Nixpkgs version

• Unstable (25.11)

Describe the bug

After enabling c-ares support in #451579 , curl no longer uses nss for resolving host names. The same can be verified using strace. curl & by extension probably libcurl no longer connects to nscd socket for hostname resolution & instead uses the dns server listed in /etc/resolv.conf.

Steps to reproduce

Before #451579

❯ nix shell "nixpkgs/932e4fee6474e2cd152d180f105a868acb9b6c6c#curl" -c strace -f -e sendto -- curl -s https://search.nixos.org 2>&1 | grep nixos.org
[pid 1016194] sendto(5, "\2\0\0\0\16\0\0\0\21\0\0\0search.nixos.org\0", 29, MSG_NOSIGNAL, NULL, 0) = 29

Now after enabling c-ares support.

❯ nix shell "nixpkgs/b5b99c73d807b4b1c1988fd360aefd5a1455769c#curl" -c strace -f -e sendto -- curl -s https://search.nixos.org 2>&1 | grep nixos.org

In the full trace it can be seen that nss is not used, and raw dns server list from /etc/resolv.conf is used instead.

Expected behaviour

libcurl should use nss by default, since it's the expected default behaviour.

Screenshots

No response

Relevant log output

Additional context

Change also seems to have caused other regressions like #462151.
And not using nss can result in failures where host name resolution is dependent on nss modules like mdns.

System metadata

❯ nix-shell -p nix-info --run "nix-info -m"

• system: "x86_64-linux"
• host os: Linux 6.17.8, NixOS, 25.11 (Xantusia), 25.11.20251116.50a96ed
• multi-user?: yes
• sandbox: yes
• version: nix-env (Nix) 2.31.2
• nixpkgs: /nix/store/qjg5hnnkydk3mri5k6rydhj08x9s7xya-source

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@lovek323 @Scrumplex

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