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This chapter describes how to establish and control network connectivity for different applications and implementations after you have completed your basic configuration, described in "Basic Firewall Configuration." This chapter contains the following sections:
Static address translation creates a permanent, one-to-one mapping between an address on an internal network (a higher security level interface) and a perimeter or external network (lower security level interface). For example, to share a web server on a perimeter interface with users on the public Internet, use static address translation to map the server's actual address to a registered IP address. Static address translation hides the actual address of the server from users on the less secure interface, making casual access by unauthorized users less likely. Unlike NAT or PAT, it requires a dedicated address on the outside network for each host, so it does not save registered IP addresses.
If you use a static command to allow inbound connections to a fixed IP address, use the access-list and access-group commands to create an access list and to bind it to the appropriate interface. For more information, refer to "Allowing Inbound Connections."
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Note The static command may not be compatible with failover. When creating a static mapping to an interface, the failover in that interface will be in the waiting state because it does not receive a response from the other PIX Firewall unit for the failover-specific protocol 105. For further information about failover, refer to "Using PIX Firewall Failover." |
The main options of the static command are as follows:
static [(internal_if_name, external_if_name)] global_ip local_ip [netmask network_mask]
[max_conns]
For example, the following command maps a server with an internal IP address of 10.1.1.3 to the registered IP address 209.165.201.12:
static (inside, outside) 209.165.201.12 10.1.1.3 netmask 255.255.255.255 0 0
This command simply maps the addresses; make sure you also configure access using the access-list and access-group commands, as described in the next section. Also, you will need to inform the DNS administrator to create an MX record for the external address so that traffic sent to the server host name is directed to the correct address.
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Note For information about how to configure static translation without NAT, refer to the static command page in the Cisco PIX Firewall Command Reference. |
By default, the PIX Firewall denies access to an internal or perimeter (more secure) network from an external (less secure) network. You specifically allow inbound connections by using access lists. Access lists work on a first-match basis, so for inbound access, you need to deny first and then permit after.
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Note Beginning with PIX Firewall version 5.3, access lists are the preferred method for managing network access. The conduit command was used in earlier versions. Access lists provide improved flexibility and greater ease of use for those familiar with Cisco IOS access control. However, the conduit command is still supported to maintain backward compatibility of configurations written for previous PIX Firewall versions. |
You use the access-list and access-group commands to permit access based on source or destination IP address, or by the protocol port number. Use the access-list command to create a single access list entry, and use the access-group command to bind one or more access list entries to a specific interface. Only specify one access-group command for each interface.
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Note To allow access only for specific users, set up authentication, as described in "Using Authentication and Authorization." |
Before you can set up an access list for a host, set up address translation by using a global or static command. Setting up address translation with the global command is described in "Basic Firewall Configuration." Setting up address translation using the static command was described earlier in the previous section "Assigning a Fixed Address to a Server."
The format for the access-list command is as follows:
access-list ID action protocol source_address port destination_address port
The format for the access-group command is as follows:
access-group ID in interface low_interface
Replace ID with the same identifier that you specified in the access-list command statement.
Replace low_interface with the lower security interface that you specified in the static command statement. This is the interface through which users will access the external (global) address.
The following example illustrates the three commands required to enable access to a web server with the external IP address 209.165.201.12:
static (inside, outside) 209.165.201.12 10.1.1.3 netmask 255.255.255.255 0 0 access-list acl_out permit tcp any host 209.165.201.12 eq www access-group acl_out in interface outside
This example uses the same static command that was shown in the previous section.
By default, all connections initiated on a network with a higher security level are allowed out, and you configure restrictions. You can control outbound access by IP address and protocol port, or combine access control with user authentication, as described in "Using Authentication and Authorization." If you are not enforcing restrictions on outbound network traffic, you do not need outbound access lists.
An outbound access list allows you to restrict users from starting outbound connections or allows you to restrict users from accessing specific destination address or networks. Access lists work on a first-match basis, so for outbound access lists, you need to permit first and then deny after.
For example, you could restrict some users from accessing web sites, permit others access, or restrict one or more users from accessing a specific web site. Define access restrictions with the access-list command, and use the access-group command to bind the access-list command statements to an interface.
When creating an outbound access list, the format for the access-list command is the same as shown earlier in "Allowing Inbound Connections":
access-list ID action protocol source_address port destination_address port
By default, outbound access is permitted, so you use the deny action to restrict access when using an outbound access list.
For example, to prevent users on the 192.168.1.0 network on the inside interface from starting connections on the outside interface and permit all others, specify the 192.168.1.0 network address as the source address and the network connected to the outside interface as the destination address. In the example that follows, the network on the outside interface is 209.165.201.0. The access-list and access-group command statements are as follows.
access-list acl_in deny tcp 192.168.1.0 255.255.255.224 209.165.201.0 255.255.255.224 access-list acl_in permit ip any any access-group acl_in in interface inside
You can use also use access lists to prevent access to a specific server. For example, if you want to restrict users on the inside interface from accessing a website at address 209.165.201.29 on the outside interface, use the following commands.
access-list acl_in deny tcp any host 209.165.201.29 eq www access-group acl_in in interface inside
These commands let any users start connections, but not to 209.165.201.29. The access-group command specifies that the users are on the inside interface.
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Note If controlling outbound access in your network is an important issue, consider using the Websense filtering application, described in "Filtering URLs with Websense." |
You can use access lists to control traffic based on IP address and protocol, but to control access and use for specific users or groups, you need to use authentication and authorization. Authentication, which is the process of identifying users, is supported by the PIX Firewall for RADIUS and TACACS+ servers. Authorization identifies the specific permissions for a given user.
If you want to apply authentication and authorization when an internal (local) host initiates a connection to an external (lower security) network, enable it on the internal (higher security) interface. To set up authentication and authorization to occur when an external host initiates a connection to an internal host, enable it on the outside interface.
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Note If you want a host on an outside (lower security level) interface to initiate connections with a host on an internal (higher security level) interface, create static and access-list command statements for the connection. |
This section includes the following topics:
To enable authentication and authorization, identify the authentication server you are using and the server encryption key on the PIX Firewall. From the configuration on the authentication server you need to determine the users that can access the network, the services that they can use, and the hosts that they can access. Once you have this information, you can configure the PIX Firewall to either enable or disable authentication or authorization.
In addition, you can configure the PIX Firewall to control user access to specific hosts or services. However, it is easier to maintain this kind of access control in a single location, at the authentication server. After you enable authentication and authorization, the PIX Firewall provides prompts inbound or outbound for users of FTP, Telnet, or HTTP (Web) access. Controlling access to a specific system or service is handled by the authentication and authorization server.
Follow these steps to enable the PIX Firewall to support TACACS+ user authentication and authorization:
Step 2 If the external network connects to the Internet, create a global address pool of registered IP addresses. Then specify the inside hosts that can start outbound connections with the nat command and with the access control lists features found in the outbound and apply commands.
Step 3 Identify the server that handles authentication or authorization using the aaa-server command. Create a unique server group name. For example:
aaa-server AuthInbound protocol tacacs+ aaa-server AuthInbound (inside) host 10.1.1.1 TheUauthKey aaa-server AuthOutbound protocol tacacs+ aaa-server AuthOutbound (inside) host 10.1.1.2 TheUauthKey
The first command statement creates the AuthInbound authentication group using TACACS+ authentication. The second command statement states that the AuthInbound server is on the inside interface, that its IP address is 10.1.1.1, and the encryption key is "TheUauthKey."
The third command statement creates the AuthOutbound authentication group using TACACS+ authentication. The fourth command statement states that the AuthOutbound server is on the inside interface, that its IP address is 10.1.1.2, and the encryption key is "TheUauthKey."
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Note RADIUS authorization is provided with the access-list command statement as described in "Configuring RADIUS Authorization." |
Step 4 Enable authentication with the aaa authentication command:
aaa authentication include ftp outbound 0 0 0 0 AuthOutbound aaa authentication include telnet outbound 0 0 0 0 AuthOutbound aaa authentication include http outbound 0 0 0 0 AuthOutbound aaa authentication include ftp inbound 0 0 0 0 AuthInbound aaa authentication include telnet inbound 0 0 0 0 AuthInbound aaa authentication include http inbound 0 0 0 0 AuthInbound
The AuthInbound and AuthOutbound groups are those you specified with the aaa-server command.
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Note Be careful to apply authentication only to protocols that can be authenticated. Applying authentication using the any keyword will prevent protocols such as SMTP or HTTPS from passing through the PIX Firewall. |
Step 5 Enable authorization with the aaa authorization command. PIX Firewall checks the authorization request with the AAA server, which makes the decision about what services a user can access. Use one or both of the following commands to specify outbound and inbound authorization.
aaa authorization include ftp outbound 0 0 0 0 aaa authorization include telnet outbound 0 0 0 0 aaa authorization include http outbound 0 0 0 0 aaa authorization include ftp inbound 0 0 0 0 aaa authorization include telnet inbound 0 0 0 0 aaa authorization include http inbound 0 0 0 0
You can specify port ranges for the aaa authorization command in the following format:
aaa authorization include | exclude author_service|[protocol/port[-port]] inbound | outbound | if_name local_ip local_mask foreign_ip foreign_mask
where:
PIX Firewall allows a RADIUS server to send user group attributes to the PIX Firewall in the RADIUS authentication response message.
The administrator first defines access lists on the PIX Firewall for each user group. For example, there could be access lists for each department in an organization, sales, marketing, engineering, and so on. The administrator then lists the access list in the group profile in CiscoSecure.
After the PIX Firewall authenticates a user, it can then use the CiscoSecure acl attribute (attribute 11, filter-id) returned by the authentication server to identify an access list for a given user group. To maintain consistency, PIX Firewall also provides the same functionality for TACACS+.
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Note Access lists can be used with either RADIUS or TACACS but authorizing FTP, HTTP, or telnet is only possible with TACACS+ |
To restrict users in a department to three servers and deny everything else, the access-list command statements are as follows:
access-list eng permit ip any server1 255.255.255.255 access-list eng permit ip any server2 255.255.255.255 access-list eng permit ip any server3 255.255.255.255 access-list eng deny ip any any
In this example, the vendor-specific attribute string in the CiscoSecure configuration has been set to acl=eng. Use this field in the CiscoSecure configuration to identify the access-list identification name. The PIX Firewall gets the acl=acl_ID from CiscoSecure and extracts the ACL number from the attribute string, which it puts in a user's uauth entry. When a user tries to open a connection, PIX Firewall checks the access list in the user's uauth entry, and depending on the permit or deny status of the access list match, permits or denies the connection. When a connection is denied, PIX Firewall generates a corresponding syslog message. If there is no match, then the implicit rule is to deny.
Because the source IP of a given user can vary depending on where they are logging in from, set the source address in the access-list command statement to any, and the destination address to identify the network services to which user is permitted or denied access.
The aaa authorization command does not require or provide a separate RADIUS option. To enable RADIUS authorization, perform the following steps.
Step 2 Create the access-list command statements to specify the services that hosts are authorized to use with RADIUS.
Step 3 Configure the authentication server with the vendor-specific acl=acl_ID identifier to specify the access-list ID.
When the PIX Firewall sends a request to the authentication server, it returns the acl=acl_ID string, which tells PIX Firewall to use the access-list command statements to determine how RADIUS users are authorized.
ActiveX objects and Java applets are security risks for outbound connections because they can contain code to attack hosts and servers. You can disable ActiveX objects and remove Java applets with the PIX Firewall filter command. In addition, you can use the filter command to work with a Websense server to remove URLs you deem inappropriate for use at your site.
This section includes the following topics:
ActiveX controls, formerly known as OLE or OCX controls, are components you can insert in a web page or other application. These controls include custom forms, calendars, or any of the extensive third-party forms for gathering or displaying information. As a technology, ActiveX creates many potential problems for the network clients including causing workstations to fail, introducing network security problems, or being used to attack servers.
The PIX Firewall ActiveX feature blocks the HTML <object> commands by commenting them out within the HTML web page. This functionality has been added to the filter command with the activex option.
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Note The <object> tag is also used for Java applets, image files, and multimedia objects, which will also be blocked by the new command. |
If the <object> or </object> HTML tags split across network packets or if the code in the tags is longer than the number of bytes in the MTU, PIX Firewall cannot block the tag.
The filter java command filters out Java applets that return to the PIX Firewall from an outbound connection. The user still receives the HTML page, but the web page source for the applet is commented out so that the applet cannot execute. Use 0 for the local_ip or foreign_ip IP addresses to mean all hosts.
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Note If Java applets are known to be in <object> tags, use the filter activex command to remove them. |
Examples
To specify that all outbound connections have Java applet blocking, use the following command:
filter java 80 0 0 0 0
This command specifies that the Java applet blocking applies to Web traffic on port 80 from any local host and for connections to any foreign host.
filter java http 192.168.3.3 255.255.255.255 0 0
This command prevents host 192.168.3.3 from downloading Java applets.
This section contains the following topics:
The filter url command allows you to prevent outbound users from accessing World Wide Web URLs that you designate using the Websense filtering application.
The allow option to the filter command determines how the PIX Firewall behaves in the event that the Websense server goes offline. If you use the allow option with the filter command and the Websense server goes offline, port 80 traffic passes through the PIX Firewall without filtering. Used without the allow option and with the server offline, PIX Firewall stops outbound port 80 (Web) traffic until the server is back online, or if another URL server is available, passes control to the next URL server.
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Note With the allow option set, PIX Firewall now passes control to an alternate server if the Websense server goes offline. |
Perform the following steps to filter URLs:
Step 2 Enable filtering with the filter command.
Step 3 If needed, improve throughput with the url-cache command. However, this command does not update Websense logs, which may affect Websense accounting reports. Accumulate Websense run logs before using the url-cache command.
Step 4 Use the show url-cache stats and the show perfmon commands to view run information.
Examples
The following example filters all outbound HTTP connections except those from the 10.0.2.54 host:
url-server (perimeter) host 10.0.1.1 filter url http 0 0 0 0 filter url except 10.0.2.54 255.255.255.255 0 0
Websense protocol version 4 enables group and username authentication between a host and a PIX Firewall. The PIX Firewall performs a username lookup, and then the Websense server handles URL filtering and username logging.
Websense protocol version 4 contains the following enhancements:
Information on Websense is available at the following website:
http://www.websense.com/products/websense/
This section provides a sample configuration for a PIX Firewall with two interfaces using NAT and PAT, illustrating the following types of configuration:
Figure 3-1 illustrates the network configuration used in this example.
The following procedure shows the basic configuration required for this example. This procedure is similar to the configuration shown in "Basic Configuration Example for Two Interfaces," in "Basic Firewall Configuration":
nameif ethernet0 outside security0 nameif ethernet1 inside security100
Step 2 Identify the line speed of each interface by entering the following commands:
interface ethernet0 10baset interface ethernet1 10baset
You may get better performance by changing the default auto option in the interface command to the specific line speed for the interface card.
Step 3 Identify the IP addresses for each interface:
ip address inside 10.1.1.1 255.255.255.0 ip address outside 209.165.201.1 255.255.255.224
Step 4 Specify the host name for the PIX Firewall:
hostname pixfirewall
This name appears in the command line prompt.
Step 5 Let inside IP addresses be recognized on the outside network and let inside users start outbound connections:
nat (inside) 1 0.0.0.0 0.0.0.0 nat (inside) 2 192.168.3.0 255.255.255.0 global (outside) 1 209.165.201.6-209.165.201.8 netmask 255.255.255.224 global (outside) 1 209.165.201.10 netmask 255.255.255.224 global (outside) 2 209.165.200.225-209.165.200.254 netmask 255.255.255.224
Step 6 Set the outside default route to the router attached to the Internet:
route outside 0 0 209.165.201.4 1
Example 3-1 shows the listing for the basic configuration required to implement a PIX Firewall with two interfaces with NAT.
nameif ethernet0 outside security0 nameif ethernet1 inside security100 interface ethernet0 10baset interface ethernet1 10baset ip address inside 10.1.1.1 255.255.255.0 ip address outside 209.165.201.1 255.255.255.224 hostname pixfirewall nat (inside) 1 0.0.0.0 0.0.0.0 nat (inside) 2 192.168.3.0 255.255.255.0 global (outside) 1 209.165.201.6-209.165.201.8 netmask 255.255.255.224 global (outside) 1 209.165.201.10 netmask 255.255.255.224 global (outside) 2 209.165.200.225-209.165.200.254 netmask 255.255.255.224 route outside 0 0 209.165.201.4 1
The following procedure shows the commands required to manage user access to H.323 and Web services. These commands are used in addition to the basic firewall configuration required, which is described in the previous section, "Basic Configuration."
access-list acl_in deny tcp host 192.168.3.3 any eq 1720 access-list acl_in deny tcp any any eq 80 access-list acl_in permit host 192.168.3.3 any eq 80 access-list acl_in permit host 10.1.1.11 any eq 80
The first access-list command statement denies host 192.168.3.3 from accessing H.323 (port 1720) services such as MS NetMeeting or InternetPhone. The next command statement denies all hosts from accessing the Web (port 80). The next command statement permits host 192.168.3.3 to use the Web. The last access-list command statement permits host 10.1.1.11 access to the Web (at port 80).
Step 2 Specify that the access-list group regulates the activities of inside hosts starting outbound connections:
access-group acl_in interface inside
Step 3 Create static address mappings:
static (inside, outside) 209.165.201.16 192.168.3.16 netmask 255.255.255.240
The static command statement creates a net static command statement, which is a static command statement for a set of IP addresses, in this case for IP addresses 209.165.201.17 through 209.165.201.30.
Step 4 Enable VoIP access:
access-list acl_out permit tcp any host 209.165.201.16 eq h323
The access-list command statement lets users on the Internet send InternetPhone (port h323) requests to users on 192.168.3.x while addressing them as 209.165.201.x.
Step 5 Establish an externally visible IP address for Web access:
static (inside, outside) 209.165.201.11 10.1.1.11 access-list acl_out permit tcp any host 209.165.201.11 eq 80
The static command statement with the access-list command statement establishes an externally visible IP address for Web access (port 80 in the access-list command statement).
Example 3-1 shows the command listing for configuring access to services for the network illustrated in Figure 3-1.
access-list acl_in deny tcp host 192.168.3.3 any eq 1720 access-list acl_in deny tcp any any eq 80 access-list acl_in permit host 192.168.3.3 any eq 80 access-list acl_in permit host 10.1.1.11 any eq 80 access-group acl_in interface inside static (inside, outside) 209.165.201.16 192.168.3.16 netmask 255.255.255.240 access-list acl_out permit tcp any host 209.165.201.16 eq h323 static (inside, outside) 209.165.201.11 10.1.1.11 access-list acl_out permit tcp any host 209.165.201.11 eq 80
This section describes how to implement authentication and authorization for traffic through the PIX Firewall, using a TACACS+ server. The commands used for this purpose are in addition to the basic firewall configuration required, which is described in the previous section, "Basic Configuration."
The aaa-server command specifies the IP address of the TACACS+ authentication server. The aaa authentication command statement specifies that users on network 192.168.3.0 starting FTP, HTTP, and Web connections from the inside interface be prompted for their usernames and passwords before being permitted to access the servers on other interfaces. The aaa authorization command statement lets the users on 192.168.3.0 access FTP, HTTP, or Telnet, and any TCP connections to anywhere as authorized by the AAA server. Even though it appears that the aaa commands let the PIX Firewall set security policy, the authentication server actually does the work to decide which users are authenticated and what services they can access when authentication is permitted.
Example 3-3 shows the command listing for configuring access to services for the network illustrated in Figure 3-1.
aaa-server TACACS+ (inside) host 10.1.1.12 1q2w3e aaa authentication include ftp inside 192.168.3.0 255.255.255.0 0 0 TACACS+ aaa authorization include ftp inside 192.168.3.0 255.255.255.0 0 0 aaa authentication include http inside 192.168.3.0255.255.255.0 0 0 TACACS+ aaa authorization include http inside 192.168.3.0 255.255.255.0 0 0 aaa authentication include telnet inside 192.168.3.0255.255.255.0 0 0 TACACS+ aaa authorization include telnet inside 192.168.3.0 255.255.255.0 0 0
The following commands demonstrate how to implement a mail server for the network shown in Figure 3-1. These commands are used in addition to the basic firewall configuration required, which is described in the previous section, "Basic Configuration."
static (inside, outside) 209.165.201.12 10.1.1.3
netmask 255.255.255.255 0 0 access-list acl_out permit tcp any host 209.165.201.12 eq smtp
The access-list command allows any outside host access to the static via SMTP (port 25). By default, PIX Firewall restricts all access to mail servers to RFC 821 section 4.5.1 commands of DATA, HELO, MAIL, NOOP, QUIT, RCPT, and RSET. This is implemented through the Mail Guard service, which is enabled by default (fixup protocol smtp 25).
Another aspect of providing access to a mail server is setting being sure that you have a DNS MX record for the static's global address, which outside users access when sending mail to your site.
Step 2 Create access to port 113, the IDENT protocol:
access-list acl_out permit tcp any host 209.165.201.12 eq 113 access-group acl_out in interface outside static (inside, outside) 209.165.201.12 10.1.1.3
netmask 255.255.255.255 0 0 access-list acl_out permit tcp any host 209.165.201.12 eq smtp access-list acl_out permit tcp any host 209.165.201.12 eq 113 access-group acl_out in interface outside
If the mail server has to talk to many mail servers on the outside which connect back with the now obsolete and highly criticized IDENT protocol, use this access-list command statement to speed up mail transmission. The access-group command statement binds the access-list command statements to the outside interface.
Example 3-4 shows the command listing for configuring access to services for the network illustrated in Figure 3-1.
static (inside, outside) 209.165.201.12 10.1.1.3
netmask 255.255.255.255 0 0 access-list acl_out permit tcp any host 209.165.201.12 eq smtp access-list acl_out permit tcp any host 209.165.201.12 eq 113 access-group acl_out in interface outside static (inside, outside) 209.165.201.12 10.1.1.3
netmask 255.255.255.255 0 0 access-list acl_out permit tcp any host 209.165.201.12 eq smtp access-list acl_out permit tcp any host 209.165.201.12 eq 113 access-group acl_out in interface outside
The following commands demonstrate how to implement Network File System (NFS) for the network shown in Figure 3-1. These commands are used in addition to the basic firewall configuration required, which is described in the previous section, "Basic Configuration":
access-list acl_out permit udp host 209.165.201.2 host 209.165.201.11 eq rpc
Refer to the UNIX /etc/rpc file and the UNIX rpc(3N) command page for more information.
Once you create an access-list command statement for RPC, you can use the following command from outside host 209.165.201.2 to track down the activity of a PCNFSD on RPC 150001.
rpcinfo -u 209.165.201.11 150001
Another use of RPC is with the following command to see the exports of 209.165.201.11 if you want to allow mounting NFS from the outside network to the inside network:
showmount -e 209.165.201.11
Many protocols based on RPC, as well as NFS, are insecure and should be used with caution. Review your security policies carefully before permitting access to RPC.
Step 2 Permit NFS access:
access-list acl_out permit udp host 209.165.201.2 host 209.165.201.11 eq 2049
NFS access occurs at port 2049 and provides access between the outside and inside, such that host 209.165.201.2 can mount 10.1.1.11 via the global address 209.165.201.11.
Example 3-5 shows the command listing for configuring access to services for the network illustrated in Figure 3-1.
access-list acl_out permit udp host 209.165.201.2 host 209.165.201.11 eq rpc access-list acl_out permit udp host 209.165.201.2 host 209.165.201.11 eq 2049
Posted: Mon Jun 10 17:50:52 PDT 2002
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