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util/dnsname: increase maxNameLength to account for trailing dot

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Fixes #17788

Signed-off-by: Fran Bull <fran@tailscale.com>
This commit is contained in:
Fran Bull 2025-11-06 14:53:22 -08:00 committed by franbull
parent e8d2f96449
commit 27a0168cdc
2 changed files with 33 additions and 1 deletions
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2
util/dnsname/dnsname.go
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@ -14,7 +14,7 @@ const (
// maxLabelLength is the maximum length of a label permitted by RFC 1035.
maxLabelLength = 63
// maxNameLength is the maximum length of a DNS name.
maxNameLength = 253
maxNameLength = 254
)
// A FQDN is a fully-qualified DNS name or name suffix.

32
util/dnsname/dnsname_test.go
View File

@ -59,6 +59,38 @@ func TestFQDN(t *testing.T) {
}
}
func TestFQDNTooLong(t *testing.T) {
// RFC 1035 says a dns name has a max size of 255 octets, and is represented as labels of len+ASCII chars so
// example.com
// is represented as
// 7example3com0
// which is to say that if we have a trailing dot then the dots cancel out all the len bytes except the first and
// we can accept 254 chars.
// This name is max length
name := "aaaaaaaaaaaaaaaaaaaaa.aaaaaaaaaaaaaaaaaaaaa.aaaaaaaaaaaaaaaaaaaaa.aaaaaaaaaaaaaaaaaaaaa.aaaaaaaaaaaaaaaaaaaaa.aaaaaaaaaaaaaaaaaaaaa.aaaaaaaaaaaaaaaaaaaaa.aaaaaaaaaaaaaaaaaaaaa.aaaaaaaaaaaaaaaaaaaaa.aaaaaaaaaaaaaaaaaaaaa.aaaaaaaaaaaaaaaaaaaaa.example.com."
if len(name) != 254 {
t.Fatalf("name should be 254 chars including trailing . (len is %d)", len(name))
}
got, err := ToFQDN(name)
if err != nil {
t.Fatalf("want: error to end with \"is too long to be a DNS name\", got: %v", err)
}
if string(got) != name {
t.Fatalf("want: %s, got: %s", name, got)
}
// This name is too long
name = "x" + name
got, err = ToFQDN(name)
if got != "" {
t.Fatalf("want: \"\", got: %s", got)
}
if err == nil || !strings.HasSuffix(err.Error(), "is too long to be a DNS name") {
t.Fatalf("want: error to end with \"is too long to be a DNS name\", got: %v", err)
}
}
func TestFQDNContains(t *testing.T) {
tests := []struct {
a, b string