caldav: add DiscoverContextURL

internal/client.go

@@ -7,6 +7,7 @@ import (
 	"fmt"
 	"io"
 	"mime"
+	"net"
 	"net/http"
 	"net/url"
 	"path"
@@ -14,6 +15,42 @@ import (
 	"unicode"
 )
 
+// DiscoverContextURL performs a DNS-based CardDAV/CalDAV service discovery as
+// described in RFC 6352 section 11. It returns the URL to the CardDAV server.
+func DiscoverContextURL(ctx context.Context, service, domain string) (string, error) {
+	var resolver net.Resolver
+
+	// Only lookup TLS records, plaintext connections are insecure
+	_, addrs, err := resolver.LookupSRV(ctx, service+"s", "tcp", domain)
+	if dnsErr, ok := err.(*net.DNSError); ok {
+		if dnsErr.IsTemporary {
+			return "", err
+		}
+	} else if err != nil {
+		return "", err
+	}
+
+	if len(addrs) == 0 {
+		return "", fmt.Errorf("webdav: domain doesn't have an SRV record")
+	}
+	addr := addrs[0]
+
+	target := strings.TrimSuffix(addr.Target, ".")
+	if target == "" {
+		return "", fmt.Errorf("webdav: empty target in SRV record")
+	}
+
+	// TODO: perform a TXT lookup, check for a "path" key in the response
+	u := url.URL{Scheme: "https"}
+	if addr.Port == 443 {
+		u.Host = target
+	} else {
+		u.Host = fmt.Sprintf("%v:%v", target, addr.Port)
+	}
+	u.Path = "/.well-known/" + service
+	return u.String(), nil
+}
+
 // HTTPClient performs HTTP requests. It's implemented by *http.Client.
 type HTTPClient interface {
 	Do(req *http.Request) (*http.Response, error)