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Knowledge Base · intrvll · Lübeck

Sysadmin guides for Linux, Windows Server, and the network

14 articles in six categories, from a disk that’s full to a certificate that’s about to expire. Each one names the versions it was written against and the day it was last checked.

Free to read, without an account. Press / on any page to search.

Articles
14
Categories
6
Commands to copy
163
Last checked
24 Oct 2026
01 / 03Categories

Articles by category with every article in each

linux/ · 3 articles

Linux

Disks that fill up, jobs that run on a schedule, and the first look at a server that has slowed down. The commands are for systemd distributions: Debian 12, Ubuntu 24.04, and RHEL 9 and its rebuilds.

All Linux articles

networking/ · 3 articles

Networking

Find out what’s listening on a port, test whether a port is reachable from somewhere else, and capture the packets when the answer still isn’t clear.

All Networking articles

dns/ · 2 articles

DNS

Query records the way a resolver does, tell an authoritative answer from a cached one, and plan a change so it takes effect when you expect it to.

All DNS articles

tls/ · 2 articles

TLS and certificates

Read what a server’s certificate says, find out when it expires, and convert keys and certificates between the formats different software asks for.

All TLS and certificates articles

ssh/ · 2 articles

SSH

Log in with keys instead of passwords, turn passwords off without locking yourself out, and keep every host’s connection details in one config file.

All SSH articles

windows/ · 2 articles

Windows Server

Read the event log, test connections, and look up DNS from PowerShell, with commands that work on Windows Server 2019, 2022, and 2025.

All Windows Server articles
02 / 03Start hereRead the article

When a server slows down start with these eight commands

Linux · 5 min

The first five minutes on a slow server

Eight commands, in the order to run them, that show whether a slow server is short of CPU, memory, or disk, or whether the problem is somewhere else, before you change anything. Read the article

  • 1. Load average with uptime
  • 2. Kernel messages with dmesg
  • 3. CPU, memory, and waits with vmstat
  • 4. Each CPU with mpstat
  • 5. Busy processes with pidstat
  • 6. Disk latency with iostat
  • 7. Available memory with free
  • 8. Network traffic with sar and ss
first five minutes
$ uptime 14:02:11 up 41 days,  3:17,  2 users,  load average: 11.84, 9.02, 4.10$ nproc4$ vmstat 1 3procs -----------memory---------- ---swap-- -----io---- -system-- ------cpu----- r  b    swpd   free   buff  cache   si   so    bi    bo   in   cs us sy id wa st 2  9 1572864  81240  10212 412880  612  904  9812  1204 2210 4120  6  4 22 68  0 1 11 1574912  79112  10212 409744  588 1012 10240   988 2301 4388  5  5 19 71  0# 71% of the time waiting for disk, and swapping: memory first
03 / 03Recently checked

Recently checked against the versions named on each

ArticleWritten againstReadingCheckedOpen
LinuxThe first five minutes on a slow serverEight commands, in the order to run them, that show whether a slow server is short of CPU, memory, or disk, or whether the problem is somewhere else, before you change anything.Debian 12, Ubuntu 24.04, RHEL 9, procps-ng 4, sysstat 125 min24 Oct 2026LinuxFind what’s filling a diskA full filesystem stops writes, logs, and often the service on top of it. Find which filesystem is full, which directory holds the space, and which deleted files a process is still holding open.Debian 12, Ubuntu 24.04, RHEL 9, GNU coreutils 8.32+4 min24 Oct 2026LinuxRun a job on a schedule with systemd timersReplace a cron line with a service and a timer. Every run then gets its own log and exit status, a run doesn’t start while the last one is still going, and a run the machine missed while it was off happens at the next boot.systemd 252 (Debian 12, RHEL 9), systemd 255 (Ubuntu 24.04)3 min24 Oct 2026NetworkingFind which process is listening on a portList every port a server listens on, read which addresses it accepts connections on, and trace a port back to its process and its systemd unit, including ports systemd holds itself.Debian 12, Ubuntu 24.04, RHEL 9, iproute2 6.1+, lsof 4.94+4 min24 Oct 2026NetworkingTest whether a TCP port is reachableCheck from another machine whether a port accepts connections, without telnet. The way a test fails, refused or timed out, tells you whether to look at the service or at a firewall.Debian 12, Ubuntu 24.04, RHEL 9, netcat-openbsd 1.219+, nmap-ncat 7.92, curl 7.76+4 min24 Oct 2026NetworkingCapture traffic with tcpdumpWatch the packets a server sends and receives, filtered down to one host or port, and read a TCP handshake, a reset, and a connection nobody answers. Then write a capture to a file for Wireshark.Debian 12, Ubuntu 24.04, RHEL 9, tcpdump 4.99, libpcap 1.105 min24 Oct 2026

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