Bind tcp connection

[Barry Margolin]

In article <dod5ao$s4u$1 at sf1.isc.org>,
 "Alex Tang" <alextang at cms.hkcable.com> wrote:

> Hi All
> 
> 
> Any one know that when clients use TCP to query dns server ? How can I 
> simulate it ? If the client use tcp query the dns server, will the server use 
> tcp to query other dns server ? and how many tcp connection to make with 
> other dns server. For example, if the dns server make a lot of dns connection 
> to other dns server , why  ?

Most clients (unless the client and server support the EDNS0 extensions) 
will switch to TCP when the response is larger than 500 bytes.  You can 
make this happen by creating a DNS entry with lots of PTR records that 
can't be merged with DNS compression:

a PTR 123456789012345678901234567890.123456789012345678901234567890.a
  PTR 123456789012345678901234567890.123456789012345678901234567890.b
  PTR 123456789012345678901234567890.123456789012345678901234567890.c
  PTR 123456789012345678901234567890.123456789012345678901234567890.d
  PTR 123456789012345678901234567890.123456789012345678901234567890.e
  PTR 123456789012345678901234567890.123456789012345678901234567890.f
  PTR 123456789012345678901234567890.123456789012345678901234567890.g
  PTR 123456789012345678901234567890.123456789012345678901234567890.h
  PTR 123456789012345678901234567890.123456789012345678901234567890.i

Doing "dig a.yourdomain.com ptr +bufsize=500" should then result in a 
switch to TCP.

I've also heard that Microsoft Exchange always uses TCP when it does its 
MX lookups.  I don't know why, it's just one of those mysterious 
Microsoft Windows things.

I don't think a client using TCP will force the server to use TCP.

I don't understand your last question.

-- 
Barry Margolin, barmar at alum.mit.edu
Arlington, MA
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