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Cisco Firewall Configuration Fundamentals

Chapter Description

This chapter describes the configuration fundamentals for IOS and ASA-based firewalls, highlighting the similarities between the product families.

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Cisco Firewalls

Cisco Firewalls


Remote Management Access to ASA and FWSM

The examples presented so far have considered that there was physical access to the console port of the appliance (or to the hosting Catalyst 6500 for the FWSM). This section examines management connections that rely on remote access protocols (Telnet, SSH, and HTTPS).

Telnet Access

Telnet is a classic terminal access protocol that has received much criticism because of its clear text nature. It is highly recommended you replace it with SSH, which provides confidentiality.

At any rate, Telnet can still be useful for testing purposes mainly during initial setup. The commands shown in Example 3-15 specify the following:

  • Telnet access is accepted only when it is initiated from source addresses on network Further, the packets must arrive through the logical interface called mgmt.
  • The authentication of users who have permission to Telnet to the firewall is done using the LOCAL database. (LOCAL is a reserved keyword for ASA and FWSM.)
  • The username admin is included in the LOCAL database.

Example 3-15 also displays a sample Telnet session coming from address

Example 3-15. Configuring and Verifying Telnet Access

! Creating a local user
username admin password cisco123 privilege 15
! Telnet access is authenticated using the LOCAL Data Base
aaa authentication telnet console LOCAL
! Defining source addresses that can initiate Telnet Access to the Firewall
telnet mgmt
! Initiating a Telnet session to ASA
DMZ# telnet
Trying ... Open
User Access Verification
Password: ********
Type help or '?' for a list of available commands.
Password: *****
! Displaying connections to the Firewall
ASA5505# show conn all | include Identity
TCP mgmt NP Identity Ifc, idle 0:00:00, bytes 1017, flags UOB
ASA5505# who

SSH Access

If remote CLI access to the firewalls is needed, SSH is the protocol of choice. It provides the same terminal services that Telnet does but with the significant advantage of encrypting traffic between client and server (the firewall receiving the connection).

Because SSH uses RSA public keys to encrypt the sessions, you need to have consistent timing information. Example 3-16 shows not only how to manually adjust and verify timing information, but also how to create a domain name and generate RSA keys.

Example 3-17 shows how to visualize SSH-related information in the Running-config. Notice that the default timeout value for SSH sessions is 5 minutes.

Example 3-16. Recommended Tasks Before Starting SSH Configuration

! Setting Local Time (before generating the cryptographic keys)
ASA5505# clock set 19:05:00 november 15 2009
! Verifying Time Information
ASA5505# show running-config clock
clock timezone BRT -3
ASA5505# show clock detail
19:18:00.569 BRT Sun  Nov 15 2009
Time source is user configuration
! Configuring a domain-name
ASA5505(config)# domain-name mylab.lab
! Removing (if needed) any previously generated RSA keys
ASA5505(config)# crypto key zeroize rsa
WARNING: All RSA keys will be removed.
WARNING: All device digital certificates issued using these keys will also be removed.
Do you really want to remove these keys? [yes/no]: yes
! Generating new RSA Cryptographic Keys
ASA5505(config)# crypto key generate rsa modulus 1024
INFO: The name for the keys will be: <Default-RSA-Key>
Keypair generation process begin. Please wait...
! Displaying the RSA Public Keys
ASA5505# show crypto key mypubkey rsa
Key pair was generated at: 19:24:29 BRT Nov  15 2009 Key name: <Default-RSA-Key>
 Usage: General Purpose Key
 Modulus Size (bits): 1024
 Key Data:
  30819f30 0d06092a 864886f7 0d010101 05000381 8d003081 89028181 008e60c4
  bce3e63a 47aa12c4 e78c0a76 f2faf41c 5d8d461a 4978a5f6 0a4ac11b 26585f61
  d6b5adcb f5ce2430 a96c6fb9 d09f2187 3525255a 349e015e 37d0dd79 90e2b2f1
  5e968993 b9bb9cde 557ba395 e0b20f7c 0049b0d8 5d901902 fe8269ce 74f06a7f
  16713eea 8fe2a0a8 9ddeb2c3 1d258249 d16e6fc4 5a3b4fb6 be977bbf 55020301 0001

Example 3-17. SSH Configuration

ASA5505# show running-config | include ssh
aaa authentication ssh console LOCAL
ssh mgmt
ssh timeout 5

HTTPS Access Using ASDM

The Adaptive Security Device Manager (ASDM) is an intuitive and easy-to-use GUI that accompanies every member of the ASA family. The interface provides a nice graphical abstraction for the actual commands that are used not only to implement the features but also to verify their operation, thus allowing users who are already familiar with classic firewall concepts (even from other vendors) to easily adapt their knowledge to the new GUI and immediately start working.

Documenting ASDM usage with its uncountable configuration and monitoring screens is beyond the scope of this book. However, the preparation of firewall devices to accommodate ASDM management is covered.

ASDM uses the HTTPS protocol for communications between the management station and the firewall. After properly loading the ASDM image on the device's flash memory, a web browser can be employed for the first access to the device, with the underlying goal of installing the ASDM launcher application on the administrator's PC.

Example 3-18 shows the preliminary tasks for enabling HTTPS access and assumes that the remote user has been granted the highest privilege level (priv-lvl = 15) and that the requests arrive though the logical interface called mgmt. In the example, the user named admin is authenticated against the LOCAL database and should start the management session from a host that belongs to the subnet. The example also contains information about location of the ASDM image in the device flash (disk0: in this case) and how to find it within the show version output.

Example 3-18. Enabling HTTPS Access on ASA

! Assigning the highest privilege level (15) to the HTTPS user
usernameadmin password ****** privilege 15
! Defining allowed source IP Addresses. Authentication using the LOCAL Data Base

   http mgmt

   aaa authentication http consoleLOCAL
! Enabling the HTTPS server

   http server enable
! Defining the location of the ASDM image

   asdm image disk0:/asdm-621.bin
! Verifying Operating System and ASDM versions
ASA5505# show version | include Version
Cisco Adaptive Security Appliance Software Version  8.2(1)
Device Manager Version 6.2(1)

Figure 3-4 portrays the web browser access to the HTPPS server on the device whose management interface is configured with the address (

Figure 3-4

Figure 3-4 First HTTPS Access and Initial ASDM Page

From this screen, select the Install ASDM Launcher and Run ASDM option and follow these steps:

  • Step 1. Authenticate with the credentials configured in Example 3-18 when the Connect to window displays.
  • Step 2. From the File Download - Security Warning window, save the ".msi" file locally.
  • Step 3. Run the ".msi" file and install the ASDM Launcher application.
  • Step 4. After starting the ASDM Launcher, fill in the IP address ( in this case) and the credentials (username/password).
  • Step 5. After accepting the device certificate, the main ASDM page displays (Figure 3-5). This screen summarizes information for the device, including available licenses, interface status, and system resources status.
    Figure 3-5

    Figure 3-5 ASDM Home Page - Device Dashboard

Figure 3-6 depicts the base ASDM screen for Interface Configuration on an ASA 5505 appliance. Notice that the full path to this particular screen, Configuration > Device Setup> Interfaces, displays on the top of the right pane.

Figure 3-6

Figure 3-6 Base ASDM Page for Interface Configuration

Figure 3-7 shows a sample ASDM screen that helps perform the Monitoring task of verifying the ARP table. The complete path for viewing this table is represented at the top of the right pane (Monitoring > Interfaces > ARP Table).

Figure 3-7

Figure 3-7 Base ASDM Page for ARP Table Monitoring

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