Thwarting Spammers

Charly's Column – grepcidr

Article from Issue 216/2018

Often it is the very simple tools that, when used appropriately, lead to the greatest success. This time, sys admin columnist Charly employs an IP address filter to count the devices in his home and trip up spammers to boot.

Although Linux has many grep variants, you can always find a new one. I only discovered grepcidr [1] a few months ago. As the name suggests, the tool filters input by IP addresses and networks. It works equally well with IPv4 and IPv6. To show grepcidr's capabilities, I will use it to compile a list of all IPv4 addresses on my home network. I got this from the Syslog on the firewall, which is also the DHCP server:

cd /var/log
grepcidr syslog|grep DHCPACK|tail -n 1500|cut -f9 -d" "|sort|uniq > 1stlist

The 1stlist file now contains 46 IP addresses:
[...41 more...]

I automated this discovery process with the following command:

grepcidr -c < ./1stlist

In this simple case, it would have been faster with wc -l, but grepcidr ultimately shows its strengths when you have to filter out different networks from such a file.

After a while, I repeat the game and write the list of IP addresses written to the 2ndlist file. For my statistics,

grepcidr -c < .2ndlist

shows that the file is one line shorter, so there is one less device on the network. I can easily find out which one is missing with diff:

diff 1stlist 2ndlist

To see how many devices in my house are run 24/7, I send the data to a round-robin database and have a history graph drawn. The interruption at about 6am in Figure 1 is a routine reboot of the DHCP server.

Figure 1: To be or not to be a device – grepcidr clarifies the question.

Fighting Off Mail Pests

I also used grepcidr to create IP blacklists on my mail server. I have set up some mail addresses that I don't use but that spammers will typically test for their existence: sales@..., for example. Twice a day, grepcidr plows through the log and extracts all the IPs from the log lines that relate to these mailboxes and writes them to the harvest file.

Because these lines also contain my own servers' IPs, I have to remove them with a whitelist, which is also a file with IP addresses.

The ready-to-use blacklist is now built by grepcidr in a single step:

cat harvest|grepcidr -vf whitelist > blacklist

This trick has helped me keep many a spammer out of my life.

The Author

Charly Kühnast manages Unix systems in the data center in the Lower Rhine region of Germany. His responsibilities include ensuring the security and availability of firewalls and the DMZ.

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