NGINX SXG module

Signed HTTP Exchange (SXG) support for nginx. Nginx will convert responses from the upstream application into SXG when client requests include the Accept: application/signed-exchange;v=b3 HTTP header with highest qvalue.

Installation

There are two options for installation: Debian package or build from source. See this article for more details.

If building from source and you have libsxg installed in a non-system directory, edit config to add ngx_module_incs=path/to/include and add -Lpath/to/lib to the existing ngx_module_libs, and launch nginx with LD_LIBRARY_PATH=path/to/lib.

Configuration

Nginx-SXG module requires configuration on nginx.

Directives

sxg

Activation flag of SXG module. This can be set or overriden inside server and location directives.

• on: Enable this plugin.
• off: Disable this plugin.

Default value is off.

sxg_certificate

Full path for the certificate file. The certificate requires all of the conditions below to match. This and all below directives can only be set inside server directives.

• Has CanSignHttpExchanges extension.
• Uses ECDSA256 or ECDSA384.

This directive is always required.

sxg_certificate_key

Full path for the private key for the certificate.

This directive is always required.

sxg_cert_url

URL for CBOR encoded certificate file. The protocol must be https.

This directive is always required.

sxg_validity_url

URL for the validity information file. It must be https and must be the same origin with the website.

This directive is always required.

sxg_max_payload

Maximum HTTP body size this module can generate SXG from. Default value is 67108864 (64 MiB).

sxg_cert_path

An absolute path in which nginx will generate and serve the CBOR-encoded certificate file. But make sure that the OCSP responder for the certificate is accessible from your nginx server to get OCSP responses. This directive is optional.

sxg_expiry_seconds

The life-span of generated SXG file in seconds. It must not be bigger than 604800 (1 week). This directive is optional. The default value is 86400 (1 day).

sxg_fallback_host

The hostname of fallback url of generated SXG file. This directive is optional. The default value is Host field parameter of HTTP request header.

Config Example

load_module "modules/ngx_http_sxg_filter_module.so";

http {
    upstream app {
        server 127.0.0.1:3000;
    }
    include       mime.types;
    default_type  application/octet-stream;
    subrequest_output_buffer_size   4096k;

    server {
        listen    80;
        server_name  example.com;

        sxg on;
        sxg_certificate     /path/to/certificate-ecdsa.pem;
        sxg_certificate_key /path/to/private-key-ecdsa.key;
        sxg_cert_url        https://cdn.test.com/example.com.cert.cbor;
        sxg_validity_url    https://example.com/validity/resource.msg;
        sxg_expiry_seconds 604800;
        sxg_fallback_host  example.com;

        location / {
            proxy_pass http://app;
        }
    }
}

Subresource support

nginx-sxg-module automatically includes signatures of subresources in its responses, allowing end users to prefetch it from distributor. When finding link: rel="preload" entry in HTTP response header from upstream, this plugin will collect the specified resource to the upstream and append rel="allowed-alt-sxg";header-integrity="sha256-...." to the original HTTP response automatically. This functionality is essential to subresource preloading for faster cross-site navigation.

• Preload URLs must be relative references of the path-absolute form, such as: Link: </app.js>;rel=preload;as=script.
• The server_name must match the externally-addressable host:port of the subresources.
• Their responses must be no larger than the configured subrequest_output_buffer_size.
• Their responses must come from an upstream server, such as via proxy_pass. The upstream may optionally be named via upstream.
• If using variables in proxy_pass, use $uri instead of $request_uri.

To ensure subresource prefetching works, verify that the header-integrity in:

curl -H 'Accept: application/signed-exchange;v=b3' https://url/of/page.html | dump-signedexchange -payload=false | grep Link:

equals the value of:

curl -H 'Accept: application/signed-exchange;v=b3' https://url/of/subresource.jpg | dump-signedexchange -headerIntegrity