Go easy dkim signer

How to use

Generate keys and add txt-record to your domain

openssl genrsa -out private 1024

openssl rsa -in private -pubout -out public

sed '1d;$d' public | tr -d '\n' > spublic; echo "" >> spublic

Then make up a selector, i.e. myselector (could be any other string)
Add txt-record to DNS of your domain:

Key: myselector._domainkey
Value: v=DKIM1; k=rsa; p=<public key from spublic here>

Use the lib in your project

Add the lib to your go module: go get github.com/metaer/go-easy-dkim-signer
Use easydkim.Sign method like this:

Example 1

package main

import (
	"log"
	"net/smtp"

	"github.com/metaer/go-easy-dkim-signer/easydkim"
)

func main() {
	var err error
	domain := "example.com"
	from := "example@example.com"
	rcpt := "rcpt@example.com"
	selector := "myselector"
	privateFileKeyPath := "private"

	message := []byte("Subject: test subject\r\n" +
		"To: " + rcpt + "\r\n" +
		"From: " + from + "\r\n" +
		"Content-Type: text/plain; charset=\"utf-8\"\r\n" +
		"\r\n" +
		"Message body")

	message, err = easydkim.Sign(message, privateFileKeyPath, selector, domain)
	if err != nil {
		log.Fatal(err)
	}

	err = smtp.SendMail("localhost:1525", nil, from, []string{rcpt}, message)
	if err != nil {
		log.Fatal(err)
	}
}

Example 2

Install go get github.com/metaer/go-easy-dkim-signer
Install go get gopkg.in/gomail.v2 for object-oriented way to create an email message

package main

import (
	"bytes"
	"log"
	"net/smtp"

	"github.com/metaer/go-easy-dkim-signer/easydkim"
	"gopkg.in/gomail.v2"
)

func main() {
	var err error
	domain := "example.com"
	from := "example@example.com"
	rcpt := "rcpt@example.com"
	selector := "myselector"
	privateFileKeyPath := "private"

	m := gomail.NewMessage()
	m.SetHeader("From", from)
	m.SetHeader("To", rcpt)
	m.SetHeader("Subject", "My subject")
	m.SetBody("text/html", "My body")

	var buffer bytes.Buffer
	_, err = m.WriteTo(&buffer)
	if err != nil {
		log.Fatal(err)
	}
	var signedMessage []byte
	signedMessage, err = easydkim.Sign(buffer.Bytes(), privateFileKeyPath, selector, domain)
	if err != nil {
		log.Fatal(err)
	}
	err = smtp.SendMail("localhost:1525", nil, from, []string{rcpt}, signedMessage)
	if err != nil {
		log.Fatal(err)
	}
}

How to test locally

Run openssl genrsa -out private 2048
Run docker-compose up -d
Run example1 or example2
Open http://localhost:1580
Check dkim signature in message source