A Nix Binary Cache Specification

Published 2021-08-12 on Farid Zakaria's Blog

I wanted to better understand how to work with Nix binary caches. Interestingly the Nix manual which usually is a great source of knowledge, has a very limited section on the Binary Cache.

It basically just points you to nix-serve, a Perl CGI script in Eelco’s (original author of Nix) personal repository.

This guide will serve to be a loose Nix Binary Cache specification. If you are interested in browsing the end result, please checkout my OpenAPI Nix HTTP Binary Cache Specification 🎆

You can also visit the GitHub repository https://github.com/fzakaria/nix-http-binary-cache-api-spec to contribute.

Since there’s no specification (aside from the code), I will use the canonical binary cache https://cache.nixos.org and using Ruby for investigation.

Let’s start off and use nix path-info to query some information. I find it always more useful to use --json to see the displayed results.

❯ nix path-info nixpkgs.ruby --store https://cache.nixos.org --json | jq
    "path": "/nix/store/p4pclmv1gyja5kzc26npqpia1qqxrf0l-ruby-2.7.3",
    "narHash": "sha256:1impfw8zdgisxkghq9a3q7cn7jb9zyzgxdydiamp8z2nlyyl0h5h",
    "narSize": 18735072,
    "references": [
    "deriver": "/nix/store/bidkcs01mww363s4s7akdhbl6ws66b0z-ruby-2.7.3.drv",
    "signatures": [
    "url": "nar/1w1fff338fvdw53sqgamddn1b2xgds473pv6y13gizdbqjv4i5p3.nar.xz",
    "downloadHash": "sha256:1w1fff338fvdw53sqgamddn1b2xgds473pv6y13gizdbqjv4i5p3",
    "downloadSize": 4029176

How did the Nix tool determine this information and what does it mean? 🕵️

Every Nix Binary Cache needs to support querying for NARInfo files for a particular store path.

For instance for our Ruby path /nix/store/p4pclmv1gyja5kzc26npqpia1qqxrf0l-ruby-2.7.3, we take the cryptographic hash portion and use that to cURL the metadata https://cache.nixos.org/p4pclmv1gyja5kzc26npqpia1qqxrf0l.narinfo.

❯ curl -i https://cache.nixos.org/p4pclmv1gyja5kzc26npqpia1qqxrf0l.narinfo
HTTP/2 200 
x-amz-id-2: Qpz5ZRR31fiF+A3lN9Gl5SVP1kC4/9jgEWUibbFX/p+rVazDVznQMtT4qgskwlkDcwOtDGtegjY=
x-amz-request-id: 5G9ZDZB9TFR665RN
last-modified: Wed, 19 May 2021 10:43:55 GMT
etag: "0faa37071023836830facecbcf99b384"
content-type: text/x-nix-narinfo
server: AmazonS3
via: 1.1 varnish, 1.1 varnish
accept-ranges: bytes
date: Thu, 12 Aug 2021 21:39:39 GMT
age: 47939
x-served-by: cache-bwi5123-BWI, cache-pao17429-PAO
x-cache: HIT, HIT
x-cache-hits: 1, 1
x-timer: S1628804380.563848,VS0,VE0
content-length: 1058

StorePath: /nix/store/p4pclmv1gyja5kzc26npqpia1qqxrf0l-ruby-2.7.3
URL: nar/1w1fff338fvdw53sqgamddn1b2xgds473pv6y13gizdbqjv4i5p3.nar.xz
Compression: xz
FileHash: sha256:1w1fff338fvdw53sqgamddn1b2xgds473pv6y13gizdbqjv4i5p3
FileSize: 4029176
NarHash: sha256:1impfw8zdgisxkghq9a3q7cn7jb9zyzgxdydiamp8z2nlyyl0h5h
NarSize: 18735072
References: 0d71ygfwbmy1xjlbj1v027dfmy9cqavy-libffi-3.3 0dbbrvlw2rahvzi69bmpqy1z9mvzg62s-gdbm-1.19 0i6vphc3vnr8mg0gxjr61564hnp0s2md-gnugrep-3.6 0vkw1m51q34dr64z5i87dy99an4hfmyg-coreutils-8.32 64ylsrpd025kcyi608w3dqckzyz57mdc-libyaml-0.2.5 65ys3k6gn2s27apky0a0la7wryg3az9q-zlib-1.2.11 9m4hy7cy70w6v2rqjmhvd7ympqkj6yxk-ncurses-6.2 a4yw1svqqk4d8lhwinn9xp847zz9gfma-bash-4.4-p23 hbm0951q7xrl4qd0ccradp6bhjayfi4b-openssl-1.1.1k hjwjf3bj86gswmxva9k40nqx6jrb5qvl-readline-6.3p08 p4pclmv1gyja5kzc26npqpia1qqxrf0l-ruby-2.7.3 sbbifs2ykc05inws26203h0xwcadnf0l-glibc-2.32-46
Deriver: bidkcs01mww363s4s7akdhbl6ws66b0z-ruby-2.7.3.drv
Sig: cache.nixos.org-1:GrGV/Ls10TzoOaCnrcAqmPbKXFLLSBDeGNh5EQGKyuGA4K1wv1LcRVb6/sU+NAPK8lDiam8XcdJzUngmdhfTBQ==

We see that it points to another URL at nar/1w1fff338fvdw53sqgamddn1b2xgds473pv6y13gizdbqjv4i5p3.nar.xz.

Let’s get some statistics about it.

NAR is the Nix ARchive. Not all archive formats are reproducible, so Nix had to create it’s own!

❯ nix-hash --type sha256 --flat \
       --base32 <(curl --silent https://cache.nixos.org/nar/1w1fff338fvdw53sqgamddn1b2xgds473pv6y13gizdbqjv4i5p3.nar.xz)


Cool 😎! Looks like we computed the correct hash for the FileHash.

How did I know it was base32! Everything in Nix is base32 😞 This is needed to shrink the necessary space to work within the POSIX filepath limits. Frustratingly, sha256sum doesn’t have a base32 option.

The NAR hash should be calcuated in a very similar fashion.

Let’s try by piping it into unxz.

❯ nix-hash --type sha256 --flat \
       --base32 <(curl --silent https://cache.nixos.org/nar/1w1fff338fvdw53sqgamddn1b2xgds473pv6y13gizdbqjv4i5p3.nar.xz | unxz)

What about References, what’s that?

We see a very similar concept in the nix-store --query CLI and it produces the same list.

❯ nix-store --query \
    --references /nix/store/p4pclmv1gyja5kzc26npqpia1qqxrf0l-ruby-2.7.3


–references Prints the set of references of the store paths paths, that is, their immediate dependencies. (For all dependencies, use –requisites.)

Looks like it just prints the immediate dependencies. In order for our Ruby to run successfully on a machine, one would have to though download the full transitive closure of runtime dependencies.

If we extract the NAR above, we see it’s just the contents of our Nix derivation.

❯ curl --silent https://cache.nixos.org/nar/1w1fff338fvdw53sqgamddn1b2xgds473pv6y13gizdbqjv4i5p3.nar.xz \
       | unxz | nix-store --restore /tmp/ruby

❯ ls -l /tmp/ruby

drwxr-xr-x - fmzakari 12 Aug 15:16 bin
drwxr-xr-x - fmzakari 12 Aug 15:16 include
drwxr-xr-x - fmzakari 12 Aug 15:16 lib
drwxr-xr-x - fmzakari 12 Aug 15:16 nix-support
drwxr-xr-x - fmzakari 12 Aug 15:16 share

Finally browsing the nix-serve source, I see there’s a /nix-cache-info path.

❯ curl https://cache.nixos.org/nix-cache-info
StoreDir: /nix/store
WantMassQuery: 1
Priority: 40

It seems to give some information about this particular cache.

The important one is the StoreDir since the store-path is part of the build recipe. When you build a derivation, the Nix store directory is encoded within.

If you pick an arbitrary Nix store dir such as ~/nix/store you will be unable to use the default NixOS binary cache.

For instance, even though above I extracted the Ruby NAR to /tmp/ruby, if we read where it expects to find the dynamic libraries, we see it still references /nix/store.

❯ ldd /tmp/ruby/bin/ruby
	linux-vdso.so.1 (0x00007ffd6e57b000)
	libruby-2.7.3.so.2.7 => /nix/store/p4pclmv1gyja5kzc26npqpia1qqxrf0l-ruby-2.7.3/lib/libruby-2.7.3.so.2.7 (0x00007f7a819e2000)
    ibz.so.1 => /nix/store/65ys3k6gn2s27apky0a0la7wryg3az9q-zlib-1.2.11/lib/libz.so.1 (0x00007f7a819c5000)

Wow! I ended up encoding this discovery into an OpenAPI specification that you can check out.

Please visit https://fzakaria.github.io/nix-http-binary-cache-api-spec/#/ or contribute to the specification at https://github.com/fzakaria/nix-http-binary-cache-api-spec.