Cisco ASA General Archives

The convenience and advantages of secure VPNs has driven the specific technology to keep evolving continuously. Several years ago we only had the standardized IPSec VPN (which still strongly exists today). IPSec is a pure IP network VPN technology for connecting distant LAN networks over unsecured paths. Also, IPSec is used for client VPNs connecting remote teleworkers to their central site network. The characteristic of IPSec VPNs is that it provides FULL network connectivity between the VPN peers. That is, a remote access client IPSec VPN will connect the remote user to the central network just like the user would be locally connected.

After IPsec, the Web SSL VPN made its appearance. The remote user needs just a Web Browser with HTTPs to connect to the central site network. After authentication, the user is presented with a Web portal with links to the applications he is allowed to run. That is, the Web SSL VPN does not provide full network visibility to the remote user. The user has access only to specific applications (like internal email, internal files etc). Both IPSec VPNs and SSL VPNs are supported by Cisco ASA 5500 firewalls.

The newest generation of remote access VPNs is offered from Cisco AnyConnect SSL VPN client. This is supported by Cisco ASA 8.x. The AnyConnect SSL VPN provides the best features from both of the other VPN technologies (IPSec and Web SSL). With AnyConnect, the remote user has full network connectivity to the central site. Also, it offers the convenience of the Web SSL since there is no need to install an IPSec VPN client permanently to the user’s computer. Instead, there is an SSL client stored in the ASA flash memory which is downloaded to the remote user’s computer on demand.

How AnyConnect SSL VPN Client works

For first time user connection, the remote teleworker just opens a browser pointing to https://<ASA-outside-public-IP>. The browser connects to the ASA firewall and presents the user with a login screen. The user enters his credentials (username/password) and the ASA identifies that the user does not have the SSL client installed. Therefore it pushes the SSL client to the user’s computer. The client installs itself to the remote PC and establishes a secure SSL VPN connection between the remote user and the ASA. The user is also assigned an IP address from an address pool configured on the ASA and has full network access to the central site. When the SSL connection is stopped, the SSL client either uninstalls itself or remains on the user’s PC (depending on the configuration of the ASA).

In the case of a previously installed client, when the user authenticates, the security appliance examines the revision of the client, and upgrades the client as necessary.

The AnyConnect SSL client can be downloaded from the security appliance, or it can be installed manually on the remote PC by the system administrator.

In another post I will explain how to configure the AnyConnect SSL VPN on a Cisco ASA 5500 firewall.

Access Control Lists (ACLs) and Network Address Translation (NAT) are two of the most common features that coexist in the configuration of a Cisco ASA appliance. For both inbound and outbound access control lists, the IP addresses specified in the ACL depend on the interface where the ACL is applied. These IP addresses must be valid on the specific interface that the ACL is attached, regardless of NAT. Keep the following statement in mind: An Access Control List takes precedence over NAT. That is, an ACL is evaluated FIRST and then a NAT rule is applied to the packet.

For example, assume an inside host with private address 10.1.1.10 is translated to a public address 200.200.200.10 for outbound traffic (inside to outside). An ACL applied to the inside interface of the ASA firewall will first be evaluated to verify if the host 10.1.1.10 can access the Internet (outbound communication) and if the ACL permits this communication, only then NAT will be performed to translate 10.1.1.10 to 200.200.200.10. This is shown in the figure below.

cisco asa nat and acl access list

See the following commands for the example above:

!The following ACL is evaluated first

ciscoasa(config)# access-list INSIDE extended permit ip host 10.1.1.10 host 100.100.100.1

ciscoasa(config)# access-group INSIDE in interface inside

!NAT can be applied only if ACL allows the communication

ciscoasa(config)# global (outside) 1 200.200.200.10 netmask 255.255.255.255

ciscoasa(config)# nat (inside) 1 0.0.0.0 0.0.0.0

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Similarly, a scenario with inbound traffic (outside to inside) works again the same way. That is, an ACL is evaluated first for inbound traffic and then a NAT translation rule is applied. For example, assume we have a Web Server located on the inside network (should be on a DMZ for better security but for the sake of simplicity we assume it is located on the inside network). The private address configured on the Web Server is 10.1.1.10. We configured also static NAT on the Firewall to map the private address of the Web Server to a public address 200.200.200.10 on the outside (see figure below). Inbound traffic coming from the Internet towards the public address of the Web Server will first go through an ACL to verify if the traffic is permitted or not. If traffic is allowed by the ACL, then the static NAT will be applied to translate the destination address from 200.200.200.10 to 10.1.1.10.

See the following commands for the example above:

!The following ACL is evaluated first

ciscoasa(config)# access-list OUTSIDE extended permit tcp any host 200.200.200.10 eq 80

ciscoasa(config)# access-group OUTSIDE in interface outside

! Static NAT can be applied only if ACL allows the communication

ciscoasa(config)# static (inside,outside) 200.200.200.10 10.1.1.10 netmask 255.255.255.255

UPDATE:

For Cisco ASA version 8.3 and later, the order of operation regarding ACL and NAT is still the same (i.e ACLs are evaluated first and then static NAT takes place), HOWEVER the ACL now must reference the real private IP of the server and NOT the public IP.

In our example above, for ASA 8.3 the ACL would look like below:

ciscoasa(config)# access-list OUTSIDE extended permit tcp any host 10.1.1.10 eq 80

Maybe the most popular and frequently used command on Cisco ASA firewalls is the one which shows the current running configuration, that is the “show run” command. However, maybe the most powerful command on Cisco ASA is the “show version” command. An example output of a show version command is shown below:

CISCO-ASA#  show version

Cisco Adaptive Security Appliance Software Version 7.2(3)
Device Manager Version 5.2(3)
Compiled on Wed 15-Aug-07 16:08 by builders
System image file is “disk0:/asa723-k8.bin”
Config file at boot was “startup-config”

CISCO-ASA up 25 mins 32 secs

Hardware:   ASA5505, 256 MB RAM, CPU Geode 500 MHz
Internal ATA Compact Flash, 128MB
BIOS Flash M50FW080 @ 0xffe00000, 1024KB
Encryption hardware device : Cisco ASA-5505 on-board accelerator (revision 0×0)
Boot microcode   : CNlite-MC-Boot-Cisco-1.2
SSL/IKE microcode: CNlite-MC-IPSEC-Admin-3.03
IPSec microcode  : CNlite-MC-IPSECm-MAIN-2.04

0: Int: Internal-Data0/0    : address is 001e.4afa.2404, irq 11
1: Ext: Ethernet0/0         : address is 001e.4afa.23fc, irq 255
2: Ext: Ethernet0/1         : address is 001e.4afa.23fd, irq 255
3: Ext: Ethernet0/2         : address is 001e.4afa.23fe, irq 255
4: Ext: Ethernet0/3         : address is 001e.4afa.23ff, irq 255
5: Ext: Ethernet0/4         : address is 001e.4afa.2400, irq 255
6: Ext: Ethernet0/5         : address is 001e.4afa.2401, irq 255
7: Ext: Ethernet0/6         : address is 001e.4afa.2402, irq 255
8: Ext: Ethernet0/7         : address is 001e.4afa.2403, irq 255
9: Int: Internal-Data0/1    : address is 0000.0003.0002, irq 255
10: Int: Not used            : irq 255
11: Int: Not used            : irq 255

Licensed features for this platform:

Maximum Physical Interfaces : 8
VLANs                       : 3, DMZ Restricted
Inside Hosts                : 10
Failover                    : Disabled
VPN-DES                     : Enabled
VPN-3DES-AES                : Enabled
VPN Peers                   : 10
WebVPN Peers                : 2
Dual ISPs                   : Disabled
VLAN Trunk Ports            : 0

This platform has a Base license.

Serial Number: 1234567890
Running Activation Key: 123456781234567812345678

Configuration register is 0×1
Configuration last modified by enable_15 at 05:35:16.773 UTC Wed Apr 2 2008

Usefulness of show version command:

The power of the show version command comes from the wealth of useful information you can obtain from the output of this command. Use the show version command to display the following information:

  • Appliance software version (in our example above it is 7.2(3)
  • Software version of ASDM GUI software (in our example above it is 5.2(3)
  • Where the appliance software image file is located (disk0:/asa723-k8.bin)
  • Appliance up-time since last reboot (25 mins 32 secs)
  • Appliance model, RAM memory and CPU type (ASA5505, 256 MB RAM, CPU Geode 500 MHz)
  • Flash Memory (Internal ATA Compact Flash, 128MB)
  • MAC Addresses of Ethernet Network Interfaces.
  • Licensed features
  • Serial Number (this can be used to order software upgrades)
  • Running activation key
  • Last time the configuration was modified.

Traffic Rate Limiting on Cisco ASA

With the new modular policy framework (MPF) introduced in ASA versions 7.x and 8.x, the firewall administrator is now able to apply policing and rate limiting to traffic passing through the ASA appliance. I got a few questions from people how this functionality works and decided to throw in a quick example below which you can easily modify accordingly to match your needs.

Scenario:

We want to rate limit a local internal host when accessing a specific external public server. The local host is 192.168.1.10 and the external public server is 100.100.100.1. We need to limit the traffic to 100kbps and burst size 8000.

Configuration Snippet:

ASA(config)#access-list rate-limit-acl extended permit ip host 192.168.1.10 host 100.100.100.1

ASA(config)#class-map rate-limit
ASA(config-cmap)#match access-list rate-limit-acl

ASA(config)#policy-map limit-policy
ASA(config-pmap)#class rate-limit
ASA(config-pmap-c)#police output 100000 8000

ASA(config)#service-policy limit-policy interface outside

In order to be able to monitor and troubleshoot your Cisco ASA firewall, you need to understand the difference between connections and translations.

Refer to the diagram above for an explanation about Connections and Translations.

A Connection works at the Transport Layer and includes the Source IP/Source Port and the Destination IP/Destination Port. Connections are subsets of Translations. You can have many connections open that are all using the same Translation. For example, a connection shown above is originated from Internal source host 192.168.1.1 with source port 1030 towards a Destination host (public Web Server) 100.100.100.1 on Destination Port 80.

A Translation works at the IP Layer and includes the Real IP Address and the Mapped (Translated) IP Address. Using NAT or PAT, a Real IP address is translated to a Mapped IP address and vice-versa. From the diagram above, the Real IP address 192.168.1.1 is translated to a Mapped IP address 20.20.20.1.

Connection Related Commands

ASA# show conn
ASA# show conn details
ASA# show local-host

The above commands will display the current active connections and information details about each connection. An example output is the following:

TCP outside:100.100.100.1/80 inside:192.168.1.1/1030 idle 0:00:05 bytes 1965 flags UIO

Translation Related Commands

ASA# show xlate
ASA# show xlate detail
ASA# clear xlate

The commands above enable you to display or clear the contents of the translation table. An example output is the following:

NAT from inside:192.168.1.1 to outside:20.20.20.1 flags i

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