03 / FIELD NOTES
Field notes
One concrete problem at a time. Know when to use a command, why it helps, and what to expect.
No command matches that filter.
Know your server before changing it
2 stepsEssential check 1 / 3
Inspect the distribution, kernel, memory, and disk without changing the system.
The SSH port that actually wins
2 stepsEssential check 2 / 3
Find every Port line under sshd_config and sshd_config.d before you reload ssh.
Check services and listening ports
3 stepsEssential check 3 / 3
List failed systemd units, listening ports, and recent errors.
A port you cannot reach from outside
4 stepsCheck the listener, the bind address, UFW, Docker, and the cloud security group, in that order.
The disk is full
4 stepsFind what takes the space, then clear logs, apt caches, and unused Docker images.
Where a service writes its logs
3 stepsRead logs for containers and systemd services, and follow them live.
Why a certificate request fails
2 stepsCheck DNS, port 80 reachability, and CDN proxying before retrying a Let’s Encrypt request.
Let nala rank apt mirrors
1 stepsInteractive nala command that ranks apt mirrors before you confirm a change.
Stop cloud-init from rewriting resolv.conf
3 stepsWrite resolvers, then make resolv.conf immutable so DHCP cannot overwrite them.
See the CPU x86-64-v level
4 stepsDownload, inspect, and run the upstream CPU check before choosing a supported kernel package.
BBRv3 still reports bbr
2 stepsInspect kernel and congestion control; the name bbr alone does not identify its version.
List CrowdSec bans
2 stepsRead-only CrowdSec decisions and metrics without inserting a ban.
Find who holds port 8080
1 stepsFind the process listening on 8080 when CrowdSec local API cannot start.
Cloud kernel and generic kernel package names
1 stepsList installed linux-image packages to compare cloud and generic metapackages.
Cancel a reinstall before reboot
1 stepsCancel a pending bin456789/reinstall before the machine reboots and erases the disk.
Fail2ban and the CrowdSec bouncer
2 stepsCheck whether Fail2ban is active before you add the CrowdSec firewall bouncer.