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Grub Fallback: Boot good kernel if new one crashes

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It’s hard to believe but I didn’t know about Grub fallback feature. So every time when I needed to reboot remote server into a new kernel I had to test it on local server to make sure it won’t panic on remote unit. And if kernel panic still happened I had to ask somebody who has physical access to the server to reboot the hardware choose proper kernel in Grub. It’s all boring and not healthful – it’s much better to use Grub’s native fallback feature. … Read more

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Why Mosh is better than SSH?

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Mosh (stands for Mobile Shell) is replacement of SSH for remote connections to Unix/Linux systems. It brings a few noticeable advantages over well known SSH connections. In brief, it’s faster and more responsive, especially on long delay and/or unreliable links. Key benefits of Mosh Stays connected if your IP is changed. Roaming feature of Mosh allows you to move between Internet connections and keep Mosh session online. For example, if your wifi connection changes IP you don’t need to reconnect. Keeps session after loosing connection. … Read more

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Attach ISO image stored in XenServer local storage

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Citrix XenServer is powerful hypervisor that is based on Linux (Redhat/Centos/Fedora family) and competes with Vmware ESXi and offer wide range of virtualization features for data centers. In general you can do a bare-metal installation of XenServer to your hardware server and create multiple virtual machines (VMs) inside XenServer host. Obviously you’ll need to have the possibility to boot VM from DVD-Rom to start installation of, let’s say, Linux Mint. You can do it using hardware DVD-Rom of the host XenServer or attach ISO image and … Read more

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Track Cisco BGP peers using Nagios

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Few will deny that monitoring of Cisco devices is essential part of sysadmin’s job. I personally use Nagios to track states of BGP neighbors on Cisco routers so if one of peers goes down I’ll receive a phone call from Nagios. You may have redundant network topology but it still makes sense to know when peer goes offline, how often it happens and how fast failover router (if any) pick-ups the traffic from failed peer. There are a few plugins for Nagios to monitoring BGP … Read more

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Top Open Source IP Address Management Software

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In this post you will find top open source software for IP address management (IPAM). If you are sysadmin at organization that holds pool of IP addresses and allocates its parts to clients then you must use IP address management tools to track used, reserved, allocated or free IP addresses. It usually prevents an overhead and allows to have clear picture of IP addresses resources usage within an organization on the whole. For example, it may be useful to know how many free IP networks are … Read more

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Top 5 Linux Monitoring Tools. Web Based.

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Linux system monitoring is one of the most important tasks for every sysadmin: it is crucial to know everything about system including CPU load, network traffic statistics, memory consumption, logged in users, availability of disk free space or service. And it’s inevitable that something breaks or goes down from time to time so usually it’s just better to know it happened from Linux monitoring system’s alert rather than from angry user. Believe me, it is true. In this article I’ve listed top 5 web based … Read more

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Wget Cookies: Download Protected Content

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Most of Linux users are using wget from time to time, sometimes even when they don’t know about it – many GUI download managers for Linux are based on wget. Anyways wget is command line tool for downloading files over HTTP, HTTPs and FTP protocols within single session. It works like a charm with default settings for downloading simple static files, at the same time if content is protected by cookies and/or referrer then wget may seem useless but it’s actually not. For example, imagine … Read more

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Fix socket timeouts in Nagios

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As any other monitoring system Nagios can produce false alarms. Usually it happens when Nagios fails to get the reply from the host being monitored during some pre-defined timeout. In order to mark service as down Nagios does three checks and if all of them are failed then the service is marked down and administrator will got an alert about its critical status. At the same time even if one of those checks fails Nagios will report administrator about it depending on configuration (e-mail, twitter, … Read more

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Nfsen: Traffic Classification

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Nfsen is open source sensor: it accepts netflow data from multiple netflow probes (servers, routers, vpn concentrators etc) and then visualizes it into human readable form. So using Nfsen you can see traffic statistics of every network device in your network in one place (actually Nfsen provides much more features).

By default Nfsen makes it possible to see only inbound and outbound traffic statistics but no protocol breakdown or any traffic classification. In the meantime it’s always useful to know what network applications are eating the bandwidth to understand if that fits baseline or not and take necessary actions. For example, if you’re monitoring Linux server which primary task is to host some website but in Nfsen you see that it generates 90% of SSH traffic and only 10% of web traffic then it would be reasonable idea to check if somebody is trying to brute force SSH password and stop that activity. In other words it’s better to have traffic statistics classified. In this article I’ll tell you how to enable traffic classification in Nfsen.

rrdgraph

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Phone call as Nagios notification

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Some time ago I found it pretty useful to configure Nagios monitoring system to send me a phone call in case of some critical problem. If some mission critical application goes down at night most probably you’ll miss an e-mail or sms notifying about that but won’t miss a telephone call to your cell phone. Honestly a telephone call is much more notorious rather than IM message notification or, again, e-mail/sms. You’re welcome to see below how to configure your Nagios for that. First of … Read more