large organization nameservers sending icmp packets to dns servers.

Patrick W. Gilmore patrick at
Tue Aug 7 18:38:06 UTC 2007

On Aug 7, 2007, at 2:14 PM, Donald Stahl wrote:

>> All things being equal (which they're usually not) you could use  
>> the ACK
>> response time of the TCP handshake if they've got TCP DNS resolution
>> available. Though again most don't for security reasons...
> Then most are incredibly stupid.

Those are impressively harsh words.

Mind if I ask what operational experience you have with name servers  
behind firewalls filtering TCP53?  I have none, so perhaps you could  
enlighten us with your vast experience?

> Several anti DoS utilities force unknown hosts to initiate a query  
> via TCP in order to be whitelisted. If the host can't perform a TCP  
> query then they get blacklisted.

That trick is so well known, most people turn it off since there has  
been more than one instance of large, well known organizations  
suffering spectacular failures by using it.  The phrase "worse than  
the disease" comes to mind.

> In addition, any UDP truncated response needs to be retried via  
> TCP- blocking it would cause a variety of problems.

Since we are talking about authorities here, one can control the size  
of ones responses.

Unless, of course, you are so incredibly stupid you can't figure out  
the difference between an authority and a caching server.


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