Cisco PIX 500 Series Security Appliances

Document ID: 13825

Interactive: This document offers customized analysis of your Cisco device.

Contents

Introduction
Prerequisites
      Requirements
      Components Used
      Conventions
Configure
      Network Diagram
      Configuration
Verify
Troubleshoot
      Troubleshooting Commands
Troubleshooting Common Problems
Information to Collect if You Open a TAC Service Request
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Related Information

Introduction

This sample configuration demonstrates how to configure a Cisco Secure PIX Firewall to separate a corporate network from the Internet.

Prerequisites

Requirements

The internal network has a Web server, a mail server, and an FTP server that users on the Internet can access. All other access to hosts on the internal network is denied from outside users.

• Real address of the Web server - 192.168.1.4; Internet address 204.69.198.3

• Real address of the Mail server - 192.168.1.15; Internet address 204.69.198.4

• Real address of the FTP server - 192.168.1.10; Internet address 204.69.198.5

All users on the internal network are allowed unrestricted access to the Internet. Internal users are allowed to ping devices on the Internet, but users on the Internet are not allowed to ping devices on the inside.

The company used in this configuration has purchased a Class C network from their ISP (204.69.198.x). The .1 and .2 addresses are reserved for the external router and the outside interface of the PIX respectively. Addresses .3 - .5 are used for internal servers that users on the Internet can access. Addresses .6 - .14 are reserved for future use for servers that external users can access.

The PIX Firewall in the example has four network interface cards, but only two of them are in use. The PIX is set up to send syslogs to a syslog server on the inside with an IP address of 192.168.1.220 (not shown in the Network Diagram).

Components Used

The information in this document is based on these software and hardware versions:

• Cisco 3640 Router

• Cisco IOS® Software Release 12.1(21)

  Note: Cisco IOS release version is not relevant within the scope of this document.

• Cisco PIX Firewall 535

• Cisco PIX Firewall Software Release 5.3.1

  Note: The configuration shown in this document has been tested and verified with version 6.3(3).

The information in this document was created from the devices in a specific lab environment. All of the devices used in this document started with a cleared (default) configuration. If your network is live, make sure that you understand the potential impact of any command.

Conventions

Refer to Cisco Technical Tips Conventions for more information on document conventions.

Configure

In this section, you are presented with the information to configure the features described in this document.

Note: Use the Command Lookup Tool ( registered customers only) to obtain more information on the commands used in this section.

Network Diagram

This document uses this network setup:

Configuration

Note: Nondefault commands are shown in bold.

Building configuration... 
: Saved 
: 
PIX Version 5.3(1) 
nameif gb-ethernet0 outside security0 
nameif gb-ethernet1 inside security100 
nameif ethernet0 intf2 security10 
nameif ethernet1 intf3 security15 
enable password 8Ry2YjIyt7RRXU24 encrypted 
passwd 2KFQnbNIdI.2KYOU encrypted 
hostname pixfirewall 
fixup protocol ftp 21 
fixup protocol http 80 
fixup protocol h323 1720 
fixup protocol rsh 514 
fixup protocol smtp 25 
fixup protocol sqlnet 1521 
fixup protocol sip 5060 
names 


!--- Create an access list to allow pings out 
!--- and return packets back in. 

access-list 100 permit icmp any any echo-reply  
access-list 100 permit icmp any any time-exceeded  
access-list 100 permit icmp any any unreachable  


!--- Allows anyone on the Internet to connect to 
!--- the web, mail, and FTP servers. 

access-list 100 permit tcp any host 204.69.198.3 eq www  
access-list 100 permit tcp any host 204.69.198.4 eq smtp  
access-list 100 permit tcp any host 204.69.198.5 eq ftp 
pager lines 24 


!--- Enable logging. 

logging on 
no logging timestamp 
no logging standby 
no logging console 
no logging monitor 


!--- Enable error and more severe syslog messages 
!--- to be saved to the local buffer. 

logging buffered errors 


!--- Send notification and more severe syslog messages
!--- to the syslog server. 

logging trap notifications 
no logging history 
logging facility 20 
logging queue 512 


!--- Send syslog messages to a syslog server 
!--- on the inside interface. 

logging host inside 192.168.1.220 


!--- All interfaces are shutdown by default. 

interface gb-ethernet0 1000auto 
interface gb-ethernet1 1000auto 
interface ethernet0 auto shutdown 
interface ethernet1 auto shutdown 
mtu outside 1500 
mtu inside 1500 
mtu intf2 1500 
mtu intf3 1500 
ip address outside 204.69.198.2 255.255.255.0 
ip address inside 192.168.1.1 255.255.255.0 
ip address intf2 127.0.0.1 255.255.255.255 
ip address intf3 127.0.0.1 255.255.255.255 
ip audit info action alarm 
ip audit attack action alarm 
no failover 
failover timeout 0:00:00 
failover poll 15 
failover ip address outside 0.0.0.0 
failover ip address inside 0.0.0.0 
failover ip address intf2 0.0.0.0 
failover ip address intf3 0.0.0.0 
arp timeout 14400 


!--- Define a Network Address Translation (NAT) pool that
!--- internal hosts use when going out to the Internet.

global (outside) 1 204.69.198.15-204.69.198.253 


!--- Define a Port Address Translation (PAT) address that 
!--- is used once the NAT pool is exhausted.

global (outside) 1 204.69.198.254 


!--- Allow all internal hosts to use 
!--- the NAT or PAT addresses specified previously.

nat (inside) 1 0.0.0.0 0.0.0.0 0 0 


!--- Define a static translation for the internal 
!--- web server to be accessible from the Internet.

static (inside,outside) 204.69.198.3 192.168.1.4 
   netmask 255.255.255.255 0 0 


!--- Define a static translation for the internal 
!--- mail server to be accessible from the Internet.

static (inside,outside) 204.69.198.4 192.168.1.15 
   netmask 255.255.255.255 0 0 


!--- Define a static translation for the internal 
!--- FTP server to be accessible from the Internet.

static (inside,outside) 204.69.198.5 192.168.1.10 
   netmask 255.255.255.255 0 0 


!--- Apply access list 100 to the outside interface.

access-group 100 in interface outside 


!--- Define a default route to the ISP router.

route outside 0.0.0.0 0.0.0.0 204.69.198.1 1 
timeout xlate 3:00:00 
timeout conn 1:00:00 half-closed 0:10:00 udp 0:02:00 rpc 0:10:00 
h323 0:05:00 sip 0:30:00 sip_media 0:02:00 
timeout uauth 0:05:00 absolute 
aaa-server TACACS+ protocol tacacs+  
aaa-server RADIUS protocol radius  
no snmp-server location 
no snmp-server contact 
snmp-server community public 
no snmp-server enable traps 
floodguard enable 
no sysopt route dnat 
isakmp identity hostname 


!--- Allow the host 192.168.1.254 to be able to 
!--- Telnet to the inside of the PIX. 

telnet 192.168.1.254 255.255.255.255 inside 
telnet timeout 5 
ssh timeout 5 
terminal width 80 
Cryptochecksum:3e7ba2f76bf9a6b42ff656d31ebda5c5 
: end 
[OK] 

Verify

There is currently no verification procedure available for this configuration.

Troubleshoot

This section provides information you can use to troubleshoot your configuration.

The Output Interpreter Tool ( registered customers only) (OIT) supports certain show commands. Use the OIT to view an analysis of show command output.

• show interface—Shows interface statistics.

• show traffic—Shows how much traffic passes through the PIX.

• show xlate—Shows the current translations built through the PIX.

• show conn—Shows the current connections through the PIX.

Troubleshooting Commands

Note: Refer to Important Information on Debug Commands before you use debug commands.

• debug icmp trace—Shows all Internet Control Message Protocol (ICMP) echo requests and replies to or through the PIX.

Troubleshooting Common Problems

If you have the output of the write terminal command from your Cisco device, you can use Output Interpreter Tool ( registered customers only) to display potential issues and fixes.

• The NAT pool (and PAT address - with the exception of interface PAT) must use IP addresses that are not used by any other device on the network. This includes static addresses (for translations) or addresses used on the interfaces.

  If you have PIX software versions 5.2 or later, the outside interface address of the PIX can be used for PAT. This is useful if you have only one external address available, or need to conserve your IP address space.

  In order to enable PAT on the outside interface address, remove the global NAT pool and PAT address from the configuration, and use the outside interface IP address as the PAT address.

  ip address outside 204.69.198.2
  nat (inside) 1 0 0
  global (outside) 1 interface

  Note: Some multimedia applications can conflict with port mappings provided by PAT. PAT does not work with the established command. PAT works with Domain Name System (DNS), FTP and passive FTP, HTTP, email, remote-procedure call (RPC), rshell, Telnet, URL filtering, and outbound traceroute. Several PIX versions support H.323 with PAT on several versions. H.323v2 with PAT support was added in version 6.2.2, while H.323v3 and v4 with PAT support was added in version 6.3.

• You must have an access list (or conduit) to permit access into your servers. Inbound access is not permitted by default.

  Note: The conduit command has been superseded by the access-list command. Cisco recommends that you migrate your configuration away from the conduit command to maintain future compatibility.

• After any access list, there is an implicit deny ip any any command.

• If the DNS server is on the outside of the PIX, and internal users want to access the internal servers with their DNS name, then the alias command must be used to doctor the DNS response from the DNS server.

• If you still have problems after you review these common problems, complete these steps:

  1. Verify that you have IP connectivity between the two devices. In order to do this, either console into the PIX, or Telnet into the PIX. Issue the terminal monitor and debug icmp trace commands.

  2. If internal users have difficulty accessing servers on the Internet, ping the server you are trying to access and see if you get a response. If you do not receive a response, look at the debug statements and make sure you see the ICMP Echo Request go out through the PIX. If you do not see the Echo Requests, then check the default gateway of the source machine. Typically, it is the PIX.

     Also, use nslookup on the client, and make sure it can resolve the IP address of the server you are trying to reach.

  3. When you have IP connectivity, turn off debug icmp trace and turn on logging console debug (if connected to the console) or logging monitor debug (if connected to the PIX via Telnet). This causes the syslog messages to be displayed to your screen. Try to connect to the server, and watch the syslogs to see if any traffic is denied. If so, the syslogs should give you a good idea of why this is happening. You can also look at the description of the syslog messages.

  4. If outside users are unable to access your internal servers:

     1. Verify the syntax of your static command.

     2. Double-check that you have permitted access with your access-list command statements.

     3. Double-check that you have applied the access list with the access-group command.

  5. If you are a registered user and you are logged in, you can troubleshoot your PIX problems with the TAC Case Collection ( registered customers only) .

Information to Collect if You Open a TAC Service Request

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Related Information

• PIX 500 Series Security Appliance Support Page
• Documentation for PIX Firewall
• Cisco Security PIX Firewall Command Reference
• Requests for Comments (RFCs)
• Technical Support & Documentation - Cisco Systems