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main.go
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main.go
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package main
import (
"bytes"
"crypto"
"crypto/x509"
"encoding/pem"
"flag"
"fmt"
"io"
"io/ioutil"
"log"
"net"
"net/mail"
"net/textproto"
"os"
"os/signal"
"strings"
"syscall"
"github.com/emersion/go-milter"
"github.com/emersion/go-msgauth/authres"
"github.com/emersion/go-msgauth/dkim"
"golang.org/x/crypto/ed25519"
)
var (
signDomains stringSliceFlag
identity string
listenURI string
privateKeyPath string
selector string
verbose bool
)
var privateKey crypto.Signer
var signHeaderKeys = []string{
"From",
"Reply-To",
"Subject",
"Date",
"To",
"Cc",
"Resent-Date",
"Resent-From",
"Resent-To",
"Resent-Cc",
"In-Reply-To",
"References",
"List-Id",
"List-Help",
"List-Unsubscribe",
"List-Subscribe",
"List-Post",
"List-Owner",
"List-Archive",
}
const maxVerifications = 5
func init() {
flag.Var(&signDomains, "d", "Domain(s) whose mail should be signed")
flag.StringVar(&identity, "i", "", "Server identity (defaults to hostname)")
flag.StringVar(&listenURI, "l", "unix:///tmp/dkim-milter.sock", "Listen URI")
flag.StringVar(&privateKeyPath, "k", "", "Private key (PEM-formatted)")
flag.StringVar(&selector, "s", "", "Selector")
flag.BoolVar(&verbose, "v", false, "Enable verbose logging")
}
type stringSliceFlag []string
func (f *stringSliceFlag) String() string {
return strings.Join(*f, ", ")
}
func (f *stringSliceFlag) Set(value string) error {
*f = append(*f, value)
return nil
}
type session struct {
authResDelete []int
headerBuf bytes.Buffer
signDomain string
signHeaderKeys []string
done <-chan error
pw *io.PipeWriter
verifs []*dkim.Verification // only valid after done is closed
signer *dkim.Signer
mw io.Writer
}
func (s *session) Connect(host string, family string, port uint16, addr net.IP, m *milter.Modifier) (milter.Response, error) {
return nil, nil
}
func (s *session) Helo(name string, m *milter.Modifier) (milter.Response, error) {
return nil, nil
}
func (s *session) MailFrom(from string, m *milter.Modifier) (milter.Response, error) {
return nil, nil
}
func (s *session) RcptTo(rcptTo string, m *milter.Modifier) (milter.Response, error) {
return nil, nil
}
func parseAddressDomain(s string) (string, error) {
addr, err := mail.ParseAddress(s)
if err != nil {
return "", err
}
parts := strings.SplitN(addr.Address, "@", 2)
if len(parts) != 2 {
return "", fmt.Errorf("dkim-milter: malformed address: missing '@'")
}
return parts[1], nil
}
func (s *session) Header(name string, value string, m *milter.Modifier) (milter.Response, error) {
if strings.EqualFold(name, "From") || strings.EqualFold(name, "Sender") {
domain, err := parseAddressDomain(value)
if err != nil {
return nil, fmt.Errorf("dkim-milter: failed to parse header field '%v': %v", name, err)
}
for _, d := range signDomains {
if strings.EqualFold(d, domain) {
s.signDomain = d
break
}
}
}
for _, k := range signHeaderKeys {
if strings.EqualFold(name, k) {
s.signHeaderKeys = append(s.signHeaderKeys, name)
}
}
field := name + ": " + value + "\r\n"
_, err := s.headerBuf.WriteString(field)
return milter.RespContinue, err
}
func getIdentity(authRes string) string {
parts := strings.SplitN(authRes, ";", 2)
return strings.TrimSpace(parts[0])
}
func shouldDeleteAuthRes(field string) bool {
id, results, err := authres.Parse(field)
if err != nil {
// Delete fields we can't parse, because other implementations might
// accept malformed fields
return true
}
if !strings.EqualFold(id, identity) {
// Not our Authentication-Results, ignore the field
return false
}
for _, res := range results {
if _, ok := res.(*authres.DKIMResult); ok {
// Delete existing DKIM Authentication-Results fields
return true
}
}
// This is our Authentication-Results field, but it isn't about DKIM. Maybe
// a previous milter has generated it (e.g. SPF), so keep it.
return false
}
func (s *session) Headers(h textproto.MIMEHeader, m *milter.Modifier) (milter.Response, error) {
// Write final CRLF to begin message body
if _, err := s.headerBuf.WriteString("\r\n"); err != nil {
return nil, err
}
// Delete any existing Authentication-Results header field with our identity
fields := h["Authentication-Results"]
for i, field := range fields {
if shouldDeleteAuthRes(field) {
s.authResDelete = append(s.authResDelete, i)
}
}
// Sign if necessary
if s.signDomain != "" {
opts := dkim.SignOptions{
Domain: s.signDomain,
Selector: selector,
Signer: privateKey,
HeaderKeys: s.signHeaderKeys,
QueryMethods: []dkim.QueryMethod{dkim.QueryMethodDNSTXT},
}
var err error
s.signer, err = dkim.NewSigner(&opts)
if err != nil {
return nil, err
}
}
// Verify existing signatures
done := make(chan error, 1)
pr, pw := io.Pipe()
s.done = done
s.pw = pw
// TODO: limit max. number of signatures
go func() {
options := dkim.VerifyOptions{MaxVerifications: maxVerifications}
var err error
s.verifs, err = dkim.VerifyWithOptions(pr, &options)
io.Copy(ioutil.Discard, pr)
pr.Close()
done <- err
close(done)
}()
// Process header
return s.BodyChunk(s.headerBuf.Bytes(), m)
}
func (s *session) BodyChunk(chunk []byte, m *milter.Modifier) (milter.Response, error) {
if _, err := s.pw.Write(chunk); err != nil {
return nil, err
}
if s.signer != nil {
if _, err := s.signer.Write(chunk); err != nil {
return nil, err
}
}
return milter.RespContinue, nil
}
func (s *session) Body(m *milter.Modifier) (milter.Response, error) {
if err := s.pw.Close(); err != nil {
return nil, err
}
for _, index := range s.authResDelete {
if err := m.ChangeHeader(index, "Authentication-Results", ""); err != nil {
return nil, err
}
}
if err := <-s.done; err == dkim.ErrTooManySignatures {
if verbose {
log.Printf("Too many signatures in message: %v", err)
}
// Ignore the error
} else if err != nil {
if verbose {
log.Printf("DKIM verification failed: %v", err)
}
return nil, err
}
if s.signer != nil {
if err := s.signer.Close(); err != nil {
if verbose {
log.Printf("DKIM signature failed: %v", err)
}
return nil, err
}
kv := s.signer.Signature()
parts := strings.SplitN(kv, ": ", 2)
if len(parts) != 2 {
panic("dkim-milter: malformed DKIM-Signature header field")
}
k, v := parts[0], strings.TrimSuffix(parts[1], "\r\n")
if err := m.InsertHeader(0, k, v); err != nil {
return nil, err
}
}
results := make([]authres.Result, 0, len(s.verifs))
if len(s.verifs) == 0 && s.signer == nil {
results = append(results, &authres.DKIMResult{
Value: authres.ResultNone,
})
}
for _, verif := range s.verifs {
if verbose {
if verif.Err != nil {
log.Printf("DKIM verification failed for %v: %v", verif.Domain, verif.Err)
} else {
log.Printf("DKIM verification succeded for %v", verif.Domain)
}
}
var val authres.ResultValue
if verif.Err == nil {
val = authres.ResultPass
} else if dkim.IsPermFail(verif.Err) {
val = authres.ResultPermError
} else if dkim.IsTempFail(verif.Err) {
val = authres.ResultTempError
} else {
val = authres.ResultFail
}
results = append(results, &authres.DKIMResult{
Value: val,
Domain: verif.Domain,
Identifier: verif.Identifier,
})
}
if len(s.verifs) > 0 || s.signer == nil {
v := authres.Format(identity, results)
if err := m.InsertHeader(0, "Authentication-Results", v); err != nil {
return nil, err
}
}
return milter.RespAccept, nil
}
func loadPrivateKey(path string) (crypto.Signer, error) {
b, err := ioutil.ReadFile(privateKeyPath)
if err != nil {
return nil, err
}
block, _ := pem.Decode(b)
if block == nil {
return nil, fmt.Errorf("no PEM data found")
}
switch strings.ToUpper(block.Type) {
case "PRIVATE KEY":
k, err := x509.ParsePKCS8PrivateKey(block.Bytes)
if err != nil {
return nil, err
}
return k.(crypto.Signer), nil
case "RSA PRIVATE KEY":
return x509.ParsePKCS1PrivateKey(block.Bytes)
case "EDDSA PRIVATE KEY":
if len(block.Bytes) != ed25519.PrivateKeySize {
return nil, fmt.Errorf("invalid Ed25519 private key size")
}
return ed25519.PrivateKey(block.Bytes), nil
default:
return nil, fmt.Errorf("unknown private key type: '%v'", block.Type)
}
}
func main() {
flag.Parse()
if identity == "" {
var err error
identity, err = os.Hostname()
if err != nil {
log.Fatal("Failed to read hostname: ", err)
}
}
if (len(signDomains) > 0 || privateKeyPath != "" || selector != "") && !(len(signDomains) > 0 && privateKeyPath != "" && selector != "") {
log.Fatal("Domain(s) (-d) and private key (-k) must be both specified")
}
if privateKeyPath != "" {
var err error
privateKey, err = loadPrivateKey(privateKeyPath)
if err != nil {
log.Fatalf("Failed to load private key from '%v': %v", privateKeyPath, err)
}
}
parts := strings.SplitN(listenURI, "://", 2)
if len(parts) != 2 {
log.Fatal("Invalid listen URI")
}
listenNetwork, listenAddr := parts[0], parts[1]
s := milter.Server{
NewMilter: func() milter.Milter {
return &session{}
},
Actions: milter.OptAddHeader | milter.OptChangeHeader,
Protocol: milter.OptNoConnect | milter.OptNoHelo | milter.OptNoMailFrom | milter.OptNoRcptTo,
}
ln, err := net.Listen(listenNetwork, listenAddr)
if err != nil {
log.Fatal("Failed to setup listener: ", err)
}
// Closing the listener will unlink the unix socket, if any
sigs := make(chan os.Signal, 1)
signal.Notify(sigs, syscall.SIGINT, syscall.SIGTERM)
go func() {
<-sigs
if err := s.Close(); err != nil {
log.Fatal("Failed to close server: ", err)
}
}()
log.Println("Milter listening at", listenURI)
if err := s.Serve(ln); err != nil && err != milter.ErrServerClosed {
log.Fatal("Failed to serve: ", err)
}
}