1 files changed, 66 insertions, 0 deletions
diff --git a/discover.go b/discover.go
new file mode 100644
index 0000000..08d4aa1
--- /dev/null
+++ b/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
+}