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Using Caddy to Indicate GitLab Repository for Go Module Path with Different Domains

January 21, 2023 #100DaysToOffload #Caddy #GitLab #Go

The self-hosted GitLab at my company, Furqan Software, is home to all the company Go projects. The domain where Furqan Software’s GitLab is accessible is different from the domain in Go module paths. That means go get doesn’t work out of the box for our Go projects.

For example, let’s say you have a Go project with the module path go.example.com/tools/aglet. And the corresponding GitLab project is at https://gitlab.example.com/tools/aglet. If you run go get go.example.com/tools/aglet, you will see an error from go get about not finding the repository.

When the go command downloads a module in direct mode, it starts by locating the repository that contains the module. […]

To allow go get go.example.com/tools/aglet to work, you need to serve a specific HTML page at https://go.example.com/tools/aglet?go-get=1 with the right meta tag.

With Caddy, you can do this easily, although with a few limitations (more on that at the end of this post).

Caddy has built-in support for interpolating Go templates. We can leverage that to serve the HTML page with the right meta tag:

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<!DOCTYPE html>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8"/>
{{$path := .Req.URL.Query.Get "p" -}}
{{$parts := splitList "/" $path -}}
{{$repo := printf "/%s/%s" (index $parts 1) (index $parts 2) -}}
<meta name="go-import" content="go.example.com{{$repo}} git https://gitlab.example.com{{$repo}}.git">
<meta name="go-source" content="go.example.com{{$repo}} https://gitlab.example.com{{$repo}} https://gitlab.example.com{{$repo}}/-/tree/master{/dir} https://gitlab.example.com{{$repo}}/-/blob/master{/dir}/{file}#L{line}">
<meta http-equiv="refresh" content="0; url=https://gitlab.example.com{{$repo}}">
</head>
<body>
<a href="https://gitlab.example.com{{$repo}}">Redirecting to GitLab...</a>
</body>
</html>

To use the template, your Caddyfile will look something like this:

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go.example.com {
  root * /etc/caddy/go-get/
  file_server
  templates
  encode gzip
  try_files index.html?p={path}
}

With this, Caddy expects the template HTML to be present in the /etc/caddy/go-get directory as index.html.

And Caddy can then start serving the required HTML with the meta tag at URLs like https://go.example.com/tools/aglet?go-get=1 for commands like go get go.example.com/tools/aglet.

The template here assumes that all Go modules will have GitLab URLs like https://gitlab.example.com/{group}/{project}. Although GitLab supports nested groups, this solution won’t be able to handle it.

Alternate and better solutions exist: like github.com/unknwon/go-import-server. And you can always use the same domain for your self-hosted GitLab instance and Go module paths to avoid relying on this quirky workaround.


This post is 12th of my #100DaysToOffload challenge. Want to get involved? Find out more at 100daystooffload.com.

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