go-gemini

go-gemini implements the Gemini protocol in Go.

It aims to provide an API similar to that of net/http to make it easy to develop Gemini clients and servers.

Examples

See examples/client and examples/server for an example client and server.

To run the examples:

go run -tags=example ./examples/server

Overview

A quick overview of the Gemini protocol:

1. Client opens connection
2. Server accepts connection
3. Client and server complete a TLS handshake
4. Client validates server certificate
5. Client sends request
6. Server sends response header
7. Server sends response body (only for successful responses)
8. Server closes connection
9. Client handles response

The way this is implemented in this package is like so:

1. Client makes a request with NewRequest. The client then sends the request with (*Client).Send(*Request) (*Response, error). The client then determines whether to trust the certificate (see TOFU).
2. Server recieves the request and constructs a response. The server calls the Serve(*ResponseWriter, *Request) method on the Handler field. The handler writes the response. The server then closes the connection.
3. Client recieves the response as a *Response. The client then handles the response.

TOFU

go-gemini makes it easy to implement Trust On First Use in your clients.

The default client loads known hosts from $XDG_DATA_HOME/gemini/known_hosts. If that is all you need, you can simply use the top-level Send function:

// Send uses the default client, which will load the default list of known hosts.
req := gemini.NewRequest("gemini://example.com")
gemini.Send(req)

Clients can also load their own list of known hosts:

client := &Client{}
if err := client.KnownHosts.LoadFrom("path/to/my/known_hosts"); err != nil {
	log.Fatal(err)
}

Clients can then specify how to trust certificates in the TrustCertificate field:

client.TrustCertificate = func(hostname string, cert *x509.Certificate, knownHosts *gemini.KnownHosts) error {
	// If the certificate is in the known hosts list, allow the connection
	return knownHosts.Lookup(hostname, cert)
}

Advanced clients can prompt the user for what to do when encountering an unknown certificate:

client.TrustCertificate = func(hostname string, cert *x509.Certificate, knownHosts *gemini.KnownHosts) error {
	err := knownHosts.Lookup(cert)
	if err != nil {
		switch err {
		case gemini.ErrCertificateNotTrusted:
			// Alert the user that the certificate is not trusted
			fmt.Printf("Warning: certificate for %s is not trusted!\n", hostname)
			fmt.Println("This could indicate a Man-in-the-Middle attack.")
		case gemini.ErrCertificateUnknown:
			// Prompt the user to trust the certificate
			if userTrustsCertificateTemporarily() {
				// Temporarily trust the certificate
				knownHosts.AddTemporary(hostname, cert)
				return nil
			} else if userTrustsCertificatePermanently() {
				// Add the certificate to the known hosts file
				knownHosts.Add(cert)
				return nil
			}
		}
	}
	return err
}

See examples/client for an example client.