Archive for category Pix

Yet another Hairpin: Internet Access from VPN Hub

Firewalls typically don’t hairpin well or at all for that matter,  unless specifically told to do so.  Hair-pinning is when a packet ultimately leaves the same interface it came into.

On a LAN it is somewhat common for packets to “bounce” off of one router interface to get to the right one, a prime candidate for the ICMP Redirect process.  Forget having your Cisco ASA or Pix participate in that little exchange of ICMP messages needed though, Cisco has long held that routing protocols are exploitable and have no place on a firewall (Yes they now speak EIGRP and OSPF, go figure).

The other example of hair-pinning that comes to mind deals with VPNs and Internet Access. The scenario is that a spoke or remote site VPNs to the hub or central site and wants to travel on to the Internet from there.  While it’s tempting to think of a VPN as originating from deep in the firewall the reality is that it is treated as coming from the outside interface.

In short you have to set up NAT for packets that arrive on the outside interface to turnaround and exit through the outside interface. Yes this is counter-intuitive, you have to apply the same NAT-Exempt and NAT statements on the interface as if friendlies were behind you and not the wild woolly Internet.

Assuming you assign VPN addresses from a  pool on 172.16.0.0/24; the CLI then looks like this:

global (outside) 1 interface
nat (inside) 0 access-list inside_nat0
nat (inside) 1 0.0.0.0 0.0.0.0

nat (outside) 0 access-list outside_nat0
nat (outside) 1 10.17.0.0 255.255.255.0

access-list outside_nat0 extended permit ip any 172.16.0.0 255.255.255.0
access-list inside_nat0_outbound extended permit ip any <your network>

Also you will need a very important sysopt:

same-security-traffic permit intra-interface

This basically turns on the ability to hair-pin.

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Upgrading your Pix to an ASA

One of the things we do is specialize in upgrading Pix configurations to ASA implementations.  We don’t run the Cisco tool as we would rather work with the network administrator to understand their policies and make sure that not only was the intent met but not fill their brand new ASA with a lot of meta-lables.

One thing that recently slipped through was the “non-printable defaults” for Pix and ASA don’t show some of the intricacies of PAP vs MS-CHAP authentication.  Be prepared to regenerate the Authentication Group from scratch if using CHAP.

I get asked a lot how to view the tunnel keys or tunnel group passwords.  To view them you can do one fo two things:

1) TFTP the config file to a server and look at it there

or

2) On ASA’s issue the command (without the quote’s y’all)
“more system:running-config”

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Fixing Static Addresses on Verizon FIOS

Verizon has a bug in their business offering for multiple static IP addresses.

When using a professional firewall that such as a Cisco ASA, I could only get 1 address to respond from offsite.

The first problem was solved by going to DSLReports.com, you have to call Verizon and convince them to instant message the group that runs the ONT’s (the termination that is onsite) to set the MAC filter to 5.

After that only 1 IP address worked per device. I could ping each other but Verizon served traffic could not see me. A quick TCP-Dump of the external segment showed the problem:

arp who-has 98.109.50.34 (00:1e:4a:87:32:59) tell 0.0.0.0
arp who-has 98.109.50.35 (00:1d:70:26:3c:53) tell 0.0.0.0

The address 0.0.0.0 is slightly illegal, the ASA ignores the ARP request and the Verizon gateway never binds the Mac to the translated IP addresses. This means that inbound static addresses didn’t work and only the physical interface address could be used for the outbound global pool.

I managed to get Verizon to admit the bug, the Alcatel equipment was partially to blame and I would imagine that the (non-professional) “firewall” that comes with the account had been modified to respond to an ARP request from 0.0.0.0 They projected it would be fixed Q1 of the next year… that was 15 months ago.

I found that the service (that I am paying for) could be made to work. I adapted a short Perl script to send ARP replies to the Verizon gateway router every 30 seconds or so, as if it was responding to an ARP request.

arp reply 98.109.50.36 is-at 00:1d:70:26:2c:53

Here I am telling the gateway that .36 is bound to the same address as .35. I was immediately able to ping the address .36 remotely, alls it took was a Linux system and the perl script below. I don’t believe that the ARP replies can be generated inside the ASA and be made to traverse the firewall; several types of lower traffic can using the ethertype command but ARP’s get absorbed. I haven’t tried proxy-arp to see if it relays the bogus advertisement as it breaks so many rules of paranoia that I doubt that the ASA would propagate it.

At the moment I have plugged in a dedicated Ethernet interface from my VMWare stack and am running a virtual Linux machine for the sole purpose of “poisoning” the ARP table. The FIOS service itself screams, though we wouldn’t ever consider using their DNS, but leave it to Verizon to pull up short on static IP address support.

Bil Herd

#!/usr/bin/perl
use Net::ARP;
use strict;
use warnings;
for (;;){
Net::ARP::send_packet(

‘eth0’, # Device
‘98.109.50.1’, # Verizon gateway, not really 0.0.0.0 of course

‘98.109.50.36’, # address that we want Verizon to respond

’00:1E:EC:9F:DB:67′, # Source MAC Mac of our address

’00:1d:70:26:cc:53, # Destinaton MAC address for ARP
‘reply’ # ARP operation
)
;
print “packet sent\n”;
sleep(30);
}

To install the Net::ARP module using CPAN:

perl -MCPAN -e ‘install Net::ARP’

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How to Install Cisco VPN Client on Windows 7

This appears to be typical of what is needed to get Windows 7 to live with Cisco VPN client:
http://weblogs.asp.net/bhouse/archive/2009/01/15/how-to-successfully-install-cisco-vpn-client-on-windows-7.aspx

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QM FSM error

Getting “QM FSM error” while establishing a Cisco VPN?  Particularly site-to-site and even more particularly with IOS on one end and a Pix/ASA on the other?

Go to the Pix/ASA side and remove Perfect Forward Secrecy (PFS).  Rather than tell you it’s incompatible, it just barfs because it can’t read it (because it’s you know… encrypted).

Example:
no cryptomap outside 1 set pfs group2

If anyone finds a better error message than the ubiquitous “QM FSM error” let me know and I will post it.

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Migrating from Cisco Pix to Cisco ASA

One of the most notable differences between Cisco ASA devices  and Cisco Pix devices  to be aware of is that the  ASA devices don’t  support the PPTP  protocol, (think of it as sanity catching up to your organization) and that the ASA5505 doesn’t support EZVpn server mode. While these may sound trivial, suddenly changing the way employees connect can feel like a cultural issue.

Another difference and one I wouldn’t  have guessed until I saw it on a customer system was that capitalization alone is not enough to distinguish names on the ASA, you cannot assign Test1 to an IP address and TEST1 to a different address, they are the same reference.  Again think of it as sanity enforcement, your organization should not be using the caplock key as an address discriminator.  The PDM “name” function is not supported at all as well as a handful of other warnings when importing.

There is a utility from Cisco available to assist in importing, alas I have never used it because I like to know exactly what is changing. Yes this can be a big  job when working with 20,000 lines of configuration but the last job I did of that size was 100% successful, not just 99%,  due to understanding the details of each conversion issue. My process is to reflect the changes back into the source and then re-import until there is a completely  clean import.

VPN organization is also very different, there are tunnel groups, isakmp definitions and group policies, these require an understanding of the intent and the security policies behind your VPN rules.

For those needing help with converting from Pix to ASA you may want to get an expert involved, especially to understand the impact on security policy as after all the firewall device is meant to be the implementation of a security policy, it should not be the security policy itself.

For help in migrating or configuring Cisco ASA security appliances or VPN connectivity and architecture email security @ idsbusiness.com

I also recommend a followup security scan if ever there is any doubt, one should be done periodically anyways so post conversion is an ideal time.

Bil Herd

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(Mostly) No split-tunnel on Cisco VPN Clients

Split tunnel is where the person connected to your corporate network via their VPN can also go willy-nilly to the Internet at the same time.  This is generally considered to be a bad thing, though I have heard a “security consultant” recommend to the customer with me in the room to go ahead and do this because it made it easier for him to connect.

Bad consultant. He should learn about ‘bots, back-doors and virus/worms in general.

However there is an issue with no-split tunnel in the Cisco world, or at least using the VPN client version 5 and the ASA, and at least for trying to be selective.  By that I mean, what if I want to get to one thing locally and tunnel the rest.

The problem appears to be as soon as you don’t “Tunnel Everything” as your selection, and instead use “Tunnel Network List Below” trying to tell it to tunnel “any”, 0.0.0.0/0 or even something like a 10.0.0.0/8 wont stop traffic from going to everything behind the local interface.It seems that the /24 of the local wins the most specific route contest against the  ” tunnel this specific /8″ command that gets pushed down.

To use “Tunnel Network List Below” to prevent local traffic, add an entry in the ACL that matches the /24 of the local LAN (assuming it’s a /24, you get the idea).  Now when you display the routing table while the VPNs connected (netstat -nr in windows or unix) you will see the local LAN but also a matching route form the VPN, and it now stands a chance of winning.

See my next post aboput why the access list seemes to be reversed when applying an ACL to a VPN as either a tunnel list or filter list.

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PDM to Cisco Pix not working

First pointed out to me by my tech Rob, some of the Cisco PIX/PDM combinations won’t make a connection on the outside interface in spite of being properly configured.  Try SSH ing to the external interface anc check the PDM again.  I have seen this almost half a dozen times in the last couple of years, the last was PIX software version 6.3(5) running PDM 3.0(1).

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