullshit. I know server is quite micro becouse they wanted to save on costs as much as possible, but it has well over half a gig free ram just before it crashes (like 800MB) with that message. Is it allocating almost gig in one call or what? Looked carefully at trusty htop while it was starting, and indeed, suddenly it just dies with quite a bit of ram free, almost as much as it weights already. And I remember booting it up when I was configuring it, and it had fair bit of headroom.

Google, help me friend... Okay, great, so apparently at some point clamav loads virus DB into ram (dafuq?), and than forks, which causes spike of 2x the ram usage, and than immidietely frees it up.

Great, that sounds like great design decision... At least I know, I can just slap on SWAP file, restart it and call it a day.

It worked, swap file is almost empty (used 15megs, 900 megs free ram, whatever).

That leaves me wandering, who figured out to load DB to ram? That means pretty much that clamav will eat a little bit more ram each vir db update, and that milisecond "double ram" spike will confuse innocent people who just wanted to run clamav and it worked last *long period of time* and now crashes without warning without any changes to configuration.

Maybe there is logical explanation, I want to know it.