Suggested prerequisite reading
»Cisco Forum FAQ »NAT, PAT, Port Forward, Internet and Server Access: Introduction and Practices
For illustration purposes, I use:
* One Cisco PIX Firewall 501 (2 Ethernet ports) and one Cisco PIX Firewall 515 (3 Ethernet ports) running PIX OS version 6.3(3) for sample configurations in OS version 6.3
* One Cisco ASA 5510 (5 Ethernet ports) running OS 7.2(3) for sample configuration in OS 7.0 or later
* ISP provided static IPs
* ISP provided DNS
* Static WAN IP addresses, provided by ISP
Note:
1) When your ISP doesn't use Static IP Address
If your ISP does not use Static IP Address, this configuration can be easily modified to suit other type of ISP connections (i.e. DHCP, PPPoE, PPPoA). Check out other part of this forum's FAQ for such situation.
Various PPPoE/PPPoA/DHCP/Static Sample Configuration with Cisco
2) For servers other than FTP
For illustration purposes, I run FTP server behind the router. This configuration can be modified to either have web server, mail server, or just any public servers that run on specific TCP or UDP port/ports. When you run web server, you can replace the TCP port 20 and 21 with TCP port 80 (the standard web port) and possibly also with TCP port 443 (the standard secure web port). As for mail server, replace with TCP port 25 (the standard mail port). For other servers, confirm your software configuration of the TCP/UDP port it uses.
3) For ASA or PIX Firewall running OS version 7.0 or later
PIX OS version 6.3(3) commands are pretty much similar to the ASA or PIX OS version 7.0 or later. Should you need more sample configurations from different angle or sample configurations on ASA or PIX running OS version 7.0 or later, keep reading to get the preliminary concept overview then proceed to the next discussion.
Background
Just a bit of explanation. Ethernet0 interface is for outside network (ISP or WAN physical interface). Ethernet1 is for inside (LAN interface).
The ISP-provided IP Block is 1.1.1.9/24 - 1.1.1.14/24, where 1.1.1.9 is for the Internet traffic, 1.1.1.13 is for the server, and 1.1.1.14 is the PIX WAN interface IP address. The default gateway (the ISP) is 1.1.1.1/24.
This configuration example is for running FTP server, which uses the standard TCP port 20 and 21. You need to permit inbound traffic from the Internet to your LAN by issuing the access-list INBOUND permit tcp any host 1.1.1.13 range 20 21 command.
The LAN uses 10.10.10.0 network with 255.255.255.0 subnet for both servers and workstations. All servers within the LAN use static IP address. The PIX is configured as DHCP server to give out IP info (IP addresses, subnet mask, DNS) to workstations that are configured as DHCP client.
This sample configuration of setup your own servers behind Cisco router suggests multiple possible network design. In general, the suggested network design are either with or without port forwarding. When there is a port forwarding in place, it means there is NAT (Network Address Translation) and/or PAT (Port Address Translation) involved. When there is no port forwarding in place, it means there is no NAT/PAT involved.
Side Note:
For more info on NAT/PAT concept, check out the following FAQ
»Cisco Forum FAQ »NAT, PAT, Port Forward, Internet and Server Access: Introduction and Practices
When there is a port forwarding in place, usually all or most of the following setup are in place.
* Servers use private IP address (typically fall under 10.0.0.0/8, 172.16.0.0/12, or 192.168.0.0/16 subnets)
* Somewhere along the line between the servers and the Internet, there is a NAT/PAT in place.
In the first part of the sample configuration, the servers use the typical Private IP address and the Cisco router performs the NAT/PAT to bridge communication between the server and the Internet.
When there is no port forwarding in place, usually all or most of the following setup are in place.
* Servers use Public Internet-routeable IP address (typically don't fall under 10.0.0.0/8, 172.16.0.0/12, or 192.168.0.0/16 subnets)
* No NAT/PAT in place between the servers and the Internet
In the second part of the sample configuration, the servers use Public IP address directly. The Cisco router does not perform NAT/PAT at all to bridge communication between the server and the Internet since there is no reason to do such.
As best practice, it is suggested not to do NAT/PAT or port forwarding between the server and the Internet when there are multiple Public IP subnets to use or when there is a large Public IP subnet that you can subnet into smaller network. When there is only one Public IP address or small-size subnet, then in general there is no other choice but to deploy NAT/PAT or port forwarding between the server and the Internet.
First Network Design: There is NAT/PAT in place between the server and the Internet (with port forwarding)
In this part of sample configuration, there will be two PAT in place between Public and Private IP addresses. One is dynamic PAT which uses single Public IP address for all local workstations. Another one is static PAT which uses single Public IP address for servers.
When using NAT/PAT, keep in mind that the Internet-accessible servers are seen from the Internet as their NAT/PAT-ed IP address (the Public IP address) and not the local IP address (not the Private IP address). The connection from the Internet users to the Public IP address will then be forwarded by the NAT/PAT device, which in this case is the ASA/PIX Firewall, according to the ASA/PIX Firewall's routing table. In other words, the ASA/PIX Firewall will (TCP/UDP) port forward connection from the Public IP address to the actual Private IP address after proper network's routing table is in place. This understanding is very important when you or someone need to test connectivity to the server and/or when you need to create filter (access list or ACL for short) to allow only certain incoming traffic from the Internet and block others.
Specifically for firewall devices such as PIX Firewall and ASA, creating such ACL to regulate incoming traffic initiated from the Internet to local LAN is required. On this sample configuration, this ACL is called INBOUND (access-list INBOUND).
Side Note:
For more info on NAT/PAT concept, check out the following FAQ
»Cisco Forum FAQ »NAT, PAT, Port Forward, Internet and Server Access: Introduction and Practices
Instruments used in this illustration are pretty much standard for running your own servers. Please note that IP addresses, username, and password are changed. However, you could always modify the configuration to suit your situation.
This sample configuration assumes that you have a block of IP addresses from ISP. There is a dedicated Public IP address for the PIX WAN interface (the Outside interface) and another dedicated Public IP address for the server PAT IP address. In this sample configuration, the network uses 1.1.1.13 IP address as the server Public IP address where the server inside Private IP address is 10.10.10.2. Any other machines use 1.1.1.9 Public IP address to go out to the Internet.
Note that the static PAT is indicated by the static command where the dynamic PAT is indicated by the pair of global and nat commands. You can change the IP address either on the static, global, or nat commands to suit your need or situation.
If you only have single Public IP address for both PIX/ASA Outside interface and server PAT IP address instead of a block of IP addresses, then you use the following commands instead.
With situation of dynamic IP address, at some point you still need to know the exact Public IP address you receive from your ISP for server connection testing and production time. To find out, you can issue show ip address command on the PIX. You will then see the associated outside interface Public IP address.
Note that the static commands above show the static PAT between outside (WAN) interface Public IP address (the one that shows on show ip address command) and Private IP address (the 10.10.10.2).
Between Static and Dynamic IP Address Assignment for LAN Machines
Typically, servers are having static IP addresses where regular workstations are having dynamic IP addresses. As mentioned, all LAN machines (including servers and workstations) are within 10.10.10.0/24 subnet. Let's say you dedicate 10.10.10.1 for the PIX, 10.10.10.2 - 10.10.10.14 for servers, and the rest (10.10.10.15 - 10.10.10.254) for workstations.
This illustration assumes all workstations are receiving dynamic IP address (as DHCP client) from the PIX which is acting as the LAN DHCP server; as indicated by the dhcpd address 10.10.10.15-10.10.10.254 inside command. Since the PIX and servers are having static IP addresses, you exclude 10.10.10.1 - 10.10.10.14 from DHCP pool. This is why the 1st DHCP IP address within the pool is 10.10.10.15 and the last one is 10.10.10.254
Tips
* You can use any computer, running any operating system with any generic FTP or other server software. There is no exclusiveness
* Make sure that the ports you plan to use are not used by other program.
* Also make sure that your ISP does not block the port you plan to use
* You can use generic port scanner software to find out if the port you plan to use is available
* Setup the FTP software to have the FTP server to use port 21 (standard port)
* FYI, port 21 only handle the connection and data is sent over port 20. With both ports PAT'd you will be able to use both PASV & PORT connections to your FTP server
* To verify the configuration, access the server from OUTSIDE network (i.e. from the Internet) and NOT from your own LAN. For this illustration, run an Internet browser (i.e. Internet Explorer, Netscape) and open ftp://1.0.0.13:21
The Sample Configuration
Finally, here is the "show running-config" output.
Note that the static command is where the port forwarding actually takes place between the Public and the Private IP address on specific TCP or UDP port. The ACL INBOUND is only a filter to permit certain incoming traffic initiated from Outside (the Internet).
Command Adjustment for Specific Needs
The above sample configuration shows how to provide FTP access to public. When you need to provide other access type such as Web or Mail, following is the command adjustment needed.
To run Public Web Server, following is the command list
To run Public Mail Server, following is the command list
To run both Public Mail and Web Server where each server resides on different LAN machines, following is the command list.
where 10.10.10.2 is the LAN Mail Server and 10.10.10.3 is the LAN Web Server.
To run both Public Mail and Web Server where each server resides on different LAN machines and each server has its own Public IP address, following is the command list.
where 10.10.10.2 is the LAN Mail Server and 1.1.1.12 is the WAN (Public) IP address. Similarly, 10.10.10.3 is the LAN Web Mail Server and 1.1.1.13 is the Public IP address.
If you like to dedicate specific Public IP address for specific LAN machines, then you can implement Static NAT instead as follows.
where you dedicate 1.1.1.12 only for 10.10.10.2 machine and dedicate 1.1.1.13 only for 10.10.10.3 machine.
Having Servers on DMZ
Scenario 1: NAT/PAT for both DMZ and Inside networks
This time there is DMZ network of 10.10.10.0/24 where the Inside network is 10.0.0.0/24. Traffic between DMZ and Inside networks pass as their original IP addresses (as in general, best practice), therefore there is no need to NAT Inside network to access DMZ and vice versa. DMZ network is only allowed to access Inside DNS server (the 10.0.0.2 IP address) and no other Inside hosts while the DMZ network is allowed to access anything Outside (the Internet).
Note that the access-list DMZ only applies when connections are initiated from DMZ machines. Any connections initiated from other network such as Inside and Outside won't be affected by this access-list DMZ.
Following is the sample configuration.
Scenario 2: NAT/PAT only for Inside network, no NAT/PAT for DMZ network
Second Network Design: There is no NAT/PAT in place between the server and the Internet (without port forwarding)
So far the presented sample configurations use 1.1.1.9/24 - 1.1.1.14/24 as the NAT/PAT IP subnet. In this second part of sample configuration, there is an addition IP subnet of 1.0.0.0/30. Instead of using 1.1.1.9/24 - 1.1.1.14/24, this sample configuration uses 1.1.1.8/29. The 1.0.0.0/30 is used as the NAT/PAT IP subnet for Inside network. DMZ network does not use NAT/PAT at all, instead the DMZ network uses the 1.1.1.8/29 directly.
With DMZ network having the Public IP address of 1.1.1.8/29 directly assigned, there are two points that are established. One is that there is no need to do NAT/PAT for DMZ network since DMZ machines already use Internet-routable Public IP address.
The other established point is to avoid unnecessary DNS BIND to resolve DMZ server name to both Public IP and Private IP addresses. With DMZ network having the Public IP address of 1.1.1.8/29 directly assigned, DMZ server DNS name resolves to always its associated Public IP address regardless of where the incoming traffic come from, either from the Internet or from the Inside network. More info on this issue can be found in the following FAQ.
»Cisco Forum FAQ »NAT, PAT, Port Forward, Internet and Server Access: Introduction and Practices
Now let's describe the network setup. The 1.1.1.8/29 IP subnet is assigned directly to all DMZ machines. A 1.1.1.14 is used as the DMZ network default gateway, which is also the PIX Firewall DMZ interface IP address. The Inside network is 10.0.0.0/24. Traffic between DMZ and Inside networks pass as their original IP addresses (as in general, best practice), therefore there is no need to NAT Inside network to access DMZ and vice versa. DMZ network is only allowed to access Inside DNS server (the 10.0.0.2 IP address) and no other Inside hosts while the DMZ network is allowed to access anything Outside (the Internet). Such access is regulated by the ACL DMZ.
There is no NAT/PAT in place for DMZ network to go out to the Internet as mentioned earlier. In other words, there is no port forwarding in place DMZ network and the Internet. Since there is no NAT/PAT between DMZ network and Inside network, there is no port forwarding in place between DMZ network and the Inside network either. This no-port-forwarding setup is due to the network design of using the actual/original IP addresses instead of using the NAT/PAT IP addresses, as described above.
There is however NAT/PAT in place for Inside network to go out to the Internet. All Inside network machines are PAT-ed to 1.0.0.2 IP address to go out to the Internet, which is the PIX Firewall Outside interface IP address.
The DMZ network still hosts FTP server, which is directly assigned 1.1.1.13 IP address. For security, there is ACL INBOUND to permit only necessary incoming traffic from the Internet and block others.
Following is the sample configuration.
1. In OS version 6.3
2. In OS version 7.0 or later
More Sample Configurations
As mentioned, PIX OS version 6.3(3) commands are pretty much similar to the ASA or PIX OS version 7.0 or later. Should you need more sample configurations from different angle or sample configurations on ASA or PIX running OS version 7.0 or later, you can check out the following links.
Run Mail Server on DMZ Network
PIX running OS 6.3 image
PIX/ASA running OS 7.0 image or newer
Run Mail Server on Inside Network
PIX running OS 6.3 image
PIX/ASA running OS 7.0 image or newer
Run Mail Server on Outside Network
PIX running OS 6.3 image
PIX/ASA running OS 7.0 image or newer
PIX Firewall - Router Combo
»Cisco Forum FAQ »Internet - PIX/ASA - Router - LAN
»Cisco Forum FAQ »Internet - Router - PIX/ASA - LAN
Some Discussions
»[Config] cisco 5520
Note on Running Microsoft Exchange Mail service with ASA/PIX Firewall running OS version 7.0 or later
The OS version 7.0 introduces a new feature called ESMTP inspection that supercede the older OS SMTP inspection. This new feature was created to inspect ESMTP (Extended SMTP) traffic in addition to standard SMTP traffic. This new inspection is based on the industry standard RFC 1869 about the ESMTP protocol definition and mechanism.
Specifically for the OS version 7.0 and newer, only the following SMTP mail commands are allowed to pass through by default. All other commands are blocked by default, based on RFC 2821 Section 4.5.1: Minimum Implementation
AUTH, DATA, EHLO, ETRN, HELO, HELP, MAIL, NOOP, QUIT, RCPT, RSET, SAML, SEND, SOML, and VRFY
The reason of such default behavior is due to security and is based on RFC requirement of minimum implementation of ESMTP mail server mechanism.
Unfortunately Microsoft Exchange ESMTP implementation does not comply with the RFC 2821 for some reason. Therefore there might be issues when MS Exchange is used to host mail server behind ASA/PIX Firewall running OS version 7.0 or later.
When you need to modify default behavior of ASA/PIX Firewall in which ESMTP commands are permitted to pass, you can check out the following official Cisco documentation to do such.
Managing SMTP and Extended SMTP Inspection
Sample Configurations of Enabling and/or Modifying Default Inspection on some protocols
Check out the following official Cisco documentations.
ESMTP TLS Configuration
PIX/ASA 7.x: Enable FTP/TFTP Services Configuration Example
Disable Default Global Inspection and Enable Non-Default Application Inspection
Troubleshooting
FTP Server related
Active FTP vs. Passive FTP, a Definitive Explanation
»FTP server doesn't work on port 21, works on other ports
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by aryoba 
last modified: 2009-10-10 05:57:22 |
