How to Fix "Permission denied (publickey)" in SSH
A step-by-step checklist to fix the SSH "Permission denied (publickey)" error: wrong user, missing key, bad permissions and server settings.
Tag: troubleshooting
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.
Linux Disk Full: How to Find What Is Using the Space
Find out why a Linux disk is full: df, du, ncdu, inodes, logs, Docker, deleted-but-open files and safe ways to clean up.
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.
Docker Cheat Sheet: Essential Commands and Troubleshooting
The Docker commands you use every day: run, logs, exec, build, volumes, cleanup, plus how to fix containers that exit or are unreachable.
Fix Corrupted Windows System Files with SFC and DISM
How to repair Windows 10 and 11 with DISM and SFC: the correct order, what the results mean, offline repair and last-resort options.
Windows "No Internet": Reset the Network Stack (ipconfig, netsh)
Fix Windows no-internet and DNS problems: find the broken layer, flush DNS, reset Winsock and TCP/IP, check proxy and drivers.
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.