Debug curl calls during a build

[jshort]

Issue description

I'm working to get nix-build to work in a corporate environment through a proxy. It seems to be able to make http/curl GETs for things in some cases and in other cases not and I get:

curl: (5) Could not resolve proxy: myproxy

I'm setting the env vars http_proxy, https_proxy, and ftp_proxy though I have confirmed through debugging it is only looking at https_proxy.

The following command works fine:

https_proxy=http://myproxy:8080 /usr/bin/curl https://sources.debian.org/data/main/s/sharutils/1:4.15.2-2+deb9u1/debian/patches/01-fix-heap-buffer-overflow-cve-2018-1000097.patch

But for some reason when nix-build runs the package build, it can't resolve myproxy from /etc/hosts.

Is there a way to debug nix-build to see all the commands being invoked for fetchurl or fetchpatch?

[jshort]

Oh yuck:

https_proxy=myproxy:8080 /usr/bin/curl -4 https://sources.debian.org/data/main/s/sharutils/1:4.15.2-2+deb9u1/debian/patches/01-fix-heap-buffer-overflow-cve-2018-1000097.patch
curl: (5) Could not resolve proxy: myproxy

So it seems that nix-build is passing -4 to force ipv4. Is there a way to override this?

[jshort]

Issue was this: #277471