Add SystemD unit file

Handle signals and gracefully shut down, waiting for clients

.github/dependabot.yml

@@ -0,0 +1,11 @@
+version: 2
+updates:
+  - package-ecosystem: "gomod"
+    directory: "/"
+    schedule:
+      interval: "daily"
+
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "daily"

.github/workflows/codeql-analysis.yml

@@ -0,0 +1,66 @@
+name: "CodeQL"
+
+on:
+  push:
+    branches: [master]
+  pull_request:
+    # The branches below must be a subset of the branches above
+    branches: [master]
+  schedule:
+    - cron: '0 15 * * 5'
+
+jobs:
+  analyze:
+    name: Analyze
+    runs-on: ubuntu-latest
+
+    strategy:
+      fail-fast: false
+      matrix:
+        # Override automatic language detection by changing the below list
+        # Supported options are ['csharp', 'cpp', 'go', 'java', 'javascript', 'python']
+        language: ['go']
+        # Learn more...
+        # https://docs.github.com/en/github/finding-security-vulnerabilities-and-errors-in-your-code/configuring-code-scanning#overriding-automatic-language-detection
+
+    steps:
+    - name: Checkout repository
+      uses: actions/checkout@v2
+      with:
+        # We must fetch at least the immediate parents so that if this is
+        # a pull request then we can checkout the head.
+        fetch-depth: 2
+
+    # If this run was triggered by a pull request event, then checkout
+    # the head of the pull request instead of the merge commit.
+    - run: git checkout HEAD^2
+      if: ${{ github.event_name == 'pull_request' }}
+
+    # Initializes the CodeQL tools for scanning.
+    - name: Initialize CodeQL
+      uses: github/codeql-action/init@v1
+      with:
+        languages: ${{ matrix.language }}
+        # If you wish to specify custom queries, you can do so here or in a config file.
+        # By default, queries listed here will override any specified in a config file. 
+        # Prefix the list here with "+" to use these queries and those in the config file.
+        # queries: ./path/to/local/query, your-org/your-repo/queries@main
+
+    # Autobuild attempts to build any compiled languages  (C/C++, C#, or Java).
+    # If this step fails, then you should remove it and run the build manually (see below)
+    - name: Autobuild
+      uses: github/codeql-action/autobuild@v1
+
+    # ℹ️ Command-line programs to run using the OS shell.
+    # 📚 https://git.io/JvXDl
+
+    # ✏️ If the Autobuild fails above, remove it and uncomment the following three lines
+    #    and modify them (or add more) to build your code if your project
+    #    uses a compiled language
+
+    #- run: |
+    #   make bootstrap
+    #   make release
+
+    - name: Perform CodeQL Analysis
+      uses: github/codeql-action/analyze@v1

.github/workflows/go.yml

@@ -0,0 +1,24 @@
+name: Go
+on: [push]
+jobs:
+
+  build:
+    name: Build
+    runs-on: ubuntu-latest
+    steps:
+
+    - name: Set up Go 1.15
+      uses: actions/setup-go@v2.1.3
+      with:
+        go-version: 1.15
+      id: go
+
+    - name: Check out code into the Go module directory
+      uses: actions/checkout@v1
+
+    - name: Get dependencies
+      run: |
+        go get -v -t -d ./...
+
+    - name: Build
+      run: go build -v .

.github/workflows/release.yaml

@@ -0,0 +1,30 @@
+name: Release Go Binaries
+
+on: 
+  release:
+    types: [created]
+
+jobs:
+  releases-matrix:
+    name: Release Go Binary
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        goos: [freebsd, linux, windows]
+        goarch: ["386", amd64]
+    steps:
+    - uses: actions/checkout@v2
+    
+    - name: Set APP_VERSION env
+      run: echo APP_VERSION=$(echo ${GITHUB_REF} | rev | cut -d'/' -f 1 | rev ) >> ${GITHUB_ENV}
+    - name: Set BUILD_TIME env
+      run: echo BUILD_TIME=$(date) >> ${GITHUB_ENV}
+      
+    - uses: wangyoucao577/go-release-action@v1.15
+      with:
+        github_token: ${{ secrets.GITHUB_TOKEN }}
+        goos: ${{ matrix.goos }}
+        goarch: ${{ matrix.goarch }}
+        goversion: "https://golang.org/dl/go1.15.8.linux-amd64.tar.gz"
+        extra_files: LICENSE README.md smtprelay.ini
+        ldflags: -s -w -X "main.appVersion=${{ env.APP_VERSION }}" -X "main.buildTime=${{ env.BUILD_TIME }}"

README.md

@@ -1,10 +1,31 @@
-# smtp-proxy
+# smtprelay
+
+[![Go Report Card](https://goreportcard.com/badge/github.com/decke/smtprelay)](https://goreportcard.com/report/github.com/decke/smtprelay)
+
+Simple Golang based SMTP relay/proxy server that accepts mail via SMTP
+and forwards it directly to another SMTP server.
+
+
+## Why another SMTP server?
+
+Outgoing mails are usually send via SMTP to an MTA (Mail Transfer Agent)
+which is one of Postfix, Exim, Sendmail or OpenSMTPD on UNIX/Linux in most
+cases. You really don't want to setup and maintain any of those full blown
+kitchensinks yourself because they are complex, fragile and hard to
+configure.
+
+My use case is simple. I need to send automatically generated mails from
+cron via msmtp/sSMTP/dma, mails from various services and network printers
+to GMail without giving away my GMail credentials to each device which
+produces mail.
 
-Simple Go SMTP relay/proxy server that accepts mails via SMTP
-and forwards directly to another SMTP server.
 
 ## Main features
 
-* STARTTLS/TLS support
+* Supports SMTPS/TLS (465), STARTTLS (587) and unencrypted SMTP (25)
+* Checks for sender, receiver, client IP
+* Authentication support with file (LOGIN, PLAIN)
+* Enforce encryption for authentication
+* Forwards all mail to a smarthost (GMail, MailGun or any other SMTP server)
+* Small codebase
 * IPv6 support
-* Forward to GMail, MailGun or any other SMTP server

auth.go

@@ -0,0 +1,100 @@
+package main
+
+import (
+	"bufio"
+	"errors"
+	"os"
+	"strings"
+
+	"golang.org/x/crypto/bcrypt"
+)
+
+var (
+	filename string
+)
+
+type AuthUser struct {
+	username         string
+	passwordHash     string
+	allowedAddresses []string
+}
+
+func AuthLoadFile(file string) error {
+	f, err := os.Open(file)
+	if err != nil {
+		return err
+	}
+	f.Close()
+
+	filename = file
+	return nil
+}
+
+func AuthReady() bool {
+	return (filename != "")
+}
+
+// Split a string and ignore empty results
+// https://stackoverflow.com/a/46798310/119527
+func splitstr(s string, sep rune) []string {
+	return strings.FieldsFunc(s, func(c rune) bool { return c == sep })
+}
+
+func parseLine(line string) *AuthUser {
+	parts := strings.Fields(line)
+
+	if len(parts) < 2 || len(parts) > 3 {
+		return nil
+	}
+
+	user := AuthUser{
+		username:         parts[0],
+		passwordHash:     parts[1],
+		allowedAddresses: nil,
+	}
+
+	if len(parts) >= 3 {
+		user.allowedAddresses = splitstr(parts[2], ',')
+	}
+
+	return &user
+}
+
+func AuthFetch(username string) (*AuthUser, error) {
+	if !AuthReady() {
+		return nil, errors.New("Authentication file not specified. Call LoadFile() first")
+	}
+
+	file, err := os.Open(filename)
+	if err != nil {
+		return nil, err
+	}
+	defer file.Close()
+
+	scanner := bufio.NewScanner(file)
+	for scanner.Scan() {
+		user := parseLine(scanner.Text())
+		if user == nil {
+			continue
+		}
+
+		if strings.ToLower(username) != strings.ToLower(user.username) {
+			continue
+		}
+
+		return user, nil
+	}
+
+	return nil, errors.New("User not found")
+}
+
+func AuthCheckPassword(username string, secret string) error {
+	user, err := AuthFetch(username)
+	if err != nil {
+		return err
+	}
+	if bcrypt.CompareHashAndPassword([]byte(user.passwordHash), []byte(secret)) == nil {
+		return nil
+	}
+	return errors.New("Password invalid")
+}

auth_test.go

@@ -0,0 +1,89 @@
+package main
+
+import (
+	"testing"
+)
+
+func stringsEqual(a, b []string) bool {
+	if len(a) != len(b) {
+		return false
+	}
+	for i := range a {
+		if a[i] != b[i] {
+			return false
+		}
+	}
+	return true
+}
+
+func TestParseLine(t *testing.T) {
+	var tests = []struct {
+		name       string
+		expectFail bool
+		line       string
+		username   string
+		addrs      []string
+	}{
+		{
+			name:       "Empty line",
+			expectFail: true,
+			line:       "",
+		},
+		{
+			name:       "Too few fields",
+			expectFail: true,
+			line:       "joe",
+		},
+		{
+			name:       "Too many fields",
+			expectFail: true,
+			line:       "joe xxx joe@example.com whatsthis",
+		},
+		{
+			name:     "Normal case",
+			line:     "joe xxx joe@example.com",
+			username: "joe",
+			addrs:    []string{"joe@example.com"},
+		},
+		{
+			name:     "No allowed addrs given",
+			line:     "joe xxx",
+			username: "joe",
+			addrs:    []string{},
+		},
+		{
+			name:     "Trailing comma",
+			line:     "joe xxx joe@example.com,",
+			username: "joe",
+			addrs:    []string{"joe@example.com"},
+		},
+		{
+			name:     "Multiple allowed addrs",
+			line:     "joe xxx joe@example.com,@foo.example.com",
+			username: "joe",
+			addrs:    []string{"joe@example.com", "@foo.example.com"},
+		},
+	}
+
+	for i, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			user := parseLine(test.line)
+			if user == nil {
+				if !test.expectFail {
+					t.Errorf("parseLine() returned nil unexpectedly")
+				}
+				return
+			}
+
+			if user.username != test.username {
+				t.Errorf("Testcase %d: Incorrect username: expected %v, got %v",
+					i, test.username, user.username)
+			}
+
+			if !stringsEqual(user.allowedAddresses, test.addrs) {
+				t.Errorf("Testcase %d: Incorrect addresses: expected %v, got %v",
+					i, test.addrs, user.allowedAddresses)
+			}
+		})
+	}
+}

cmd/README.md

@@ -0,0 +1,6 @@
+
+To run the hasher, do like this
+
+```bash
+$ go run hasher.go hunter2
+```

cmd/hasher.go

@@ -0,0 +1,22 @@
+package main
+
+import (
+	"fmt"
+	"os"
+
+	"golang.org/x/crypto/bcrypt"
+)
+
+func main() {
+	if len(os.Args) != 2 {
+		fmt.Fprintln(os.Stderr, "Usage: hasher PASSWORD")
+		os.Exit(1)
+	}
+	password := os.Args[1]
+
+	hash, err := bcrypt.GenerateFromPassword([]byte(password), bcrypt.DefaultCost)
+	if err != nil {
+		fmt.Fprintln(os.Stderr, "Error generating hash: %s", err)
+	}
+	fmt.Println(string(hash))
+}

config.go

@@ -0,0 +1,146 @@
+package main
+
+import (
+	"flag"
+	"net"
+	"regexp"
+	"net/smtp"
+
+	"github.com/vharitonsky/iniflags"
+	"github.com/sirupsen/logrus"
+)
+
+var (
+	appVersion = "unknown"
+	buildTime  = "unknown"
+)
+
+var (
+	logFile           = flag.String("logfile", "", "Path to logfile")
+	logFormat         = flag.String("log_format", "default", "Log output format")
+	logLevel          = flag.String("log_level", "info", "Minimum log level to output")
+	hostName          = flag.String("hostname", "localhost.localdomain", "Server hostname")
+	welcomeMsg        = flag.String("welcome_msg", "", "Welcome message for SMTP session")
+	listen            = flag.String("listen", "127.0.0.1:25 [::1]:25", "Address and port to listen for incoming SMTP")
+	localCert         = flag.String("local_cert", "", "SSL certificate for STARTTLS/TLS")
+	localKey          = flag.String("local_key", "", "SSL private key for STARTTLS/TLS")
+	localForceTLS     = flag.Bool("local_forcetls", false, "Force STARTTLS (needs local_cert and local_key)")
+	allowedNetsStr    = flag.String("allowed_nets", "127.0.0.0/8 ::1/128", "Networks allowed to send mails")
+	allowedNets       = []*net.IPNet{}
+	allowedSenderStr  = flag.String("allowed_sender", "", "Regular expression for valid FROM EMail addresses")
+	allowedSender     *regexp.Regexp
+	allowedRecipStr   = flag.String("allowed_recipients", "", "Regular expression for valid TO EMail addresses")
+	allowedRecipients *regexp.Regexp
+	allowedUsers      = flag.String("allowed_users", "", "Path to file with valid users/passwords")
+	remoteHost        = flag.String("remote_host", "", "Outgoing SMTP server")
+	remoteUser        = flag.String("remote_user", "", "Username for authentication on outgoing SMTP server")
+	remotePass        = flag.String("remote_pass", "", "Password for authentication on outgoing SMTP server")
+	remoteAuthStr     = flag.String("remote_auth", "none", "Auth method on outgoing SMTP server (none, plain, login)")
+	remoteAuth        smtp.Auth
+	remoteSender      = flag.String("remote_sender", "", "Sender e-mail address on outgoing SMTP server")
+	versionInfo       = flag.Bool("version", false, "Show version information")
+)
+
+
+func setupAllowedNetworks() {
+	for _, netstr := range splitstr(*allowedNetsStr, ' ') {
+		baseIP, allowedNet, err := net.ParseCIDR(netstr)
+		if err != nil {
+			log.WithField("netstr", netstr).
+				WithError(err).
+				Fatal("Invalid CIDR notation in allowed_nets")
+		}
+
+		// Reject any network specification where any host bits are set,
+		// meaning the address refers to a host and not a network.
+		if !allowedNet.IP.Equal(baseIP) {
+			log.WithFields(logrus.Fields{
+				"given_net": netstr,
+				"proper_net": allowedNet,
+			}).Fatal("Invalid network in allowed_nets (host bits set)")
+		}
+
+		allowedNets = append(allowedNets, allowedNet)
+	}
+}
+
+func setupAllowedPatterns() {
+	var err error
+
+	if (*allowedSenderStr != "") {
+		allowedSender, err = regexp.Compile(*allowedSenderStr)
+		if err != nil {
+			log.WithField("allowed_sender", *allowedSenderStr).
+				WithError(err).
+				Fatal("allowed_sender pattern invalid")
+		}
+	}
+
+	if (*allowedRecipStr != "") {
+		allowedRecipients, err = regexp.Compile(*allowedRecipStr)
+		if err != nil {
+			log.WithField("allowed_recipients", *allowedRecipStr).
+				WithError(err).
+				Fatal("allowed_recipients pattern invalid")
+		}
+	}
+}
+
+
+func setupRemoteAuth() {
+	logger := log.WithField("remote_auth", *remoteAuthStr)
+
+	// Remote auth disabled?
+	if *remoteAuthStr == "" || *remoteAuthStr == "none" {
+		if *remoteUser != "" {
+			logger.Fatal("remote_user given but not used")
+		}
+		if *remotePass != "" {
+			logger.Fatal("remote_pass given but not used")
+		}
+
+		// No auth; use empty default
+		return
+	}
+
+	// We need a username, password, and remote host
+	if *remoteUser == "" {
+		logger.Fatal("remote_user required but empty")
+	}
+	if *remotePass == "" {
+		logger.Fatal("remote_pass required but empty")
+	}
+	if *remoteHost == "" {
+		logger.Fatal("remote_auth without remote_host is pointless")
+	}
+
+	host, _, err := net.SplitHostPort(*remoteHost)
+	if err != nil {
+		logger.WithField("remote_host", *remoteHost).
+			   Fatal("Invalid remote_host")
+	}
+
+	switch *remoteAuthStr {
+	case "plain":
+		remoteAuth = smtp.PlainAuth("", *remoteUser, *remotePass, host)
+	case "login":
+		remoteAuth = LoginAuth(*remoteUser, *remotePass)
+	default:
+		logger.Fatal("Invalid remote_auth type")
+	}
+}
+
+func ConfigLoad() {
+	iniflags.Parse()
+
+	// Set up logging as soon as possible
+	setupLogger()
+
+	if (*remoteHost == "") {
+		log.Warn("remote_host not set; mail will not be forwarded!")
+	}
+
+	setupAllowedNetworks()
+	setupAllowedPatterns()
+	setupRemoteAuth()
+}

go.mod

@@ -1,6 +1,11 @@
-module code.bluelife.at/decke/smtp-proxy
+module github.com/decke/smtprelay
 
 require (
-	github.com/chrj/smtpd v0.1.1
+	github.com/chrj/smtpd v0.3.0
+	github.com/google/uuid v1.2.0
+	github.com/sirupsen/logrus v1.8.1
 	github.com/vharitonsky/iniflags v0.0.0-20180513140207-a33cd0b5f3de
+	golang.org/x/crypto v0.0.0-20201221181555-eec23a3978ad
 )
+
+go 1.13

go.sum

@@ -1,5 +1,24 @@
-github.com/chrj/smtpd v0.1.1 h1:Jk9ZimOh4njDpMbDLPmdQX+x9AUYGrfE68EyjuQDPjk=
-github.com/chrj/smtpd v0.1.1/go.mod h1:jt4ydELuZmqhn9hn3YpEPV1dY00aOB+Q1nWXnBDFKeY=
+github.com/chrj/smtpd v0.3.0 h1:cw1LSHDOz7N3XbkcZSF/bue9dh7ATKk5ZksfBztV6b0=
+github.com/chrj/smtpd v0.3.0/go.mod h1:1hmG9KbrE10JG1SmvG79Krh4F6713oUrw2+gRp1oSYk=
+github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c=
+github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
 github.com/eaigner/dkim v0.0.0-20150301120808-6fe4a7ee9cfb/go.mod h1:FSCIHbrqk7D01Mj8y/jW+NS1uoCerr+ad+IckTHTFf4=
+github.com/google/uuid v1.2.0 h1:qJYtXnJRWmpe7m/3XlyhrsLrEURqHRM2kxzoxXqyUDs=
+github.com/google/uuid v1.2.0/go.mod h1:TIyPZe4MgqvfeYDBFedMoGGpEw/LqOeaOT+nhxU+yHo=
+github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
+github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
+github.com/sirupsen/logrus v1.8.1 h1:dJKuHgqk1NNQlqoA6BTlM1Wf9DOH3NBjQyu0h9+AZZE=
+github.com/sirupsen/logrus v1.8.1/go.mod h1:yWOB1SBYBC5VeMP7gHvWumXLIWorT60ONWic61uBYv0=
+github.com/stretchr/testify v1.2.2 h1:bSDNvY7ZPG5RlJ8otE/7V6gMiyenm9RtJ7IUVIAoJ1w=
+github.com/stretchr/testify v1.2.2/go.mod h1:a8OnRcib4nhh0OaRAV+Yts87kKdq0PP7pXfy6kDkUVs=
 github.com/vharitonsky/iniflags v0.0.0-20180513140207-a33cd0b5f3de h1:fkw+7JkxF3U1GzQoX9h69Wvtvxajo5Rbzy6+YMMzPIg=
 github.com/vharitonsky/iniflags v0.0.0-20180513140207-a33cd0b5f3de/go.mod h1:irMhzlTz8+fVFj6CH2AN2i+WI5S6wWFtK3MBCIxIpyI=
+golang.org/x/crypto v0.0.0-20190308221718-c2843e01d9a2/go.mod h1:djNgcEr1/C05ACkg1iLfiJU5Ep61QUkGW8qpdssI0+w=
+golang.org/x/crypto v0.0.0-20201221181555-eec23a3978ad h1:DN0cp81fZ3njFcrLCytUHRSUkqBjfTo4Tx9RJTWs0EY=
+golang.org/x/crypto v0.0.0-20201221181555-eec23a3978ad/go.mod h1:jdWPYTVW3xRLrWPugEBEK3UY2ZEsg3UU495nc5E+M+I=
+golang.org/x/net v0.0.0-20190404232315-eb5bcb51f2a3/go.mod h1:t9HGtf8HONx5eT2rtn7q6eTqICYqUVnKs3thJo3Qplg=
+golang.org/x/sys v0.0.0-20190215142949-d0b11bdaac8a/go.mod h1:STP8DvDyc/dI5b8T5hshtkjS+E42TnysNCUPdjciGhY=
+golang.org/x/sys v0.0.0-20191026070338-33540a1f6037 h1:YyJpGZS1sBuBCzLAR1VEpK193GlqGZbnPFnPV/5Rsb4=
+golang.org/x/sys v0.0.0-20191026070338-33540a1f6037/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
+golang.org/x/term v0.0.0-20201117132131-f5c789dd3221/go.mod h1:Nr5EML6q2oocZ2LXRh80K7BxOlk5/8JxuGnuhpl+muw=
+golang.org/x/text v0.3.0/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=

logger.go

@@ -0,0 +1,60 @@
+package main
+
+import (
+	"fmt"
+	"os"
+	"time"
+
+	"github.com/sirupsen/logrus"
+)
+
+var (
+	log *logrus.Logger
+)
+
+func setupLogger() {
+	log = logrus.New()
+
+	// Handle logfile
+	if (*logFile == "") {
+		log.SetOutput(os.Stderr)
+	} else {
+		writer, err := os.OpenFile(*logFile, os.O_CREATE|os.O_RDWR|os.O_APPEND, 0600)
+		if err != nil {
+			fmt.Printf("cannot open log file: %s\n", err)
+			os.Exit(1)
+		}
+
+		log.SetOutput(writer)
+	}
+
+	// Handle log_format
+	switch *logFormat {
+	case "json":
+		log.SetFormatter(&logrus.JSONFormatter{
+			TimestampFormat:   time.RFC3339Nano,
+			DisableHTMLEscape: true,
+		})
+	case "plain":
+		log.SetFormatter(&logrus.TextFormatter{
+			DisableTimestamp: true,
+		})
+	case "", "default":
+		log.SetFormatter(&logrus.TextFormatter{
+			FullTimestamp: true,
+		})
+	default:
+		fmt.Fprintf(os.Stderr, "Invalid log_format: %s\n", *logFormat)
+		os.Exit(1)
+	}
+
+	// Handle log_level
+	level, err := logrus.ParseLevel(*logLevel)
+	if err != nil {
+		level = logrus.InfoLevel
+
+		log.WithField("given_level", *logLevel).
+			Warn("could not parse log level, defaulting to 'info'")
+	}
+	log.SetLevel(level)
+}

main.go

@@ -2,91 +2,381 @@ package main
 
 import (
 	"crypto/tls"
-	"flag"
-	"log"
+	"fmt"
 	"net"
-	"net/smtp"
+	"net/textproto"
+	"os"
+	"os/signal"
 	"strings"
-	"time"
+	"syscall"
 
 	"github.com/chrj/smtpd"
-	"github.com/vharitonsky/iniflags"
+	"github.com/google/uuid"
+	"github.com/sirupsen/logrus"
 )
 
-var (
-	hostName   = flag.String("hostname", "localhost.localdomain", "Server hostname")
-	welcomeMsg = flag.String("welcome_msg", "", "Welcome message for SMTP session")
-	listen     = flag.String("listen", "127.0.0.1:25 [::1]:25", "Address and port to listen for incoming SMTP")
-	localCert  = flag.String("local_cert", "", "SSL certificate for STARTTLS/TLS")
-	localKey   = flag.String("local_key", "", "SSL private key for STARTTLS/TLS")
-	localForceTLS = flag.Bool("local_forcetls", false, "Force STARTTLS (needs local_cert and local_key)")
-	remoteHost = flag.String("remote_host", "smtp.gmail.com:587", "Outgoing SMTP server")
-	remoteUser = flag.String("remote_user", "", "Username for authentication on outgoing SMTP server")
-	remotePass = flag.String("remote_pass", "", "Password for authentication on outgoing SMTP server")
-)
+func connectionChecker(peer smtpd.Peer) error {
+	// This can't panic because we only have TCP listeners
+	peerIP := peer.Addr.(*net.TCPAddr).IP
 
-func handler(peer smtpd.Peer, env smtpd.Envelope) error {
-
-	var auth smtp.Auth
-	host, _, _ := net.SplitHostPort(*remoteHost)
-
-	if *remoteUser != "" && *remotePass != "" {
-		auth = smtp.PlainAuth("", *remoteUser, *remotePass, host)
+	if len(allowedNets) == 0 {
+		// Special case: empty string means allow everything
+		return nil
 	}
 
-	return smtp.SendMail(
+	for _, allowedNet := range allowedNets {
+		if allowedNet.Contains(peerIP) {
+			return nil
+		}
+	}
+
+	log.WithFields(logrus.Fields{
+		"ip": peerIP,
+	}).Warn("Connection refused from address outside of allowed_nets")
+	return smtpd.Error{Code: 421, Message: "Denied"}
+}
+
+func addrAllowed(addr string, allowedAddrs []string) bool {
+	if allowedAddrs == nil {
+		// If absent, all addresses are allowed
+		return true
+	}
+
+	addr = strings.ToLower(addr)
+
+	// Extract optional domain part
+	domain := ""
+	if idx := strings.LastIndex(addr, "@"); idx != -1 {
+		domain = strings.ToLower(addr[idx+1:])
+	}
+
+	// Test each address from allowedUsers file
+	for _, allowedAddr := range allowedAddrs {
+		allowedAddr = strings.ToLower(allowedAddr)
+
+		// Three cases for allowedAddr format:
+		if idx := strings.Index(allowedAddr, "@"); idx == -1 {
+			// 1. local address (no @) -- must match exactly
+			if allowedAddr == addr {
+				return true
+			}
+		} else {
+			if idx != 0 {
+				// 2. email address (user@domain.com) -- must match exactly
+				if allowedAddr == addr {
+					return true
+				}
+			} else {
+				// 3. domain (@domain.com) -- must match addr domain
+				allowedDomain := allowedAddr[idx+1:]
+				if allowedDomain == domain {
+					return true
+				}
+			}
+		}
+	}
+
+	return false
+}
+
+func senderChecker(peer smtpd.Peer, addr string) error {
+	// check sender address from auth file if user is authenticated
+	if *allowedUsers != "" && peer.Username != "" {
+		user, err := AuthFetch(peer.Username)
+		if err != nil {
+			// Shouldn't happen: authChecker already validated username+password
+			log.WithFields(logrus.Fields{
+				"peer": peer.Addr,
+				"username": peer.Username,
+			}).WithError(err).Warn("could not fetch auth user")
+			return smtpd.Error{Code: 451, Message: "Bad sender address"}
+		}
+
+		if !addrAllowed(addr, user.allowedAddresses) {
+			log.WithFields(logrus.Fields{
+				"peer": peer.Addr,
+				"username": peer.Username,
+				"sender_address": addr,
+			}).Warn("sender address not allowed for authenticated user")
+			return smtpd.Error{Code: 451, Message: "Bad sender address"}
+		}
+	}
+
+	if allowedSender == nil {
+		// Any sender is permitted
+		return nil
+	}
+
+	if allowedSender.MatchString(addr) {
+		// Permitted by regex
+		return nil
+	}
+
+	log.WithFields(logrus.Fields{
+		"sender_address": addr,
+		"peer": peer.Addr,
+	}).Warn("sender address not allowed by allowed_sender pattern")
+	return smtpd.Error{Code: 451, Message: "Bad sender address"}
+}
+
+func recipientChecker(peer smtpd.Peer, addr string) error {
+	if allowedRecipients == nil {
+		// Any recipient is permitted
+		return nil
+	}
+
+	if allowedRecipients.MatchString(addr) {
+		// Permitted by regex
+		return nil
+	}
+
+	log.WithFields(logrus.Fields{
+		"peer": peer.Addr,
+		"recipient_address": addr,
+	}).Warn("recipient address not allowed by allowed_recipients pattern")
+	return smtpd.Error{Code: 451, Message: "Bad recipient address"}
+}
+
+func authChecker(peer smtpd.Peer, username string, password string) error {
+	err := AuthCheckPassword(username, password)
+	if err != nil {
+		log.WithFields(logrus.Fields{
+			"peer": peer.Addr,
+			"username": username,
+		}).WithError(err).Warn("auth error")
+		return smtpd.Error{Code: 535, Message: "Authentication credentials invalid"}
+	}
+	return nil
+}
+
+func mailHandler(peer smtpd.Peer, env smtpd.Envelope) error {
+	peerIP := ""
+	if addr, ok := peer.Addr.(*net.TCPAddr); ok {
+		peerIP = addr.IP.String()
+	}
+
+	logger := log.WithFields(logrus.Fields{
+		"from": env.Sender,
+		"to": env.Recipients,
+		"peer": peerIP,
+		"host": *remoteHost,
+		"uuid": generateUUID(),
+	})
+
+	if (*remoteHost == "") {
+		logger.Warning("remote_host not set; discarding mail")
+		return nil
+	}
+
+	logger.Info("delivering mail from peer using smarthost")
+
+	env.AddReceivedLine(peer)
+
+	var sender string
+
+	if *remoteSender == "" {
+		sender = env.Sender
+	} else {
+		sender = *remoteSender
+	}
+
+	err := SendMail(
 		*remoteHost,
-		auth,
-		env.Sender,
+		remoteAuth,
+		sender,
 		env.Recipients,
 		env.Data,
 	)
+	if err != nil {
+		var smtpError smtpd.Error
+
+		switch err.(type) {
+		case *textproto.Error:
+			err := err.(*textproto.Error)
+			smtpError = smtpd.Error{Code: err.Code, Message: err.Msg}
+
+			logger.WithFields(logrus.Fields{
+				"err_code": err.Code,
+				"err_msg": err.Msg,
+			}).Error("delivery failed")
+		default:
+			smtpError = smtpd.Error{Code: 554, Message: "Forwarding failed"}
+
+			logger.WithError(err).
+				Error("delivery failed")
+		}
+
+		return smtpError
+	}
+
+	logger.Debug("delivery successful")
+	return nil
+}
+
+func generateUUID() string {
+	uniqueID, err := uuid.NewRandom()
+
+	if err != nil {
+		log.WithError(err).
+			Error("could not generate UUIDv4")
+
+		return ""
+	}
+
+	return uniqueID.String()
+}
+
+func getTLSConfig() *tls.Config {
+	// Ciphersuites as defined in stock Go but without 3DES and RC4
+	// https://golang.org/src/crypto/tls/cipher_suites.go
+	var tlsCipherSuites = []uint16{
+		tls.TLS_AES_128_GCM_SHA256,
+		tls.TLS_AES_256_GCM_SHA384,
+		tls.TLS_CHACHA20_POLY1305_SHA256,
+		tls.TLS_ECDHE_RSA_WITH_CHACHA20_POLY1305_SHA256,
+		tls.TLS_ECDHE_ECDSA_WITH_CHACHA20_POLY1305_SHA256,
+		tls.TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256,
+		tls.TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256,
+		tls.TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384,
+		tls.TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384,
+		tls.TLS_RSA_WITH_AES_128_GCM_SHA256, // does not provide PFS
+		tls.TLS_RSA_WITH_AES_256_GCM_SHA384, // does not provide PFS
+	}
+
+	if *localCert == "" || *localKey == "" {
+		log.WithFields(logrus.Fields{
+			"cert_file": *localCert,
+			"key_file": *localKey,
+		}).Fatal("TLS certificate/key file not defined in config")
+	}
+
+	cert, err := tls.LoadX509KeyPair(*localCert, *localKey)
+	if err != nil {
+		log.WithField("error", err).
+			Fatal("cannot load X509 keypair")
+	}
+
+	return &tls.Config{
+		PreferServerCipherSuites: true,
+		MinVersion:               tls.VersionTLS12,
+		CipherSuites:             tlsCipherSuites,
+		Certificates:             []tls.Certificate{cert},
+	}
 }
 
 func main() {
+	ConfigLoad()
 
-	iniflags.Parse()
+	if *versionInfo {
+		fmt.Printf("smtprelay/%s (%s)\n", appVersion, buildTime)
+		os.Exit(0)
+	}
 
-	listeners := strings.Split(*listen, " ")
+	log.WithField("version", appVersion).
+		Debug("starting smtprelay")
 
-	for i := range(listeners) {
-		listener := listeners[i]
+	// Load allowed users file
+	if *allowedUsers != "" {
+		err := AuthLoadFile(*allowedUsers)
+		if err != nil {
+			log.WithField("file", *allowedUsers).
+				WithError(err).
+				Fatal("cannot load allowed users file")
+		}
+	}
 
+	var servers []*smtpd.Server
+
+	// Create a server for each desired listen address
+	for _, listenAddr := range strings.Split(*listen, " ") {
 		server := &smtpd.Server{
-			Hostname:	*hostName,
-			WelcomeMessage: *welcomeMsg,
-			Handler:        handler,
+			Hostname:          *hostName,
+			WelcomeMessage:    *welcomeMsg,
+			ConnectionChecker: connectionChecker,
+			SenderChecker:     senderChecker,
+			RecipientChecker:  recipientChecker,
+			Handler:           mailHandler,
 		}
 
-		if strings.Index(listeners[i], "://") == -1 {
-			;
-		} else if strings.HasPrefix(listeners[i], "tls://") || strings.HasPrefix(listeners[i], "starttls://") {
+		if *allowedUsers != "" {
+			server.Authenticator = authChecker
+		}
 
-			listener = strings.TrimPrefix(listener, "tls://")
-			listener = strings.TrimPrefix(listener, "starttls://")
+		var lsnr net.Listener
+		var err error
 
-			if *localCert == "" || *localKey == "" {
-				log.Fatal("TLS certificate/key not defined in config")
-			}
+		if strings.Index(listenAddr, "://") == -1 {
+			log.WithField("address", listenAddr).
+				Info("listening on address")
 
-			cert, err := tls.LoadX509KeyPair(*localCert, *localKey)
-			if err != nil {
-				log.Fatal(err)
-			}
+			lsnr, err = net.Listen("tcp", listenAddr)
+		} else if strings.HasPrefix(listenAddr, "starttls://") {
+			listenAddr = strings.TrimPrefix(listenAddr, "starttls://")
 
+			server.TLSConfig = getTLSConfig()
 			server.ForceTLS = *localForceTLS
-			server.TLSConfig = &tls.Config {
-				Certificates: [] tls.Certificate{cert},
-			}
+
+			log.WithField("address", listenAddr).
+				Info("listening on address (STARTTLS)")
+			lsnr, err = net.Listen("tcp", listenAddr)
+		} else if strings.HasPrefix(listenAddr, "tls://") {
+			listenAddr = strings.TrimPrefix(listenAddr, "tls://")
+
+			server.TLSConfig = getTLSConfig()
+
+			log.WithField("address", listenAddr).
+				Info("listening on address (TLS)")
+			lsnr, err = tls.Listen("tcp", listenAddr, server.TLSConfig)
 		} else {
-			log.Fatal("Unknown protocol in listener ", listener)
+			log.WithField("address", listenAddr).
+				Fatal("unknown protocol in listen address")
 		}
 
-		log.Printf("Listen on %s ...\n", listener)
-		go server.ListenAndServe(listener)
+		if err != nil {
+			log.WithFields(logrus.Fields{
+				"address": listenAddr,
+			}).WithError(err).Fatal("error starting listener")
+		}
+		servers = append(servers, server)
+
+		go func() {
+			server.Serve(lsnr)
+		}()
 	}
 
-	for true {
-		time.Sleep(time.Minute)
+	handleSignals()
+
+	// First close the listeners
+	for _, server := range servers {
+		logger := log.WithField("address", server.Address())
+		logger.Debug("Shutting down server")
+		err := server.Shutdown(false)
+		if err != nil {
+			logger.WithError(err).
+				Warning("Shutdown failed")
+		}
 	}
+
+	// Then wait for the clients to exit
+	for _, server := range servers {
+		logger := log.WithField("address", server.Address())
+		logger.Debug("Waiting for server")
+		err := server.Wait()
+		if err != nil {
+			logger.WithError(err).
+				Warning("Wait failed")
+		}
+	}
+
+	log.Debug("done")
+}
+
+func handleSignals() {
+	// Wait for SIGINT, SIGQUIT, or SIGTERM
+	sigs := make(chan os.Signal, 1)
+	signal.Notify(sigs, syscall.SIGINT, syscall.SIGQUIT, syscall.SIGTERM)
+	sig := <-sigs
+
+	log.WithField("signal", sig).
+		Info("shutting down in response to received signal")
 }

main_test.go

@@ -0,0 +1,94 @@
+package main
+
+import (
+	"testing"
+)
+
+func TestAddrAllowedNoDomain(t *testing.T) {
+	allowedAddrs := []string{"joe@abc.com"}
+	if addrAllowed("bob.com", allowedAddrs) {
+		t.FailNow()
+	}
+}
+
+func TestAddrAllowedSingle(t *testing.T) {
+	allowedAddrs := []string{"joe@abc.com"}
+
+	if !addrAllowed("joe@abc.com", allowedAddrs) {
+		t.FailNow()
+	}
+	if addrAllowed("bob@abc.com", allowedAddrs) {
+		t.FailNow()
+	}
+}
+
+func TestAddrAllowedDifferentCase(t *testing.T) {
+	allowedAddrs := []string{"joe@abc.com"}
+	testAddrs := []string{
+		"joe@ABC.com",
+		"Joe@abc.com",
+		"JOE@abc.com",
+		"JOE@ABC.COM",
+	}
+	for _, addr := range testAddrs {
+		if !addrAllowed(addr, allowedAddrs) {
+			t.Errorf("Address %v not allowed, but should be", addr)
+		}
+	}
+}
+
+func TestAddrAllowedLocal(t *testing.T) {
+	allowedAddrs := []string{"joe"}
+
+	if !addrAllowed("joe", allowedAddrs) {
+		t.FailNow()
+	}
+	if addrAllowed("bob", allowedAddrs) {
+		t.FailNow()
+	}
+}
+
+func TestAddrAllowedMulti(t *testing.T) {
+	allowedAddrs := []string{"joe@abc.com", "bob@def.com"}
+	if !addrAllowed("joe@abc.com", allowedAddrs) {
+		t.FailNow()
+	}
+	if !addrAllowed("bob@def.com", allowedAddrs) {
+		t.FailNow()
+	}
+	if addrAllowed("bob@abc.com", allowedAddrs) {
+		t.FailNow()
+	}
+}
+
+func TestAddrAllowedSingleDomain(t *testing.T) {
+	allowedAddrs := []string{"@abc.com"}
+	if !addrAllowed("joe@abc.com", allowedAddrs) {
+		t.FailNow()
+	}
+	if addrAllowed("joe@def.com", allowedAddrs) {
+		t.FailNow()
+	}
+}
+
+func TestAddrAllowedMixed(t *testing.T) {
+	allowedAddrs := []string{"app", "app@example.com", "@appsrv.example.com"}
+	if !addrAllowed("app", allowedAddrs) {
+		t.FailNow()
+	}
+	if !addrAllowed("app@example.com", allowedAddrs) {
+		t.FailNow()
+	}
+	if addrAllowed("ceo@example.com", allowedAddrs) {
+		t.FailNow()
+	}
+	if !addrAllowed("root@appsrv.example.com", allowedAddrs) {
+		t.FailNow()
+	}
+	if !addrAllowed("dev@appsrv.example.com", allowedAddrs) {
+		t.FailNow()
+	}
+	if addrAllowed("appsrv@example.com", allowedAddrs) {
+		t.FailNow()
+	}
+}

service_files/smtprelay.service

@@ -0,0 +1,11 @@
+[Unit]
+Description=SMTP Relay
+After=network-online.target
+Wants=network-online.target
+
+[Service]
+Type=simple
+ExecStart=/opt/smtprelay/bin/smtprelay -config /etc/smtprelay/smtprelay.ini
+
+[Install]
+WantedBy=multi-user.target

smtp-proxy.ini

@@ -1,37 +0,0 @@
-; smtp-proxy configuration
-
-; Hostname for this SMTP server
-;hostname = "localhost.localdomain"
-
-; Welcome message for clients
-;welcome_msg = "<hostname> ESMTP ready."
-
-; Listen on the following addresses for incoming
-; unencrypted connections.
-;listen = 127.0.0.1:25 [::1]:25
-
-; STARTTLS and TLS are also supported but need a
-; SSL certificate and key.
-;listen = tls://127.0.0.1:465 tls://[::1]:465
-;listen = starttls://127.0.0.1:587 starttls://[::1]:587
-;local_cert = smtpd.pem
-;local_key  = smtpd.key
-
-; Enforce encrypted connection on STARTTLS ports before
-; accepting mails from client.
-;local_forcetls = false
-
-; Relay all mails to this SMTP server
-
-; GMail
-;remote_host = smtp.gmail.com:587
-
-; Mailgun.org
-;remote_host = smtp.mailgun.org:587
-
-; Mailjet.com
-;remote_host = in-v3.mailjet.com:587
-
-; Authentication credentials on outgoing SMTP server
-;remote_user =
-;remote_pass =

smtp.go

@@ -0,0 +1,484 @@
+// Copyright 2010 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package smtp implements the Simple Mail Transfer Protocol as defined in RFC 5321.
+// It also implements the following extensions:
+//	8BITMIME  RFC 1652
+//	AUTH      RFC 2554
+//	STARTTLS  RFC 3207
+// Additional extensions may be handled by clients.
+//
+// The smtp package is frozen and is not accepting new features.
+// Some external packages provide more functionality. See:
+//
+//   https://godoc.org/?q=smtp
+package main
+
+import (
+	"crypto/tls"
+	"encoding/base64"
+	"errors"
+	"fmt"
+	"io"
+	"net"
+	"net/smtp"
+	"net/textproto"
+	"strings"
+)
+
+// A Client represents a client connection to an SMTP server.
+type Client struct {
+	// Text is the textproto.Conn used by the Client. It is exported to allow for
+	// clients to add extensions.
+	Text *textproto.Conn
+	// keep a reference to the connection so it can be used to create a TLS
+	// connection later
+	conn net.Conn
+	// whether the Client is using TLS
+	tls        bool
+	serverName string
+	// map of supported extensions
+	ext map[string]string
+	// supported auth mechanisms
+	auth       []string
+	localName  string // the name to use in HELO/EHLO
+	didHello   bool   // whether we've said HELO/EHLO
+	helloError error  // the error from the hello
+}
+
+// Dial returns a new Client connected to an SMTP server at addr.
+// The addr must include a port, as in "mail.example.com:smtp".
+func Dial(addr string) (*Client, error) {
+	conn, err := net.Dial("tcp", addr)
+	if err != nil {
+		return nil, err
+	}
+	host, _, _ := net.SplitHostPort(addr)
+	return NewClient(conn, host)
+}
+
+// NewClient returns a new Client using an existing connection and host as a
+// server name to be used when authenticating.
+func NewClient(conn net.Conn, host string) (*Client, error) {
+	text := textproto.NewConn(conn)
+	_, _, err := text.ReadResponse(220)
+	if err != nil {
+		text.Close()
+		return nil, err
+	}
+	c := &Client{Text: text, conn: conn, serverName: host, localName: *hostName}
+	_, c.tls = conn.(*tls.Conn)
+	return c, nil
+}
+
+// Close closes the connection.
+func (c *Client) Close() error {
+	return c.Text.Close()
+}
+
+// hello runs a hello exchange if needed.
+func (c *Client) hello() error {
+	if !c.didHello {
+		c.didHello = true
+		err := c.ehlo()
+		if err != nil {
+			c.helloError = c.helo()
+		}
+	}
+	return c.helloError
+}
+
+// Hello sends a HELO or EHLO to the server as the given host name.
+// Calling this method is only necessary if the client needs control
+// over the host name used. The client will introduce itself as "localhost"
+// automatically otherwise. If Hello is called, it must be called before
+// any of the other methods.
+func (c *Client) Hello(localName string) error {
+	if err := validateLine(localName); err != nil {
+		return err
+	}
+	if c.didHello {
+		return errors.New("smtp: Hello called after other methods")
+	}
+	c.localName = localName
+	return c.hello()
+}
+
+// cmd is a convenience function that sends a command and returns the response
+func (c *Client) cmd(expectCode int, format string, args ...interface{}) (int, string, error) {
+	id, err := c.Text.Cmd(format, args...)
+	if err != nil {
+		return 0, "", err
+	}
+	c.Text.StartResponse(id)
+	defer c.Text.EndResponse(id)
+	code, msg, err := c.Text.ReadResponse(expectCode)
+	return code, msg, err
+}
+
+// helo sends the HELO greeting to the server. It should be used only when the
+// server does not support ehlo.
+func (c *Client) helo() error {
+	c.ext = nil
+	_, _, err := c.cmd(250, "HELO %s", c.localName)
+	return err
+}
+
+// ehlo sends the EHLO (extended hello) greeting to the server. It
+// should be the preferred greeting for servers that support it.
+func (c *Client) ehlo() error {
+	_, msg, err := c.cmd(250, "EHLO %s", c.localName)
+	if err != nil {
+		return err
+	}
+	ext := make(map[string]string)
+	extList := strings.Split(msg, "\n")
+	if len(extList) > 1 {
+		extList = extList[1:]
+		for _, line := range extList {
+			args := strings.SplitN(line, " ", 2)
+			if len(args) > 1 {
+				ext[args[0]] = args[1]
+			} else {
+				ext[args[0]] = ""
+			}
+		}
+	}
+	if mechs, ok := ext["AUTH"]; ok {
+		c.auth = strings.Split(mechs, " ")
+	}
+	c.ext = ext
+	return err
+}
+
+// StartTLS sends the STARTTLS command and encrypts all further communication.
+// Only servers that advertise the STARTTLS extension support this function.
+func (c *Client) StartTLS(config *tls.Config) error {
+	if err := c.hello(); err != nil {
+		return err
+	}
+	_, _, err := c.cmd(220, "STARTTLS")
+	if err != nil {
+		return err
+	}
+	c.conn = tls.Client(c.conn, config)
+	c.Text = textproto.NewConn(c.conn)
+	c.tls = true
+	return c.ehlo()
+}
+
+// TLSConnectionState returns the client's TLS connection state.
+// The return values are their zero values if StartTLS did
+// not succeed.
+func (c *Client) TLSConnectionState() (state tls.ConnectionState, ok bool) {
+	tc, ok := c.conn.(*tls.Conn)
+	if !ok {
+		return
+	}
+	return tc.ConnectionState(), true
+}
+
+// Verify checks the validity of an email address on the server.
+// If Verify returns nil, the address is valid. A non-nil return
+// does not necessarily indicate an invalid address. Many servers
+// will not verify addresses for security reasons.
+func (c *Client) Verify(addr string) error {
+	if err := validateLine(addr); err != nil {
+		return err
+	}
+	if err := c.hello(); err != nil {
+		return err
+	}
+	_, _, err := c.cmd(250, "VRFY %s", addr)
+	return err
+}
+
+// Auth authenticates a client using the provided authentication mechanism.
+// A failed authentication closes the connection.
+// Only servers that advertise the AUTH extension support this function.
+func (c *Client) Auth(a smtp.Auth) error {
+	if err := c.hello(); err != nil {
+		return err
+	}
+	encoding := base64.StdEncoding
+	mech, resp, err := a.Start(&smtp.ServerInfo{c.serverName, c.tls, c.auth})
+	if err != nil {
+		c.Quit()
+		return err
+	}
+	resp64 := make([]byte, encoding.EncodedLen(len(resp)))
+	encoding.Encode(resp64, resp)
+	code, msg64, err := c.cmd(0, strings.TrimSpace(fmt.Sprintf("AUTH %s %s", mech, resp64)))
+	for err == nil {
+		var msg []byte
+		switch code {
+		case 334:
+			msg, err = encoding.DecodeString(msg64)
+		case 235:
+			// the last message isn't base64 because it isn't a challenge
+			msg = []byte(msg64)
+		default:
+			err = &textproto.Error{Code: code, Msg: msg64}
+		}
+		if err == nil {
+			resp, err = a.Next(msg, code == 334)
+		}
+		if err != nil {
+			// abort the AUTH
+			c.cmd(501, "*")
+			c.Quit()
+			break
+		}
+		if resp == nil {
+			break
+		}
+		resp64 = make([]byte, encoding.EncodedLen(len(resp)))
+		encoding.Encode(resp64, resp)
+		code, msg64, err = c.cmd(0, string(resp64))
+	}
+	return err
+}
+
+// Mail issues a MAIL command to the server using the provided email address.
+// If the server supports the 8BITMIME extension, Mail adds the BODY=8BITMIME
+// parameter. If the server supports the SMTPUTF8 extension, Mail adds the
+// SMTPUTF8 parameter.
+// This initiates a mail transaction and is followed by one or more Rcpt calls.
+func (c *Client) Mail(from string) error {
+	if err := validateLine(from); err != nil {
+		return err
+	}
+	if err := c.hello(); err != nil {
+		return err
+	}
+	cmdStr := "MAIL FROM:<%s>"
+	if c.ext != nil {
+		if _, ok := c.ext["8BITMIME"]; ok {
+			cmdStr += " BODY=8BITMIME"
+		}
+		if _, ok := c.ext["SMTPUTF8"]; ok {
+			cmdStr += " SMTPUTF8"
+		}
+	}
+	_, _, err := c.cmd(250, cmdStr, from)
+	return err
+}
+
+// Rcpt issues a RCPT command to the server using the provided email address.
+// A call to Rcpt must be preceded by a call to Mail and may be followed by
+// a Data call or another Rcpt call.
+func (c *Client) Rcpt(to string) error {
+	if err := validateLine(to); err != nil {
+		return err
+	}
+	_, _, err := c.cmd(25, "RCPT TO:<%s>", to)
+	return err
+}
+
+type dataCloser struct {
+	c *Client
+	io.WriteCloser
+}
+
+func (d *dataCloser) Close() error {
+	d.WriteCloser.Close()
+	_, _, err := d.c.Text.ReadResponse(250)
+	return err
+}
+
+// Data issues a DATA command to the server and returns a writer that
+// can be used to write the mail headers and body. The caller should
+// close the writer before calling any more methods on c. A call to
+// Data must be preceded by one or more calls to Rcpt.
+func (c *Client) Data() (io.WriteCloser, error) {
+	_, _, err := c.cmd(354, "DATA")
+	if err != nil {
+		return nil, err
+	}
+	return &dataCloser{c, c.Text.DotWriter()}, nil
+}
+
+var testHookStartTLS func(*tls.Config) // nil, except for tests
+
+// SendMail connects to the server at addr with TLS when port 465 or
+// smtps is specified or unencrypted otherwise and switches to TLS if
+// possible, authenticates with the optional mechanism a if possible,
+// and then sends an email from address from, to addresses to, with
+// message msg.
+// The addr must include a port, as in "mail.example.com:smtp".
+//
+// The addresses in the to parameter are the SMTP RCPT addresses.
+//
+// The msg parameter should be an RFC 822-style email with headers
+// first, a blank line, and then the message body. The lines of msg
+// should be CRLF terminated. The msg headers should usually include
+// fields such as "From", "To", "Subject", and "Cc".  Sending "Bcc"
+// messages is accomplished by including an email address in the to
+// parameter but not including it in the msg headers.
+//
+// The SendMail function and the net/smtp package are low-level
+// mechanisms and provide no support for DKIM signing, MIME
+// attachments (see the mime/multipart package), or other mail
+// functionality. Higher-level packages exist outside of the standard
+// library.
+func SendMail(addr string, a smtp.Auth, from string, to []string, msg []byte) error {
+	if err := validateLine(from); err != nil {
+		return err
+	}
+	for _, recp := range to {
+		if err := validateLine(recp); err != nil {
+			return err
+		}
+	}
+	host, port, err := net.SplitHostPort(addr)
+	if err != nil {
+		return err
+	}
+	var c *Client
+	if port == "465" || port == "smtps" {
+		config := &tls.Config{ServerName: host}
+		conn, err := tls.Dial("tcp", addr, config)
+		if err != nil {
+			return err
+		}
+		defer conn.Close()
+		c, err = NewClient(conn, host)
+		if err != nil {
+			return err
+		}
+		if err = c.hello(); err != nil {
+			return err
+		}
+	} else {
+		c, err = Dial(addr)
+		if err != nil {
+			return err
+		}
+		defer c.Close()
+		if err = c.hello(); err != nil {
+			return err
+		}
+		if ok, _ := c.Extension("STARTTLS"); ok {
+			config := &tls.Config{ServerName: c.serverName}
+			if testHookStartTLS != nil {
+				testHookStartTLS(config)
+			}
+			if err = c.StartTLS(config); err != nil {
+				return err
+			}
+		}
+	}
+	if a != nil && c.ext != nil {
+		if _, ok := c.ext["AUTH"]; !ok {
+			return errors.New("smtp: server doesn't support AUTH")
+		}
+		if err = c.Auth(a); err != nil {
+			return err
+		}
+	}
+	if err = c.Mail(from); err != nil {
+		return err
+	}
+	for _, addr := range to {
+		if err = c.Rcpt(addr); err != nil {
+			return err
+		}
+	}
+	w, err := c.Data()
+	if err != nil {
+		return err
+	}
+	_, err = w.Write(msg)
+	if err != nil {
+		return err
+	}
+	err = w.Close()
+	if err != nil {
+		return err
+	}
+	return c.Quit()
+}
+
+// Extension reports whether an extension is support by the server.
+// The extension name is case-insensitive. If the extension is supported,
+// Extension also returns a string that contains any parameters the
+// server specifies for the extension.
+func (c *Client) Extension(ext string) (bool, string) {
+	if err := c.hello(); err != nil {
+		return false, ""
+	}
+	if c.ext == nil {
+		return false, ""
+	}
+	ext = strings.ToUpper(ext)
+	param, ok := c.ext[ext]
+	return ok, param
+}
+
+// Reset sends the RSET command to the server, aborting the current mail
+// transaction.
+func (c *Client) Reset() error {
+	if err := c.hello(); err != nil {
+		return err
+	}
+	_, _, err := c.cmd(250, "RSET")
+	return err
+}
+
+// Noop sends the NOOP command to the server. It does nothing but check
+// that the connection to the server is okay.
+func (c *Client) Noop() error {
+	if err := c.hello(); err != nil {
+		return err
+	}
+	_, _, err := c.cmd(250, "NOOP")
+	return err
+}
+
+// Quit sends the QUIT command and closes the connection to the server.
+func (c *Client) Quit() error {
+	if err := c.hello(); err != nil {
+		return err
+	}
+	_, _, err := c.cmd(221, "QUIT")
+	if err != nil {
+		return err
+	}
+	return c.Text.Close()
+}
+
+// validateLine checks to see if a line has CR or LF as per RFC 5321
+func validateLine(line string) error {
+	if strings.ContainsAny(line, "\n\r") {
+		return errors.New("smtp: A line must not contain CR or LF")
+	}
+	return nil
+}
+
+// LOGIN authentication
+type loginAuth struct {
+	username, password string
+}
+
+func LoginAuth(username, password string) smtp.Auth {
+	return &loginAuth{username, password}
+}
+
+func (a *loginAuth) Start(server *smtp.ServerInfo) (string, []byte, error) {
+	return "LOGIN", []byte{}, nil
+}
+
+func (a *loginAuth) Next(fromServer []byte, more bool) ([]byte, error) {
+	if more {
+		switch string(fromServer) {
+		case "Username:":
+			return []byte(a.username), nil
+		case "Password:":
+			return []byte(a.password), nil
+		default:
+			return nil, errors.New("Unknown fromServer")
+		}
+	}
+	return nil, nil
+}

smtprelay.ini

@@ -0,0 +1,80 @@
+; smtprelay configuration
+
+; Logfile (blank/default is stderr)
+;logfile = 
+
+; Log format: default, plain (no timestamp), json
+;log_format = "default"
+
+; Log level: panic, fatal, error, warn, info, debug, trace
+;log_level = "info"
+
+; Hostname for this SMTP server
+;hostname = "localhost.localdomain"
+
+; Welcome message for clients
+;welcome_msg = "<hostname> ESMTP ready."
+
+; Listen on the following addresses for incoming
+; unencrypted connections.
+;listen = 127.0.0.1:25 [::1]:25
+
+; STARTTLS and TLS are also supported but need a
+; SSL certificate and key.
+;listen = tls://127.0.0.1:465 tls://[::1]:465
+;listen = starttls://127.0.0.1:587 starttls://[::1]:587
+;local_cert = smtpd.pem
+;local_key  = smtpd.key
+
+; Enforce encrypted connection on STARTTLS ports before
+; accepting mails from client.
+;local_forcetls = false
+
+; Networks that are allowed to send mails to us
+; Defaults to localhost. If set to "", then any address is allowed.
+;allowed_nets = 127.0.0.0/8 ::1/128
+
+; Regular expression for valid FROM EMail addresses
+; If set to "", then any sender is permitted.
+; Example: ^(.*)@localhost.localdomain$
+;allowed_sender =
+
+; Regular expression for valid TO EMail addresses
+; If set to "", then any recipient is permitted.
+; Example: ^(.*)@localhost.localdomain$
+;allowed_recipients =
+
+; File which contains username and password used for
+; authentication before they can send mail.
+; File format: username bcrypt-hash [email[,email[,...]]]
+;   username: The SMTP auth username
+;   bcrypt-hash: The bcrypt hash of the pasword (generate with "./hasher password")
+;   email: Comma-separated list of allowed "from" addresses:
+;          - If omitted, user can send from any address
+;          - If @domain.com is given, user can send from any address @domain.com
+;          - Otherwise, email address must match exactly (case-insensitive)
+;          E.g. "app@example.com,@appsrv.example.com"
+;allowed_users =
+
+; Relay all mails to this SMTP server.
+; If not set, mails are discarded.
+
+; GMail
+;remote_host = smtp.gmail.com:587
+
+; Mailgun.org
+;remote_host = smtp.mailgun.org:587
+
+; Mailjet.com
+;remote_host = in-v3.mailjet.com:587
+
+; Authentication credentials on outgoing SMTP server
+;remote_user =
+;remote_pass =
+
+; Authentication method on outgoing SMTP server
+; (none, plain, login)
+;remote_auth = none
+
+; Sender e-mail address on outgoing SMTP server
+;remote_sender =