"bad rdlength" error in dynamic update request

[freb]

I'm porting some dynamic DNS code from python to this dns library and I'm getting an error. I have a small server that listens for DNS updates. The code works fine when requesting with nsupdate, but errors when I try with a FortiGate firewall.

The requests are not identical between the two clients but I am not sure why all other systems I've tested against seem to work fine with the FortiGate. The only difference that I can tell is that the FortiGate sends two updates (summary from wireshark):

testhost.dns.com: type A, class ANY
testhost.dns.com: type A, class IN, addr 3.3.3.3

and nsupdate just sends one:

testhost.dns.com: type A, class IN, addr 3.3.3.3

The part of the library that throws the error is in the UnpackRR function in msg.go where it throws "bad rdlength". The first update from the FortiGate has data length of 0 but I've tried deletes from nsupdate as well that have length 0 with no problem.

I dug into the RFC but didn't find anything that would make me think what the FortiGate is trying to do is illegal.

[freb]

After thinking about this some more I realized that the first update from the FortiGate is just a delete request followed by an update/add request. After reproducing this request with nsupdate I get the same "bad rdlength" error which leads me to believe there is a bug. Here is the exact nsupdate command I used:

$ nsupdate -d -y key.:a2V5
> server 127.0.0.1
> zone dns.com
> update delete testhost.dns.com A
> update add testhost.dns.com 60 A 3.3.3.3
> send

It apears to only happen when you specify the record type for the delete as this will produce the error:

update delete testhost.dns.com A

but this will success:

update delete testhost.dns.com ANY

Thanks.

[andrewtj]

While writing a test case for this I've found RemoveRRset() doesn't work as advertised: it removes a specific RR rather than an RRset. I'm out of time to look at this now but should be able to take another look later.

[miekg]

Can you post where it breaks?

[andrewtj]

This sample returns bad rdlength:

 []byte{171, 68, 40, 0, 0, 1, 0, 0, 0, 2, 0, 0, 7, 101, 120, 97, 109, 112, 108, 101, 0, 0, 6, 0, 1, 192, 12, 0, 1, 0, 255, 0, 0, 0, 0, 0, 0, 192, 12, 0, 1, 0, 1, 0, 0, 0, 0, 0, 4, 127, 0, 0, 1}

It should be an update message for the zone "example.", deleting the A RRset "example." and then adding an A record at "example.".

[miekg]

Ah, thanks. Test added in update_test.go.

I think this is order dependent :( i.e. what RR are before and/or after, which makes it annoying. I may have some time today to look into it.

[miekg]

The fundamental problem here is that unpackStructValue has no concept of rdlength. A better longer term fix would be to rewrite this function entirely and maybe drop reflection: basically adapt something as zscan_rr.go... but Meh.

If have a small patch the makes the current testcase at least parse.

[miekg]

See 34f43d3

[andrewtj]

go generate might provide some more options for generating pack functions, maybe even the structs themselves.

More examples can be raised with something like:

package main

import "github.com/miekg/dns"

func main() {
    s := &dns.Server{Addr: ":5030", Net: "tcp"}
    dns.HandleFunc(".", func(w dns.ResponseWriter, r *dns.Msg) {
        m := new(dns.Msg)
        m.SetReply(r)
        w.WriteMsg(m)
    })
    s.ListenAndServe()
}

#!/bin/sh
set -e
(
    echo server 127.0.0.1 5030
    echo zone example.
    for i in `seq 1 4`; do
        echo update delete $i.example. TYPE$i
    done
    echo show
    echo send
    echo answer
) | nsupdate -v

[miekg]

[ Quoting notifications@github.com in "Re: [dns] "bad rdlength" error in d..." ]

go generate might provide some more options for generating pack functions, maybe even the structs themselves.

Yeah, from the types and tags most stuff can be inferred. Seems like a good
think to do (and speed up the parsing). I won't have a lot of time for this
though :(

More examples can be raised with something like:

package main

import "github.com/miekg/dns"

func main() {
  s := &dns.Server{Addr: ":5030", Net: "tcp"}
  dns.HandleFunc(".", func(w dns.ResponseWriter, r *dns.Msg) {
      m := new(dns.Msg)
      m.SetReply(r)
      w.WriteMsg(m)
  })
  s.ListenAndServe()
}

#!/bin/sh
set -e
(
   echo server 127.0.0.1 5030
   echo zone example.
   for i in `seq 1 4`; do
       echo update delete $i.example. TYPE$i
   done
   echo show
   echo send
   echo answer
) | nsupdate -v

Hmm. These should all be relatively easy to fix. I'll give it a whack.

#150 (comment)
/Miek

Miek Gieben

[andrewtj]

I've tried to provoke this behaviour again and been unable to—call it fixed?