Tag Archives: linux

Flushing dns caching in linux

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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)

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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

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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

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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

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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

Using strace for analyzing performance

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There is a very nice example on the O’reilly ‘Linux Under The Hood’ training about using strace to compare performance of two commands that provide the same output but with different calls. The two different commands were: ls and echo *, that they both provide the listing of the current directory.

First using the strace with ls:

kosmas:$ strace -c ls
abi-4.13.0-45-generic      config-4.4.0-31-generic        initrd.img-4.4.0-31-generic   System.map-4.15.0-123-generic         vmlinuz-4.15.0-123-generic
abi-4.13.0-46-generic      efi                            memtest86+.bin                System.map-4.15.0-124-generic         vmlinuz-4.15.0-124-generic
abi-4.4.0-31-generic       grub                           memtest86+.elf                System.map-4.15.0-126-generic         vmlinuz-4.15.0-126-generic
config-4.13.0-45-generic   initrd.img-4.13.0-45-generic   memtest86+_multiboot.bin      System.map-4.4.0-31-generic           vmlinuz-4.4.0-31-generic
config-4.13.0-46-generic   initrd.img-4.13.0-46-generic   retpoline-4.13.0-45-generic   vmlinuz-4.13.0-45-generic             vmlinuz-4.4.0-31-generic.efi.signed
config-4.15.0-123-generic  initrd.img-4.15.0-123-generic  retpoline-4.13.0-46-generic   vmlinuz-4.13.0-45-generic.efi.signed
config-4.15.0-124-generic  initrd.img-4.15.0-124-generic  System.map-4.13.0-45-generic  vmlinuz-4.13.0-46-generic
config-4.15.0-126-generic  initrd.img-4.15.0-126-generic  System.map-4.13.0-46-generic  vmlinuz-4.13.0-46-generic.efi.signed
% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ----------------
 15.81    0.000095          12         8           write
 14.64    0.000088           7        12           mprotect
 13.48    0.000081           5        17           mmap
  9.98    0.000060           7         9           openat
  7.82    0.000047           4        11           close
  7.32    0.000044          22         2           getdents
  6.82    0.000041           6         7           read
  5.66    0.000034           3        10           fstat
  4.99    0.000030           4         8         8 access
  3.16    0.000019          10         2         2 statfs
  2.00    0.000012          12         1           munmap
  1.83    0.000011           4         3           brk
  1.66    0.000010           5         2           ioctl
  1.50    0.000009           5         2           rt_sigaction
  0.67    0.000004           4         1           rt_sigprocmask
  0.67    0.000004           4         1           arch_prctl
  0.67    0.000004           4         1           set_tid_address
  0.67    0.000004           4         1           set_robust_list
  0.67    0.000004           4         1           prlimit64
  0.00    0.000000           0         1           execve
------ ----------- ----------- --------- --------- ----------------
100.00    0.000601                   100        10 total

And using the same with echo *

kosmas:$ strace -c echo *
abi-4.13.0-45-generic abi-4.13.0-46-generic abi-4.4.0-31-generic config-4.13.0-45-generic config-4.13.0-46-generic config-4.15.0-123-generic config-4.15.0-124-generic config-4.15.0-126-generic config-4.4.0-31-generic efi grub initrd.img-4.13.0-45-generic initrd.img-4.13.0-46-generic initrd.img-4.15.0-123-generic initrd.img-4.15.0-124-generic initrd.img-4.15.0-126-generic initrd.img-4.4.0-31-generic memtest86+.bin memtest86+.elf memtest86+_multiboot.bin retpoline-4.13.0-45-generic retpoline-4.13.0-46-generic System.map-4.13.0-45-generic System.map-4.13.0-46-generic System.map-4.15.0-123-generic System.map-4.15.0-124-generic System.map-4.15.0-126-generic System.map-4.4.0-31-generic vmlinuz-4.13.0-45-generic vmlinuz-4.13.0-45-generic.efi.signed vmlinuz-4.13.0-46-generic vmlinuz-4.13.0-46-generic.efi.signed vmlinuz-4.15.0-123-generic vmlinuz-4.15.0-124-generic vmlinuz-4.15.0-126-generic vmlinuz-4.4.0-31-generic vmlinuz-4.4.0-31-generic.efi.signed
% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ----------------
  0.00    0.000000           0         1           read
  0.00    0.000000           0         1           write
  0.00    0.000000           0         5           close
  0.00    0.000000           0         4           fstat
  0.00    0.000000           0         6           mmap
  0.00    0.000000           0         4           mprotect
  0.00    0.000000           0         1           munmap
  0.00    0.000000           0         3           brk
  0.00    0.000000           0         3         3 access
  0.00    0.000000           0         1           execve
  0.00    0.000000           0         1           arch_prctl
  0.00    0.000000           0         3           openat
------ ----------- ----------- --------- --------- ----------------
100.00    0.000000                    33         3 total

So the difference in time is 0.00000 for the echo and 0.000601 for the ls.

Similarly the number of calls for the echo is 33, but ls is using 100.