Query source port 53

[Barry Margolin]

In article <bmk0iq$16j2$1 at sf1.isc.org>,
Nico Kadel-Garcia  <nkadel at comcast.net> wrote:
>Barry Margolin wrote:
>> It's never been used as the source port for zone transfers, not even by
>> BIND 4.  That uses TCP, and has always used an ephemeral source port.
>> Otherwise, you wouldn't be able to have multiple concurrent zone transfers
>> between the same master and slave.
>
>
>I'm not expert enough to know that you're completely incorrect, but am 
>expert enough in networks to know that your reasoning is deeply flawed. 

Sounds like you don't know much about the internal operation of TCP, though.

>If network servers of various sorts could only ever open one connection 
>at a time for an open service port, then SSH, HTTP, FTP and other 
>services could not have simultaneous connections from the same client.

I didn't say they could only open one connection.  I said that if the
previous poster's assertion were true, they wouldn't be able to open
multiple connections.  Since they obviously *can* open multiple
connections, his claim must be false.

>Do those negotiate the use of other ports and spin off multiple daemons 
>and services to support these transfers? Why, yes they do: but you can 
>certainly look at the network negotiations for other such services to 
>see that if you block their primary port, life gets rather odd.....

TCP connections are identified by four parameters: local-address,
local-port, remote-address, remote-port.  Each connection to a particular
TCP-based server will have the same local-port, but the remote-address
and/or remote-port will be different.  If the connections are from the same
client, the remote-addresses will be the same, but the remote-ports will be
different, and this is what allows multiple connections from the same
client.

This is why they can't use port 53 as the source port in zone transfer
connections -- it's already required to be used as the destination port.
If one client tried to open multiple zone transfer connections to the same
server, they would all have the same local-port and remote-port, and there
wouldn't be any way to distinguish the connections from each other.

>I think you mean somthing by "ephemereal source port" that I'm missing here.

The high-numbered ports that are used temporarily as the source ports in
TCP connections are customarily called "ephemeral ports".

-- 
Barry Margolin, barry.margolin at level3.com
Level(3), Woburn, MA
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