Cloudflare 521, 522 and 525 Errors: How to Fix Them
What Cloudflare 521, 522, 523, 524, 525 and 526 errors mean and how to find the cause: firewall, SSL mode, certificates and DNS.
Category: Networking
Which Process Is Using a Port on Linux? (ss, lsof, fuser)
Find and stop the process holding a port on Linux with ss, lsof and fuser, and fix "Address already in use" errors.
Subnetting and CIDR Explained with Simple Examples
Learn subnetting fast: CIDR prefixes, masks, block-size trick, splitting networks, private ranges and tools to check your math.
DNS Troubleshooting Guide: dig, nslookup and Common Problems
Diagnose DNS issues with dig, nslookup and Resolve-DnsName: record types, status codes, TTL, flushing caches and the usual causes.
Diagnose Network Connectivity Step by Step: ping, mtr, nc, tcpdump
A layered method to find network problems: what refused and timed out really mean, mtr and traceroute, MTU tests and packet capture.
Set Up a WireGuard VPN Server on Ubuntu 24.04
Build a WireGuard VPN on Ubuntu 24.04: keys, server and client configs, NAT and firewall rules, full or split tunnel, and troubleshooting.