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implement "requiretls", rfc 8689

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with requiretls, the tls verification mode/rules for email deliveries can be
changed by the sender/submitter. in two ways:

1. "requiretls" smtp extension to always enforce verified tls (with mta-sts or
dnssec+dane), along the entire delivery path until delivery into the final
destination mailbox (so entire transport is verified-tls-protected).

2. "tls-required: no" message header, to ignore any tls and tls verification
errors even if the recipient domain has a policy that requires tls verification
(mta-sts and/or dnssec+dane), allowing delivery of non-sensitive messages in
case of misconfiguration/interoperability issues (at least useful for sending
tls reports).

we enable requiretls by default (only when tls is active), for smtp and
submission. it can be disabled through the config.

for each delivery attempt, we now store (per recipient domain, in the account
of the sender) whether the smtp server supports starttls and requiretls. this
support is shown (after having sent a first message) in the webmail when
sending a message (the previous 3 bars under the address input field are now 5
bars, the first for starttls support, the last for requiretls support). when
all recipient domains for a message are known to implement requiretls,
requiretls is automatically selected for sending (instead of "default" tls
behaviour). users can also select the "fallback to insecure" to add the
"tls-required: no" header.

new metrics are added for insight into requiretls errors and (some, not yet
all) cases where tls-required-no ignored a tls/verification error.

the admin can change the requiretls status for messages in the queue. so with
default delivery attempts, when verified tls is required by failing, an admin
could potentially change the field to "tls-required: no"-behaviour.

messages received (over smtp) with the requiretls option, get a comment added
to their Received header line, just before "id", after "with".
This commit is contained in:
Mechiel Lukkien
2023-10-24 10:06:16 +02:00
parent 0e5e16b3d0
commit 2f5d6069bf
31 changed files with 1102 additions and 274 deletions
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96
smtpclient/client.go
View File

@ -44,17 +44,27 @@ var (
"secode",
},
)
metricTLSRequiredNoIgnored = promauto.NewCounterVec(
prometheus.CounterOpts{
Name: "mox_smtpclient_tlsrequiredno_ignored_total",
Help: "Connection attempts with TLS policy findings ignored due to message with TLS-Required: No header. Does not cover case where TLS certificate cannot be PKIX-verified.",
},
[]string{
"ignored", // daneverification (no matching tlsa record)
},
)
)
var (
ErrSize = errors.New("message too large for remote smtp server") // SMTP server announced a maximum message size and the message to be delivered exceeds it.
Err8bitmimeUnsupported = errors.New("remote smtp server does not implement 8bitmime extension, required by message")
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 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")
ErrSize = errors.New("message too large for remote smtp server") // SMTP server announced a maximum message size and the message to be delivered exceeds it.
Err8bitmimeUnsupported = errors.New("remote smtp server does not implement 8bitmime extension, required by message")
ErrSMTPUTF8Unsupported = errors.New("remote smtp server does not implement smtputf8 extension, required by message")
ErrRequireTLSUnsupported = errors.New("remote smtp server does not implement requiretls extension, required for delivery")
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 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")
)
// TLSMode indicates if TLS must, should or must not be used.
@ -67,7 +77,8 @@ const (
// 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).
// (e.g. with unknown parameters in the TLSA records). Also for allowing
// verification errors with DANE with message header TLS-Required no.
TLSUnverifiedStartTLS TLSMode = "unverifiedstarttls"
// TLS immediately ("implicit TLS"), with either verified DANE TLSA records or a
@ -105,6 +116,7 @@ type Client struct {
lastlog time.Time // For adding delta timestamps between log lines.
cmds []string // Last or active command, for generating errors and metrics.
cmdStart time.Time // Start of command.
tls bool // Whether connection is TLS protected.
botched bool // If set, protocol is out of sync and no further commands can be sent.
needRset bool // If set, a new delivery requires an RSET command.
@ -117,6 +129,7 @@ type Client struct {
extPipelining bool // Remote server supports command pipelining.
extSMTPUTF8 bool // Remote server supports SMTPUTF8 extension.
extAuthMechanisms []string // Supported authentication mechanisms.
extRequireTLS bool // Remote supports REQUIRETLS extension.
}
// Error represents a failure to deliver a message.
@ -185,7 +198,9 @@ func (e Error) Error() string {
// 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.
// without TLS. For messages with header TLS-Required no, DANE records may be
// passed along with tlsMode TLSUnverifiedStartTLS. In that case, failing DANE
// verification causes an error to be logged, but the connection won't be aborted.
//
// 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
@ -212,13 +227,14 @@ func New(ctx context.Context, log *mlog.Log, conn net.Conn, tlsMode TLSMode, ehl
if tlsMode == TLSStrictImmediate {
// todo: we could also verify DANE here. not applicable to SMTP delivery.
config := c.tlsConfig(tlsMode)
tlsconn := tls.Client(conn, &config)
tlsconn := tls.Client(conn, config)
if err := tlsconn.HandshakeContext(ctx); err != nil {
return nil, err
}
c.conn = tlsconn
tlsversion, ciphersuite := mox.TLSInfo(tlsconn)
c.log.Debug("tls client handshake done", mlog.Field("tls", tlsversion), mlog.Field("ciphersuite", ciphersuite), mlog.Field("servername", remoteHostname))
c.tls = true
} else {
c.conn = conn
}
@ -239,12 +255,26 @@ func New(ctx context.Context, log *mlog.Log, conn net.Conn, tlsMode TLSMode, ehl
return c, nil
}
func (c *Client) tlsConfig(tlsMode TLSMode) tls.Config {
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)
config := dane.TLSClientConfig(c.log, c.daneRecords, c.remoteHostname, c.moreRemoteHostnames, c.verifiedRecord)
if tlsMode == TLSUnverifiedStartTLS {
// In case of delivery with header "TLS-Required: No", the connection should not be
// aborted.
origVerify := config.VerifyConnection
config.VerifyConnection = func(cs tls.ConnectionState) error {
err := origVerify(cs)
if err != nil {
c.log.Infox("verifying dane failed, continuing due to tls mode unverified starttls, due to tls-required-no message header", err)
metricTLSRequiredNoIgnored.WithLabelValues("daneverification").Inc()
}
return nil
}
}
return &config
}
// todo: possibly accept older TLS versions for TLSOpportunistic?
return tls.Config{
return &tls.Config{
ServerName: c.remoteHostname.ASCII,
RootCAs: mox.Conf.Static.TLS.CertPool,
InsecureSkipVerify: tlsMode == TLSOpportunistic || tlsMode == TLSUnverifiedStartTLS,
@ -538,6 +568,8 @@ func (c *Client) hello(ctx context.Context, tlsMode TLSMode, ehloHostname dns.Do
c.ext8bitmime = true
case "PIPELINING":
c.extPipelining = true
case "REQUIRETLS":
c.extRequireTLS = true
default:
// For SMTPUTF8 we must ignore any parameter. ../rfc/6531:207
if s == "SMTPUTF8" || strings.HasPrefix(s, "SMTPUTF8 ") {
@ -592,7 +624,7 @@ func (c *Client) hello(ctx context.Context, tlsMode TLSMode, ehloHostname dns.Do
// For TLSStrictStartTLS, the Go TLS library performs the checks needed for MTA-STS.
// ../rfc/8461:646
tlsConfig := c.tlsConfig(tlsMode)
nconn := tls.Client(conn, &tlsConfig)
nconn := tls.Client(conn, tlsConfig)
c.conn = nconn
nctx, cancel := context.WithTimeout(ctx, time.Minute)
@ -609,6 +641,7 @@ func (c *Client) hello(ctx context.Context, tlsMode TLSMode, ehloHostname dns.Do
tlsversion, ciphersuite := mox.TLSInfo(nconn)
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))
c.tls = true
hello(false)
}
@ -720,6 +753,24 @@ func (c *Client) SupportsSMTPUTF8() bool {
return c.extSMTPUTF8
}
// SupportsStartTLS returns whether the SMTP server supports the STARTTLS
// extension.
func (c *Client) SupportsStartTLS() bool {
return c.extStartTLS
}
// SupportsRequireTLS returns whether the SMTP server supports the REQUIRETLS
// extension. The REQUIRETLS extension is only announced after enabling
// STARTTLS.
func (c *Client) SupportsRequireTLS() bool {
return c.extRequireTLS
}
// TLSEnabled returns whether TLS is enabled for this connection.
func (c *Client) TLSEnabled() bool {
return c.tls
}
// Deliver attempts to deliver a message to a mail server.
//
// mailFrom must be an email address, or empty in case of a DSN. rcptTo must be
@ -733,12 +784,15 @@ func (c *Client) SupportsSMTPUTF8() bool {
// character, or when UTF-8 is used in a localpart, reqSMTPUTF8 must be true. If set,
// the remote server must support the SMTPUTF8 extension or delivery will fail.
//
// If requireTLS is true, the remote server must support the REQUIRETLS
// extension, or delivery will fail.
//
// Deliver uses the following SMTP extensions if the remote server supports them:
// 8BITMIME, SMTPUTF8, SIZE, PIPELINING, ENHANCEDSTATUSCODES, STARTTLS.
//
// Returned errors can be of type Error, one of the Err-variables in this package
// or other underlying errors, e.g. for i/o. Use errors.Is to check.
func (c *Client) Deliver(ctx context.Context, mailFrom string, rcptTo string, msgSize int64, msg io.Reader, req8bitmime, reqSMTPUTF8 bool) (rerr error) {
func (c *Client) Deliver(ctx context.Context, mailFrom string, rcptTo string, msgSize int64, msg io.Reader, req8bitmime, reqSMTPUTF8, requireTLS bool) (rerr error) {
defer c.recover(&rerr)
if c.origConn == nil {
@ -761,6 +815,9 @@ func (c *Client) Deliver(ctx context.Context, mailFrom string, rcptTo string, ms
// ../rfc/6531:313
c.xerrorf(false, 0, "", "", "%w", ErrSMTPUTF8Unsupported)
}
if !c.extRequireTLS && requireTLS {
c.xerrorf(false, 0, "", "", "%w", ErrRequireTLSUnsupported)
}
if c.extSize && msgSize > c.maxSize {
c.xerrorf(true, 0, "", "", "%w: message is %d bytes, remote has a %d bytes maximum size", ErrSize, msgSize, c.maxSize)
@ -782,12 +839,17 @@ func (c *Client) Deliver(ctx context.Context, mailFrom string, rcptTo string, ms
// ../rfc/6531:213
smtputf8Arg = " SMTPUTF8"
}
var requiretlsArg string
if requireTLS {
// ../rfc/8689:155
requiretlsArg = " REQUIRETLS"
}
// Transaction overview: ../rfc/5321:1015
// MAIL FROM: ../rfc/5321:1879
// RCPT TO: ../rfc/5321:1916
// DATA: ../rfc/5321:1992
lineMailFrom := fmt.Sprintf("MAIL FROM:<%s>%s%s%s", mailFrom, mailSize, bodyType, smtputf8Arg)
lineMailFrom := fmt.Sprintf("MAIL FROM:<%s>%s%s%s%s", mailFrom, mailSize, bodyType, smtputf8Arg, requiretlsArg)
lineRcptTo := fmt.Sprintf("RCPT TO:<%s>", rcptTo)
// We are going into a transaction. We'll clear this when done.