falco is VCL parser and linter optimized for Fastly.

This is a VCL parser, but dedicated to Fastly's VCL (version 2.x), so we don't care about the latest Varnish (7.x or later) syntax. The Varnish may have additional syntax, builtin function, predefined variables, but this tool may not parse correctly.

Additionally, Fastly provides its special builtin function, predefined variables. It's not compatible with Varnish. But this tool is optimized for them, we could parse and lint their declarations.

Fastly is a really fantastic CDN, but sometimes we have problems with deployment operations. On deploy custom VCL to the Fastly, VCLs are validated when activating a new service version. Typically our deployment flow using custom VCLs is following:

1. Clone active service and create new version
2. Delete existing custom VCLs
3. Upload new VCL files to the Fastly
4. Activate new device version <= Validate VCLs on the Fastly cloud

Above flows take a time, and then if we have some mistakes on VCL e.g. missing semicolon X(, the deployment will fail. Additionally, unnecessary service versions will be created by our trivial issue.

To solve them, we made a Fastly dedicated VCL parser and linter tool to notice syntax errors and unexpected mistakes before starting the deployment flow.

Download binary from releases page according to your platform and place it under the $PATH.

Command help displays following:

falco -h
=======================================
  falco: Fastly VCL parser / linter
=======================================
Usage:
    falco [main vcl file]

Flags:
    -I, --include_path : Add include path
    -t, --transformer  : Specify transformer
    -h, --help         : Show this help
    -r, --remote       : Communicate with Fastly API
    -V, --version      : Display build version
    -v,                : Verbose warning lint result
    -vv,               : Verbose all lint result

Example:
    falco -I . -vv /path/to/vcl/main.vcl

Your VCL will have dependent modules loaded via include [module]. falco accept include path from -I, --include_path flag and search and load destination module from include path.

On linting, falco could not recognize when the user-defined subroutine is called, so you should apply the subroutine scope by adding annotation or its subroutine name. falco understands call scope by following rules:

If the subroutine name has a suffix of _[scope], falco lint within that scope.

sub custom_recv { // name has `_recv` suffix, lint with RECV scope
  ...
}

sub custom_fetch { // name has `_fetch` suffix, lint with FETCH scope
  ...
}

Following table describes subroutine name and recognizing scope:

For some reasons, the subroutine name could not be changed. Then, if you apply a hint of scope on annotation, falco also understands scope:

// @recv
sub custom_process { // subroutine has `recv` annotation, lint with RECV scope
  ...
}

// @fetch
sub custom_request { // subroutine has `fetch` annotation, lint with FETCH scope
  ...
}

Following table describes annotation name and recognizing scope:

Partially supports fetching Fastly managed VCL snippets. See remote.md in detail.

falco has built in lint rules. see rules in detail. falco may report lots of errors and warnings because falco lints with strict type checks, disallows implicit type conversions even VCL is fuzzy typed language.

To avoid them, you can override severity levels by putting a configuration file named .falcorc on working directory. the configuration file contents format is following:

## /path/to/working/directory/.falcorc
regex/matched-value-override: IGNORE
...

Format is simply a yaml key-value object. The key is rule name, see rules.md and value should be one of IGNORE, INFO, WARNING and ERROR, case insensitive.

In the above case, the rule of regex/matched-value-override reports INFO as default, but overrides IGNORE which does not report it.

falco reports three of severity on linting:

VCL may cause errors on Fastly, or may cause unexpected behavior for actual works.

VCL could work, but may have potential bug and cause unexpected behavior for actual works.

falco does not output warnings as default. To see them, run with -v option.

VCL is fine, but we suggest you improve your VCL considering Fastly recommendation.

falco does not output information as default. To see them, run with -vv option.

falco is planning to transpile Fastly VCL to the other programming language e.g Go (HTTP service), node.js (Lambda@Edge) to use temporal CDN instead of Fastly.

• Fork this repository
• Customize / Fix problem
• Send PR :-)
• Or feel free to create issues for us. We'll look into it

MIT License

• @ysugimoto
• @smaeda-ks