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do not lookup cname after looking up the txt for mta-sts, and follow cnames for mocks

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because the txt would already follow cnames.
the additional cname lookup didn't hurt, it just didn't do anything.
i probably didn't realize that before looking deeper into dns.
This commit is contained in:
Mechiel Lukkien
2023-10-14 22:42:26 +02:00
parent 8ca198882e
commit 101c2703d2
8 changed files with 43 additions and 76 deletions
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51
mtasts/mtasts.go
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@ -162,45 +162,26 @@ var (
)
// LookupRecord looks up the MTA-STS TXT DNS record at "_mta-sts.<domain>",
// following CNAME records, and returns the parsed MTA-STS record, the DNS TXT
// record and any CNAMEs that were followed.
func LookupRecord(ctx context.Context, resolver dns.Resolver, domain dns.Domain) (rrecord *Record, rtxt string, rcnames []string, rerr error) {
// following CNAME records, and returns the parsed MTA-STS record and the DNS TXT
// record.
func LookupRecord(ctx context.Context, resolver dns.Resolver, domain dns.Domain) (rrecord *Record, rtxt string, rerr error) {
log := xlog.WithContext(ctx)
start := time.Now()
defer func() {
log.Debugx("mtasts lookup result", rerr, mlog.Field("domain", domain), mlog.Field("record", rrecord), mlog.Field("cnames", rcnames), mlog.Field("duration", time.Since(start)))
log.Debugx("mtasts lookup result", rerr, mlog.Field("domain", domain), mlog.Field("record", rrecord), mlog.Field("duration", time.Since(start)))
}()
// ../rfc/8461:289
// ../rfc/8461:351
// We lookup the txt record, but must follow CNAME records when the TXT does not exist.
var cnames []string
// We lookup the txt record, but must follow CNAME records when the TXT does not
// exist. LookupTXT follows CNAMEs.
name := "_mta-sts." + domain.ASCII + "."
var txts []string
for {
var err error
txts, _, err = dns.WithPackage(resolver, "mtasts").LookupTXT(ctx, name)
if dns.IsNotFound(err) {
// DNS has no specified limit on how many CNAMEs to follow. Chains of 10 CNAMEs
// have been seen on the internet.
if len(cnames) > 16 {
return nil, "", cnames, fmt.Errorf("too many cnames")
}
cname, _, err := dns.WithPackage(resolver, "mtasts").LookupCNAME(ctx, name)
if dns.IsNotFound(err) {
return nil, "", cnames, ErrNoRecord
}
if err != nil {
return nil, "", cnames, fmt.Errorf("%w: %s", ErrDNS, err)
}
cnames = append(cnames, cname)
name = cname
continue
} else if err != nil {
return nil, "", cnames, fmt.Errorf("%w: %s", ErrDNS, err)
} else {
break
}
txts, _, err := dns.WithPackage(resolver, "mtasts").LookupTXT(ctx, name)
if dns.IsNotFound(err) {
return nil, "", ErrNoRecord
} else if err != nil {
return nil, "", fmt.Errorf("%w: %s", ErrDNS, err)
}
var text string
@ -215,18 +196,18 @@ func LookupRecord(ctx context.Context, resolver dns.Resolver, domain dns.Domain)
continue
}
if err != nil {
return nil, "", cnames, err
return nil, "", err
}
if record != nil {
return nil, "", cnames, ErrMultipleRecords
return nil, "", ErrMultipleRecords
}
record = r
text = txt
}
if record == nil {
return nil, "", cnames, ErrNoRecord
return nil, "", ErrNoRecord
}
return record, text, cnames, nil
return record, text, nil
}
// Policy fetch errors.
@ -330,7 +311,7 @@ func Get(ctx context.Context, resolver dns.Resolver, domain dns.Domain) (record
log.Debugx("mtasts get result", err, mlog.Field("domain", domain), mlog.Field("record", record), mlog.Field("policy", policy), mlog.Field("duration", time.Since(start)))
}()
record, _, _, err = LookupRecord(ctx, resolver, domain)
record, _, err = LookupRecord(ctx, resolver, domain)
if err != nil {
return nil, nil, err
}