how to create barcodes in excel 2016 Figure 7-13: Using a PIX for securely connecting an internal LAN to the Internet in Software

Printer GS1 - 13 in Software Figure 7-13: Using a PIX for securely connecting an internal LAN to the Internet

Figure 7-13: Using a PIX for securely connecting an internal LAN to the Internet
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The PIX has two Ethernet interfaces, one for connection to the internal LAN, and one for connection to the router that interfaces to the Internet or other external network The external interface has a range of addresses that it can use to communicate with the external network The internal interface is configured with an IP address appropriate for the internal network numbering scheme The main job of the PIX is to map internal to external addresses whenever internal computers need to communicate with the external network This makes internal computers appear to the external world as if they are directly connected to the external interface of the PIX Because the external interface of the PIX is Ethernet, MAC addresses are needed to deliver packets to hosts To make the internal hosts appear as if they are on the external interface at the Data Link as well as the Network layer, the PIX runs Proxy ARP Proxy ARP assigns Data Link MAC addresses to the external Network layer IP addresses, making internal computers look as if they are on the external interface to the Data Link layer protocols In most cases, the communication with the external network originates from within the internal network As the PIX operates on the packet rather than application level (the way proxy servers do), the PIX can keep track of UDP conversations as well as TCP connections When an internal computer wants to communicate with an external computer, the PIX logs the internal source address and dynamically allocates an address from the external pool of addresses and logs the translation This is known as stateful NAT, as the PIX remembers to whom it is talking, and which computer originated the conversation Packets coming into the internal network from the external network are permitted only if they belong to an already identified conversation The source address of the internal computer is compared to a table of existing translations, and if an existing translation exists, the external address already assigned is used If an existing entry is not present, a new external address is allocated Entries in this translation time out with a preconfigured timeout value
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This mechanism is efficient for most normal implementations In this fashion, a small number of assigned Internet numbers can service a large community of internal computers, as not all internal computers will want to connect to the Internet at the same time There are instances, however, when you need to allow external computers to initiate a conversation with selected internal computers Typically these include services such as e-mail, WWW servers, and FTP hosts The PIX allows you to hard-code an external address to an internal address that does not time out In this case, the usual filtering on destination address and port numbers can be applied An external user still cannot gain any knowledge of the internal network without cracking into the PIX itself Without knowledge of the internal network, a malicious user cannot stage attacks on the internal network from internal hosts With the PIX protecting your internal LAN, you might want to locate e-mail, WWW servers, and FTP hosts on the outside network The PIX then can give secure access to these machines for internal users, and external users have access to these services without having to traverse the internal LAN Another key security feature of the PIX is that it randomizes the sequence numbers of TCP packets Since IP address spoofing was documented in 1985, it has been possible for intruders to take control of an existing TCP connection and, using that, send their own data to computers on an internal LAN To do this, the intruder has to guess the correct sequence number With normal TCP/IP implementations this is easy, because most start a conversation with the same number each time a connection is initiated The PIX, however, uses an algorithm to randomize the generation of sequence numbers, making it virtually impossible for an attacker to guess the sequence numbers in use for existing connections A Simple PIX Configuration Configuring the PIX is a relatively straightforward task It is much simpler than setting up a proxy server and multiple DNS machines to provide an equivalent level of security In concept, what you need to do is assign an IP address and pool of addresses to use for access on the outside, and an IP address and netmask for the internal connection, RIP, timeout, and additional security information Figure 7-14 shows a sample configuration
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