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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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93
webmail/api.go
View File

@ -16,6 +16,7 @@ import (
"net/mail"
"net/textproto"
"os"
"runtime/debug"
"sort"
"strings"
"sync"
@ -33,6 +34,7 @@ import (
"github.com/mjl-/mox/dkim"
"github.com/mjl-/mox/dns"
"github.com/mjl-/mox/message"
"github.com/mjl-/mox/metrics"
"github.com/mjl-/mox/mlog"
"github.com/mjl-/mox/mox-"
"github.com/mjl-/mox/moxio"
@ -162,6 +164,7 @@ type SubmitMessage struct {
ResponseMessageID int64 // If set, this was a reply or forward, based on IsForward.
ReplyTo string // If non-empty, Reply-To header to add to message.
UserAgent string // User-Agent header added if not empty.
RequireTLS *bool // For "Require TLS" extension during delivery.
}
// ForwardAttachments references attachments by a list of message.Part paths.
@ -522,6 +525,9 @@ func (w Webmail) MessageSubmit(ctx context.Context, m SubmitMessage) {
if m.UserAgent != "" {
header("User-Agent", m.UserAgent)
}
if m.RequireTLS != nil && !*m.RequireTLS {
header("TLS-Required", "No")
}
header("MIME-Version", "1.0")
if len(m.Attachments) > 0 || len(m.ForwardAttachments.Paths) > 0 {
@ -685,7 +691,7 @@ func (w Webmail) MessageSubmit(ctx context.Context, m SubmitMessage) {
Localpart: rcpt.Localpart,
IPDomain: dns.IPDomain{Domain: rcpt.Domain},
}
_, err := queue.Add(ctx, log, reqInfo.AccountName, fromPath, toPath, has8bit, smtputf8, msgSize, messageID, []byte(rcptMsgPrefix), dataFile, nil)
_, err := queue.Add(ctx, log, reqInfo.AccountName, fromPath, toPath, has8bit, smtputf8, msgSize, messageID, []byte(rcptMsgPrefix), dataFile, nil, m.RequireTLS)
if err != nil {
metricSubmission.WithLabelValues("queueerror").Inc()
}
@ -1635,12 +1641,27 @@ const (
SecurityResultUnknown SecurityResult = "unknown"
)
// RecipientSecurity is a quick analysis of the security properties of delivery to the recipient (domain).
// Fields are nil when an error occurred during analysis.
// RecipientSecurity is a quick analysis of the security properties of delivery to
// the recipient (domain).
type RecipientSecurity struct {
MTASTS SecurityResult // Whether we have a stored enforced MTA-STS policy, or domain has MTA-STS DNS record.
DNSSEC SecurityResult // Whether MX lookup response was DNSSEC-signed.
DANE SecurityResult // Whether first delivery destination has DANE records.
// Whether recipient domain supports (opportunistic) STARTTLS, as seen during most
// recent delivery attempt. Will be "unknown" if no delivery to the domain has been
// attempted yet.
STARTTLS SecurityResult
// Whether we have a stored enforced MTA-STS policy, or domain has MTA-STS DNS
// record.
MTASTS SecurityResult
// Whether MX lookup response was DNSSEC-signed.
DNSSEC SecurityResult
// Whether first delivery destination has DANE records.
DANE SecurityResult
// Whether recipient domain is known to implement the REQUIRETLS SMTP extension.
// Will be "unknown" if no delivery to the domain has been attempted yet.
RequireTLS SecurityResult
}
// RecipientSecurity looks up security properties of the address in the
@ -1650,6 +1671,18 @@ func (Webmail) RecipientSecurity(ctx context.Context, messageAddressee string) (
return recipientSecurity(ctx, resolver, messageAddressee)
}
// logPanic can be called with a defer from a goroutine to prevent the entire program from being shutdown in case of a panic.
func logPanic(ctx context.Context) {
x := recover()
if x == nil {
return
}
log := xlog.WithContext(ctx)
log.Error("recover from panic", mlog.Field("panic", x))
debug.PrintStack()
metrics.PanicInc(metrics.Webmail)
}
// separate function for testing with mocked resolver.
func recipientSecurity(ctx context.Context, resolver dns.Resolver, messageAddressee string) (RecipientSecurity, error) {
log := xlog.WithContext(ctx)
@ -1658,6 +1691,8 @@ func recipientSecurity(ctx context.Context, resolver dns.Resolver, messageAddres
SecurityResultUnknown,
SecurityResultUnknown,
SecurityResultUnknown,
SecurityResultUnknown,
SecurityResultUnknown,
}
msgAddr, err := mail.ParseAddress(messageAddressee)
@ -1675,6 +1710,7 @@ func recipientSecurity(ctx context.Context, resolver dns.Resolver, messageAddres
// MTA-STS.
wg.Add(1)
go func() {
defer logPanic(ctx)
defer wg.Done()
policy, _, err := mtastsdb.Get(ctx, resolver, addr.Domain)
@ -1690,6 +1726,7 @@ func recipientSecurity(ctx context.Context, resolver dns.Resolver, messageAddres
// DNSSEC and DANE.
wg.Add(1)
go func() {
defer logPanic(ctx)
defer wg.Done()
_, origNextHopAuthentic, expandedNextHopAuthentic, _, hosts, _, err := smtpclient.GatherDestinations(ctx, log, resolver, dns.IPDomain{Domain: addr.Domain})
@ -1737,7 +1774,51 @@ func recipientSecurity(ctx context.Context, resolver dns.Resolver, messageAddres
}
}()
// STARTTLS and RequireTLS
reqInfo := ctx.Value(requestInfoCtxKey).(requestInfo)
acc, err := store.OpenAccount(reqInfo.AccountName)
xcheckf(ctx, err, "open account")
defer func() {
if acc != nil {
err := acc.Close()
log.Check(err, "closing account")
}
}()
err = acc.DB.Read(ctx, func(tx *bstore.Tx) error {
q := bstore.QueryTx[store.RecipientDomainTLS](tx)
q.FilterNonzero(store.RecipientDomainTLS{Domain: addr.Domain.Name()})
rd, err := q.Get()
if err == bstore.ErrAbsent {
return nil
} else if err != nil {
rs.STARTTLS = SecurityResultError
rs.RequireTLS = SecurityResultError
log.Errorx("looking up recipient domain", err, mlog.Field("domain", addr.Domain))
return nil
}
if rd.STARTTLS {
rs.STARTTLS = SecurityResultYes
} else {
rs.STARTTLS = SecurityResultNo
}
if rd.RequireTLS {
rs.RequireTLS = SecurityResultYes
} else {
rs.RequireTLS = SecurityResultNo
}
return nil
})
xcheckf(ctx, err, "lookup recipient domain")
// Close account as soon as possible, not after waiting for MTA-STS/DNSSEC/DANE
// checks to complete, which can take a while.
err = acc.Close()
log.Check(err, "closing account")
acc = nil
wg.Wait()
return rs, nil
}