This is Day 8 of Butter Days, from Badfish Coffee and Tea in Orangevale, CA.

Last week I got started writing a proxy in Rust that would add the proper signature headers to an AWS request, so that I could hide the non-standardness of its signing process and write everything else using good standards.

First I ran the simple_proxy crate to see if I could rewrite headers, and then I copied the unit tests that I wrote for the awscurl project.

This week, my goal is to finish that proxy and send my first request!

See Day 1 of Butter Days for context on what I’m ultimately trying to build.

The Story So Far

Last week I was able to only finish one of the four functions, but I actually finished the rest off screen the next day.

Here it is in all its glory. It’s a mess but it passes the tests, which is all I’m aiming for at the moment! Because the whole file is pretty big I’m just going to paste the top level function and you can check the pull request that I’ll put up if you want to see all the code.

pub fn generate_aws_signature_headers(
    query: String,
    headers: HashMap<String, String>,
    port: Option<u16>,
    host: String,
    amzdate: String,
    method: String,
    data: Vec<u8>,
    security_token: Option<String>,
    data_binary: bool,
    datestamp: String,
    service: String,
    region: String,
    access_key: String,
    secret_key: String,
    canonical_uri: String) -> HashMap<String, String> {
    let (canonical_request,
         signed_headers) = task_1_create_a_canonical_request(
         query, headers, port, host, amzdate.clone(), method, data,
         security_token.clone(), data_binary, canonical_uri);
    let (string_to_sign,
         credential_scope) = task_2_create_the_string_to_sign(
         amzdate.clone(), datestamp.clone(), canonical_request, service.clone(),
    let signature = task_3_calculate_the_signature(
        datestamp, string_to_sign, service, region, secret_key);
    let new_headers = task_4_build_auth_headers_for_the_request(
        amzdate, payload_hash, algorithm, credential_scope, signed_headers,
        signature, access_key, security_token);
    return new_headers;

The next step is to understand how I can add new headers using the simple_proxy library, and hook all this up!

Adding Headers

I’m going to copy an existing middleware and see if I can figure out how to change it to add my arbitrary headers.

Most of my programming experience with rust has been doing whatever the compiler tells me to do. I’ve had very few real errors once I get rid of all the compiler errors. Let’s see if that’s the case here.

Wow, the hyper library has inline examples!

/// Returns a mutable reference to the associated header field map.
/// # Examples
/// ```
/// # use http::*;
/// # use http::header::*;
/// let mut request: Request<()> = Request::default();
/// request.headers_mut().insert(HOST, HeaderValue::from_static("world"));
/// assert!(!request.headers().is_empty());
/// ```
pub fn headers_mut(&mut self) -> &mut HeaderMap<HeaderValue> {
    &mut self.head.headers

After gutting one of the other middlewares (and of course satisfying the all knowing compiler), I ended up with this.

pub struct AwsSignatureHeaders {}

impl AwsSignatureHeaders {
    pub fn new() -> Self {

impl Middleware for AwsSignatureHeaders {
    fn name() -> String {

    fn before_request(
        &mut self,
        req: &mut Request<Body>,
        _context: &ServiceContext,
        _state: &State,
    ) -> Result<MiddlewareResult, MiddlewareError> {
        req.headers_mut().insert("foo", HeaderValue::from_static("bar"));

Let’s see if that works using the echo endpoint we used before.

$ http_proxy=localhost:8080 curl -s | jq
  "args": {},
  "headers": {
    "x-forwarded-proto": "https",
    "host": "",
    "accept": "*/*",
    "foo": "bar",
    "proxy-connection": "Keep-Alive",
    "user-agent": "curl/7.59.0",
    "x-forwarded-port": "80"
  "url": ""

Nice! This is starting to look good. Now let’s add our signature headers.

Signing The Request

At this point, I spent a lot of time banging on type conversions, and learning about how to actually set headers in hyper.

One difference that tripped me up is that they don’t let you set the header name from a dynamic string, but I’m returning a hash map of dynamic strings from my signing function to represent the headers. Ultimately that’s a deliberate choice made by the library maintainers so that we get static type checking. Those docs are old though, and they took any mention of it out of the docs in recent versions, but it’s fun to know why they did it.

Ultimately, this was easy to fix. I just defined the custom AWS headers as static strings, following the library’s example.

const XAMZCONTENTSHA256: &str = "x-amz-content-sha256";
const XAMZSECURITYTOKEN: &str = "x-amz-security-token";
const XAMZDATE: &str = "x-amz-date";

Then I called aws_signature_builder::generate_aws_signature_headers to get my new headers and ended up with lines that looked like this:

if new_headers.contains_key(XAMZCONTENTSHA256) {

This is so close… The headers are being set, the signature is being generated, and the tests are all passing.

At this point, I got impatient. I wanted to know the real answer to “will this even work”. So, I cheated and hard coded my secret key and a bunch of other arguments to the signature generation. After that, I started the proxy, and ran this:

$ http_proxy=localhost:8080 curl -s "" 
<?xml version="1.0" encoding="UTF-8"?>
<DescribeInstancesResponse xmlns="">

It’s… alive! That’s definitely my instance. I almost can’t believe it was that easy. Unit tests are amazing.

All that’s left now is a bunch of random cleanup, like not hard coding my secret keys and finding a good way to force the proxy to talk https to Amazon, but this is it! I wonder how many people have gotten results back from the Amazon API using a regular old curl command.

This is actually great, I just want to play around with it. Check it out:

$ http_proxy=localhost:8080 curl -s \
    "" \
    | xq .DescribeInstancesResponse.reservationSet.item[].instancesSet[].tagSet
  "item": {
    "key": "aws:autoscaling:groupName",
    "value": "cloudless.web-cd7e33a"
  "item": {
    "key": "aws:autoscaling:groupName",
    "value": "cloudless.consul-1"

Here I’m filtering the output using the beautiful xq command, which is a simple but amazing command line xml parser based on jq.

Anything Is Possible

If I can actually use this to talk to AWS using a real OpenAPI spec, that opens up a huge range of possibilities. Here are some examples:

I don’t want to oversell it, because there can always be challenges, but if this works, it will be something.

Cleaning Up

Now I’m trying to stop hard coding things, so I’m using the rusoto credential library to load my credentials.

I hit an issue where I was calling a method that I could see was implemented, but I was getting errors that the method wasn’t defined and didn’t have any idea why. Yet again, the compiler comes to the rescue:

 $ cargo build
   Compiling aws-signature-proxy v0.1.0 (/home/sverch/projects/aws-signature-proxy)
error[E0599]: no method named `credentials` found for type `rusoto_credential::DefaultCredentialsProvider` in the current scope
  --> src/
61 |         let credentials = provider.credentials().wait().unwrap();
   |                                    ^^^^^^^^^^^
   = help: items from traits can only be used if the trait is in scope
help: the following trait is implemented but not in scope, perhaps add a `use` for it:
5  | use rusoto_credential::ProvideAwsCredentials;

error: aborting due to previous error

For more information about this error, try `rustc --explain E0599`.
error: Could not compile `aws-signature-proxy`.

To learn more, run the command again with --verbose.

This is the most helpful compiler I’ve ever used in my life. Of course, just listening to exactly what the compiler was telling me to do fixed the problem.

After integrating that, all my normal AWS credential configuration methods worked no problem! I was looking for a way to get the region from my profile as well, but looks like it’s not implemented yet. Maybe at some point I can contribute to that, but for now I’ll just work around it and not bother loading the region in the normal way.

As for using https, I know I want to do that, but I might have to do it next time. I realized that unlike with http where I can intercept traffic and change whatever I want, when I’m using https I actually have to terminate the connection and man in the middle. Now that I’ve done this, maybe I can use this proxy that says it does that. This is why I need a man in the middle proxy:

$ http_proxy=localhost:8080 curl -s "" | xq .
  "Response": {
    "Errors": {
      "Error": {
        "Code": "MissingParameter",
        "Message": "The request must contain the parameter AWSAccessKeyId"
    "RequestID": "b9cdc176-4f60-42db-aca7-a8290732a377"

My proxy can’t change the headers if I’m using https, which makes sense. That’s the whole point.

Now that the cleanup is all done, here’s the first pull request for the aws-signature-proxy!

Now What?

This was one of the big things preventing me from auto generating the client. Now that I’ve gotten this working (except for the https support, which I need), I can pop back up to my original goal of dumping IAM information in such a way that makes it easier to detect misconfigurations.

I’m also not currently able to get IAM information using my proxy, which awscurl of course (the real MVP) helped me confirm was because of the lack of https support:

$ awscurl --service iam ''
requests.exceptions.ConnectionError: HTTPConnectionPool(host='', port=80): Max retries exceeded with url: /?Action=ListUsers&Version=2010-05-08 (Caused by NewConnectionError('<urllib3.connection.HTTPConnection object at 0x7f417a0ca950>: Failed to establish a new connection: [Errno 111] Connection refused',))

$ awscurl --service iam ''
(returns my iam users)

This means that next time my goal is to add https support, and if I have time I want to start doing more interesting things with it, like generating a CLI tool or an export tool.

Thanks for reading! Feedback and issues are appreciated on that project.