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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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80
smtpserver/server.go
View File

@ -17,6 +17,7 @@ import (
"io"
"math"
"net"
"net/textproto"
"os"
"runtime/debug"
"sort"
@ -201,7 +202,7 @@ func Listen() {
port := config.Port(listener.SMTP.Port, 25)
for _, ip := range listener.IPs {
firstTimeSenderDelay := durationDefault(listener.SMTP.FirstTimeSenderDelay, firstTimeSenderDelayDefault)
listen1("smtp", name, ip, port, hostname, tlsConfig, false, false, maxMsgSize, false, listener.SMTP.RequireSTARTTLS, listener.SMTP.DNSBLZones, firstTimeSenderDelay)
listen1("smtp", name, ip, port, hostname, tlsConfig, false, false, maxMsgSize, false, listener.SMTP.RequireSTARTTLS, !listener.SMTP.NoRequireTLS, listener.SMTP.DNSBLZones, firstTimeSenderDelay)
}
}
if listener.Submission.Enabled {
@ -211,7 +212,7 @@ func Listen() {
}
port := config.Port(listener.Submission.Port, 587)
for _, ip := range listener.IPs {
listen1("submission", name, ip, port, hostname, tlsConfig, true, false, maxMsgSize, !listener.Submission.NoRequireSTARTTLS, !listener.Submission.NoRequireSTARTTLS, nil, 0)
listen1("submission", name, ip, port, hostname, tlsConfig, true, false, maxMsgSize, !listener.Submission.NoRequireSTARTTLS, !listener.Submission.NoRequireSTARTTLS, true, nil, 0)
}
}
@ -222,7 +223,7 @@ func Listen() {
}
port := config.Port(listener.Submissions.Port, 465)
for _, ip := range listener.IPs {
listen1("submissions", name, ip, port, hostname, tlsConfig, true, true, maxMsgSize, true, true, nil, 0)
listen1("submissions", name, ip, port, hostname, tlsConfig, true, true, maxMsgSize, true, true, true, nil, 0)
}
}
}
@ -230,7 +231,7 @@ func Listen() {
var servers []func()
func listen1(protocol, name, ip string, port int, hostname dns.Domain, tlsConfig *tls.Config, submission, xtls bool, maxMessageSize int64, requireTLSForAuth, requireTLSForDelivery bool, dnsBLs []dns.Domain, firstTimeSenderDelay time.Duration) {
func listen1(protocol, name, ip string, port int, hostname dns.Domain, tlsConfig *tls.Config, submission, xtls bool, maxMessageSize int64, requireTLSForAuth, requireTLSForDelivery, requireTLS bool, dnsBLs []dns.Domain, firstTimeSenderDelay time.Duration) {
addr := net.JoinHostPort(ip, fmt.Sprintf("%d", port))
if os.Getuid() == 0 {
xlog.Print("listening for smtp", mlog.Field("listener", name), mlog.Field("address", addr), mlog.Field("protocol", protocol))
@ -252,7 +253,7 @@ func listen1(protocol, name, ip string, port int, hostname dns.Domain, tlsConfig
continue
}
resolver := dns.StrictResolver{} // By leaving Pkg empty, it'll be set by each package that uses the resolver, e.g. spf/dkim/dmarc.
go serve(name, mox.Cid(), hostname, tlsConfig, conn, resolver, submission, xtls, maxMessageSize, requireTLSForAuth, requireTLSForDelivery, dnsBLs, firstTimeSenderDelay)
go serve(name, mox.Cid(), hostname, tlsConfig, conn, resolver, submission, xtls, maxMessageSize, requireTLSForAuth, requireTLSForDelivery, requireTLS, dnsBLs, firstTimeSenderDelay)
}
}
@ -277,6 +278,7 @@ type conn struct {
conn net.Conn
tls bool
extRequireTLS bool // Whether to announce and allow the REQUIRETLS extension.
resolver dns.Resolver
r *bufio.Reader
w *bufio.Writer
@ -316,8 +318,9 @@ type conn struct {
// Message transaction.
mailFrom *smtp.Path
has8bitmime bool // If MAIL FROM parameter BODY=8BITMIME was sent. Required for SMTPUTF8.
smtputf8 bool // todo future: we should keep track of this per recipient. perhaps only a specific recipient requires smtputf8, e.g. due to a utf8 localpart. we should decide ourselves if the message needs smtputf8, e.g. due to utf8 header values.
requireTLS *bool // MAIL FROM with REQUIRETLS set.
has8bitmime bool // If MAIL FROM parameter BODY=8BITMIME was sent. Required for SMTPUTF8.
smtputf8 bool // todo future: we should keep track of this per recipient. perhaps only a specific recipient requires smtputf8, e.g. due to a utf8 localpart. we should decide ourselves if the message needs smtputf8, e.g. due to utf8 header values.
recipients []rcptAccount
}
@ -353,6 +356,7 @@ func (c *conn) reset() {
// ../rfc/5321:2502
func (c *conn) rset() {
c.mailFrom = nil
c.requireTLS = nil
c.has8bitmime = false
c.smtputf8 = false
c.recipients = nil
@ -524,7 +528,7 @@ func (c *conn) writelinef(format string, args ...any) {
var cleanClose struct{} // Sentinel value for panic/recover indicating clean close of connection.
func serve(listenerName string, cid int64, hostname dns.Domain, tlsConfig *tls.Config, nc net.Conn, resolver dns.Resolver, submission, tls bool, maxMessageSize int64, requireTLSForAuth, requireTLSForDelivery bool, dnsBLs []dns.Domain, firstTimeSenderDelay time.Duration) {
func serve(listenerName string, cid int64, hostname dns.Domain, tlsConfig *tls.Config, nc net.Conn, resolver dns.Resolver, submission, tls bool, maxMessageSize int64, requireTLSForAuth, requireTLSForDelivery, requireTLS bool, dnsBLs []dns.Domain, firstTimeSenderDelay time.Duration) {
var localIP, remoteIP net.IP
if a, ok := nc.LocalAddr().(*net.TCPAddr); ok {
localIP = a.IP
@ -545,6 +549,7 @@ func serve(listenerName string, cid int64, hostname dns.Domain, tlsConfig *tls.C
conn: nc,
submission: submission,
tls: tls,
extRequireTLS: requireTLS,
resolver: resolver,
lastlog: time.Now(),
tlsConfig: tlsConfig,
@ -821,6 +826,10 @@ func (c *conn) cmdHello(p *parser, ehlo bool) {
if !c.tls && c.tlsConfig != nil {
// ../rfc/3207:90
c.bwritelinef("250-STARTTLS")
} else if c.extRequireTLS {
// ../rfc/8689:202
// ../rfc/8689:143
c.bwritelinef("250-REQUIRETLS")
}
if c.submission {
// ../rfc/4954:123
@ -1285,6 +1294,15 @@ func (c *conn) cmdMail(p *parser) {
case "SMTPUTF8":
// ../rfc/6531:213
c.smtputf8 = true
case "REQUIRETLS":
// ../rfc/8689:155
if !c.tls {
xsmtpUserErrorf(smtp.C530SecurityRequired, smtp.SePol7EncNeeded10, "requiretls only allowed on tls-encrypted connections")
} else if !c.extRequireTLS {
xsmtpUserErrorf(smtp.C555UnrecognizedAddrParams, smtp.SeSys3NotSupported3, "REQUIRETLS not allowed for this connection")
}
v := true
c.requireTLS = &v
default:
// ../rfc/5321:2230
xsmtpUserErrorf(smtp.C555UnrecognizedAddrParams, smtp.SeSys3NotSupported3, "unrecognized parameter %q", key)
@ -1645,7 +1663,12 @@ func (c *conn) cmdData(p *parser) {
recvHdr := &message.HeaderWriter{}
// For additional Received-header clauses, see:
// https://www.iana.org/assignments/mail-parameters/mail-parameters.xhtml#table-mail-parameters-8
recvHdr.Add(" ", "Received:", "from", recvFrom, "by", recvBy, "via", "tcp", "with", with, "id", mox.ReceivedID(c.cid)) // ../rfc/5321:3158
withComment := ""
if c.requireTLS != nil && *c.requireTLS {
// Comment is actually part of ID ABNF rule. ../rfc/5321:3336
withComment = " (requiretls)"
}
recvHdr.Add(" ", "Received:", "from", recvFrom, "by", recvBy, "via", "tcp", "with", with+withComment, "id", mox.ReceivedID(c.cid)) // ../rfc/5321:3158
if c.tls {
tlsConn := c.conn.(*tls.Conn)
tlsComment := message.TLSReceivedComment(c.log, tlsConn.ConnectionState())
@ -1665,6 +1688,23 @@ func (c *conn) cmdData(p *parser) {
}
}
// Check if a message has unambiguous "TLS-Required: No" header. Messages must not
// contain multiple TLS-Required headers. The only valid value is "no". But we'll
// accept multiple headers as long as all they are all "no".
// ../rfc/8689:223
func hasTLSRequiredNo(h textproto.MIMEHeader) bool {
l := h.Values("Tls-Required")
if len(l) == 0 {
return false
}
for _, v := range l {
if !strings.EqualFold(v, "no") {
return false
}
}
return true
}
// submit is used for mail from authenticated users that we will try to deliver.
func (c *conn) submit(ctx context.Context, recvHdrFor func(string) string, msgWriter *message.Writer, dataFile *os.File) {
// Similar between ../smtpserver/server.go:/submit\( and ../webmail/webmail.go:/MessageSubmit\(
@ -1692,6 +1732,14 @@ func (c *conn) submit(ctx context.Context, recvHdrFor func(string) string, msgWr
xsmtpUserErrorf(smtp.C550MailboxUnavail, smtp.SePol7DeliveryUnauth1, "must match authenticated user")
}
// TLS-Required: No header makes us not enforce recipient domain's TLS policy.
// ../rfc/8689:206
// Only when requiretls smtp extension wasn't used. ../rfc/8689:246
if c.requireTLS == nil && hasTLSRequiredNo(header) {
v := false
c.requireTLS = &v
}
// Outgoing messages should not have a Return-Path header. The final receiving mail
// server will add it.
// ../rfc/5321:3233
@ -1787,7 +1835,7 @@ func (c *conn) submit(ctx context.Context, recvHdrFor func(string) string, msgWr
xmsgPrefix := append([]byte(recvHdrFor(rcptAcc.rcptTo.String())), msgPrefix...)
msgSize := int64(len(xmsgPrefix)) + msgWriter.Size
if _, err := queue.Add(ctx, c.log, c.account.Name, *c.mailFrom, rcptAcc.rcptTo, msgWriter.Has8bit, c.smtputf8, msgSize, messageID, xmsgPrefix, dataFile, nil); err != nil {
if _, err := queue.Add(ctx, c.log, c.account.Name, *c.mailFrom, rcptAcc.rcptTo, msgWriter.Has8bit, c.smtputf8, msgSize, messageID, xmsgPrefix, dataFile, nil, c.requireTLS); err != nil {
// Aborting the transaction is not great. But continuing and generating DSNs will
// probably result in errors as well...
metricSubmission.WithLabelValues("queueerror").Inc()
@ -1872,6 +1920,16 @@ func (c *conn) deliver(ctx context.Context, recvHdrFor func(string) string, msgW
xsmtpUserErrorf(smtp.C550MailboxUnavail, smtp.SeNet4Loop6, "loop detected, more than 100 Received headers")
}
// TLS-Required: No header makes us not enforce recipient domain's TLS policy.
// Since we only deliver locally at the moment, this won't influence our behaviour.
// Once we forward, it would our delivery attempts.
// ../rfc/8689:206
// Only when requiretls smtp extension wasn't used. ../rfc/8689:246
if c.requireTLS == nil && hasTLSRequiredNo(headers) {
v := false
c.requireTLS = &v
}
// We'll be building up an Authentication-Results header.
authResults := message.AuthResults{
Hostname: mox.Conf.Static.HostnameDomain.XName(c.smtputf8),
@ -2547,7 +2605,7 @@ func (c *conn) deliver(ctx context.Context, recvHdrFor func(string) string, msgW
if Localserve {
c.log.Error("not queueing dsn for incoming delivery due to localserve")
} else if err := queueDSN(context.TODO(), c, *c.mailFrom, dsnMsg); err != nil {
} else if err := queueDSN(context.TODO(), c, *c.mailFrom, dsnMsg, c.requireTLS != nil && *c.requireTLS); err != nil {
metricServerErrors.WithLabelValues("queuedsn").Inc()
c.log.Errorx("queuing DSN for incoming delivery, no DSN sent", err)
}