No Release file

E: The repository no longer has a release file

TCP autotuning

.https://fasterdata.es.net/host-tuning/background/#t1

TCP auto tuning on linux

For example, if your ping time is 50 ms, and the end-to-end network consists of all 1G or 10G Ethernet, the TCP buffers should be:
.05 sec * (1 Gbit / 8 bits) = 6.25 MBytes.

The repository 'http://mariadb.nethub.com.hk/repo/10.3/ubuntu bionic Release' no longer has a Release file. N: Updating from such a repository can't be done securely, and is therefore disabled by default. N: See apt-secure(8) manpage for repository creation and user configuration details. W: An error occurred during the signature verification.

Error message:

E: The repository 'http://mariadb.nethub.com.hk/repo/10.3/ubuntu bionic Release' no longer has a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.

Ubuntu release version

$ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description:    Ubuntu 18.04.1 LTS
Release:        18.04
Codename:       bionic

Few other error messages:

Err:15 https://mariadb.nethub.com.hk/repo/10.3/ubuntu bionic Release
  Redirection from https to 'http://ftp.yz.yamagata-u.ac.jp/pub/dbms/mariadb/repo/10.3/ubuntu/dists/bionic/Release' is forbidden [IP: 218.213.216.19 443]
Err:16 https://mirrors.shu.edu.cn/mariadb/repo/10.3/ubuntu bionic InRelease
  Could not connect to mirrors.shu.edu.cn:443 (202.121.199.235). - connect (111: Connection refused)
Err:17 https://mirrors.shu.edu.cn/mariadb/repo/10.2/ubuntu bionic InRelease
  Unable to connect to mirrors.shu.edu.cn:https:

Resolution

https://downloads.mariadb.org/mariadb/repositories/#distro=Ubuntu&distro_release=bionic--ubuntu_bionic&mirror=piconets&version=10.5

second

sudo apt install netcat

Error

W: Failed to fetch https://packages.sury.org/php/dists/buster/InRelease  Temporary failure resolving 'packages.sury.org'
W: Some index files failed to download. They have been ignored, or old ones used instead.