I'm working with Level3 on a similar problem. They filter both UDP and TCP
port 1900 on our peer to them. This is blocking all connections that randomly
use ephemeral tcp port 1900.
They are refusing to remove the tcp port 1900 filter without dispensation from
the DDoS security gods. I understand blocking UDP 1900, what is the purpose of
Level3 filtering tcp port 1900?
On 3/25/19, 12:44 PM, "NANOG on behalf of Saku Ytti"
<nanog-boun...@nanog.org on behalf of s...@ytti.fi> wrote:
Hey Tom,
> If your edge ingress ACLs are not 100% in sync all the time, you will
inevitably have Really Weird Stuff happen that will end up taking forever to
diagnose.
You may at some cases have hard to troubleshoot issues, which is true
for everything, even when perfectly configured, because software is
not perfect. However choosing to do iACL is still something many
networks choose to do, because the upside is worth the complexity to
them.
> Packet filtering is more computationally taxing than just routing is.
Your edge equipment is likely going to be built for maximum routing efficiency.
Trying to bite off too much filtering there increases your risk of legit
traffic being tossed on the floor.
Depends on implementation, on some implementations it is zero-cost on
some it is not. On most implementations it's very cheap, particularly
compared to say uRPF. It seems your position is 'i don't know how ACL
works on my platforms and i don't trust myself to write ACL, so i
should not do them', which is perfectly valid position under those
constrains, but other networks have other constrains under which it is
no longer valid proposal to omit doing iACL.
I would encourage networks to continue deploying iACL and consider it
BCP. iACL removes attack surface and protects you from host of known
and unknown SIRT issues.
--
++ytti
