Cisco IOS Easy VPN

Cisco IOS Easy VPN De-mystified

At first, there seems to be nothing “easy” about configuring “Easy VPN” for IOS! But, don’t be alarmed, tenacious technician!

The critical thing to grasp is that this method requires IKE “group2” Diffie-Hellman exchange for a 1024bit modulus. Your VPN server has to have a IKE policy with this enabled.

Also, on the EZ client side, you’ll notice that the ISAKMP policies starting with priority 65515 are used from the client’s perspective. The first one specifies AES, SHA, and Group2. I recommend that you specify a matching policy on the server side first.

The server side configuration is classic remote access VPN type.

You will begin with a “crypto isakmp client configuration group ” and work your way to “crypto isakmp profile” and “crypto ipsec profile” where all the elements are tied together, so that they can be applied to the dynamic map or virtual-template.

Consider:

crypto isakmp client configuration group iosvpn
key cisco50
pool iosvpnpool
acl 144
banner ^CWelcome to R6 IOS remote access VPN ^C

crypto isakmp profile iosvpn
match identity group iosvpn
client authentication list LOCO
isakmp authorization list LOCO
client configuration address respond
virtual-template 20

crypto ipsec transform-set ccie esp-3des esp-sha-hmac

crypto ipsec profile iosvpn
set transform-set ccie
set reverse-route tag 23501
set isakmp-profile iosvpn

aaa new-model
aaa authentication login LOCO local
aaa authorization network LOCO local

ip local pool iosvpnpool 192.168.99.1 192.168.99.10

DMVPN through firewalls in virtual multi-context mode

When you have an ASA running in multi-context mode, you have to pay extra attention to the flow classifier.

Consider:

ccie-asa1/india# packet-tracer input out icmp 192.168.70.1 8 0 10.7.1.2
Result:input-interface: outinput-status: upinput-line-status: upAction: dropDrop-reason: (ifc-classify) Virtual firewall classification failed

Let's work backwards. Here, we are dealing with an external network 192.168.70.0 /24 trying t reach a internal network 10.7.1.0/24 in a "no nat-control" setup. The multi-context ASA here has multiple contexts sharing an outside interface (common in SP and hosting environments).

Here is the challenge:
When a packet arrives at the outside interface destined for a destination inside one of the contexts, how does the firewall know which one to forward to? Without unique mac-addresses, the layer 2 address does not give any clues. The firewall relies on NAT configuration to determine the context.

For example:
In the "ccie-asa1/india" context
static (inside,out) 10.7.1.1 10.7.1.1 netmask 255.255.255.255

Now, the classifier has information to act on the packet is sent to the "india" context.

This behavior is critical to grasp when it comes to configuring DMVPN through the firewall. Here, we have a need to allow IP/47 (GRE) traffic to the 10.7.1.1 address, or IP/50 (ESP) or IP/51 (AH). You can be very specific and allow just the protocol you are using. Don't forget udp/500 and if necessary, udp/4500 for IKE.

ASA - Active/Standby with IPSec

With a pair of ASA appliances, you can configure an active/standby pair with full IPSec support, including remote access VPN failover.

Proceed with configuring the active unit first. Specify the failover addresses for each interface and don't forget to include the "management-only" interface! This is crucial to allow remote access VPN connections to fail over (it took me a while to finally solve this issue, and it is not documented anywhere!)

interface GigabitEthernet 0/1
nameif inside
ip address 10.35.0.1 255.255.255.0 standby 10.35.0.2



failover lan unit primary
failover lan interface failcheck GigabitEthernet1/0
failover key *****
failover replication http
failover link failcheck GigabitEthernet1/0
failover interface ip failcheck 10.10.10.1 255.255.255.0 standby 10.10.10.2
failover

If you wish, you can dedicate an interface to failover "link" for stateful failover purposes. It will need its own IP addressing of course.

On the backup appliance, you need only configure the failover interface(s) and addressing and enable failover.

Cisco IPS - VLAN Groups

With VLAN groups, you can assign specific policies to different sets of VLANs. Here is a practical example. Note that the "group" mode work best when used in-line between two switches.

Switch A (dot1q) <----> Gi0/0 IPS Gi0/1 <----> (dot1q) Switch B

Let's say your trunk carries vlans 600 to 609. You can configure an "interface pair" on the IPS and call it "outside_pair" for example.

Then, proceed to configure VLAN groups -> create a group, specify subinterface "1" and select VLANs 600 to 604. Next, specify subinterface "2" and specify VLANs 605 to 609.

You can apply separate policies to outside_pair.1 (to handle the first 5 VLANs) and outside_pair.2 for the last 5 VLANs.

ASA/PIX: Cut-thru authentication/authorization with RADIUS

Cut-thru authentication provides an added layer of security and a central point to manage access-lists, the ACS in this case.

First, create a "downloadable IP ACL" within the ACS framework.
Shared Profile Components -> Downloadable IP ACLs -> Add (ACL and ACE's)

Next, associate this ACL with an ACS user or group. Note that you might have to modify "Interface Configuration" and "Advanced Options" to make "downloadable ACLs" appear in the user definition pages.

Now, on the ASA:

First create an access-list to define interesting traffic:
access-list redzone permit tcp any any eq telnet

Next permit access to either the appliance's virtual telnet addresses or at least one inside target host via telnet in the access-list applied on the outside interface:
access-list outside_in permit tcp any host 10.35.0.252 eq telnet

Next, configure AAA authentication:
aaa authentication match redzone outside myRadiusServer

Note that you should configure your RADIUS server:
aaa-server myRaidiusServer protocol radius
aaa-server myRadiusServer (outside) host H.O.S.T key secret

You can also configure local authentication with the appliance:
virtual telnet h.0.s.t

Remember to allow telnet access to the this virtual telnet IP address via the outside_in access-list.

Once the user is authenticated, you can verify by:
show uauth
show access-lists

You will see the temporary ACL in the output of the show command. Note that no "authorization" configuration was necessary when using RADIUS (contrast with the post that describes TACACS+ authentication and authorizaton).

Smart filtering with SNMP v3

SNMP version 3 provides the wonderful benefits of strong authentication, privacy and tight control of what information you allow the device to reveal. With Cisco IOS 12.4T, the configuration tasks are quite simple:

First, define a "view" thusly:
snmp-server view NOC interfaces included

You can exclude specific interfaces. For example, "26" represents "Gi0/0.500" interface and you want to exclude it entirely:
snnp-server view NOC ifEntry.*.26 excluded

Note the "*" which is a wildcard to exclude all elements for ifIndex 26. This of course can be more specific to limit exactly the elements you choose to hide. Or you can very specific and only allow access to certain interfaces only.

Next, configure a SNMP group:
snmp-server group NOC v3 auth read NOC

Finally configure the user:
snmp-server user noc NOC v3 auth md5 soeasytosee

Or you can take a step further and enable privacy with DES as follows:
snmp-server user noc NOC v3 auth md4 soeasytosee priv des hidemeplease

You can test with "snmpwalk"
snmpwalk -v3 -aMD5 -Asoeasytosee -unoc -lauthNopriv host.ip.address

When privacy is enabled:
snmpwalk -v3 -unoc -aMD5 -Asoeasytosee -xDES -Xhidemeplease -lauthpriv host.ip.address

ASA: AAA authorization with TACACS+

I have to admit, this was a tricky one! I have scoured the web for sometime now in an effort to accomplish AAA authorization, but using TACACS. While "downloadables ACLs" with RADIUS accomplish this and more relatively easily, getting TACACS to authorize is a bit more obscure.

access-list redzone permit tcp any host 10.35.0.0 255.255.255.0 eq telnet
access-list redzone permit tcp any host 10.35.0.0.0 255.255.255.0 eq http

aaa-server taca1 protocol tacacs
aaa-server taca1 (outside) host 172.30.3.100
key xxxx

aaa authentication match redzone outside taca1
aaa authorization match redzone outside taca1

Now for the interesting part:

On the ACS server, create a user and assign a "shell command authorization set" which specifes, for example here:

command: telnet
arg: permit 10.35.0.100

command: http
arg: permit 10.35.0.100

Please make sure you use the regular "shell command authorization and not the PIX/IOS set.