Networking

Find processes related to a specific connection.

Discover hosts on local network to quickly locate new devices.

Perform reverse DNS lookup using dig command-line utility.

List outgoing connections.

Filter specific IP address.

Display default gateway.

Use the native IPsec stack to connect multiple networks over the internet.

Perform hostname canonicalization to simplify your configuration and ease work with multiple servers.

Match specific network inside the SSH client configuration file using Python helper script.

Define multiple configurations for the same host using the SSH client configuration file.

Use Python script to determine if the hostname belongs to a specific network.

Keep track of network latency using SmokePing on Debian Buster.

SmokePing 2.7.3 needs an update to use the SSH probe on Debian Buster.

Determine available authentication methods on the specified OpenSSH server.

Unmount not accessible NFS share that still has open files.

I have already described how to create VLAN interface, but things have changed over time, so I decided to update the know-how.

This is an extension to an older blog post on how to mount WebDAV share using systemd as this proved to be a very reliable method that is easy to mark as a dependency for other services.

Update the system time using systemd as a NTP client.

Sometimes it is very convenient to define shell function and execute it remotely.

Use simple shell function to instantly create known_hosts entries for specified host, including its IP address and remove outdated public host keys.

Global known_hosts file located in /etc/ssh/ssh_known_hosts and user managed ~/.ssh/known_hosts contain known host public keys. Due to its nature these files can contain multiple entries using the same key. It is not a problem, but merging entries using the same key is the first step that is necessary to inspect it visually and to verify host entries.

Disconnect idle SSH clients using built-in client alive mechanism.

You can check my earlier blog post on how to mount WebDAV share using command-line or fstab (file systems table), but you are not limited to these two options as you can use systemd to take care of the whole process and automount WebDAV resource on demand.

I am an enthusiast of the uWSGI project. You are still not limited to it as you can use fcgiwrap, a very lightweight and straightforward FastCGI wrapper for CGI scripts that do not require additional configuration.

Keep track of network latency using SmokePing, uWSGI application server, and Nginx HTTP server. SmokePing supports many configurable probes, alerting, and live charts accessible through a web browser. It was created by Tobi Oetiker, which is known for RRDtool an Open Source data logging and graphing system for time series data.

Nextcloud is a handy self hosted web-based application that can be used to store files, contacts and calendars. It provides the desktop and mobile client. In addition, it supports Web Distributed Authoring and Versioning protocol, so let’s mount a WebDAV share.

I am using curl to verify redirects and display real destination URL, and you should try it too.

Use curl utility to verify the HTTP response code for the given URL.

Two years ago, I described a simple way to display established TCP connections using ss command. Today I will use lsof and gawk to pretty print network connections.

Currently, I am working over an unstable network connection, so my SSH session is constantly disconnected. It is a very uncomfortable situation as a dropped SSH connection freezes the whole terminal.

The easiest way to reliably use Serial port within the local network is to use RS-232/422/485 NPort Device Server produced by Moxa company. I will shortly describe how to install NPort 5110 (1-port RS-232 serial device server) on Debian Wheezy.

A couple of days on the road made me realize that I need a simple desktop utility that will download files when I sleep.

Last year I wrote a short article about how to set up SSH port forwarding but I forgot to mention that you can quickly terminate existing port forwardings and create new ones during the SSH connection.

What to do when you want to analyze data contained in database without overloading the server? The answer is simple as you can use MySQL Master-Slave replication to offload this task. I will describe it briefly here.

Yesterday I was playing with Ubiquiti devices and noticed that there is now an option to set management VLAN directly using simple configuration mode.

Change the MAC address of an Ethernet interface and make this change permanent. I will briefly explain how to do this using standard commands and one particularly useful tool MAC Changer.

Yesterday, I wrote a short note about VLAN interfaces. Today, I want to write a short entry on how to bridge Ethernet interfaces to keep things complete as I need to mention creating an ACCESS port to tag packets on desired interface.

VLAN (virtual local area network) is a very useful concept as you can easily separate device management from users by using appropriate network devices and configuration. I will describe here in the form of a short note how to create a VLAN interface using a Debian system.

Simple shell script to backup MySQL databases.

I am playing with DHCP option 82 so I need to read DHCP packets. The easiest way to do this is to use dhcpdump by Edwin Groothuis because of very simple usage and friendly output.

Recently I needed to replicate data between two Linux servers so I could mitigate potential hardware failure. To achieve this goal I used GlusterFS – an Open Source distributed file system. In this example I will use Debian Wheezy as I want to try more recent version of GlusterFS.

SSH port forwarding is a nice feature that allows you to create encrypted tunnels over an unsecured network. It is easy and straightforward to remember and use in daily work so I will describe it here with couple of examples.

Recently I was playing with Growl notifications and I thought it would be a cool idea to see on the desktop notifications sent from my phone.

Signal strength on Mikrotik devices can be monitored by Cacti using SNMP protocol. It is straightforward if you have just a couple of those devices but you can be easily overwhelmed by dozens of data templates because the wireless card’s MAC address is part of the OID.

Important notice – Jump directly to more recent Ubiquiti and management VLAN blog post. This post is now somewhat obsolete as you don’t need to perform steps described here because there is an option to set management VLAN using simple configuration mode.

The fastest way to download or upload directory tree over FTP using just plain console is to execute lftp command.

Display established TCP connections inside the terminal.

My ISP provides a dynamically assigned IP address using DHCP. This solution has a couple of drawbacks but it comes in handy if I need to change the IP address to overcome time limit restrictions on streaming services…

Using firmware version v5.3.3 there is no way to set management VLAN using a browser, but it can be easily set using SSH in a couple of steps as described below.

It’s easy to miss something when you need to monitor a dozen logs on different servers and some other devices. Fortunately, there is a simple solution as you can use rsyslog with PostgreSQL or MySQL support to store all local and remote events on your network and then visualize results using LogAnalyzer.

If you need to test router configuration, prepare a small lab, or just learn something new then download and use GNS3.

Managing SPS switches could be real fun if you use expect to automate such tasks.

I prefer to use a small netbook over a desktop PC for light web browsing at home. It’s very comfortable but I need something to filter out advertisements.

Perform HTTP compression test using curl utility.