Your DHCP server must be in only one VLAN

Recently, we have made a major upgrade of the network infrastructure for one customer. We replaced the old switches and introduced the complete VLAN infrastructure. At the end of the day, everything seemed to be good.

Then, after two days, DUDE started to raise alarms related to DHCP service. We received a bunch of e-mails related to an empty DHCP pool. It was a mystery how this DHCP pool emptied when there are only 10 workstations and this happened during a public holiday when the offices were empty. Continue reading

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Duplicate V3 extension in a X.509 certificate

Recently, I got a report that one application can’t connect to the Exchange server. The error message was:

Exchange server error: The request failed. javax.net.ssl.SSLProtocolException: java.io.IOException: Duplicate extensions not allowed.

The first Google search revealed to me that this error message is related to the error inside the SSL certificate and the java based library can’t override such situation. Continue reading

Advanced disk repairing option in Windows 8 and later

A few months ago, I encountered a problem with one USB hard disk. Whenever it was attached to any PC, Windows will prompt that volume has errors. As always, we click on Scan and Fix option expecting that that would do the job.

This disk was one of a kind and now you can read about it. I tried a few times to repair it. Every time, the disk check passed and all was fine, until I again needed to reconnect the disk. Then the story repeated. Continue reading

Tiny Core Linux on a USB stick

The Linux live CD or USB stick is not hot news. But, having the full installation running from a USB flash drive that acts like a hard drive is the difference. All you need is any PC with the boot from USB capability and 128+ Megabytes of RAM.

Are you interested? Continue reading

Configuring a FloppyFW router

I successfully built a new VM that will serve as the ADSL router in my virtual lab. As the next step, I needed to configure it with specific parameters for my virtual lab. I will guide you through the configuration process for this scenario.

The good news is that the whole configuration for FloppyFW is stored in only one single file named config. This file is placed in the root of the floppy image file. Continue reading

A new “ADSL” router in my virtual lab

I keep looking for ways to improve my virtual lab and made it as realistic as possible. Although using Mikrotik routers is enough to build a large infrastructure, it’s not completely accurate with the real world experience – Mikrotik routers always work.

In the real world, we often need to connect a Mikrotik router either directly to other types of devices or even behind an exotic grandma’s home appliance. On the other hand, this is the lab and I don’t need any fancy features in such software. I want to save every megabyte of RAM or disk space on my laptop.

My search was fruitful and I found the perfect candidate – FloppyFW. This is the fully functional Linux-based router/firewall that can fit on a single 1.44” floppy disk.

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Trojan on my Android tablet after the factory data reset

At the end of 2017 I decided to reset my Android tablet. I used it as an auxiliary on-field device. There was no important data on it, but it could do with a clear down, a fresh start. The factory reset is a very easy task on Android devices.

I ran the reset task, it restored my device to a clean state and everything looked fine. I spotted McAfee anti-virus on it and ran it just for fun. It updated itself, started the scanning process and alerted me of the threat. I had a Trojan virus on the factory reset device.

Continue reading