security
Security-relevant commands and practices
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chmod
Tested chmod examples: numeric and symbolic modes, recursive changes, setuid/setgid/sticky bits, permission audits, and troubleshooting.
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ssh
Tested ssh examples: key-based auth, the config file, host key verification, port forwarding, multiplexing, and reading a failed connection.
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sudo
Tested sudo and su examples: running one command as root, what the environment loses on the way, the redirect that still runs as you, and reading sudoers.
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The repository is not signed
apt cannot verify a repository's signature: the NO_PUBKEY and Missing key errors. What Debian 13 changed, and why apt-key adv is no longer the fix.
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Installing a .deb by hand
Install a downloaded .deb the way that resolves its dependencies, see what dpkg -i leaves behind when you don't, and read the package before you trust it.
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Adding a third-party repository safely
Why apt-key is gone, how Signed-By scopes a vendor's key to their repository alone, and how to stop that repository replacing the rest of your system.