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:


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