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How to remove old kernels from Ubuntu

This method should work for both Ubuntu server OS (instances) and desktops that we use.
Use case: Automatic or manual updates to the OS may result in multiple kernels installed in your system. This is usually alright, but it can become a problem if your the OS has a small /boot partition, and it runs out of space because of a large number of old kernels installed there. You could follow these simple steps to delete the older kernels
1. Verify the kernel that is currently booted and is in use:
uname -r4.4.0-79-generic
Whatever you do, don't delete this kernel.

2. Finding out what kernels are currently installed in the system:
dpkg —list | grep linux-imagelinux-image-4.4.0-72-generic               4.4.0-72.93                                   amd64        Linux kernel image for version 4.4.0 on 64 bit x86 SMP linux-image-4.4.0-75-generic               4.4.0-75.96                                   amd64        Linux kernel image for version 4.4.0 on 64 bit x86 SMP linux-image-4.4.0-77-gene…

Useful tcpdump commands and examples

How to enable DNSBL or RBL on Zimbra to fight against spam

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DNS-based Blackhole List (DNSBL) or Real-time Blackhole List (RBL) is an effort to fight spam emails. It is a blacklist of source IP addresses that have a reputation of sending spam emails. Most email systems can be configured to check these lists and block or flag emails that were sent from domains/IPs listed there. The ‘Blackhole List’ is sometimes called ‘blacklist’ by email admins.