Setup Caddy as a reverse proxy on NixOS (Part 3: Caddy)

Part 3: Configure Caddy

  1. Background
  2. caddyProxy.nix
  3. Caddyfile
    1. Cloudflare origin cert
    2. Download certs
    3. Initial setup
    4. Enable HTTPS
    5. Redirect www to apex
    6. Reverse proxy
    7. Host header
    8. Add or remove headers
    9. Cache-Control
    10. Complete Caddyfile
  4. configuration.nix

8 Jul 2022: Updated to Caddy 2.5 syntax.

In this segment, I show you how I set up this website (mdleom.com) to reverse proxy to curben.netlify.app using Caddy on NixOS (see above diagram). If you’re not using NixOS, simply skip to the Caddyfile section.

This post is Part 2 of a series of articles that show you how I set up Caddy and Tor hidden service on NixOS:

Architecture behind mdleom.com

Background §

In NixOS, Caddy can be easily configured through “configuration.nix”, without even touching a Caddyfile, if you have a rather simple setup. For example, to serve static files from “/var/www/“ folder,

configuration.nix
services.caddy = {
  enable = true;
  email = [email protected];
  config =
    ''
      example.com {
        root /var/www/
      }
    '';
}

Once rebuild, caddy will run as a systemd service. This config also automatically enable HTTPS on example.com using Let’s Encrypt cert which will be stored in “/var/lib/caddy/“ folder by default.

The magic behind the option is “caddy.nix“ which exposes the services.caddy option. It also take care of creating a systemd unit file and installation the caddy package, so you don’t need to install it beforehand. caddy.nix is bundled with NixOS so you can use services.caddy straightaway.

This shows the declarative property of NixOS. Nix, the package manager behind NixOS, also enables the system to be atomic. Imagine putting the whole system binaries under Git or file system snapshot. If you botch the system upgrade, you can easily rollback to previous state (usually via Grub menu).

A package is installed in /nix/store/<hash>/ folder and that hash is what makes Nix atomic. I mention this atomic thing because a package’s binary is only symlink to $PATH (“/usr/bin”) when installed using environment.systemPackages option or nix-env. In this case, “caddy.nix” simply specify the required binary “pkgs.caddy/bin/caddy” and NixOS will automatically install the required package. Since the caddy binary is not available under $PATH, running $ caddy command will return “command not found” error. If you need to use the caddy binary, you have three options:

  1. Locate the binary in “/nix/store” by checking $ systemctl status caddy. This is only available when caddy service is enabled in “configuration.nix”. Disabling the service will remove the package.
  2. Install it as a system package using environment.systemPackages.
  3. Install it as a user package using $ nix-env -f '<nixpkgs>' -iA caddy.

caddy.nix grants CAP_NET_BIND_SERVICE capability which is not needed in my use case because I’m not binding caddy to port < 1024.

caddyProxy.nix §

I created another nix file which is similar to “caddy.nix”, but without CAP_NET_BIND_SERVICE capability. I also removed Let’s Encrypt-related options since I’m using Cloudflare origin certificate. I renamed the options.services.caddy to options.services.caddyProxy to avoid clash with “caddy.nix”. Save the file to “/etc/caddy/caddyProxy.nix” with root as owner. We’ll revisit this file in “configuration.nix“ section later in this guide.

/etc/caddy/caddyProxy.nix
{ config, lib, pkgs, ... }:

with lib;

let
  cfg = config.services.caddyProxy;
in {
  options.services.caddyProxy = {
    enable = mkEnableOption "Caddy web server";

    config = mkOption {
      default = "/etc/caddy/caddyProxy.conf";
      type = types.str;
      description = "Path to Caddyfile";
    };

    adapter = mkOption {
      default = "caddyfile";
      example = "nginx";
      type = types.str;
      description = ''
        Name of the config adapter to use.
        See https://caddyserver.com/docs/config-adapters for the full list.
      '';
    };

    dataDir = mkOption {
      default = "/var/lib/caddyProxy";
      type = types.path;
      description = ''
        The data directory, for storing certificates. Before 17.09, this
        would create a .caddy directory. With 17.09 the contents of the
        .caddy directory are in the specified data directory instead.
      '';
    };

    package = mkOption {
      default = pkgs.caddy;
      defaultText = "pkgs.caddy";
      type = types.package;
      description = "Caddy package to use.";
    };
  };

  config = mkIf cfg.enable {
    systemd.services.caddyProxy = {
      description = "Caddy web server";
      after = [ "network-online.target" ];
      wants = [ "network-online.target" ]; # systemd-networkd-wait-online.service
      wantedBy = [ "multi-user.target" ];
      startLimitIntervalSec = 14400;
      startLimitBurst = 10;
      serviceConfig = {
        ExecStart = "${cfg.package}/bin/caddy run --config ${cfg.config} --adapter ${cfg.adapter}";
        ExecReload = "${cfg.package}/bin/caddy reload --config ${cfg.config} --adapter ${cfg.adapter}";
        Type = "simple";
        User = "caddyProxy";
        Group = "caddyProxy";
        Restart = "on-abnormal";
        # < 20.09
        # https://github.com/NixOS/nixpkgs/pull/97512
        # StartLimitIntervalSec = 14400;
        # StartLimitBurst = 10;
        NoNewPrivileges = true;
        LimitNPROC = 512;
        LimitNOFILE = 1048576;
        PrivateTmp = true;
        PrivateDevices = true;
        ProtectHome = true;
        ProtectSystem = "full";
        ReadWriteDirectories = cfg.dataDir;
        KillMode = "mixed";
        KillSignal = "SIGQUIT";
        TimeoutStopSec = "5s";
      };
    };

    users.users.caddyProxy = {
      home = cfg.dataDir;
      createHome = true;
    };

    users.groups.caddyProxy = {
      members = [ "caddyProxy" ];
    };
  };
}

Caddyfile §

Caddy web server is configured using a Caddyfile. The Caddyfile format I’m using is only compatible with Caddy v1. I will update to v2 once the stable version is released on NixOS. Note that v1 and v2 are incompatible with each other.

Cloudflare origin cert §

For TLS setup, I’m using Cloudflare Origin Certificate. This cert is only valid for connection between Cloudflare and the origin server (i.e. my web server) because it’s not signed by a CA. The cert is only signed by Cloudflare and does not have a valid chain of trust. TLS connection between a visitor and Cloudflare is enabled by Cloudflare Universal SSL which has a valid cert.

I’m using “Full (strict)” mode which requires either origin cert or a valid cert signed by a trusted CA. This mode forbids self-signed cert unlike “Full” mode. Let’s Encrypt cert is compatible with “Full (strict)”. However, putting a web server behind a CDN means that Caddy could only obtain a Let’s Encrypt using DNS challenge not the default HTTP challenge. Setting up the DNS challenge requires installing tls.dns.cloudflare Caddy plugin which is not included in the NixOS package. The plugin also requires access to my Cloudflare’s API key which I’m not really comfortable with. Hence, the use of Origin Certificate.

Download certs §

Generate and download the cert from Cloudflare Dash → SSL/TLS → Origin Server → Create Certificate. You can choose the validity from 1 week to 15 years. I choose 1 year so I need to repeat this process every year. Make sure you have both certificate (.pem) and private key (.key).

Cloudflare Origin Certificate

I also use Authenticated Origin Pull which utilize TLS client authentication. A client must present a client certificate that is signed by a private key; in this case, it is signed by Cloudflare itself. The client certificate can be verified using Cloudflare’s public key available here.

By now, you should have three files:

  1. <domain>.pem
  2. <domain>.key
  3. origin-pull-ca.pem

Move the files to home folder of “caddyProxy” user, which is “/var/lib/caddyProxy” in this case. Set the files’ owner and group to caddyProxy and permission to 600.

# chown caddyProxy:caddyProxy /var/lib/caddyProxy/*
# chmod 600 /var/lib/caddyProxy/*

If you followed my Part 2 guide, you should have caddyProxy user and group before executing chown and chmod. If you haven’t, check out this section of Part 2.

Initial setup §

Set up Caddy to listen on apex domain and www.* on port 4430

Caddyfile
mdleom.com:4430 www.mdleom.com:4430 {

}

Enable HTTPS §

Subsequent configurations (directives) shall be inside the curly braces. Let’s start with tls directive.

mdleom.com:4430 www.mdleom.com:4430 {
  tls /var/lib/caddyProxy/mdleom.com.pem /var/lib/caddyProxy/mdleom.com.key {
    protocols tls1.3
    client_auth {
      mode require_and_verify
      trusted_ca_cert_file /var/lib/caddyProxy/origin-pull-ca.pem
    }
  }
}

Redirect www to apex §

Connection to www.mdleom.com is redirected to mdleom.com with HTTP 301 status.

@www host www.mdleom.com
redir @www https://mdleom.com{uri} permanent

{label1} placeholder refers to the first part of the request hostname, e.g. if hostname is foo.bar.com, {label1} is foo, {label2} is bar and so on.

{uri} is used to retain the path when redirecting. www.mdleom.com/foo/bar is redirected to mdleom.com/foo/bar.

If you prefer to redirect apex to www,

@www host mdleom.com
redir @www https://www.mdleom.com{uri} permanent

Reverse proxy §

Aside from reverse proxy to curben.netlify.app, I also configured my Netlify website to use Statically CDN for on-the-fly image processing. My current config is:

source/_redirects_redirects
/img/* https://cdn.statically.io/img/:splat 200
/screenshot/* https://cdn.statically.io/screenshot/curben.netlify.app/:splat 200
/files/* https://gitlab.com/curben/blog/-/raw/site/:splat 200

In Caddyfile, the config can be expressed as:

handle /img/* {
  reverse_proxy https://cdn.statically.io
}

handle_path /screenshot/* {
  # "curben.netlify.app" is updated to "mdleom.com"
  rewrite * /screenshot/mdleom.com{path}

  reverse_proxy https://cdn.statically.io
}

handle_path /files/* {
  rewrite * /curben/blog/-/raw/site{path}

  reverse_proxy https://gitlab.com
}

reverse_proxy https://curben.netlify.app

rewrite directive is necessary to remove img/ and screenshot/* from the path, so that “mdleom.com/img/foo.jpg” is linked to “https://cdn.statically.io/img/foo.jpg“, not “https://cdn.statically.io/img/img/foo.jpg“.

Host header §

To make sure Caddy sends the correct Host: header to the upstream/backend locations, I use header_up option,

handle /img/* {
  reverse_proxy https://cdn.statically.io {
    header_up Host cdn.statically.io
  }
}

handle_path /screenshot/* {
  rewrite * /screenshot/mdleom.com{path}

  reverse_proxy https://cdn.statically.io {
    header_up Host cdn.statically.io
  }
}

reverse_proxy https://curben.netlify.app {
  header_up Host curben.netlify.app
}

If there are multiple backends for the reverse_proxy, it’s better to use a placeholder instead of hardcording the Host header.

reverse_proxy https://curben.pages.dev https://curben.netlify.app {
  header_up Host {http.reverse_proxy.upstream.host}
}

Add or remove headers §

To prevent any unnecessary request headers from being sent to the upstreams, I use header_up. I use it to remove cookie, referer and other headers added by Cloudflare. Since there are many headers to remove, I group them as a global variable. I apply it to all reverse_proxy directives.

(removeHeaders) {
  header_up -cdn-loop
  header_up -cf-cache-status
  header_up -cf-connecting-ip
  header_up -cf-ipcountry
  header_up -cf-ray
  header_up -cf-request-id
  header_up -cf-visitor
  header_up -cf-worker
  header_up -client-ip
  header_up -cookie
  header_up -forwarded
  header_up -referer
  # https://user-agent-client-hints.glitch.me/
  header_up -sec-ch-ua-arch
  header_up -sec-ch-ua-bitness
  header_up -sec-ch-ua-full-version
  header_up -sec-ch-ua-ua
  header_up -sec-ch-ua-ua-mobile
  header_up -sec-ch-ua-ua-model
  header_up -sec-ch-ua-ua-platform
  header_up -sec-ch-ua-ua-platform-version
  header_up -true-client-ip
  header_up -via
  header_up -x-forwarded-for
  header_up -x-forwarded-proto
  header_up -x-proxyuser-ip
  header_up Host {http.reverse_proxy.upstream.host}
  header_up User-Agent "Mozilla/5.0 (compatible; Googlebot/2.1; +http://www.google.com/bot.html)"
}

mdleom.com {
  handle /img/* {
    reverse_proxy https://cdn.statically.io {
      import removeHeaders
    }
  }

  handle_path /screenshot/* {
    rewrite * /screenshot/mdleom.com{path}

    reverse_proxy https://cdn.statically.io {
      import removeHeaders
    }
  }

  reverse_proxy https://curben.pages.dev https://curben.netlify.app {
    import removeHeaders
  }
}

The upstream locations insert some information into the response headers that are irrelevant to the site visitors. I use header directive to filter them out. It also applies to all reverse_proxy directives.

header {
  -access-control-allow-origin
  -access-control-expose-headers
  -alt-svc
  -cdn-cache
  -cdn-cachedat
  -cdn-edgestorageid
  -cdn-pullzone
  -cdn-requestcountrycode
  -cdn-requestid
  -cdn-uid
  -cf-bgj
  -cf-cache-status
  -cf-polished
  -cf-ray
  -cf-request-id
  -content-disposition
  -etag
  -expect-ct
  -server
  -set-cookie
  -timing-allow-origin
  -via
  -x-bytes-saved
  -x-cache
  -x-cache-hits
  -x-nf-request-id
  -x-served-by
  -x-timer
  Content-Security-Policy "default-src 'self'; child-src 'none'; connect-src 'none'; font-src 'none'; frame-src 'none'; img-src 'self'; manifest-src 'self'; media-src 'none'; object-src 'none'; prefetch-src 'none'; script-src 'self'; style-src 'self'; worker-src 'none'; base-uri 'none'; form-action https://duckduckgo.com https://duckduckgogg42xjoc72x3sjasowoarfbgcmvfimaftt6twagswzczad.onion; frame-ancestors 'none'; block-all-mixed-content"
  Expires "0"
  Permissions-Policy "accelerometer 'none'; ambient-light-sensor 'none'; autoplay 'none'; camera 'none'; display-capture 'none'; document-domain 'none'; encrypted-media 'none'; fullscreen 'none'; geolocation 'none'; gyroscope 'none'; magnetometer 'none'; microphone 'none'; midi 'none'; payment 'none'; picture-in-picture 'none'; speaker 'none'; sync-xhr 'none'; usb 'none'; vibrate 'none'; vr 'none'; wake-lock 'none'; webauthn 'none'; xr-spatial-tracking 'none'; interest-cohort=()"
  Referrer-Policy "no-referrer"
  X-Content-Type-Options "nosniff"
  X-Frame-Options "DENY"
  X-XSS-Protection "1; mode=block"
  defer
}

I also add the Cache-Control and Referrer-Policy to the response header. Use minus (-) sign before each option to remove particular header. Without minus sign, the specified header is either added or replacing an existing one.

Cache-Control §

/libs folder contains third-party libraries. Since the library is usually requested by a specific version, we can safely assume that the response would remain the same. This means I can set long expiration and immutable on the response. immutable is to tell the browser that revalidation is not needed.

header {
  Cache-Control "max-age=86400, public"
}

header /libs/* {
  Cache-Control "public, max-age=31536000, immutable"
}

Complete Caddyfile §

Since I also set up reverse proxy for Tor Onion and I2P Eepsite, I refactor most of the configuration into “common.conf” and import it into “caddyProxy.conf”.

common.conf
{
  ## disable admin endpoint
  # admin off
  ## http->https redirect
  # auto_https disable_redirects
  ## Remove PII from error log
  log {
    level ERROR
    format filter {
      wrap json {
        time_format iso8601
      }
      fields {
        request>remote_ip delete
        request>remote_port delete
        request>headers>CDN-Loop delete
        request>headers>CF-Cache-Status delete
        request>headers>CF-Connecting-IP delete
        request>headers>CF-IPCountry delete
        request>headers>CF-RAY delete
        request>headers>CF-Request-ID delete
        request>headers>CF-Visitor delete
        request>headers>CF-Worker delete
        request>headers>Client-IP delete
        request>headers>Cookie delete
        request>headers>Forwarded delete
        request>headers>Referer delete
        request>headers>Sec-CH-UA-Arch delete
        request>headers>Sec-CH-UA-Bitness delete
        request>headers>Sec-CH-UA-Full-Version delete
        request>headers>Sec-CH-UA-UA delete
        request>headers>Sec-CH-UA-UA-Mobile delete
        request>headers>Sec-CH-UA-UA-Model delete
        request>headers>Sec-CH-UA-UA-Platform delete
        request>headers>Sec-CH-UA-UA-Platform-Version delete
        request>headers>True-Client-IP delete
        request>headers>User-Agent delete
        request>headers>Via delete
        request>headers>X-Forwarded-For delete
        request>headers>X-Forwarded-Proto delete
        request>headers>X-ProxyUser-Ip delete
      }
    }
  }
}

(setHeaders) {
  -access-control-allow-origin
  -access-control-expose-headers
  -alt-svc
  -cdn-cache
  -cdn-cachedat
  -cdn-edgestorageid
  -cdn-pullzone
  -cdn-requestcountrycode
  -cdn-requestid
  -cdn-uid
  -cf-bgj
  -cf-cache-status
  -cf-polished
  -cf-ray
  -cf-request-id
  -content-disposition
  -etag
  -expect-ct
  -gitlab-lb
  -gitlab-sv
  -server
  -set-cookie
  -timing-allow-origin
  -via
  -x-bytes-saved
  -x-cache
  -x-cache-hits
  -x-download-options
  -x-gitlab-feature-category
  -x-nf-request-id
  -x-permitted-cross-domain-policies
  -x-request-id
  -x-runtime
  -x-served-by
  -x-timer
  -x-ua-compatible
  Cache-Control "max-age=86400, public"
  Content-Security-Policy "default-src 'self'; child-src 'none'; connect-src 'none'; font-src 'none'; frame-src 'none'; img-src 'self'; manifest-src 'self'; media-src 'none'; object-src 'none'; prefetch-src 'none'; script-src 'self'; style-src 'self'; worker-src 'none'; base-uri 'none'; form-action https://duckduckgo.com https://duckduckgogg42xjoc72x3sjasowoarfbgcmvfimaftt6twagswzczad.onion; frame-ancestors 'none'; block-all-mixed-content"
  Expires "0"
  Permissions-Policy "accelerometer 'none'; ambient-light-sensor 'none'; autoplay 'none'; camera 'none'; display-capture 'none'; document-domain 'none'; encrypted-media 'none'; fullscreen 'none'; geolocation 'none'; gyroscope 'none'; magnetometer 'none'; microphone 'none'; midi 'none'; payment 'none'; picture-in-picture 'none'; speaker 'none'; sync-xhr 'none'; usb 'none'; vibrate 'none'; vr 'none'; wake-lock 'none'; webauthn 'none'; xr-spatial-tracking 'none'; interest-cohort=()"
  Referrer-Policy "no-referrer"
  X-Content-Type-Options "nosniff"
  X-Frame-Options "DENY"
  X-XSS-Protection "1; mode=block"
}

(removeHeaders) {
  header_up -cdn-loop
  header_up -cf-cache-status
  header_up -cf-connecting-ip
  header_up -cf-ipcountry
  header_up -cf-ray
  header_up -cf-request-id
  header_up -cf-visitor
  header_up -cf-worker
  header_up -client-ip
  header_up -cookie
  header_up -forwarded
  header_up -referer
  # https://user-agent-client-hints.glitch.me/
  header_up -sec-ch-ua-arch
  header_up -sec-ch-ua-bitness
  header_up -sec-ch-ua-full-version
  header_up -sec-ch-ua-ua
  header_up -sec-ch-ua-ua-mobile
  header_up -sec-ch-ua-ua-model
  header_up -sec-ch-ua-ua-platform
  header_up -sec-ch-ua-ua-platform-version
  header_up -true-client-ip
  header_up -via
  header_up -x-forwarded-for
  header_up -x-forwarded-proto
  header_up -x-proxyuser-ip
  header_up Host {http.reverse_proxy.upstream.host}
  header_up User-Agent "Mozilla/5.0 (compatible; Googlebot/2.1; +http://www.google.com/bot.html)"
}

(reverseProxy) {
  reverse_proxy https://{args.0} {
    import removeHeaders
    header_up Host {http.reverse_proxy.upstream.host}
  }
}

(pathProxy) {
  @staticFiles {
    path *.css *.gif *.ico *.jpg *.js *.pdf *.png *.svg *.webp
  }
  header @staticFiles {
    Cache-Control "max-age=604800, public"
    defer
  }

  header /libs/* {
    Cache-Control "max-age=31536000, public, immutable"
    defer
  }

  handle /img/* {
    import reverseProxy cdn.statically.io
  }

  handle_path /screenshot/* {
    rewrite * /screenshot/mdleom.com{path}

    import reverseProxy cdn.statically.io
  }

  handle_path /files/* {
    rewrite * /curben/blog/-/raw/site{path}

    import reverseProxy gitlab.com
  }

  # Multiple mirrors
  reverse_proxy https://curben.pages.dev https://curben.netlify.app https://curben.gitlab.io https://curbengh.github.io {
    import removeHeaders
    lb_policy first
  }
}
caddyProxy.conf
import common.conf

## mdleom.com
mdleom.com:4430 www.mdleom.com:4430 {
  tls /var/lib/caddyProxy/mdleom.com.pem /var/lib/caddyProxy/mdleom.com.key {
    protocols tls1.3
    client_auth {
      mode require_and_verify
      trusted_ca_cert_file /var/lib/caddyProxy/origin-pull-ca.pem
    }
  }

  # www -> apex
  @www host www.mdleom.com
  redir @www https://mdleom.com{uri} permanent

  header {
    import setHeaders
    Onion-Location "http://xw226dvxac7jzcpsf4xb64r4epr6o5hgn46dxlqk7gnjptakik6xnzqd.onion"
    Strict-Transport-Security "max-age=31536000; includeSubDomains; preload"
    defer
  }

  import pathProxy
}

configuration.nix §

One last thing to do is to import “caddyProxy.nix“ and enable services.caddyProxy.

/etc/nixos/configuration.nix
require = [ /etc/caddy/caddyProxy.nix ];
services.caddyProxy = {
  enable = true;
  config = "/etc/caddy/caddyProxy.conf";
};