Captain's Log

How to clear DNS cache everywhere

Jon M — Thu, 26 Mar 2026 00:00:00 -0500

All the ways and places to clear DNS cache

Update /etc/hosts to use auto-apt-proxy

Jon M — Thu, 19 Mar 2026 00:00:00 -0500

Quickly add apt-proxy to /etc/hosts

Create Floppy Disk Images for 86Box

Jon M — Fri, 05 Sep 2025 00:00:00 -0500

Create a floppy disk image for use with 86box and others

Create a local dnf repo from a directory of packages

Jon M — Thu, 14 Aug 2025 00:00:00 -0500

Create a local dnf repository from a directory of packages.

How to push to a different remote branch with Git

Jon M — Wed, 19 Feb 2025 00:00:00 -0600

Create a firewalld service for Plex Media Server

Jon M — Mon, 27 Jan 2025 00:00:00 -0600

How to create a firewalld service definition for Plex Media Server

Printing only headers with cURL

Jon M — Mon, 20 Jan 2025 00:00:00 -0600

Using cURL only print headers and not entire page content

Delete lines in Vim beginning with a specific character

Jon M — Mon, 13 Jan 2025 00:00:00 -0600

Delete lines in Vim beginning with a specific character like the hash or blank lines

Using grep with tail

Jon M — Thu, 09 Jan 2025 00:00:00 -0600

Using grep --line-buffered to filter tail

End an active telnet session

Jon M — Mon, 21 Oct 2024 00:00:00 -0500

Use CTRL+] and q to end an active telnet session

Swap Floppy Disks or CD-ROMs using QEMU monitor

Jon M — Thu, 12 Sep 2024 00:00:00 -0500

Use QEMU monitor to swap floppy or CD images

Enable SSH Port Forwarding after opening session

Jon M — Fri, 23 Aug 2024 00:00:00 -0500

Enabling Port Forwarding during an active SSH session.

Installing Webmin on AlmaLinux

Jon M — Wed, 21 Aug 2024 00:00:00 -0500

Notes on install Webmin on AlmaLinux

Determine Raspberry Pi model from command line

Jon M — Mon, 19 Aug 2024 00:00:00 -0500

Determine Raspberry Pi model from command line

Cisco: Disconnect from service-module session

Jon M — Mon, 12 Aug 2024 00:00:00 -0500

When connected to a service module you need to press Ctrl+Shift+6, then X in order to go back to the host router.

Auto-restart a crashed systemd service

Jon M — Wed, 10 Jul 2024 14:20:00 -0500

With systemd, it is easy to configure a service to automatically restart when it's crashed.

Installing qemu-guest-agent with virt-customize

Jon M — Tue, 21 May 2024 10:00:00 -0500

When using cloud-images from various distrobutions, sometimes the qemu-guest-agent isn't installed. Using virt-customize you config modify the image to include this package as well as enable it boot.

$ virt-customize -a /path/to/image --install qemu-guest-agent --run-command '/usr/bin/systemctl enable qemu-guest-agent'

The guest should not be running when you do …

Useful OpenSSL Tips

Jon M — Sat, 16 Mar 2024 12:30:00 -0500

Examine a single certificate
Examine a file that contains multiple certificates
Verify a certificate is valid using a specific root.
Check that the certificate, and chain, being served is valid

Just writing down some common openssl command I often find myself using. One day this collection might be more complete …

Generate Public SSH Key From Private SSH Key

Jon M — Fri, 12 Jan 2024 14:30:00 -0600

If you have a private SSH key but need to regenerate the public key again, use ssh-keygen with -y. This option will read a private OpenSSH format file and print an OpenSSH public key to stdout.

ssh-keygen -y -f ~/.ssh/id_ed25519 > ~/.ssh/id_ed25519.pub
Enter passphrase:

This will save the …

PXE netboot.xyz with Unifi UDM-Pro and Synology TFTP

Jon M — Fri, 15 Dec 2023 07:45:00 -0600

netboot.xyz lets you PXE boot various operating system installers or utilities from a single tool over the network.

— netboot.xyz

Enable TFPT service on Synology DSM

Control Panel > File Services > Advanced
Check "Enable TFTP service"
Select TFPT root folder
Apply

You can edit advanced settings to enable logging, configure …

Synology Storage Analyzer A data write error

Jon M — Tue, 09 May 2023 07:45:00 -0500

After many months of using Synology Storage Analyzer to create automatic reports recently these have been failing with the error

Storage Analyzer: A data write error occurred while generating the usage report.

A bit of searching online and this seems to be common for a few reasons. In my case …

Printing line numbers with cat and grep

Jon M — Thu, 29 Sep 2022 07:45:00 -0500

Both grep and cat can include line numbers.

With cat:

:::text
[root@mx1 ~]# cat -n /etc/hosts 
   1  # Loopback entries; do not change.
   2  # For historical reasons, localhost precedes localhost.localdomain:
   3  127.0.0.1   localhost localhost.localdomain localhost4
   4  ::1         localhost localhost.localdomain localhost6
   5  # See hosts(5 …

Debug logging with systemd-resolved

Jon M — Fri, 23 Sep 2022 18:01:00 -0500

Add an override. sudo systemctl edit systemd-resolved

[Service]
Environment=SYSTEMD_LOG_LEVEL=debug

Make changes active. sudo systemctl restart systemd-resolved
View logs. journalctl -u systemd-resolved -f

And when you're done, undo all these changes.

sudo systemctl revert systemd-resolved
sudo systemctl restart systemd-resolved`

Retrieve Existing Keytabs from FreeIPA

Jon M — Tue, 30 Aug 2022 09:00:00 -0500

This is a work in progress.

Introduction

In this example, s1.prod.example.com is a real server and an IPA client. The name ipa.example.com is an alias.

The same process can be repeated to allow additional real hosts to retrieve a keytab for ipa.example.com, simply …

Using Healthchecks.io with systemd timers

Jon M — Sun, 01 May 2022 09:00:00 -0500

I use healthchecks.io to monitor several cronjobs and this works great.

Recently I have been using systemd timers instead of cronjobs but wanted to continue to use healthchecks with them. I accomplished this using ExecStartPre and ExecStartPost in the service file.

To add these checks to the dnf-automatic.service …

Kill a hung SSH session

Jon M — Mon, 25 Apr 2022 08:00:00 -0500

End a hung ssh sessions with ~ + ..

In practice this doesn't also ways work for me. I've found that sometimes you must press Enter a couple of times before ~. seems to do anything.

Octoprint Plugins

Jon M — Wed, 20 Apr 2022 08:00:00 -0500

Just a list of the 3rd party plugins I have installed in Octoprint. Keeping these here for my own future reference.

ABL Expert Plugin
Bed Visualizer
DiscordRemote
Filament Manager
OctoDash Companion
Octolapse
PrettyGCode
Print History Plugin
PrintTimeGenius
Slicer Thumbnails
UI Customizer

View Markdown from Terminal

Jon M — Tue, 23 Nov 2021 10:00:00 -0600

You can view markdown files using any file viewer. However, to get a slightly more pleasent view you can use pandoc to first convert to HTML and then render that with either lynx or w3m (any any other browser.)

Examples:

# Using pandoc with Lynx
$ pandoc file.md | lynx -stdin

# Using …

Making a temporary home directory

Jon M — Tue, 14 Sep 2021 00:00:00 -0500

There are times when your home directory might not be available. This can be due to your homedir coming from LDAP or another network service and the local directory not being available. When logging in you'll see something similar to

Last login: Mon Sep 13 21:03:37 from foo …

Using yum with only the cache

Jon M — Wed, 08 Sep 2021 00:00:00 -0500

Yum keeps a local cache. To use yum with only the cache use yum -C or yum --cacheonly.

Reference

man yum

Check SSH Key Fingerprints

Jon M — Thu, 19 Aug 2021 10:00:00 -0500

Show the fingerprint of an SSH key

$ ssh-keygen -l -f ~/.ssh/id_rsa

The default hashing is sha256, however it might be more useful to see the fingerprint in md5.

$ ssh-keygen -l -f ~/.ssh/id_rsa

Less useful, but sometimes fun, is showing the randomart assocated with the key

~ $ ssh-keygen -l -v …

Force chrony to update time

Jon M — Wed, 28 Jul 2021 10:00:00 -0500

$ chronyc -a 'burst 4/10'
$ sleep 5
$ chronyc -a makestep

References

chrony - chronyc(1)

Update libvirt/KVM guest virtual machine templates

Jon M — Sun, 01 Mar 2020 20:28:00 -0600

virt-customize can be used to customize a virtual machine image by installing packages, editing configuration files, and so on. This is done by modifying the disk image in place.

Update the template

Run yum update to update the packages already installed in the template to their latest versions.

$ virt-customize --connect …

Reset and Clone libvirt/KVM guest virtual machines

Jon M — Thu, 20 Feb 2020 19:50:00 -0600

Prepare the clone

Use the virt-sysprep command to reset a virtual machine. The virtual machine must be shut down before you use this command.

virt-sysprep --connect qemu:///system -d centos7-gold

Create the clone

Use the virt-clone command to create a clone of an existing virtual machine.

virt-clone  --original centos7-gold --name …

Automatic DNS resolution for libvirt guests using a local domain

Jon M — Wed, 19 Feb 2020 20:50:00 -0600

Configuring libvirt

First libvirt needs to be configured. Modify the default network using virsh net-edit default.

$ sudo virsh net-dumpxml default
<network connections='1'>
  <name>default</name>
  <uuid>3a3627c2-15a7-4e7f-b7b6-0c2310ba9427</uuid>
  <forward mode='nat'>
    <nat>
      <port start='1024' end='65535'/>
    </nat>
  </forward>
  <bridge name='virbr0' stp='on' delay='0'/>
  <mac address='52 …

Clear systemd journal

Jon M — Mon, 09 Apr 2018 12:03:00 -0500

If the journal is configured to archive entires these can take up unnecessary disk space when not properly maintained. The current disk usage can be seen with journalctl --disk-usage. This includes both active and archived journal files.

Clear journal entries older then X number of days

$ journalctl --vacuum-time=5d

Clear …

Reset root password on RHEL7

Jon M — Wed, 21 Mar 2018 00:00:00 -0500

How to reset an unknown root password using rd.break on RHEL 7.

Start by booting until the GRUB2 menu appears then edit the boot loader menu with the e key. Find the line beginning with linux and make then follow these steps.

add rd.break
run mount -o remount …

Using screen as a serial console

Jon M — Tue, 20 Feb 2018 11:59:00 -0600

Using screen you can connect to any serial device in /dev. A typical USB-to-RS-232 adapter will usually be /dev/ttyUSB0. A physical serial port will usually be /dev/ttyS0.

screen /dev/ttyUSB0 9600

Discover IPv6 Neighbors

Jon M — Tue, 20 Feb 2018 00:00:00 -0600

Ping nodes on the local network segment using multicast¹

ping6 -c2 -I <interface> ff02::1

Then, view IPv6 neighbors

ip -f inet6 neigh

References

https://en.wikipedia.org/wiki/Multicast_address ↩

Renew Expired GPG Keys

Jon M — Mon, 22 Jan 2018 00:00:00 -0600

To renew an expiring, or extend the expiration of a gpg key edit the key and set a new expiration date.

gpg --edit-key <keyid>
command> list
command> key [keyid]
command> expire
command> save

Where keyid above is the key that is expiring.

Set time with NTPd

Jon M — Mon, 01 Jan 2018 00:00:00 -0600

Use ntpd -gc to start the NTP daemon and set the time regardless of what the offset between the local and remote clock is.

Modify iptables rules

Jon M — Tue, 03 Oct 2017 00:00:00 -0500

Show all iptables rules and include line numbers

iptables -L INPUT --line-numbers

Replace iptables rule at a specific line number

iptables -R X INPUT <new rule here>

Grow LVM physical volume

Jon M — Sun, 19 Feb 2017 00:00:00 -0600

The pvresize command can be used to grow an existing physical volume so long as you grow the parition on the underlying device.

Use fdisk -l /dev/sdc1 to find the start sector of the partition.
Delete existing partition and create a new partition with the same start sector.
Use …

Using ssmtp with FastMail

Jon M — Fri, 10 Feb 2017 00:00:00 -0600

#  a config file for sSMTP sendmail.

# The user that gets all mail for userids less than 1000. If blank, address
# rewriting is disabled.
Root=roots-mail@example.com

# the smarthost or relay or ...
Mailhub=smtp.fastmail.com:465

# TLS Config
UseTLS=YES
UseSTARTTLS=NO

RewriteDomain=example.com

# The full hostname
Hostname …

Signing GnuPG keys with caff

Jon M — Wed, 16 Nov 2016 00:00:00 -0600

These are my notes on getting caff configured and working in order to send signed keys after the OhioLinuxFest keysigning. All of this was done on a Fedora 24 Workstation using only the standard repositories.

Installation

The pgp-tools package provides caff. It can be easily installed with yum. I also …

Configuring ipmi using ipmitool

Jon M — Wed, 10 Aug 2016 00:00:00 -0500

Packages and Services

On RHEL7Server the OpenIPMI and ipmitool are necessary to configure and use IPMI.

$ yum install OpenIMPI ipmitool

Networking

Configure a static ip address on the first port. For dynamic configuration set ipsrc to dynamic.

$ sudo ipmitool lan set 1 ipsrc static
$ sudo ipmitool lan set 1 ipaddr …

Reinstall the Kernel with yum

Jon M — Wed, 20 Jul 2016 00:00:00 -0500

After failing to update the initrd after making changes to the plymouth boot I was unable to boot into the kernel. The yum reinstall fixed this

$ yum reinstall kernel

However, this doesn't always work. If yum feels the kernel is already installed and healthy this will return 'nothing to do' …

Have dovecot logins update lastlog

Jon M — Wed, 13 Jul 2016 00:00:00 -0500

Two changes are needed to have dovecot update lastlog when a user logins with either imap or pop3.

In /etc/dovecot/cond.d/auth-system.conf.ext enable PAM sessions

passdb {
  driver = pam
  args = session=yes dovecot

Then, enable the lastlog module in the dovecot PAM configuration at /etc/pam.d …

SELinux Permissive Domains

Jon M — Fri, 18 Mar 2016 00:00:00 -0500

When SELinux is configured to run in Permissive mode it will only log when an action would be denied using Enforcing. Using permissive domains a single process and be configured to run in Permissive mode while leaving the rest of the system in Enforcing.

Make a domain permissive with semanage …

Change network device name after restore

Jon M — Mon, 11 Jan 2016 00:00:00 -0600

Restoring a server using an image based backup product can result in network inteface naming changing. Here is a short procedure to changes those names back.

If the MAC address of a physical or virtual network interface changes then the device name will often change as well. This is common …

Interface teaming with nmcli

Jon M — Mon, 28 Dec 2015 00:00:00 -0600

Create the team interface

This creates a new team interface named team0 and sets runner (bond mode) to lacp. In these examples the name of the interface is team0 - this is entirely for personal tastes. The connection name can be anything, using team simply makes things easier to follow.

$ nmcli …

Hammer notes

Jon M — Wed, 09 Dec 2015 00:00:00 -0600

Here is a collection of my notes on using hammer cli with Red Hat Satellite. Most of these things probably work with Katello also, but all of my testing is with Satellite.

Add new Product

Products are a group of repositories. Content Hosts can subscribe to a product to make …

RPM Notes

Jon M — Sun, 22 Nov 2015 00:00:00 -0600

Here is a collection of my notes on using RPM. Most of these are well documented in man pages and other sources around the web. I\'ve collected the ones I\'ve used most here as a reference for myself and others.

Query package information

Show package information

$ rpm -qi …