Flushing dns caching in linux

Problem

You want to update your dns resolution locally after some change in DNS.

You want to do this because your local dns cache still holds the old information about the domain. For example using your local dns and the google one returns two different results

host changed_domain.com
returns the old ip

host changed_domain.com 8.8.8.8
returns the new ip.

Solution

Check your systemd-resolved is active:

sudo systemctl is-active systemd-resolved
active

Get some statistics:

sudo systemd-resolve --statistics

Transactions
Current Transactions: 0
  Total Transactions: 38818

Cache
  Current Cache Size: 73
          Cache Hits: 21120
        Cache Misses: 19745

DNSSEC Verdicts
              Secure: 0
            Insecure: 0
               Bogus: 0
       Indeterminate: 0

Flush the cache (make the Current Cache Size above 0)

sudo systemd-resolve --flush-caches

Check the cache again (should be 0 now):

sudo systemd-resolve --statistics

ransactions
Current Transactions: 0
  Total Transactions: 38818

Cache
  Current Cache Size: 0
          Cache Hits: 21120
        Cache Misses: 19745

DNSSEC Verdicts
              Secure: 0
            Insecure: 0
               Bogus: 0
       Indeterminate: 0

Information taken from https://www.techrepublic.com/article/how-to-flush-the-dns-cache-on-linux/

asdf and terraform (or vault or packer)

Using the asdf version manager to manage versions of various binaries like terraform, vault or packer is easily done by following the instructions below:

  1. Download and install asdf if you haven’t got it already (https://asdf-vm.com/#/core-manage-asdf)
  2. If you want to install the latest version of terraform for example do the following:
asdf plugin add terraform
asdf install terraform latest
asdf global terraform 0.14.9
asdf list
terraform
  0.14.9
terraform --version
Terraform v0.14.9

Get/Set platform information

To get or set in environment variables the platform information of a system you can use the following:

export ARCH=$(case $(arch) in x86_64) echo -n amd64 ;; aarch64) echo -n arm64 ;; *) echo -n $(arch) ;; esac)
export OS=$(uname | awk '{print tolower($0)}')

Taken from the installation instructions of the Operator SKD here: https://master.sdk.operatorframework.io/docs/installation/

Failed to start docker.service: Unit is masked

Problem

Trying to start the docker service after some upgrades fails with the following message:

Failed to start docker.service: Unit is masked.

Solution

It turns out that after upgrading or more specifically removing and then upgrading the docker installation in ubuntu (in this particular case in raspberry 4 with Ubuntu 20.04 installed), results in this error.

A search brings up the following:

https://forums.docker.com/t/failed-to-start-docker-service-unit-is-masked/67413

and from that the following bug post:

https://bugs.launchpad.net/ubuntu/+source/docker.io/+bug/1844894

So the solution is to run the following to be able to start the docker service (described in the first link above):

sudo systemctl unmask docker
sudo systemctl start docker

Getting detailed information about your Linux distribution

To be able to get a more detailed information about your current linux distribution, including the code names use the following:

kosmas:$ cat /etc/os-release
NAME="Ubuntu"
VERSION="18.04.5 LTS (Bionic Beaver)"
ID=ubuntu
ID_LIKE=debian
PRETTY_NAME="Ubuntu 18.04.5 LTS"
VERSION_ID="18.04"
HOME_URL="https://www.ubuntu.com/"
SUPPORT_URL="https://help.ubuntu.com/"
BUG_REPORT_URL="https://bugs.launchpad.net/ubuntu/"
PRIVACY_POLICY_URL="https://www.ubuntu.com/legal/terms-and-policies/privacy-policy"
VERSION_CODENAME=bionic
UBUNTU_CODENAME=bionic