Support .env files if it exists

Bump github.com/peterbourgon/ff/v3 from 3.1.2 to 3.3.0

.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

@@ -25,7 +25,7 @@ jobs:
 
     steps:
     - name: Checkout repository
-      uses: actions/checkout@v2
+      uses: actions/checkout@v3
       with:
         # We must fetch at least the immediate parents so that if this is
         # a pull request then we can checkout the head.
@@ -38,7 +38,7 @@ jobs:
 
     # Initializes the CodeQL tools for scanning.
     - name: Initialize CodeQL
-      uses: github/codeql-action/init@v1
+      uses: github/codeql-action/init@v2
       with:
         languages: ${{ matrix.language }}
         # If you wish to specify custom queries, you can do so here or in a config file.
@@ -49,7 +49,7 @@ jobs:
     # 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
+      uses: github/codeql-action/autobuild@v2
 
     # ℹ️ Command-line programs to run using the OS shell.
     # 📚 https://git.io/JvXDl
@@ -63,4 +63,4 @@ jobs:
     #   make release
 
     - name: Perform CodeQL Analysis
-      uses: github/codeql-action/analyze@v1
+      uses: github/codeql-action/analyze@v2

.github/workflows/go.yml

@@ -1,5 +1,5 @@
 name: Go
-on: [push]
+on: [push, pull_request]
 jobs:
 
   build:
@@ -7,14 +7,14 @@ jobs:
     runs-on: ubuntu-latest
     steps:
 
-    - name: Set up Go 1.14
-      uses: actions/setup-go@v1
+    - name: Set up Go 1.18
+      uses: actions/setup-go@v3
       with:
-        go-version: 1.14
+        go-version: 1.18
       id: go
 
     - name: Check out code into the Go module directory
-      uses: actions/checkout@v1
+      uses: actions/checkout@v3
 
     - name: Get dependencies
       run: |
@@ -22,3 +22,6 @@ jobs:
 
     - name: Build
       run: go build -v .
+
+    - name: Test
+      run: go test -v .

.github/workflows/release.yaml

@@ -11,20 +11,20 @@ jobs:
     strategy:
       matrix:
         goos: [freebsd, linux, windows]
-        goarch: ["386", amd64]
+        goarch: [amd64, arm64]
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v3
     
     - 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@v1.14
+    - uses: wangyoucao577/go-release-action@v1.30
       with:
         github_token: ${{ secrets.GITHUB_TOKEN }}
         goos: ${{ matrix.goos }}
         goarch: ${{ matrix.goarch }}
-        goversion: "https://golang.org/dl/go1.15.8.linux-amd64.tar.gz"
+        goversion: 1.18
         extra_files: LICENSE README.md smtprelay.ini
         ldflags: -s -w -X "main.appVersion=${{ env.APP_VERSION }}" -X "main.buildTime=${{ env.BUILD_TIME }}"

README.md

@@ -16,16 +16,17 @@ 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.
+via a remote SMTP server without giving away my mail credentials to each
+device which produces mail.
 
 
 ## Main features
 
+* Simple configuration with ini file .env file or environment variables
 * 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)
+* Forwards all mail to a smarthost (any SMTP server)
 * Small codebase
 * IPv6 support

config.go

@@ -1,9 +1,18 @@
 package main
 
 import (
+	"bufio"
 	"flag"
+	"fmt"
+	"io"
+	"net"
+	"os"
+	"regexp"
+	"strings"
+	"time"
 
-	"github.com/vharitonsky/iniflags"
+	"github.com/peterbourgon/ff/v3"
+	"github.com/sirupsen/logrus"
 )
 
 var (
@@ -12,25 +21,255 @@ var (
 )
 
 var (
-	logFile           = flag.String("logfile", "/var/log/smtprelay.log", "Path to logfile")
-	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)")
-	allowedNets       = flag.String("allowed_nets", "127.0.0.1/8 ::1/128", "Networks allowed to send mails")
-	allowedSender     = flag.String("allowed_sender", "", "Regular expression for valid FROM EMail addresses")
-	allowedRecipients = flag.String("allowed_recipients", "", "Regular expression for valid TO EMail addresses")
-	allowedUsers      = flag.String("allowed_users", "", "Path to file with valid users/passwords")
-	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")
-	remoteAuth        = flag.String("remote_auth", "plain", "Auth method on outgoing SMTP server (plain, login)")
-	remoteSender      = flag.String("remote_sender", "", "Sender e-mail address on outgoing SMTP server")
-	versionInfo       = flag.Bool("version", false, "Show version information")
+	flagset = flag.NewFlagSet("smtprelay", flag.ContinueOnError)
+
+	// config flags
+	logFile          = flagset.String("logfile", "", "Path to logfile")
+	logFormat        = flagset.String("log_format", "default", "Log output format")
+	logLevel         = flagset.String("log_level", "info", "Minimum log level to output")
+	hostName         = flagset.String("hostname", "localhost.localdomain", "Server hostname")
+	welcomeMsg       = flagset.String("welcome_msg", "", "Welcome message for SMTP session")
+	listenStr        = flagset.String("listen", "127.0.0.1:25 [::1]:25", "Address and port to listen for incoming SMTP")
+	localCert        = flagset.String("local_cert", "", "SSL certificate for STARTTLS/TLS")
+	localKey         = flagset.String("local_key", "", "SSL private key for STARTTLS/TLS")
+	localForceTLS    = flagset.Bool("local_forcetls", false, "Force STARTTLS (needs local_cert and local_key)")
+	readTimeoutStr   = flagset.String("read_timeout", "60s", "Socket timeout for read operations")
+	writeTimeoutStr  = flagset.String("write_timeout", "60s", "Socket timeout for write operations")
+	dataTimeoutStr   = flagset.String("data_timeout", "5m", "Socket timeout for DATA command")
+	maxConnections   = flagset.Int("max_connections", 100, "Max concurrent connections, use -1 to disable")
+	maxMessageSize   = flagset.Int("max_message_size", 10240000, "Max message size in bytes")
+	maxRecipients    = flagset.Int("max_recipients", 100, "Max RCPT TO calls for each envelope")
+	allowedNetsStr   = flagset.String("allowed_nets", "127.0.0.0/8 ::1/128", "Networks allowed to send mails")
+	allowedSenderStr = flagset.String("allowed_sender", "", "Regular expression for valid FROM EMail addresses")
+	allowedRecipStr  = flagset.String("allowed_recipients", "", "Regular expression for valid TO EMail addresses")
+	allowedUsers     = flagset.String("allowed_users", "", "Path to file with valid users/passwords")
+	command          = flagset.String("command", "", "Path to pipe command")
+	remotesStr       = flagset.String("remotes", "", "Outgoing SMTP servers")
+
+	// additional flags
+	_           = flagset.String("config", "", "Path to config file (ini format)")
+	versionInfo = flagset.Bool("version", false, "Show version information")
+
+	// internal
+	listenAddrs       = []protoAddr{}
+	readTimeout       time.Duration
+	writeTimeout      time.Duration
+	dataTimeout       time.Duration
+	allowedNets       = []*net.IPNet{}
+	allowedSender     *regexp.Regexp
+	allowedRecipients *regexp.Regexp
+	remotes           = []*Remote{}
 )
 
-func ConfigLoad() {
-	iniflags.Parse()
+func localAuthRequired() bool {
+	return *allowedUsers != ""
+}
+
+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 setupRemotes() {
+	logger := log.WithField("remotes", *remotesStr)
+
+	if *remotesStr != "" {
+		for _, remoteURL := range strings.Split(*remotesStr, " ") {
+			r, err := ParseRemote(remoteURL)
+			if err != nil {
+				logger.Fatal(fmt.Sprintf("error parsing url: '%s': %v", remoteURL, err))
+			}
+
+			remotes = append(remotes, r)
+		}
+	}
+}
+
+type protoAddr struct {
+	protocol string
+	address  string
+}
+
+func splitProto(s string) protoAddr {
+	idx := strings.Index(s, "://")
+	if idx == -1 {
+		return protoAddr{
+			address: s,
+		}
+	}
+	return protoAddr{
+		protocol: s[0:idx],
+		address:  s[idx+3:],
+	}
+}
+
+func setupListeners() {
+	for _, listenAddr := range strings.Split(*listenStr, " ") {
+		pa := splitProto(listenAddr)
+
+		if localAuthRequired() && pa.protocol == "" {
+			log.WithField("address", pa.address).
+				Fatal("Local authentication (via allowed_users file) " +
+					"not allowed with non-TLS listener")
+		}
+
+		listenAddrs = append(listenAddrs, pa)
+	}
+}
+
+func setupTimeouts() {
+	var err error
+
+	readTimeout, err = time.ParseDuration(*readTimeoutStr)
+	if err != nil {
+		log.WithField("read_timeout", *readTimeoutStr).
+			WithError(err).
+			Fatal("read_timeout duration string invalid")
+	}
+	if readTimeout.Seconds() < 1 {
+		log.WithField("read_timeout", *readTimeoutStr).
+			Fatal("read_timeout less than one second")
+	}
+
+	writeTimeout, err = time.ParseDuration(*writeTimeoutStr)
+	if err != nil {
+		log.WithField("write_timeout", *writeTimeoutStr).
+			WithError(err).
+			Fatal("write_timeout duration string invalid")
+	}
+	if writeTimeout.Seconds() < 1 {
+		log.WithField("write_timeout", *writeTimeoutStr).
+			Fatal("write_timeout less than one second")
+	}
+
+	dataTimeout, err = time.ParseDuration(*dataTimeoutStr)
+	if err != nil {
+		log.WithField("data_timeout", *dataTimeoutStr).
+			WithError(err).
+			Fatal("data_timeout duration string invalid")
+	}
+	if dataTimeout.Seconds() < 1 {
+		log.WithField("data_timeout", *dataTimeoutStr).
+			Fatal("data_timeout less than one second")
+	}
+}
+
+func ConfigLoad() {
+	// use .env file if it exists
+	if _, err := os.Stat(".env"); err == nil {
+		if err := ff.Parse(flagset, os.Args[1:],
+			ff.WithEnvVarPrefix("smtprelay"),
+			ff.WithConfigFile(".env"),
+			ff.WithConfigFileParser(ff.EnvParser),
+		); err != nil {
+			fmt.Fprintf(os.Stderr, "error: %v\n", err)
+			os.Exit(1)
+		}
+	} else {
+		// use env variables and smtprelay.ini file
+		if err := ff.Parse(flagset, os.Args[1:],
+			ff.WithEnvVarPrefix("smtprelay"),
+			ff.WithConfigFileFlag("config"),
+			ff.WithConfigFileParser(IniParser),
+		); err != nil {
+			fmt.Fprintf(os.Stderr, "error: %v\n", err)
+			os.Exit(1)
+		}
+	}
+
+	// Set up logging as soon as possible
+	setupLogger()
+
+	if *versionInfo {
+		fmt.Printf("smtprelay/%s (%s)\n", appVersion, buildTime)
+		os.Exit(0)
+	}
+
+	if *remotesStr == "" && *command == "" {
+		log.Warn("no remotes or command set; mail will not be forwarded!")
+	}
+
+	setupAllowedNetworks()
+	setupAllowedPatterns()
+	setupRemotes()
+	setupListeners()
+	setupTimeouts()
+}
+
+// IniParser is a parser for config files in classic key/value style format. Each
+// line is tokenized as a single key/value pair. The first "=" delimited
+// token in the line is interpreted as the flag name, and all remaining tokens
+// are interpreted as the value. Any leading hyphens on the flag name are
+// ignored.
+func IniParser(r io.Reader, set func(name, value string) error) error {
+	s := bufio.NewScanner(r)
+	for s.Scan() {
+		line := strings.TrimSpace(s.Text())
+		if line == "" {
+			continue // skip empties
+		}
+
+		if line[0] == '#' || line[0] == ';' {
+			continue // skip comments
+		}
+
+		var (
+			name  string
+			value string
+			index = strings.IndexRune(line, '=')
+		)
+		if index < 0 {
+			name, value = line, "true" // boolean option
+		} else {
+			name, value = strings.TrimSpace(line[:index]), strings.Trim(strings.TrimSpace(line[index+1:]), "\"")
+		}
+
+		if i := strings.Index(value, " #"); i >= 0 {
+			value = strings.TrimSpace(value[:i])
+		}
+
+		if err := set(name, value); err != nil {
+			return err
+		}
+	}
+	return nil
 }

config_test.go

@@ -0,0 +1,44 @@
+package main
+
+import (
+	"testing"
+)
+
+func TestSplitProto(t *testing.T) {
+	var tests = []struct {
+		input string
+		proto string
+		addr  string
+	}{
+		{
+			input: "localhost",
+			proto: "",
+			addr:  "localhost",
+		},
+		{
+			input: "tls://my.local.domain",
+			proto: "tls",
+			addr:  "my.local.domain",
+		},
+		{
+			input: "starttls://my.local.domain",
+			proto: "starttls",
+			addr:  "my.local.domain",
+		},
+	}
+
+	for i, test := range tests {
+		testName := test.input
+		t.Run(testName, func(t *testing.T) {
+			pa := splitProto(test.input)
+			if pa.protocol != test.proto {
+				t.Errorf("Testcase %d: Incorrect proto: expected %v, got %v",
+					i, test.proto, pa.protocol)
+			}
+			if pa.address != test.addr {
+				t.Errorf("Testcase %d: Incorrect addr: expected %v, got %v",
+					i, test.addr, pa.address)
+			}
+		})
+	}
+}

go.mod

@@ -1,9 +1,19 @@
 module github.com/decke/smtprelay
 
 require (
-	github.com/chrj/smtpd v0.2.0
-	github.com/vharitonsky/iniflags v0.0.0-20180513140207-a33cd0b5f3de
-	golang.org/x/crypto v0.0.0-20201221181555-eec23a3978ad
+	github.com/chrj/smtpd v0.3.1
+	github.com/google/uuid v1.3.0
+	github.com/peterbourgon/ff/v3 v3.3.0
+	github.com/sirupsen/logrus v1.9.0
+	github.com/stretchr/testify v1.8.0
+	golang.org/x/crypto v0.0.0-20220411220226-7b82a4e95df4
 )
 
-go 1.13
+require (
+	github.com/davecgh/go-spew v1.1.1 // indirect
+	github.com/pmezard/go-difflib v1.0.0 // indirect
+	golang.org/x/sys v0.0.0-20220715151400-c0bba94af5f8 // indirect
+	gopkg.in/yaml.v3 v3.0.1 // indirect
+)
+
+go 1.18

go.sum

@@ -1,13 +1,28 @@
-github.com/chrj/smtpd v0.2.0 h1:QGbE4UQz7sKjvXpRgNLuiBOjcWTzBKu/dj0hyDLpD14=
-github.com/chrj/smtpd v0.2.0/go.mod h1:1hmG9KbrE10JG1SmvG79Krh4F6713oUrw2+gRp1oSYk=
-github.com/eaigner/dkim v0.0.0-20150301120808-6fe4a7ee9cfb/go.mod h1:FSCIHbrqk7D01Mj8y/jW+NS1uoCerr+ad+IckTHTFf4=
-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/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=
+github.com/chrj/smtpd v0.3.1 h1:kogHFkbFdKaoH3bgZkqNC9uVtKYOFfM3uV3rroBdooE=
+github.com/chrj/smtpd v0.3.1/go.mod h1:JtABvV/LzvLmEIzy0NyDnrfMGOMd8wy5frAokwf6J9Q=
+github.com/davecgh/go-spew v1.1.0/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
+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/google/uuid v1.3.0 h1:t6JiXgmwXMjEs8VusXIJk2BXHsn+wx8BZdTaoZ5fu7I=
+github.com/google/uuid v1.3.0/go.mod h1:TIyPZe4MgqvfeYDBFedMoGGpEw/LqOeaOT+nhxU+yHo=
+github.com/peterbourgon/ff/v3 v3.3.0 h1:PaKe7GW8orVFh8Unb5jNHS+JZBwWUMa2se0HM6/BI24=
+github.com/peterbourgon/ff/v3 v3.3.0/go.mod h1:zjJVUhx+twciwfDl0zBcFzl4dW8axCRyXE/eKY9RztQ=
+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 v1.9.0 h1:trlNQbNUG3OdDrDil03MCb1H2o9nJ1x4/5LYw7byDE0=
+github.com/sirupsen/logrus v1.9.0/go.mod h1:naHLuLoDiP4jHNo9R0sCBMtWGeIprob74mVsIT4qYEQ=
+github.com/stretchr/objx v0.1.0/go.mod h1:HFkY916IF+rwdDfMAkV7OtwuqBVzrE8GR6GFx+wExME=
+github.com/stretchr/objx v0.4.0/go.mod h1:YvHI0jy2hoMjB+UWwv71VJQ9isScKT/TqJzVSSt89Yw=
+github.com/stretchr/testify v1.7.0/go.mod h1:6Fq8oRcR53rry900zMqJjRRixrwX3KX962/h/Wwjteg=
+github.com/stretchr/testify v1.7.1/go.mod h1:6Fq8oRcR53rry900zMqJjRRixrwX3KX962/h/Wwjteg=
+github.com/stretchr/testify v1.8.0 h1:pSgiaMZlXftHpm5L7V1+rVB+AZJydKsMxsQBIJw4PKk=
+github.com/stretchr/testify v1.8.0/go.mod h1:yNjHg4UonilssWZ8iaSj1OCr/vHnekPRkoO+kdMU+MU=
+golang.org/x/crypto v0.0.0-20220411220226-7b82a4e95df4 h1:kUhD7nTDoI3fVd9G4ORWrbV5NY0liEs/Jg2pv5f+bBA=
+golang.org/x/crypto v0.0.0-20220411220226-7b82a4e95df4/go.mod h1:IxCIyHEi3zRg3s0A5j5BB6A9Jmi73HwBIUl50j+osU4=
+golang.org/x/sys v0.0.0-20220715151400-c0bba94af5f8 h1:0A+M6Uqn+Eje4kHMK80dtF3JCXC4ykBgQG4Fe06QRhQ=
+golang.org/x/sys v0.0.0-20220715151400-c0bba94af5f8/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
+gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405 h1:yhCVgyC4o1eVCa2tZl7eS0r+SDo693bJlVdllGtEeKM=
+gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
+gopkg.in/yaml.v3 v3.0.0-20200313102051-9f266ea9e77c/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=
+gopkg.in/yaml.v3 v3.0.1 h1:fxVm/GzAzEWqLHuvctI91KS9hhNmmWOoWu0XTYJS7CA=
+gopkg.in/yaml.v3 v3.0.1/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=

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

@@ -1,39 +1,39 @@
 package main
 
 import (
+	"bytes"
 	"crypto/tls"
-	"fmt"
-	"io"
-	"log"
 	"net"
-	"net/smtp"
+	"net/textproto"
 	"os"
-	"regexp"
+	"os/exec"
+	"os/signal"
 	"strings"
-	"time"
+	"syscall"
 
 	"github.com/chrj/smtpd"
+	"github.com/google/uuid"
+	"github.com/sirupsen/logrus"
 )
 
 func connectionChecker(peer smtpd.Peer) error {
-	var peerIP net.IP
-	if addr, ok := peer.Addr.(*net.TCPAddr); ok {
-		peerIP = net.ParseIP(addr.IP.String())
-	} else {
-		return smtpd.Error{Code: 421, Message: "Denied"}
+	// This can't panic because we only have TCP listeners
+	peerIP := peer.Addr.(*net.TCPAddr).IP
+
+	if len(allowedNets) == 0 {
+		// Special case: empty string means allow everything
+		return nil
 	}
 
-	nets := strings.Split(*allowedNets, " ")
-
-	for i := range nets {
-		_, allowedNet, _ := net.ParseCIDR(nets[i])
-
+	for _, allowedNet := range allowedNets {
 		if allowedNet.Contains(peerIP) {
 			return nil
 		}
 	}
 
-	log.Printf("Connection from peer=[%s] denied: Not in allowed_nets\n", peerIP)
+	log.WithFields(logrus.Fields{
+		"ip": peerIP,
+	}).Warn("Connection refused from address outside of allowed_nets")
 	return smtpd.Error{Code: 421, Message: "Denied"}
 }
 
@@ -82,63 +82,69 @@ func addrAllowed(addr string, allowedAddrs []string) bool {
 
 func senderChecker(peer smtpd.Peer, addr string) error {
 	// check sender address from auth file if user is authenticated
-	if *allowedUsers != "" && peer.Username != "" {
+	if localAuthRequired() && 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.Printf("Mail from=<%s> not allowed for authenticated user %s (%v)\n",
-				addr, peer.Username, peer.Addr)
+			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 == "" {
+	if allowedSender == nil {
+		// Any sender is permitted
 		return nil
 	}
 
-	re, err := regexp.Compile(*allowedSender)
-	if err != nil {
-		log.Printf("allowed_sender invalid: %v\n", err)
-		return smtpd.Error{Code: 451, Message: "Bad sender address"}
-	}
-
-	if re.MatchString(addr) {
+	if allowedSender.MatchString(addr) {
+		// Permitted by regex
 		return nil
 	}
 
-	log.Printf("Mail from=<%s> not allowed by allowed_sender pattern for peer %v\n",
-		addr, peer.Addr)
+	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 == "" {
+	if allowedRecipients == nil {
+		// Any recipient is permitted
 		return nil
 	}
 
-	re, err := regexp.Compile(*allowedRecipients)
-	if err != nil {
-		log.Printf("allowed_recipients invalid: %v\n", err)
-		return smtpd.Error{Code: 451, Message: "Bad recipient address"}
-	}
-
-	if re.MatchString(addr) {
+	if allowedRecipients.MatchString(addr) {
+		// Permitted by regex
 		return nil
 	}
 
-	log.Printf("Mail to=<%s> not allowed by allowed_recipients pattern for peer %v\n",
-		addr, peer.Addr)
+	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.Printf("Auth error for peer %v: %v\n", peer.Addr, err)
+		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
@@ -150,52 +156,90 @@ func mailHandler(peer smtpd.Peer, env smtpd.Envelope) error {
 		peerIP = addr.IP.String()
 	}
 
-	log.Printf("new mail from=<%s> to=%s peer=[%s]\n", env.Sender,
-		env.Recipients, peerIP)
+	logger := log.WithFields(logrus.Fields{
+		"from": env.Sender,
+		"to":   env.Recipients,
+		"peer": peerIP,
+		"uuid": generateUUID(),
+	})
 
-	var auth smtp.Auth
-	host, _, _ := net.SplitHostPort(*remoteHost)
-
-	if *remoteUser != "" && *remotePass != "" {
-		switch *remoteAuth {
-		case "plain":
-			auth = smtp.PlainAuth("", *remoteUser, *remotePass, host)
-		case "login":
-			auth = LoginAuth(*remoteUser, *remotePass)
-		default:
-			return smtpd.Error{Code: 530, Message: "Authentication method not supported"}
-		}
+	if *remotesStr == "" && *command == "" {
+		logger.Warning("no remote_host or command set; discarding mail")
+		return nil
 	}
 
 	env.AddReceivedLine(peer)
 
-	log.Printf("delivering using smarthost %s\n", *remoteHost)
+	if *command != "" {
+		cmdLogger := logger.WithField("command", *command)
 
-	var sender string
+		var stdout bytes.Buffer
+		var stderr bytes.Buffer
 
-	if *remoteSender == "" {
-		sender = env.Sender
-	} else {
-		sender = *remoteSender
+		cmd := exec.Command(*command)
+		cmd.Stdin = bytes.NewReader(env.Data)
+		cmd.Stdout = &stdout
+		cmd.Stderr = &stderr
+
+		err := cmd.Run()
+		if err != nil {
+			cmdLogger.WithError(err).Error(stderr.String())
+			return smtpd.Error{Code: 554, Message: "External command failed"}
 		}
 
+		cmdLogger.Info("pipe command successful: " + stdout.String())
+	}
+
+	for _, remote := range remotes {
+		logger = logger.WithField("host", remote.Addr)
+		logger.Info("delivering mail from peer using smarthost")
+
 		err := SendMail(
-		*remoteHost,
-		auth,
-		sender,
+			remote,
+			env.Sender,
 			env.Recipients,
 			env.Data,
 		)
 		if err != nil {
-		log.Printf("delivery failed: %v\n", err)
-		return smtpd.Error{Code: 554, Message: "Forwarding failed"}
+			var smtpError smtpd.Error
+
+			switch err := err.(type) {
+			case *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")
 			}
 
-	log.Printf("%s delivery successful\n", env.Recipients)
+			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
@@ -214,12 +258,16 @@ func getTLSConfig() *tls.Config {
 	}
 
 	if *localCert == "" || *localKey == "" {
-		log.Fatal("TLS certificate/key not defined in config")
+		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.Fatal(err)
+		log.WithField("error", err).
+			Fatal("cannot load X509 keypair")
 	}
 
 	return &tls.Config{
@@ -233,79 +281,113 @@ func getTLSConfig() *tls.Config {
 func main() {
 	ConfigLoad()
 
-	if *versionInfo {
-		fmt.Printf("smtprelay/%s (%s)\n", appVersion, buildTime)
-		os.Exit(0)
-	}
-
-	if *logFile != "" {
-		f, err := os.OpenFile(*logFile, os.O_WRONLY|os.O_CREATE|os.O_APPEND, 0600)
-		if err != nil {
-			log.Fatalf("Error opening logfile: %v", err)
-		}
-		defer f.Close()
-
-		log.SetOutput(io.MultiWriter(os.Stdout, f))
-	}
+	log.WithField("version", appVersion).
+		Debug("starting smtprelay")
 
 	// Load allowed users file
-	if *allowedUsers != "" {
+	if localAuthRequired() {
 		err := AuthLoadFile(*allowedUsers)
 		if err != nil {
-			log.Fatalf("Authentication file: %s\n", err)
+			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, " ") {
+	for _, listen := range listenAddrs {
+		logger := log.WithField("address", listen.address)
+
 		server := &smtpd.Server{
 			Hostname:          *hostName,
 			WelcomeMessage:    *welcomeMsg,
+			ReadTimeout:       readTimeout,
+			WriteTimeout:      writeTimeout,
+			DataTimeout:       dataTimeout,
+			MaxConnections:    *maxConnections,
+			MaxMessageSize:    *maxMessageSize,
+			MaxRecipients:     *maxRecipients,
 			ConnectionChecker: connectionChecker,
 			SenderChecker:     senderChecker,
 			RecipientChecker:  recipientChecker,
 			Handler:           mailHandler,
 		}
 
-		if *allowedUsers != "" {
+		if localAuthRequired() {
 			server.Authenticator = authChecker
 		}
 
 		var lsnr net.Listener
 		var err error
 
-		if strings.Index(listenAddr, "://") == -1 {
-			log.Printf("Listen on %s ...\n", listenAddr)
-
-			lsnr, err = net.Listen("tcp", listenAddr)
-		} else if strings.HasPrefix(listenAddr, "starttls://") {
-			listenAddr = strings.TrimPrefix(listenAddr, "starttls://")
+		switch listen.protocol {
+		case "":
+			logger.Info("listening on address")
+			lsnr, err = net.Listen("tcp", listen.address)
 
+		case "starttls":
 			server.TLSConfig = getTLSConfig()
 			server.ForceTLS = *localForceTLS
 
-			log.Printf("Listen on %s (STARTTLS) ...\n", listenAddr)
-			lsnr, err = net.Listen("tcp", listenAddr)
-		} else if strings.HasPrefix(listenAddr, "tls://") {
-			listenAddr = strings.TrimPrefix(listenAddr, "tls://")
+			logger.Info("listening on address (STARTTLS)")
+			lsnr, err = net.Listen("tcp", listen.address)
 
+		case "tls":
 			server.TLSConfig = getTLSConfig()
 
-			log.Printf("Listen on %s (TLS) ...\n", listenAddr)
-			lsnr, err = tls.Listen("tcp", listenAddr, server.TLSConfig)
-		} else {
-			log.Fatal("Unknown protocol in listen address ", listenAddr)
+			logger.Info("listening on address (TLS)")
+			lsnr, err = tls.Listen("tcp", listen.address, server.TLSConfig)
+
+		default:
+			logger.WithField("protocol", listen.protocol).
+				Fatal("unknown protocol in listen address")
 		}
 
 		if err != nil {
-			log.Fatal(err)
+			logger.WithError(err).Fatal("error starting listener")
 		}
-		defer lsnr.Close()
+		servers = append(servers, server)
 
-		go server.Serve(lsnr)
+		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")
+}

remotes.go

@@ -0,0 +1,83 @@
+package main
+
+import (
+	"fmt"
+	"net/smtp"
+	"net/url"
+)
+
+type Remote struct {
+	SkipVerify bool
+	Auth       smtp.Auth
+	Scheme     string
+	Hostname   string
+	Port       string
+	Addr       string
+	Sender     string
+}
+
+// ParseRemote creates a remote from a given url in the following format:
+//
+// smtp://[user[:password]@][netloc][:port][/remote_sender][?param1=value1&...]
+// smtps://[user[:password]@][netloc][:port][/remote_sender][?param1=value1&...]
+// starttls://[user[:password]@][netloc][:port][/remote_sender][?param1=value1&...]
+//
+// Supported Params:
+// - skipVerify: can be "true" or empty to prevent ssl verification of remote server's certificate.
+// - auth: can be "login" to trigger "LOGIN" auth instead of "PLAIN" auth
+func ParseRemote(remoteURL string) (*Remote, error) {
+	u, err := url.Parse(remoteURL)
+	if err != nil {
+		return nil, err
+	}
+
+	if u.Scheme != "smtp" && u.Scheme != "smtps" && u.Scheme != "starttls" {
+		return nil, fmt.Errorf("'%s' is not a supported relay scheme", u.Scheme)
+	}
+
+	hostname, port := u.Hostname(), u.Port()
+
+	if port == "" {
+		switch u.Scheme {
+		case "smtp":
+			port = "25"
+		case "smtps":
+			port = "465"
+		case "starttls":
+			port = "587"
+		}
+	}
+
+	q := u.Query()
+	r := &Remote{
+		Scheme:   u.Scheme,
+		Hostname: hostname,
+		Port:     port,
+		Addr:     fmt.Sprintf("%s:%s", hostname, port),
+	}
+
+	if u.User != nil {
+		pass, _ := u.User.Password()
+		user := u.User.Username()
+
+		if hasAuth, authVal := q.Has("auth"), q.Get("auth"); hasAuth {
+			if authVal != "login" {
+				return nil, fmt.Errorf("Auth must be login or not present, received '%s'", authVal)
+			}
+
+			r.Auth = LoginAuth(user, pass)
+		} else {
+			r.Auth = smtp.PlainAuth("", user, pass, u.Hostname())
+		}
+	}
+
+	if hasVal, skipVerify := q.Has("skipVerify"), q.Get("skipVerify"); hasVal && skipVerify != "false" {
+		r.SkipVerify = true
+	}
+
+	if u.Path != "" {
+		r.Sender = u.Path[1:]
+	}
+
+	return r, nil
+}

remotes_test.go

@@ -0,0 +1,114 @@
+package main
+
+import (
+	"net/smtp"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func AssertRemoteUrlEquals(t *testing.T, expected *Remote, remotUrl string) {
+	actual, err := ParseRemote(remotUrl)
+	assert.Nil(t, err)
+	assert.NotNil(t, actual)
+	assert.Equal(t, expected.Scheme, actual.Scheme, "Scheme %s", remotUrl)
+	assert.Equal(t, expected.Addr, actual.Addr, "Addr %s", remotUrl)
+	assert.Equal(t, expected.Hostname, actual.Hostname, "Hostname %s", remotUrl)
+	assert.Equal(t, expected.Port, actual.Port, "Port %s", remotUrl)
+	assert.Equal(t, expected.Sender, actual.Sender, "Sender %s", remotUrl)
+	assert.Equal(t, expected.SkipVerify, actual.SkipVerify, "SkipVerify %s", remotUrl)
+
+	if expected.Auth != nil || actual.Auth != nil {
+		assert.NotNil(t, expected, "Auth %s", remotUrl)
+		assert.NotNil(t, actual, "Auth %s", remotUrl)
+		assert.IsType(t, expected.Auth, actual.Auth)
+	}
+}
+
+func TestValidRemoteUrls(t *testing.T) {
+	AssertRemoteUrlEquals(t, &Remote{
+		Scheme:     "smtp",
+		SkipVerify: false,
+		Auth:       nil,
+		Hostname:   "email.com",
+		Port:       "25",
+		Addr:       "email.com:25",
+		Sender:     "",
+	}, "smtp://email.com")
+
+	AssertRemoteUrlEquals(t, &Remote{
+		Scheme:     "smtp",
+		SkipVerify: true,
+		Auth:       nil,
+		Hostname:   "email.com",
+		Port:       "25",
+		Addr:       "email.com:25",
+		Sender:     "",
+	}, "smtp://email.com?skipVerify")
+
+	AssertRemoteUrlEquals(t, &Remote{
+		Scheme:     "smtp",
+		SkipVerify: false,
+		Auth:       smtp.PlainAuth("", "user", "pass", ""),
+		Hostname:   "email.com",
+		Port:       "25",
+		Addr:       "email.com:25",
+		Sender:     "",
+	}, "smtp://user:pass@email.com")
+
+	AssertRemoteUrlEquals(t, &Remote{
+		Scheme:     "smtp",
+		SkipVerify: false,
+		Auth:       LoginAuth("user", "pass"),
+		Hostname:   "email.com",
+		Port:       "25",
+		Addr:       "email.com:25",
+		Sender:     "",
+	}, "smtp://user:pass@email.com?auth=login")
+
+	AssertRemoteUrlEquals(t, &Remote{
+		Scheme:     "smtp",
+		SkipVerify: false,
+		Auth:       LoginAuth("user", "pass"),
+		Hostname:   "email.com",
+		Port:       "25",
+		Addr:       "email.com:25",
+		Sender:     "sender@website.com",
+	}, "smtp://user:pass@email.com/sender@website.com?auth=login")
+
+	AssertRemoteUrlEquals(t, &Remote{
+		Scheme:     "smtps",
+		SkipVerify: false,
+		Auth:       LoginAuth("user", "pass"),
+		Hostname:   "email.com",
+		Port:       "465",
+		Addr:       "email.com:465",
+		Sender:     "sender@website.com",
+	}, "smtps://user:pass@email.com/sender@website.com?auth=login")
+
+	AssertRemoteUrlEquals(t, &Remote{
+		Scheme:     "smtps",
+		SkipVerify: true,
+		Auth:       LoginAuth("user", "pass"),
+		Hostname:   "email.com",
+		Port:       "8425",
+		Addr:       "email.com:8425",
+		Sender:     "sender@website.com",
+	}, "smtps://user:pass@email.com:8425/sender@website.com?auth=login&skipVerify")
+
+	AssertRemoteUrlEquals(t, &Remote{
+		Scheme:     "starttls",
+		SkipVerify: true,
+		Auth:       LoginAuth("user", "pass"),
+		Hostname:   "email.com",
+		Port:       "8425",
+		Addr:       "email.com:8425",
+		Sender:     "sender@website.com",
+	}, "starttls://user:pass@email.com:8425/sender@website.com?auth=login&skipVerify")
+}
+
+func TestMissingScheme(t *testing.T) {
+	_, err := ParseRemote("http://user:pass@email.com:8425/sender@website.com")
+	assert.NotNil(t, err, "Err must be present")
+	assert.Equal(t, err.Error(), "'http' is not a supported relay scheme")
+}

smtp.go

@@ -4,9 +4,11 @@
 
 // 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.
@@ -106,7 +108,7 @@ func (c *Client) Hello(localName string) error {
 }
 
 // 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) {
+func (c *Client) cmd(expectCode int, format string, args ...any) (int, string, error) {
 	id, err := c.Text.Cmd(format, args...)
 	if err != nil {
 		return 0, "", err
@@ -137,12 +139,8 @@ func (c *Client) ehlo() error {
 	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]] = ""
-			}
+			k, v, _ := strings.Cut(line, " ")
+			ext[k] = v
 		}
 	}
 	if mechs, ok := ext["AUTH"]; ok {
@@ -242,7 +240,8 @@ func (c *Client) Auth(a smtp.Auth) error {
 
 // 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.
+// 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 {
@@ -256,6 +255,9 @@ func (c *Client) Mail(from string) error {
 		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
@@ -318,7 +320,11 @@ var testHookStartTLS func(*tls.Config) // nil, except for tests
 // 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 {
+func SendMail(r *Remote, from string, to []string, msg []byte) error {
+	if r.Sender != "" {
+		from = r.Sender
+	}
+
 	if err := validateLine(from); err != nil {
 		return err
 	}
@@ -327,19 +333,19 @@ func SendMail(addr string, a smtp.Auth, from string, to []string, msg []byte) er
 			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)
+	var err error
+	if r.Scheme == "smtps" {
+		config := &tls.Config{
+			ServerName:         r.Hostname,
+			InsecureSkipVerify: r.SkipVerify,
+		}
+		conn, err := tls.Dial("tcp", r.Addr, config)
 		if err != nil {
 			return err
 		}
 		defer conn.Close()
-		c, err = NewClient(conn, host)
+		c, err = NewClient(conn, r.Hostname)
 		if err != nil {
 			return err
 		}
@@ -347,7 +353,7 @@ func SendMail(addr string, a smtp.Auth, from string, to []string, msg []byte) er
 			return err
 		}
 	} else {
-		c, err = Dial(addr)
+		c, err = Dial(r.Addr)
 		if err != nil {
 			return err
 		}
@@ -356,20 +362,25 @@ func SendMail(addr string, a smtp.Auth, from string, to []string, msg []byte) er
 			return err
 		}
 		if ok, _ := c.Extension("STARTTLS"); ok {
-			config := &tls.Config{ServerName: c.serverName}
+			config := &tls.Config{
+				ServerName:         c.serverName,
+				InsecureSkipVerify: r.SkipVerify,
+			}
 			if testHookStartTLS != nil {
 				testHookStartTLS(config)
 			}
 			if err = c.StartTLS(config); err != nil {
 				return err
 			}
+		} else if r.Scheme == "starttls" {
+			return errors.New("starttls: server does not support extension, check remote scheme")
 		}
 	}
-	if a != nil && c.ext != nil {
+	if r.Auth != 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 {
+		if err = c.Auth(r.Auth); err != nil {
 			return err
 		}
 	}

smtprelay.ini

@@ -1,13 +1,23 @@
 ; smtprelay configuration
+;
+; All config parameters can also be provided as environment
+; variables in uppercase and the prefix "SMTPRELAY_".
+; (eg. SMTPRELAY_LOGFILE, SMTPRELAY_LOG_FORMAT)
 
-; Logfile
-;logfile = /var/log/smtprelay.log
+; 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"
+;hostname = localhost.localdomain
 
 ; Welcome message for clients
-;welcome_msg = "<hostname> ESMTP ready."
+;welcome_msg = <hostname> ESMTP ready.
 
 ; Listen on the following addresses for incoming
 ; unencrypted connections.
@@ -24,14 +34,44 @@
 ; accepting mails from client.
 ;local_forcetls = false
 
+; Socket timeout for read operations
+; Duration string as sequence of decimal numbers,
+; each with optional fraction and a unit suffix.
+; Valid time units are "ns", "us", "ms", "s", "m", "h".
+;read_timeout = 60s
+
+; Socket timeout for write operations
+; Duration string as sequence of decimal numbers,
+; each with optional fraction and a unit suffix.
+; Valid time units are "ns", "us", "ms", "s", "m", "h".
+;write_timeout = 60s
+
+; Socket timeout for DATA command
+; Duration string as sequence of decimal numbers,
+; each with optional fraction and a unit suffix.
+; Valid time units are "ns", "us", "ms", "s", "m", "h".
+;data_timeout = 5m
+
+; Max concurrent connections, use -1 to disable
+;max_connections = 100
+
+; Max message size in bytes
+;max_message_size = 10240000
+
+; Max RCPT TO calls for each envelope
+;max_recipients = 100
+
 ; Networks that are allowed to send mails to us
-;allowed_nets = 127.0.0.1/8 ::1/128
+; 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 =
 
@@ -47,24 +87,40 @@
 ;          E.g. "app@example.com,@appsrv.example.com"
 ;allowed_users =
 
-; Relay all mails to this SMTP server
+; Relay all mails to this SMTP servers.
+; If not set, mails are discarded.
+;
+; Format:
+;   protocol://[user[:password]@][netloc][:port][/remote_sender][?param1=value1&...]
+;
+;   protocol: smtp (unencrypted), smtps (TLS), starttls (STARTTLS)
+;   user: Username for authentication
+;   password: Password for authentication
+;   remote_sender: Email address to use as FROM
+;   params:
+;     skipVerify: "true" or empty to prevent ssl verification of remote server's certificate
+;     auth: "login" to use LOGIN authentication
 
 ; GMail
-;remote_host = smtp.gmail.com:587
+;remotes = starttls://user:pass@smtp.gmail.com:587
 
 ; Mailgun.org
-;remote_host = smtp.mailgun.org:587
+;remotes = starttls://user:pass@smtp.mailgun.org:587
 
 ; Mailjet.com
-;remote_host = in-v3.mailjet.com:587
+;remotes = starttls://user:pass@in-v3.mailjet.com:587
 
-; Authentication credentials on outgoing SMTP server
-;remote_user =
-;remote_pass =
+; Ignore remote host certificates
+;remotes = starttls://user:pass@server:587?skipVerify
 
-; Authentication method on outgoing SMTP server
-; (plain, login)
-;remote_auth = plain
+; Login Authentication method on outgoing SMTP server
+;remotes = smtp://user:pass@server:2525?auth=login
 
 ; Sender e-mail address on outgoing SMTP server
-;remote_sender = 
+;remotes = smtp://user:pass@server:2525/overridden@email.com?auth=login
+
+; Multiple remotes, space delimited
+;remotes = smtp://127.0.0.1:1025 starttls://user:pass@smtp.mailgun.org:587
+
+; Pipe messages to external command
+;command = /usr/local/bin/script