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X-Forwarded-Host, Rack & Hasura: good deeds never go unpunished

I had to track down a bug at work this week, and it was caused by the unpredicted interaction between two components that both do something that makes sense in their own context.

Understanding why the problem happens requires to know a few things about enterprise networking, so it’s a great excuse to write a blog post.

X-Forwarded-Host & networking

In many organizations, application servers like web servers are not directly exposed on the internet but are isolated through a layer of firewalls:

firewall1

From a browser, the requests first goes through the firewall then reach the server behind them (if the firewalls decide they should let the request pass).

The firewall are reachable on the www.domain.cute domain, and the application servers are reachable on the web.domain.cute domain from within the network.

(Note: using a domain to reach the application server is not the only approach, for example you can use a virtual IP address, but it’s not the case here.)

When a query reach the proxy, the URL is like https://www.domain.cute/cats, and when it is transmitted to the the application servers, it will become https://web.domain.cute/cats.

And sometimes this happens:

firewall2

When the same application servers are used for several domains with different contents, from their point of view the request is targeted at https://web.domain.cute/cats, but they need to know if the request was originally targeted to https://www.domain.cute/cats or to https://secret.domain.cute/cats.

A possible solution would be to have a separate domain for the application servers for each public domain so you could do a kind of matching between them, like www.app.domain.cute for www.domain.cute and secret.app.domain.cute for secret.domain.cute but it increases the infrastructure’s complexity.

The de-facto standard for this is to use the X-Forwarded-Host header: when a proxy passes the query to an application server, it adds a X-Forwarded-Host header containing the original host that has been queried, in our case www.domain.cute or secret.domain.cute. The application server can then use this header to decide which content to serve.

Rack wants to help you

Rack is a Ruby framework that “provides a minimal, modular, and adaptable interface for developing web applications in Ruby”. It provides a middleware between web servers and web framework in Ruby, which means that if you decide to write a new Ruby web servers and make it Rack-compatible, it should works with all Rack-compatible web frameworks like Rails, or if you decide to write your own web framework, doing it using the Rack API should make it work with all the Rack-compatible web servers.

Rack is mostly an (well-designed) API, but it also does a few plumbing-related things that are generally useful for all web-related things, like normalizing the requests headers. One of these things is to use the X-Forwarded-Host header — when it’s available — to replace the host of the request.

In the previous example, it means that if your application server is in Ruby and use Rack, when the proxied request has a X-Forwarded-Host header with the value www.domain.cute, from the application code it looks like the request that hit you is targeted at www.domain.cute instead of web.domain.cute. This way, the proxy layer is kind of hidden and you can write your code like it doesn’t exist and your application is accessed on the initial domain.

Hasura also wants to help you

Hasura is a tool that enable you to expose GraphQL APIs from a database. This kind of tool makes some people angry but it can be handy when you want to share data between components of your system without connecting directly to their database but don’t want to invest into designing custom APIs.

To secure the requests, Hasura can use webhooks, in this case the Hasura server calls an endpoint you specified to validate that the request is allowed.

hasura1

In some case, you could want to use the same Hasura server to expose several APIs to several consumers. For example you could have a private API for internal consumers so can expose more data to them. And in this case you may want to make the Hasura server available on several domains:

hasura2

You probably guess what will happen: sometimes the authentication rules may rely on the domain the request comes from (pub-api.domain.cute or priv-api.domain.cute) so you need to transmit this data to the authentication server.

There’s no specified way to do this, but there’s an existing mechanism that could be repurposed because this use case is not so different: the X-Forwarded-Host header. So when the Hasura server call the authentication server, it will put the original host in the X-Forwarded-Host header.

Hasura + Rack: well yes, but actually no

In our application server, we use the request domain to decide which endpoint to call with Rails' request-based constraints.

We don’t use the kind of proxy described above: our servers acts like their directly reached on the public-facing domains.

And when I tried to deploy our new Hasura authentication endpoint on auth.website.cute, the authentication failed because it got an unexpected answer from the authentication server. When looking at the authentication server logs, I saw 404 errors.

After adding more logs, it seemed like the requests reached our servers with a api.domain.cute domain, which didn’t make any sense at first, especially since we don’t use any private DNS setup.

As the server is set up to answer to auth.website.cute, calling it on api.website.cute returns 404 since it didn’t have any endpoint configured on this domain.

If you read the article until here, who can understand what was happening: Rack and Hasura conspired together in an unexpected way.

The Rack solution is not a bad idea, and Hasura’s way to repurpose the X-Forwarded-Host header kind of make sense, but together the result was unexpected, and I needed a few hours to figure the whole explanation.

There is no configuration to disable the X-Forwarded-Host trick on Rack or to tell Hasura to not add the header, so I see two ways to deal with the issue:

  1. Add a routing rule based on the Hasura domain on the application server, even if it doesn’t make any real sense.

  2. Add a Rack middleware to remove the header before it can be processed to avoid the problem.

I’m not sure yet which one I’ll choose.