Confusing AXFR errors

[kalfeher]

Description

When attempting an xfr from zonedata.iis.se, for se. using a non-tsig signed transfer I see the following error consistently:
dns: overflowing header size

When attempting an xfr from zonedata.iis.se, for nu. using a non-tsig signed transfer I see the following two errors occasionally:
dns: overflow unpacking uint16
dns: buffer size too small
However the transfer attempt for nu. will also work as expected about half of the time.

I'm using just the high level APIs for a simple transfer. AXFR via dig @zonedata.iis.se axfr se works as expected.

Steps to reproduce

func main() {
	name := "se."
	primary := "zonedata.iis.se:53"
	t := new(dns.Transfer)
	m := new(dns.Msg)
	m.SetAxfr(name)
	c, err := t.In(m, primary)
	if err != nil {
		fmt.Println(err)
		return
	}
	for r := range c {
		if r.Error != nil {
			fmt.Println(r.Error)
			return
		}
		for _, a := range r.RR {
			fmt.Println(a.String() + "\n")
		}
	}
}

Expected Behaviour

All records received for zone file (~8106000 RRs), finishing with the SOA RR.

Actual Behaviour

For SE zone I receive the error dns: overflowing header size at some point during the transfer. Number of records received before the error is not consistent.

For NU zone I receive the one of the following errors at some point during the transfer dns: overflow unpacking uint16 or dns: buffer size too small. As with the SE zone, the number of records received before the error is not consistent. Approx half of my transfer attempts succeed for the NU zone.