Re: DNS ACL ?

[Lynx <lynx () enemy org>]

On Nov 11, 2005 at 0835 -0500, John Hally appeared and said:
> [...]
> I need a sanity check regarding DNS ACLs.  For external facing DNS servers
> you need to allow only udp/53 inbound, correct?

This should be ok. DNS traffic also has 53/UDP -> 53/UDP, so you have to
take that into account when doing source port filtering.

> I know tcp/53 is used for zone transfers and requests/replies greater
> than a certain size, but they shouldn't typically happen for general
> dns queries correct?  

I think it is ok for pure DNS caches to use UDP only. DNS authorities
need TCP for zone transfers. A cache can do without zone transfers. It
is also a good idea to do some packet length limiting when designing
filters, because there are ways to tunnel arbitrary traffic through DNS.

Best,
Lynx.


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