[cisco-bba] ACLs on Virtual-Access templates

Tue Feb 3 02:31:06 EST 2009

Arie:

I did some hunting around and the only stuff I did see showed that uRPF is
processed before ACLs, so I'm a bit confused in that regard.  If you can
verify, that would be appreciated.

Using some debug commands I did identify a PPPoA connection that was
spoofing traffic.  I removed off the inbound ACL on the Virtual-Template and
I see that the strict uRPF is working:

Router#sh ip interface Vi1.1120 | inc verif

  IP verify source reachable-via RX

   214 verification drops

   0 suppressed verification drops

   0 verification drop-rate

Today I tried adding logging to uRPF:

interface Virtual-Template1

 mtu 1492

 ip unnumbered Loopback11

 ip verify unicast source reachable-via rx 126

 ip mtu 1492

 ip tcp adjust-mss 1452

 no logging event link-status

 peer default ip address dhcp

 ppp mtu adaptive

 ppp authentication pap

 ppp ipcp dns a.b.c.d e.f.g.h

end

access-list 126 deny   ip any any log

Unfortunately, it does not appear to be logging anything.  There's no 126
lines, and I know that there's spoofed traffic coming through.  I re-applied
my inbound ACL on the Virtual-Template, and I immediately saw this:

Feb  3 01:27:45.326 CST: %SEC-6-IPACCESSLOGP: list 125 denied tcp
10.0.0.2(53070) -> 189.92.5.249(16016), 1 packet

Feb  3 01:27:46.482 CST: %SEC-6-IPACCESSLOGP: list 125 denied tcp
10.0.0.2(52461) -> 24.82.113.224(22019), 1 packet 

It's strange.

In regards to how I would like ACL-based logs to be documented; here's a
typical log entry:

Jan 31 15:23:21 a.b.c.d 38279: Jan 31 15:23:20.964 CST: %SEC-6-IPACCESSLOGP:
list 125 denied udp 80.212.149.228(55190) -> 192.168.0.0(19427), 1 packet

Jan 31 15:23:32 a.b.c.d 38287: Jan 31 15:23:31.476 CST: %SEC-6-IPACCESSLOGP:
list 125 denied tcp 222.172.244.3(2047) -> 192.168.0.0(19427), 1 packet

Jan 31 15:23:33 a.b.c.d 38288: Jan 31 15:23:32.784 CST: %SEC-6-IPACCESSLOGP:
list 125 denied udp 151.48.173.200(25235) -> 192.168.0.0(19427), 1 packet

Jan 31 15:23:36 a.b.c.d 38290: Jan 31 15:23:34.884 CST: %SEC-6-IPACCESSLOGP:
list 125 denied udp 58.108.93.71(13502) -> 192.168.0.0(19427), 1 packet

The feature request would be to log the interfaces, something like this:

Jan 31 15:23:21 a.b.c.d 38279: Jan 31 15:23:20.964 CST: %SEC-6-IPACCESSLOGP:
list 125 denied udp 80.212.149.228(55190) [ATM4/0.100 1/405 Vi1.1120] ->
192.168.0.0(19427) [Fa0/1], 1 packet

Jan 31 15:23:21 e.f.g.g 38281: Jan 31 15:23:21.123 CST: %SEC-6-IPACCESSLOGP:
list 125 denied udp 199.120.70.3(55190) [Fa0/1] -> 192.168.0.0(19427)
[ATM4/0.100 1/899 Vi1.923], 1 packet

Regards,

Frank

From: Arie Vayner [mailto:arievayner at gmail.com] 
Sent: Sunday, February 01, 2009 1:19 AM
To: Frank Bulk
Cc: cisco-bba at puck.nether.net
Subject: Re: [cisco-bba] ACLs on Virtual-Access templates

Frank,

So, just to be sure - if you remove the ACL, does uRPF work as it should?
I would assume (I don't have the time right now to make sure) that the ACL
is evaluated before as it is part of the ingress packet processing, while
uRPF is done later (close to the time when you also do a route lookup).
Therefore, it would make sense for the ACL to take precedence.

With regards to the log message, can you show me some example of what you
actually get? I can then try and see if we can file an enhancement request.

Thanks
Arie

On Sun, Feb 1, 2009 at 6:58 AM, Frank Bulk <frnkblk at iname.com> wrote:

Just to add to that, is there a way that the Virtual-interface that's doing
the spoofing can be identified?  The log entries for the ACL hits don't show
anything but the spoofed IP, but I don't know which connection is doing it.
Perhaps it's one PPPoA/E connection that's generating those spoofed packets.

Frank

From: Arie Vayner [mailto:arievayner at gmail.com] 
Sent: Saturday, January 31, 2009 3:49 PM
To: Frank Bulk
Cc: cisco-bba at puck.nether.net
Subject: Re: [cisco-bba] ACLs on Virtual-Access templates

Frank,

uRFP should be the right way to block packets from the client as a source...
After you connect, do you see the uRPF feature enabled on the Virtual-Access
(show run interface and show ip interface)?

Arie

On Sat, Jan 31, 2009 at 11:35 PM, Frank Bulk <frnkblk at iname.com> wrote:

Is there a way to build an ACL on a Virtual-Access template such that the
connection can only use the IP address given to it by IPCP?

I applied strict uRPF to the Virtual-Access template, but that didn't stop
this kind of traffic:

Jan 31 15:23:21 a.b.c.d 38279: Jan 31 15:23:20.964 CST: %SEC-6-IPACCESSLOGP:
list 125 denied udp 80.212.149.228(55190) -> 192.168.0.0(19427), 1 packet
Jan 31 15:23:32 a.b.c.d 38287: Jan 31 15:23:31.476 CST: %SEC-6-IPACCESSLOGP:
list 125 denied tcp 222.172.244.3(2047) -> 192.168.0.0(19427), 1 packet
Jan 31 15:23:33 a.b.c.d 38288: Jan 31 15:23:32.784 CST: %SEC-6-IPACCESSLOGP:
list 125 denied udp 151.48.173.200(25235) -> 192.168.0.0(19427), 1 packet
Jan 31 15:23:36 a.b.c.d 38290: Jan 31 15:23:34.884 CST: %SEC-6-IPACCESSLOGP:
list 125 denied udp 58.108.93.71(13502) -> 192.168.0.0(19427), 1 packet

Those source IPs aren't mine, and are targeting an RFC1918 address.  I'm
blocking traffic originating from my PPPoA/E customers that use a source IP
address outside my netblock or are targeting an RFC198 address using an
inbound ACL on the Virtual-Access template, but it doesn't stop a a customer
from spoofing their neighbor's IP address.

I've had a basic ACL in place on our internet-facing Ethernet port (Cisco
7206VXR with NPE-400) for a long time, but I didn't having anything in place
to block RFC 1918 addresses.  I could have applied the rules to the ACL on
the Ethernet interface, but I've been told to apply an ACL as close as
possible to the source of the traffic.

To further complicate matters, I also use this router to route RFC 1918
space for corporate needs.  I keep that "separate" by using source-based
routing, but that didn't prevent PPPoA/E customers from sending a packet to
the RFC 1918 space, even if the return packet never got back to them.
Perhaps I should use a VRF for handling corporate, traffic, except that I've
never done that before and I would need to spend some time learning.

Frank

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