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implement dnssec-awareness throughout code, and dane for incoming/outgoing mail delivery

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the vendored dns resolver code is a copy of the go stdlib dns resolver, with
awareness of the "authentic data" (i.e. dnssec secure) added, as well as support
for enhanced dns errors, and looking up tlsa records (for dane). ideally it
would be upstreamed, but the chances seem slim.

dnssec-awareness is added to all packages, e.g. spf, dkim, dmarc, iprev. their
dnssec status is added to the Received message headers for incoming email.

but the main reason to add dnssec was for implementing dane. with dane, the
verification of tls certificates can be done through certificates/public keys
published in dns (in the tlsa records). this only makes sense (is trustworthy)
if those dns records can be verified to be authentic.

mox now applies dane to delivering messages over smtp. mox already implemented
mta-sts for webpki/pkix-verification of certificates against the (large) pool
of CA's, and still enforces those policies when present. but it now also checks
for dane records, and will verify those if present. if dane and mta-sts are
both absent, the regular opportunistic tls with starttls is still done. and the
fallback to plaintext is also still done.

mox also makes it easy to setup dane for incoming deliveries, so other servers
can deliver with dane tls certificate verification. the quickstart now
generates private keys that are used when requesting certificates with acme.
the private keys are pre-generated because they must be static and known during
setup, because their public keys must be published in tlsa records in dns.
autocert would generate private keys on its own, so had to be forked to add the
option to provide the private key when requesting a new certificate. hopefully
upstream will accept the change and we can drop the fork.

with this change, using the quickstart to setup a new mox instance, the checks
at internet.nl result in a 100% score, provided the domain is dnssec-signed and
the network doesn't have any issues.
This commit is contained in:
Mechiel Lukkien
2023-10-10 12:09:35 +02:00
parent c4324fdaa1
commit daa908e9f4
177 changed files with 12907 additions and 3131 deletions
Download Patch File Download Diff File Expand all files Collapse all files

108
smtpclient/client.go
View File

@ -17,6 +17,9 @@ import (
"github.com/prometheus/client_golang/prometheus"
"github.com/prometheus/client_golang/prometheus/promauto"
"github.com/mjl-/adns"
"github.com/mjl-/mox/dane"
"github.com/mjl-/mox/dns"
"github.com/mjl-/mox/metrics"
"github.com/mjl-/mox/mlog"
@ -49,7 +52,7 @@ var (
ErrSMTPUTF8Unsupported = errors.New("remote smtp server does not implement smtputf8 extension, required by message")
ErrStatus = errors.New("remote smtp server sent unexpected response status code") // Relatively common, e.g. when a 250 OK was expected and server sent 451 temporary error.
ErrProtocol = errors.New("smtp protocol error") // After a malformed SMTP response or inconsistent multi-line response.
ErrTLS = errors.New("tls error") // E.g. handshake failure, or hostname validation was required and failed.
ErrTLS = errors.New("tls error") // E.g. handshake failure, or hostname verification was required and failed.
ErrBotched = errors.New("smtp connection is botched") // Set on a client, and returned for new operations, after an i/o error or malformed SMTP response.
ErrClosed = errors.New("client is closed")
)
@ -58,13 +61,20 @@ var (
type TLSMode string
const (
// TLS with STARTTLS for MX SMTP servers, with validated certificate is required: matching name, not expired, trusted by CA.
// Required TLS with STARTTLS for SMTP servers, with either verified DANE TLSA
// record, or a WebPKI-verified certificate (with matching name, not expired, etc).
TLSStrictStartTLS TLSMode = "strictstarttls"
// TLS immediately ("implicit TLS"), with validated certificate is required: matching name, not expired, trusted by CA.
// Required TLS with STARTTLS for SMTP servers, without verifiying the certificate.
// This mode is needed to fallback after only unusable DANE records were found
// (e.g. with unknown parameters in the TLSA records).
TLSUnverifiedStartTLS TLSMode = "unverifiedstarttls"
// TLS immediately ("implicit TLS"), with either verified DANE TLSA records or a
// verified certificate: matching name, not expired, trusted by CA.
TLSStrictImmediate TLSMode = "strictimmediate"
// Use TLS if remote claims to support it, but do not validate the certificate
// Use TLS if remote claims to support it, but do not verify the certificate
// (not trusted by CA, different host name or expired certificate is accepted).
TLSOpportunistic TLSMode = "opportunistic"
@ -80,8 +90,12 @@ type Client struct {
// can be wrapped in a tls.Client. We close origConn instead of conn because
// closing the TLS connection would send a TLS close notification, which may block
// for 5s if the server isn't reading it (because it is also sending it).
origConn net.Conn
conn net.Conn
origConn net.Conn
conn net.Conn
remoteHostname dns.Domain // TLS with SNI and name verification.
daneRecords []adns.TLSA // For authenticating (START)TLS connection.
moreRemoteHostnames []dns.Domain // Additional allowed names in TLS certificate.
verifiedRecord *adns.TLSA // If non-nil, then will be set to verified DANE record if any.
r *bufio.Reader
w *bufio.Writer
@ -164,21 +178,27 @@ func (e Error) Error() string {
// records with preferences, other DNS records, MTA-STS, retries and special
// cases into account.
//
// tlsMode indicates if TLS is required, optional or should not be used. A
// certificate is only validated (trusted, match remoteHostname and not expired)
// for the strict tls modes. By default, SMTP does not verify TLS for
// interopability reasons, but MTA-STS or DANE can require it. If opportunistic TLS
// is used, and a TLS error is encountered, the caller may want to try again (on a
// new connection) without TLS.
// tlsMode indicates if TLS is required, optional or should not be used. Only for
// strict TLS modes is the certificate verified: Either with DANE, or through
// the trusted CA pool with matching remoteHostname and not expired. For DANE,
// additional host names in moreRemoteHostnames are allowed during TLS certificate
// verification. By default, SMTP does not verify TLS for interopability reasons,
// but MTA-STS or DANE can require it. If opportunistic TLS is used, and a TLS
// error is encountered, the caller may want to try again (on a new connection)
// without TLS.
//
// If auth is non-empty, authentication will be done with the first algorithm
// supported by the server. If none of the algorithms are supported, an error is
// returned.
func New(ctx context.Context, log *mlog.Log, conn net.Conn, tlsMode TLSMode, ourHostname, remoteHostname dns.Domain, auth []sasl.Client) (*Client, error) {
func New(ctx context.Context, log *mlog.Log, conn net.Conn, tlsMode TLSMode, ehloHostname, remoteHostname dns.Domain, auth []sasl.Client, daneRecords []adns.TLSA, moreRemoteHostnames []dns.Domain, verifiedRecord *adns.TLSA) (*Client, error) {
c := &Client{
origConn: conn,
lastlog: time.Now(),
cmds: []string{"(none)"},
origConn: conn,
remoteHostname: remoteHostname,
daneRecords: daneRecords,
moreRemoteHostnames: moreRemoteHostnames,
verifiedRecord: verifiedRecord,
lastlog: time.Now(),
cmds: []string{"(none)"},
}
c.log = log.Fields(mlog.Field("smtpclient", "")).MoreFields(func() []mlog.Pair {
now := time.Now()
@ -190,12 +210,9 @@ func New(ctx context.Context, log *mlog.Log, conn net.Conn, tlsMode TLSMode, our
})
if tlsMode == TLSStrictImmediate {
tlsconfig := tls.Config{
ServerName: remoteHostname.ASCII,
RootCAs: mox.Conf.Static.TLS.CertPool,
MinVersion: tls.VersionTLS12, // ../rfc/8996:31 ../rfc/8997:66
}
tlsconn := tls.Client(conn, &tlsconfig)
// todo: we could also verify DANE here. not applicable to SMTP delivery.
config := c.tlsConfig(tlsMode)
tlsconn := tls.Client(conn, &config)
if err := tlsconn.HandshakeContext(ctx); err != nil {
return nil, err
}
@ -216,12 +233,25 @@ func New(ctx context.Context, log *mlog.Log, conn net.Conn, tlsMode TLSMode, our
c.tw = moxio.NewTraceWriter(c.log, "LC: ", timeoutWriter{c.conn, 30 * time.Second, c.log})
c.w = bufio.NewWriter(c.tw)
if err := c.hello(ctx, tlsMode, ourHostname, remoteHostname, auth); err != nil {
if err := c.hello(ctx, tlsMode, ehloHostname, auth); err != nil {
return nil, err
}
return c, nil
}
func (c *Client) tlsConfig(tlsMode TLSMode) tls.Config {
if c.daneRecords != nil {
return dane.TLSClientConfig(c.log, c.daneRecords, c.remoteHostname, c.moreRemoteHostnames, c.verifiedRecord)
}
// todo: possibly accept older TLS versions for TLSOpportunistic?
return tls.Config{
ServerName: c.remoteHostname.ASCII,
RootCAs: mox.Conf.Static.TLS.CertPool,
InsecureSkipVerify: tlsMode == TLSOpportunistic || tlsMode == TLSUnverifiedStartTLS,
MinVersion: tls.VersionTLS12, // ../rfc/8996:31 ../rfc/8997:66
}
}
// xbotchf generates a temporary error and marks the client as botched. e.g. for
// i/o errors or invalid protocol messages.
func (c *Client) xbotchf(code int, secode string, lastLine, format string, args ...any) {
@ -463,7 +493,7 @@ func (c *Client) recover(rerr *error) {
*rerr = cerr
}
func (c *Client) hello(ctx context.Context, tlsMode TLSMode, ourHostname, remoteHostname dns.Domain, auth []sasl.Client) (rerr error) {
func (c *Client) hello(ctx context.Context, tlsMode TLSMode, ehloHostname dns.Domain, auth []sasl.Client) (rerr error) {
defer c.recover(&rerr)
// perform EHLO handshake, falling back to HELO if server does not appear to
@ -474,7 +504,7 @@ func (c *Client) hello(ctx context.Context, tlsMode TLSMode, ourHostname, remote
c.cmds[0] = "ehlo"
c.cmdStart = time.Now()
// Syntax: ../rfc/5321:1827
c.xwritelinef("EHLO %s", ourHostname.ASCII)
c.xwritelinef("EHLO %s", ehloHostname.ASCII)
code, _, lastLine, remains := c.xreadecode(false)
switch code {
// ../rfc/5321:997
@ -486,7 +516,7 @@ func (c *Client) hello(ctx context.Context, tlsMode TLSMode, ourHostname, remote
// ../rfc/5321:996
c.cmds[0] = "helo"
c.cmdStart = time.Now()
c.xwritelinef("HELO %s", ourHostname.ASCII)
c.xwritelinef("HELO %s", ehloHostname.ASCII)
code, _, lastLine, _ = c.xreadecode(false)
if code != smtp.C250Completed {
c.xerrorf(code/100 == 5, code, "", lastLine, "%w: expected 250 to HELO, got %d", ErrStatus, code)
@ -536,8 +566,8 @@ func (c *Client) hello(ctx context.Context, tlsMode TLSMode, ourHostname, remote
hello(true)
// Attempt TLS if remote understands STARTTLS and we aren't doing immediate TLS or if caller requires it.
if c.extStartTLS && (tlsMode != TLSSkip && tlsMode != TLSStrictImmediate) || tlsMode == TLSStrictStartTLS {
c.log.Debug("starting tls client", mlog.Field("tlsmode", tlsMode), mlog.Field("servername", remoteHostname))
if c.extStartTLS && (tlsMode != TLSSkip && tlsMode != TLSStrictImmediate) || tlsMode == TLSStrictStartTLS || tlsMode == TLSUnverifiedStartTLS {
c.log.Debug("starting tls client", mlog.Field("tlsmode", tlsMode), mlog.Field("servername", c.remoteHostname))
c.cmds[0] = "starttls"
c.cmdStart = time.Now()
c.xwritelinef("STARTTLS")
@ -561,14 +591,8 @@ func (c *Client) hello(ctx context.Context, tlsMode TLSMode, ourHostname, remote
// For TLSStrictStartTLS, the Go TLS library performs the checks needed for MTA-STS.
// ../rfc/8461:646
// todo: possibly accept older TLS versions for TLSOpportunistic?
tlsConfig := &tls.Config{
ServerName: remoteHostname.ASCII,
RootCAs: mox.Conf.Static.TLS.CertPool,
InsecureSkipVerify: tlsMode != TLSStrictStartTLS,
MinVersion: tls.VersionTLS12, // ../rfc/8996:31 ../rfc/8997:66
}
nconn := tls.Client(conn, tlsConfig)
tlsConfig := c.tlsConfig(tlsMode)
nconn := tls.Client(conn, &tlsConfig)
c.conn = nconn
nctx, cancel := context.WithTimeout(ctx, time.Minute)
@ -584,7 +608,7 @@ func (c *Client) hello(ctx context.Context, tlsMode TLSMode, ourHostname, remote
c.w = bufio.NewWriter(c.tw)
tlsversion, ciphersuite := mox.TLSInfo(nconn)
c.log.Debug("starttls client handshake done", mlog.Field("tls", tlsversion), mlog.Field("ciphersuite", ciphersuite), mlog.Field("servername", remoteHostname), mlog.Field("insecureskipverify", tlsConfig.InsecureSkipVerify))
c.log.Debug("starttls client handshake done", mlog.Field("tlsmode", tlsMode), mlog.Field("tls", tlsversion), mlog.Field("ciphersuite", ciphersuite), mlog.Field("servername", c.remoteHostname), mlog.Field("danerecord", c.verifiedRecord))
hello(false)
}
@ -916,3 +940,13 @@ func (c *Client) Close() (rerr error) {
}
return
}
// Conn returns the connection with initialized SMTP session. Once the caller uses
// this connection it is in control, and responsible for closing the connection,
// and other functions on the client must not be called anymore.
func (c *Client) Conn() (net.Conn, error) {
if err := c.conn.SetDeadline(time.Time{}); err != nil {
return nil, fmt.Errorf("clearing io deadlines: %w", err)
}
return c.conn, nil
}