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Date: Wed Feb 05 2003 - 11:02:06 CST
> -----Original Message-----
> From: Paul Schmehl [mailto:paulsutdallas.edu]
> Sent: 05 February 2003 15:38
> To: John.Aireyrnib.org.uk
> Cc: full-disclosurelists.netsys.com
> Subject: RE: [Full-Disclosure] SQL Slammer - lessons learned
> On Wed, 2003-02-05 at 06:55, John.Aireyrnib.org.uk wrote:
> > How the ports are managed by the ISPs is up to them. We
> have a managed
> > router where we block everything we can without breaking
> legitimate access.
> > However, not having a practical option to block certain
> ports is a problem.
> > My point was on the allocation and use by TCP/IP stacks.
> Can you think of a legitimate reason why ISPs should allow ports
> 135-139/TCP/UDP to be open to the Internet? How about port 445/UDP?
> Many ISPs now block port 25/TCP (for obvious reasons.) Why not other
> service ports? What about the ISPs whose policy it is to not allow
> customers to run servers? Why should they allow any traffic
> at all from
> the service ports?
Indeed they should, but many don't block them. This is not what I'm talking
about here though.
> > Sure, you can block 1434 udp inbound, but what if your DNS
> server (that
> > doesn't run SQL server) picks that port randomly for
> incoming data from
> > other DNS servers? You'll get failures when you shouldn't.
> No, you wouldn't, because DNS servers talk on port 53, and
> they wouldn't
> negotiate port 1434 because it's reserved for SQL.
Not necessarily. An authoritative server may be listening on port 53, but
use a different port for requests to other DNS servers. This is useful if
you have split-horizon DNS, where the inside servers either cannot or do not
wish to be open to the world on port 53. Some firewalls actually block
inbound UDP to privileged ports, so you have to use higher ports.
Reserved is only true in the port numbers list, not on the implementation of
TCP/IP (which is what I've been getting at all along!). You'll find your
workstation is using "reserved" ports regularly. For example, I've just
connected to a website, and port 3672 was used by my machine, but according
lispworks-orb 3672/tcp LispWorks ORB
lispworks-orb 3672/udp LispWorks ORB
This was chosen at random.
I hope you are beginning to understand my point, that you can't just block
individual ports outright without breaking something. This is why you
currently need "stateful" inspection to determine whether the connection to
that port is a response to an internal request or whether it is a request
from outside (ie a SYN,RST or SYN-ACK packet)
John Airey, BSc (Jt Hons), CNA, RHCE
Internet systems support officer, ITCSD, Royal National Institute of the
Bakewell Road, Peterborough PE2 6XU,
Tel.: +44 (0) 1733 375299 Fax: +44 (0) 1733 370848 John.Aireyrnib.org.uk
Am I the only person in the UK who finds it strange that our Prime Minister
complains of Human Rights abuses around the world, yet wishes to opt out of
the European Convention of Human Rights?
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