agent_gateway

An mTLS HTTP/2 CONNECT proxy that authorizes connections based on custom X.509 certificate extensions.

The proxy accepts incoming mTLS connections, extracts a custom extension value from the client certificate, and checks it against a TOML policy file to decide whether the client may connect to the requested destination. If allowed, it opens a raw TCP connection to the destination and tunnels data bidirectionally. The client is responsible for establishing its own TLS session to the destination through the tunnel.

Building

cargo build --release

Configuration

Copy config.example.toml to config.toml and edit it. Key sections:

[server] -- Listen address and TLS material.

Field	Required	Description
listen_addr	yes	host:port to bind (e.g. 0.0.0.0:8443)
tls_cert_path	yes	PEM server certificate
tls_key_path	yes	PEM private key for the server cert
client_ca_path	yes	PEM CA that issued client certificates

[policy] -- Maps certificate extension values to allowed destinations.

Field	Description
client_ext_oid	OID of the custom X.509 extension to extract (dotted notation)
rules[].extension_value	Value to match in the extension
rules[].allowed_destinations	List of host or host:port entries. Port defaults to 443 if omitted.

[observability] -- Logging, tracing, and metrics.

Field	Required	Description
log_level	yes	tracing filter (e.g. info, debug, agent_gateway=debug)
otlp_endpoint	no	OTLP gRPC endpoint for distributed tracing
metrics_bind	no	host:port for Prometheus metrics scraping

Running

agent_gateway --config config.toml

Shut down cleanly with Ctrl-C.

Client requirements

Clients must:

1. Connect over HTTP/2 with mTLS. Present a client certificate signed by the CA specified in client_ca_path. The certificate must contain a custom X.509 extension at the OID configured in policy.client_ext_oid, with a DER-encoded UTF8String value.

2. Use the HTTP CONNECT method. The request authority must be host:port (port defaults to 443 if omitted). Example using hyper:

   let req = Request::builder()
       .method(Method::CONNECT)
       .uri("api.example.com:443")
       .body(Empty::<Bytes>::new())?;

3. Handle TLS to the destination. After receiving 200 OK, the tunnel is an opaque TCP pipe. The client must perform its own TLS handshake with the destination through the tunnel.

Response codes

Status	Meaning
200	Tunnel established -- begin sending data
400	Malformed request (missing/invalid authority)
403	Policy denied the connection
405	Non-CONNECT method used
502	Could not reach the destination

Client certificate extension

The extension value is matched exactly (case-sensitive) against policy rules. The extension must be an X.509 extension at the configured OID containing a single DER-encoded ASN.1 UTF8String.

Example certificate generation with rcgen:

use rcgen::{CertificateParams, CustomExtension};

let oid: &[u64] = &[1, 3, 6, 1, 4, 1, 57264, 1, 1];
let value = der_encode_utf8_string("agent-alpha");
params.custom_extensions.push(
    CustomExtension::from_oid_content(oid, value),
);

Tests

cargo test

38 integration tests cover policy evaluation, destination normalization (including IPv6 and default port), config validation, TLS PKI generation, and proxy request parsing.