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nixpkgs/doc/packages/etc-files.section.md
Yueh-Shun Li a12beafc57 doc: move doc/builders/packages -> doc/packages
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/etc files

Certain calls in glibc require access to runtime files found in /etc such as /etc/protocols or /etc/services -- getprotobyname is one such function.

On non-NixOS distributions these files are typically provided by packages (i.e., netbase) if not already pre-installed in your distribution. This can cause non-reproducibility for code if they rely on these files being present.

If iana-etc is part of your buildInputs, then it will set the environment variables NIX_ETC_PROTOCOLS and NIX_ETC_SERVICES to the corresponding files in the package through a setup hook.

> nix-shell -p iana-etc

[nix-shell:~]$ env | grep NIX_ETC
NIX_ETC_SERVICES=/nix/store/aj866hr8fad8flnggwdhrldm0g799ccz-iana-etc-20210225/etc/services
NIX_ETC_PROTOCOLS=/nix/store/aj866hr8fad8flnggwdhrldm0g799ccz-iana-etc-20210225/etc/protocols

Nixpkg's version of glibc has been patched to check for the existence of these environment variables. If the environment variables are not set, then it will attempt to find the files at the default location within /etc.