mirror of
https://github.com/mjl-/mox.git
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d1e93020d8
the underscores aren't valid, but have been seen in the wild, so we have a workaround for them. there are limitations, it won't work with idna domains. and if the domain has other policies, like mta-sts, the mx host won't pass either. after report from richard g about delivery issue, thanks!
420 lines
17 KiB
Go
420 lines
17 KiB
Go
package smtpclient
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import (
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"context"
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"crypto/sha256"
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"crypto/sha512"
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"crypto/x509"
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"errors"
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"fmt"
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"net"
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"sort"
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"strings"
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"time"
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"github.com/mjl-/adns"
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"github.com/mjl-/mox/dns"
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"github.com/mjl-/mox/mlog"
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)
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var (
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errCNAMELoop = errors.New("cname loop")
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errCNAMELimit = errors.New("too many cname records")
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errDNS = errors.New("dns lookup error")
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errNoMail = errors.New("domain does not accept email as indicated with single dot for mx record")
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)
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// GatherDestinations looks up the hosts to deliver email to a domain ("next-hop").
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// If it is an IP address, it is the only destination to try. Otherwise CNAMEs of
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// the domain are followed. Then MX records for the expanded CNAME are looked up.
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// If no MX record is present, the original domain is returned. If an MX record is
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// present but indicates the domain does not accept email, ErrNoMail is returned.
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// If valid MX records were found, the MX target hosts are returned.
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//
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// haveMX indicates if an MX record was found.
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//
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// origNextHopAuthentic indicates if the DNS record for the initial domain name was
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// DNSSEC secure (CNAME, MX).
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//
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// expandedNextHopAuthentic indicates if the DNS records after following CNAMEs were
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// DNSSEC secure.
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//
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// These authentic flags are used by DANE, to determine where to look up TLSA
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// records, and which names to allow in the remote TLS certificate. If MX records
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// were found, both the original and expanded next-hops must be authentic for DANE
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// to apply. For a non-IP with no MX records found, the authentic result can be
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// used to decide which of the names to use as TLSA base domain.
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func GatherDestinations(ctx context.Context, log *mlog.Log, resolver dns.Resolver, origNextHop dns.IPDomain) (haveMX, origNextHopAuthentic, expandedNextHopAuthentic bool, expandedNextHop dns.Domain, hosts []dns.IPDomain, permanent bool, err error) {
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// ../rfc/5321:3824
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// IP addresses are dialed directly, and don't have TLSA records.
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if len(origNextHop.IP) > 0 {
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return false, false, false, expandedNextHop, []dns.IPDomain{origNextHop}, false, nil
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}
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// We start out assuming the result is authentic. Updated with each lookup.
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origNextHopAuthentic = true
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expandedNextHopAuthentic = true
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// We start out delivering to the recipient domain. We follow CNAMEs.
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rcptDomain := origNextHop.Domain
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// Domain we are actually delivering to, after following CNAME record(s).
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expandedNextHop = rcptDomain
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// Keep track of CNAMEs we have followed, to detect loops.
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domainsSeen := map[string]bool{}
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for i := 0; ; i++ {
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if domainsSeen[expandedNextHop.ASCII] {
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// todo: only mark as permanent failure if TTLs for all records are beyond latest possibly delivery retry we would do.
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err := fmt.Errorf("%w: recipient domain %s: already saw %s", errCNAMELoop, rcptDomain, expandedNextHop)
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return false, origNextHopAuthentic, expandedNextHopAuthentic, expandedNextHop, nil, false, err
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}
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domainsSeen[expandedNextHop.ASCII] = true
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// note: The Go resolver returns the requested name if the domain has no CNAME
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// record but has a host record.
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if i == 16 {
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// We have a maximum number of CNAME records we follow. There is no hard limit for
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// DNS, and you might think folks wouldn't configure CNAME chains at all, but for
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// (non-mail) domains, CNAME chains of 10 records have been encountered according
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// to the internet.
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// todo: only mark as permanent failure if TTLs for all records are beyond latest possibly delivery retry we would do.
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err := fmt.Errorf("%w: recipient domain %s, last resolved domain %s", errCNAMELimit, rcptDomain, expandedNextHop)
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return false, origNextHopAuthentic, expandedNextHopAuthentic, expandedNextHop, nil, false, err
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}
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// Do explicit CNAME lookup. Go's LookupMX also resolves CNAMEs, but we want to
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// know the final name, and we're interested in learning if the first vs later
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// results were DNSSEC-(in)secure.
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// ../rfc/5321:3838 ../rfc/3974:197
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cctx, ccancel := context.WithTimeout(ctx, 30*time.Second)
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defer ccancel()
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cname, cnameResult, err := resolver.LookupCNAME(cctx, expandedNextHop.ASCII+".")
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ccancel()
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if i == 0 {
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origNextHopAuthentic = origNextHopAuthentic && cnameResult.Authentic
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}
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expandedNextHopAuthentic = expandedNextHopAuthentic && cnameResult.Authentic
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if err != nil && !dns.IsNotFound(err) {
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err = fmt.Errorf("%w: cname lookup for %s: %v", errDNS, expandedNextHop, err)
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return false, origNextHopAuthentic, expandedNextHopAuthentic, expandedNextHop, nil, false, err
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}
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if err == nil && cname != expandedNextHop.ASCII+"." {
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d, err := dns.ParseDomain(strings.TrimSuffix(cname, "."))
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if err != nil {
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// todo: only mark as permanent failure if TTLs for all records are beyond latest possibly delivery retry we would do.
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err = fmt.Errorf("%w: parsing cname domain %s: %v", errDNS, expandedNextHop, err)
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return false, origNextHopAuthentic, expandedNextHopAuthentic, expandedNextHop, nil, false, err
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}
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expandedNextHop = d
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// Start again with new domain.
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continue
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}
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// Not a CNAME, so lookup MX record.
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mctx, mcancel := context.WithTimeout(ctx, 30*time.Second)
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defer mcancel()
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// Note: LookupMX can return an error and still return records: Invalid records are
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// filtered out and an error returned. We must process any records that are valid.
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// Only if all are unusable will we return an error. ../rfc/5321:3851
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mxl, mxResult, err := resolver.LookupMX(mctx, expandedNextHop.ASCII+".")
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mcancel()
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if i == 0 {
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origNextHopAuthentic = origNextHopAuthentic && mxResult.Authentic
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}
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expandedNextHopAuthentic = expandedNextHopAuthentic && mxResult.Authentic
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if err != nil && len(mxl) == 0 {
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if !dns.IsNotFound(err) {
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err = fmt.Errorf("%w: mx lookup for %s: %v", errDNS, expandedNextHop, err)
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return false, origNextHopAuthentic, expandedNextHopAuthentic, expandedNextHop, nil, false, err
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}
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// No MX record, attempt delivery directly to host. ../rfc/5321:3842
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hosts = []dns.IPDomain{{Domain: expandedNextHop}}
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return false, origNextHopAuthentic, expandedNextHopAuthentic, expandedNextHop, hosts, false, nil
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} else if err != nil {
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log.Infox("mx record has some invalid records, keeping only the valid mx records", err)
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}
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// ../rfc/7505:122
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if err == nil && len(mxl) == 1 && mxl[0].Host == "." {
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// Note: Depending on MX record TTL, this record may be replaced with a more
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// receptive MX record before our final delivery attempt. But it's clearly the
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// explicit desire not to be bothered with email delivery attempts, so mark failure
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// as permanent.
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return true, origNextHopAuthentic, expandedNextHopAuthentic, expandedNextHop, nil, true, errNoMail
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}
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// The Go resolver already sorts by preference, randomizing records of same
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// preference. ../rfc/5321:3885
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for _, mx := range mxl {
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// Parsing lax (unless pedantic mode) for MX targets with underscores as seen in the wild.
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host, err := dns.ParseDomainLax(strings.TrimSuffix(mx.Host, "."))
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if err != nil {
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// note: should not happen because Go resolver already filters these out.
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err = fmt.Errorf("%w: invalid host name in mx record %q: %v", errDNS, mx.Host, err)
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return true, origNextHopAuthentic, expandedNextHopAuthentic, expandedNextHop, nil, true, err
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}
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hosts = append(hosts, dns.IPDomain{Domain: host})
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}
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if len(hosts) > 0 {
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err = nil
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}
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return true, origNextHopAuthentic, expandedNextHopAuthentic, expandedNextHop, hosts, false, err
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}
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}
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// GatherIPs looks up the IPs to try for connecting to host, with the IPs ordered
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// to take previous attempts into account. For use with DANE, the CNAME-expanded
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// name is returned, and whether the DNS responses were authentic.
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func GatherIPs(ctx context.Context, log *mlog.Log, resolver dns.Resolver, host dns.IPDomain, dialedIPs map[string][]net.IP) (authentic bool, expandedAuthentic bool, expandedHost dns.Domain, ips []net.IP, dualstack bool, rerr error) {
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if len(host.IP) > 0 {
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return false, false, dns.Domain{}, []net.IP{host.IP}, false, nil
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}
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authentic = true
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expandedAuthentic = true
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// The Go resolver automatically follows CNAMEs, which is not allowed for host
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// names in MX records, but seems to be accepted and is documented for DANE SMTP
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// behaviour. We resolve CNAMEs explicitly, so we can return the final name, which
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// DANE needs. ../rfc/7671:246
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// ../rfc/5321:3861 ../rfc/2181:661 ../rfc/7672:1382 ../rfc/7671:1030
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name := host.Domain.ASCII + "."
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for i := 0; ; i++ {
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cname, result, err := resolver.LookupCNAME(ctx, name)
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if i == 0 {
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authentic = result.Authentic
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}
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expandedAuthentic = expandedAuthentic && result.Authentic
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if dns.IsNotFound(err) {
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break
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} else if err != nil {
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return authentic, expandedAuthentic, dns.Domain{}, nil, dualstack, err
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} else if strings.TrimSuffix(cname, ".") == strings.TrimSuffix(name, ".") {
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break
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}
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if i > 10 {
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return authentic, expandedAuthentic, dns.Domain{}, nil, dualstack, fmt.Errorf("mx lookup: %w", errCNAMELimit)
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}
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name = strings.TrimSuffix(cname, ".") + "."
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}
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if name == host.Domain.ASCII+"." {
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expandedHost = host.Domain
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} else {
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var err error
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expandedHost, err = dns.ParseDomain(strings.TrimSuffix(name, "."))
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if err != nil {
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return authentic, expandedAuthentic, dns.Domain{}, nil, dualstack, fmt.Errorf("parsing cname-resolved domain: %w", err)
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}
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}
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ipaddrs, result, err := resolver.LookupIPAddr(ctx, name)
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authentic = authentic && result.Authentic
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expandedAuthentic = expandedAuthentic && result.Authentic
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if err != nil || len(ipaddrs) == 0 {
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return authentic, expandedAuthentic, expandedHost, nil, false, fmt.Errorf("looking up %q: %w", name, err)
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}
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var have4, have6 bool
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for _, ipaddr := range ipaddrs {
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ips = append(ips, ipaddr.IP)
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if ipaddr.IP.To4() == nil {
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have6 = true
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} else {
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have4 = true
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}
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}
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dualstack = have4 && have6
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prevIPs := dialedIPs[host.String()]
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if len(prevIPs) > 0 {
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prevIP := prevIPs[len(prevIPs)-1]
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prevIs4 := prevIP.To4() != nil
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sameFamily := 0
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for _, ip := range prevIPs {
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is4 := ip.To4() != nil
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if prevIs4 == is4 {
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sameFamily++
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}
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}
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preferPrev := sameFamily == 1
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// We use stable sort so any preferred/randomized listing from DNS is kept intact.
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sort.SliceStable(ips, func(i, j int) bool {
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aIs4 := ips[i].To4() != nil
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bIs4 := ips[j].To4() != nil
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if aIs4 != bIs4 {
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// Prefer "i" if it is not same address family.
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return aIs4 != prevIs4
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}
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// Prefer "i" if it is the same as last and we should be preferring it.
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return preferPrev && ips[i].Equal(prevIP)
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})
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log.Debug("ordered ips for dialing", mlog.Field("ips", ips))
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}
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return
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}
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// GatherTLSA looks up TLSA record for either expandedHost or host, and returns
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// records usable for DANE with SMTP, and host names to allow in DANE-TA
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// certificate name verification.
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//
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// If no records are found, this isn't necessarily an error. It can just indicate
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// the domain/host does not opt-in to DANE, and nil records and a nil error are
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// returned.
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//
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// Only usable records are returned. If any record was found, DANE is required and
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// this is indicated with daneRequired. If no usable records remain, the caller
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// must do TLS, but not verify the remote TLS certificate.
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func GatherTLSA(ctx context.Context, log *mlog.Log, resolver dns.Resolver, host dns.Domain, expandedAuthentic bool, expandedHost dns.Domain) (daneRequired bool, daneRecords []adns.TLSA, tlsaBaseDomain dns.Domain, err error) {
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// ../rfc/7672:912
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// This function is only called when the lookup of host was authentic.
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var l []adns.TLSA
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if host == expandedHost || !expandedAuthentic {
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tlsaBaseDomain = host
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l, err = lookupTLSACNAME(ctx, log, resolver, 25, "tcp", host)
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} else if expandedAuthentic {
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// ../rfc/7672:934
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tlsaBaseDomain = expandedHost
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l, err = lookupTLSACNAME(ctx, log, resolver, 25, "tcp", expandedHost)
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if err == nil && len(l) == 0 {
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tlsaBaseDomain = host
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l, err = lookupTLSACNAME(ctx, log, resolver, 25, "tcp", host)
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}
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}
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if len(l) == 0 || err != nil {
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daneRequired = err != nil
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log.Debugx("gathering tlsa records failed", err, mlog.Field("danerequired", daneRequired))
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return daneRequired, nil, dns.Domain{}, err
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}
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daneRequired = len(l) > 0
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l = filterUsableTLSARecords(log, l)
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log.Debug("tlsa records exist", mlog.Field("danerequired", daneRequired), mlog.Field("records", l), mlog.Field("basedomain", tlsaBaseDomain))
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return daneRequired, l, tlsaBaseDomain, err
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}
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// lookupTLSACNAME composes a TLSA domain name to lookup, follows CNAMEs and looks
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// up TLSA records. no TLSA records exist, a nil error is returned as it means
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// the host does not opt-in to DANE.
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func lookupTLSACNAME(ctx context.Context, log *mlog.Log, resolver dns.Resolver, port int, protocol string, host dns.Domain) (l []adns.TLSA, rerr error) {
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name := fmt.Sprintf("_%d._%s.%s", port, protocol, host.ASCII+".")
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for i := 0; ; i++ {
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cname, result, err := resolver.LookupCNAME(ctx, name)
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if dns.IsNotFound(err) {
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if !result.Authentic {
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log.Debugx("cname nxdomain result during tlsa lookup not authentic, not doing dane for host", err, mlog.Field("host", host), mlog.Field("name", name))
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return nil, nil
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}
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break
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} else if err != nil {
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return nil, fmt.Errorf("looking up cname for tlsa candidate base domain: %w", err)
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} else if !result.Authentic {
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log.Debugx("cname result during tlsa lookup not authentic, not doing dane for host", err, mlog.Field("host", host), mlog.Field("name", name))
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return nil, nil
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}
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if i == 10 {
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return nil, fmt.Errorf("looking up cname for tlsa candidate base domain: %w", errCNAMELimit)
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}
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name = strings.TrimSuffix(cname, ".") + "."
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}
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var result adns.Result
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var err error
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l, result, err = resolver.LookupTLSA(ctx, 0, "", name)
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if dns.IsNotFound(err) || err == nil && len(l) == 0 {
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log.Debugx("no tlsa records for host, not doing dane", err, mlog.Field("host", host), mlog.Field("name", name), mlog.Field("authentic", result.Authentic))
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return nil, nil
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} else if err != nil {
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return nil, fmt.Errorf("looking up tlsa records for tlsa candidate base domain: %w", err)
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} else if !result.Authentic {
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log.Debugx("tlsa lookup not authentic, not doing dane for host", err, mlog.Field("host", host), mlog.Field("name", name))
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return nil, err
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}
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return l, nil
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}
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func filterUsableTLSARecords(log *mlog.Log, l []adns.TLSA) []adns.TLSA {
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// Gather "usable" records. ../rfc/7672:708
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o := 0
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for _, r := range l {
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// A record is not usable when we don't recognize parameters. ../rfc/6698:649
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switch r.Usage {
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case adns.TLSAUsageDANETA, adns.TLSAUsageDANEEE:
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default:
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// We can regard PKIX-TA and PKIX-EE as "unusable" with SMTP DANE. ../rfc/7672:1304
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continue
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}
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switch r.Selector {
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case adns.TLSASelectorCert, adns.TLSASelectorSPKI:
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default:
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continue
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}
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switch r.MatchType {
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case adns.TLSAMatchTypeFull:
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if r.Selector == adns.TLSASelectorCert {
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if _, err := x509.ParseCertificate(r.CertAssoc); err != nil {
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log.Debugx("parsing certificate in dane tlsa record, ignoring", err)
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continue
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}
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} else if r.Selector == adns.TLSASelectorSPKI {
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if _, err := x509.ParsePKIXPublicKey(r.CertAssoc); err != nil {
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log.Debugx("parsing certificate in dane tlsa record, ignoring", err)
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continue
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}
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}
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case adns.TLSAMatchTypeSHA256:
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if len(r.CertAssoc) != sha256.Size {
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log.Debug("dane tlsa record with wrong data size for sha2-256", mlog.Field("got", len(r.CertAssoc)), mlog.Field("expect", sha256.Size))
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continue
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}
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case adns.TLSAMatchTypeSHA512:
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if len(r.CertAssoc) != sha512.Size {
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log.Debug("dane tlsa record with wrong data size for sha2-512", mlog.Field("got", len(r.CertAssoc)), mlog.Field("expect", sha512.Size))
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continue
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}
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default:
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continue
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}
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l[o] = r
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o++
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}
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return l[:o]
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}
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// GatherTLSANames returns the allowed names in TLS certificates for verification
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// with PKIX-* or DANE-TA. The first name should be used for SNI.
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//
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// If there was no MX record, the next-hop domain parameters (i.e. the original
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// email destination host, and its CNAME-expanded host, that has MX records) are
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// ignored and only the base domain parameters are taken into account.
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func GatherTLSANames(haveMX, expandedNextHopAuthentic, expandedTLSABaseDomainAuthentic bool, origNextHop, expandedNextHop, origTLSABaseDomain, expandedTLSABaseDomain dns.Domain) []dns.Domain {
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// Gather the names to check against TLS certificate. ../rfc/7672:1318
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if !haveMX {
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// ../rfc/7672:1336
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if !expandedTLSABaseDomainAuthentic || origTLSABaseDomain == expandedTLSABaseDomain {
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return []dns.Domain{origTLSABaseDomain}
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}
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return []dns.Domain{expandedTLSABaseDomain, origTLSABaseDomain}
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} else if expandedNextHopAuthentic {
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// ../rfc/7672:1326
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var l []dns.Domain
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if expandedTLSABaseDomainAuthentic {
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l = []dns.Domain{expandedTLSABaseDomain}
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}
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if expandedTLSABaseDomain != origTLSABaseDomain {
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l = append(l, origTLSABaseDomain)
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}
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l = append(l, origNextHop)
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if origNextHop != expandedNextHop {
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l = append(l, expandedNextHop)
|
|
}
|
|
return l
|
|
} else {
|
|
// We don't attempt DANE after insecure MX, but behaviour for it is specified.
|
|
// ../rfc/7672:1332
|
|
return []dns.Domain{origNextHop}
|
|
}
|
|
}
|