Auto-discover upstream SMTP and IMAP servers

Closes: https://todo.sr.ht/~sircmpwn/koushin/49

README.md

@@ -4,6 +4,12 @@
 
 ## Usage
 
+Assuming SRV DNS records are properly set up (see [RFC 6186]):
+
+    go run example.org
+
+To manually specify upstream servers:
+
     go run ./cmd/koushin imaps://mail.example.org:993 smtps://mail.example.org:465
 
 See `-h` for more information.
@@ -55,6 +61,7 @@ Send patches on the [mailing list], report bugs on the [issue tracker].
 
 MIT
 
+[RFC 6186]: https://tools.ietf.org/html/rfc6186
 [Go plugin helpers]: https://godoc.org/git.sr.ht/~emersion/koushin#GoPlugin
 [mailing list]: https://lists.sr.ht/~sircmpwn/koushin
 [issue tracker]: https://todo.sr.ht/~sircmpwn/koushin

discover.go

@@ -0,0 +1,66 @@
+package koushin
+
+import (
+	"fmt"
+	"net"
+	"net/url"
+	"strings"
+)
+
+func discoverTCP(service, name string) (string, error) {
+	_, addrs, err := net.LookupSRV(service, "tcp", name)
+	if dnsErr, ok := err.(*net.DNSError); ok {
+		if dnsErr.IsTemporary {
+			return "", err
+		}
+	} else if err != nil {
+		return "", err
+	}
+
+	if len(addrs) == 0 {
+		return "", nil
+	}
+	addr := addrs[0]
+
+	target := strings.TrimSuffix(addr.Target, ".")
+	if target == "" {
+		return "", nil
+	}
+
+	return fmt.Sprintf("%v:%v", target, addr.Port), nil
+}
+
+// discoverIMAP performs a DNS-based IMAP service discovery, as defined in
+// RFC 6186 section 3.2.
+func discoverIMAP(domain string) (*url.URL, error) {
+	imapsHost, err := discoverTCP("imaps", domain)
+	if err != nil {
+		return nil, err
+	}
+	if imapsHost != "" {
+		return &url.URL{Scheme: "imaps", Host: imapsHost}, nil
+	}
+
+	imapHost, err := discoverTCP("imap", domain)
+	if err != nil {
+		return nil, err
+	}
+	if imapHost != "" {
+		return &url.URL{Scheme: "imap", Host: imapHost}, nil
+	}
+
+	return nil, fmt.Errorf("IMAP service discovery not configured for domain %q", domain)
+}
+
+// discoverSMTP performs a DNS-based SMTP submission service discovery, as
+// defined in RFC 6186 section 3.1.
+func discoverSMTP(domain string) (*url.URL, error) {
+	host, err := discoverTCP("submission", domain)
+	if err != nil {
+		return nil, err
+	}
+	if host == "" {
+		return nil, fmt.Errorf("SMTP service discovery not configured for domain %q", domain)
+	}
+	return &url.URL{Scheme: "smtp", Host: host}, nil
+}

server.go

@@ -108,12 +108,18 @@ func (s *Server) parseIMAPUpstream() error {
 		return fmt.Errorf("failed to parse upstream IMAP server: %v", err)
 	}
 
+	if u.Scheme == "" {
+		u, err = discoverIMAP(u.Host)
+		if err != nil {
+			return fmt.Errorf("failed to discover IMAP server: %v", err)
+		}
+	}
+
 	s.imap.host = u.Host
 	switch u.Scheme {
 	case "imap":
 		// This space is intentionally left blank
-	case "imaps", "":
-		// TODO: auto-discovery for empty scheme
+	case "imaps":
 		s.imap.tls = true
 	case "imap+insecure":
 		s.imap.insecure = true
@@ -133,12 +139,19 @@ func (s *Server) parseSMTPUpstream() error {
 		return fmt.Errorf("failed to parse upstream SMTP server: %v", err)
 	}
 
+	if u.Scheme == "" {
+		u, err = discoverSMTP(u.Host)
+		if err != nil {
+			s.e.Logger.Printf("Failed to discover SMTP server: %v", err)
+			return nil
+		}
+	}
+
 	s.smtp.host = u.Host
 	switch u.Scheme {
 	case "smtp":
 		// This space is intentionally left blank
-	case "smtps", "":
-		// TODO: auto-discovery for empty scheme
+	case "smtps":
 		s.smtp.tls = true
 	case "smtp+insecure":
 		s.smtp.insecure = true