smtp-check: Add a small utility to check SMTP setups

This patch adds a small utility called "smtp-check" that will perform basic
checks on the SMTP setup for the given domain.

Only basic things are implemented for now.

cmd/smtp-check/smtp-check.go

@@ -0,0 +1,70 @@
+// smtp-check is a command-line too for checking SMTP setups.
+package main
+
+import (
+	"crypto/tls"
+	"flag"
+	"log"
+	"net"
+	"net/smtp"
+
+	"blitiri.com.ar/go/chasquid/internal/tlsconst"
+
+	"golang.org/x/net/idna"
+)
+
+var (
+	port = flag.String("port", "smtp",
+		"port to use for connecting to the MX servers")
+)
+
+func main() {
+	flag.Parse()
+
+	domain := flag.Arg(0)
+	if domain == "" {
+		log.Fatal("Use: smtp-check <domain>")
+	}
+
+	domain, err := idna.ToASCII(domain)
+	if err != nil {
+		log.Fatal("IDNA conversion failed: %v", err)
+	}
+
+	mxs, err := net.LookupMX(domain)
+	if err != nil {
+		log.Fatalf("MX lookup: %v", err)
+	}
+
+	if len(mxs) == 0 {
+		log.Fatalf("MX lookup returned no results")
+	}
+
+	for _, mx := range mxs {
+		log.Printf("=== Testing MX: %2d  %s", mx.Pref, mx.Host)
+
+		c, err := smtp.Dial(mx.Host + ":" + *port)
+		if err != nil {
+			log.Fatal(err)
+		}
+
+		config := &tls.Config{
+			// Expect the server to have a certificate valid for the MX
+			// we're connecting to.
+			ServerName: mx.Host,
+		}
+		err = c.StartTLS(config)
+		if err != nil {
+			log.Fatal(err)
+		}
+
+		cstate, _ := c.TLSConnectionState()
+		log.Printf("%s - %s", tlsconst.VersionName(cstate.Version),
+			tlsconst.CipherSuiteName(cstate.CipherSuite))
+
+		log.Printf("")
+		c.Close()
+	}
+
+	log.Printf("=== Success")
+}