go-chasquid-smtp/cmd/smtp-check/smtp-check.go

// 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/spf"
	"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")
	skipTLSCheck = flag.Bool("skip_tls_check", false,
		"skip TLS check (useful if connections are blocked)")
)

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.Fatalf("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)

		ips, err := net.LookupIP(mx.Host)
		if err != nil {
			log.Fatal(err)
		}
		for _, ip := range ips {
			result, err := spf.CheckHost(ip, domain)
			if result != spf.Pass {
				log.Printf("SPF check != pass for IP %s: %s - %s",
					ip, result, err)
			}
		}

		if *skipTLSCheck {
			log.Printf("TLS check skipped")
		} else {
			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.Fatalf("TLS error: %v", err)
			}

			cstate, _ := c.TLSConnectionState()
			log.Printf("TLS OK: %s - %s", tlsconst.VersionName(cstate.Version),
				tlsconst.CipherSuiteName(cstate.CipherSuite))

			c.Close()
		}

		log.Printf("")
	}

	log.Printf("=== Success")
}