ciscoasa(config)# static (inside,outside) tcp interface 80 192168120 80 netmask 255255255255 in Software

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ciscoasa(config)# static (inside,outside) tcp interface 80 192168120 80 netmask 255255255255
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In this example, web traffic sent to port 80 to the IP address on the outside interface of the appliance will be redirected to 192168120 on port 80 of the inside interface
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An address translation is configured for every source and destination interface pair: this allows the appliance to translate a source address to something different depending on the destination interface the source is trying to reach When address translation is required (NAT control is enabled), there must be an existing entry in the xlate table, or the
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Figure 5-20 Static PAT example
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appliance must be able to build an entry before the appliance will switch a packet between interfaces: Building translation policies can be done dynamically (nat and global commands) or statically (static command) The exceptions to this rule are the nat 0 commands, which create exemptions to the address translation process However, when multiple translation policies are configured, the question is which translation policy should be used by the appliance When looking for a matching translation policy, the appliance goes through the following steps: 1 The appliance looks for an existing translation in the translation table; sometimes Cisco will refer to this as trying to find a matching xlate slot in the translation table 2 If no entry exists in the translation table, the appliance looks for address translation exceptions in the nat 0 commands on a best-match basis 3 If there are no matches on the Identity NAT commands, the appliance will try to find a match against the configured static NAT commands based on a best-match basis 4 If there are no matches on the static NAT commands, the appliance will try to find a match against the configured static PAT (PAR) policies on a bestmatch basis 5 If no match is found within the PAR translation policies, the appliance then looks for a match in its policy nat and global commands with a corresponding ACL 6 If there is not a match on a policy translation configuration, the appliance then looks for a match in its normal nat and global commands 7 If a translation or translation policy doesn t exist for the packet, the appliance will drop the packet if NAT control is enabled; if NAT control is not enabled, then the packet is not translated, but can flow through the appliance, assuming other appliance policies allow it
5:
Address Translation
TCP SYN FLOOD ATTACKS
Some types of traffic are malicious One example is the weakness that TCP has during the three-way handshake when establishing a connection: the destination assumes that when it receives a SYN, it is a legitimate connection attempt However, attackers could use this to their advantage and spoof thousands of TCP SYNs, making it look like there are thousands of legitimate connection requests The issue the destination has is that since the destination assumes the connection requests are valid, it must maintain them for a period before determining that they aren t going to complete the three-way handshake and removing them from the local connection table This can have a devastating impact on the destination, since most operating systems will keep the connection in their local table from 30 to 60 seconds, and the connection table has finite resources to store connections So an attacker could easily fill up the connection table and deny legitimate connection attempts while the attack is ongoing
The Original TCP Intercept
Cisco introduced the TCP Intercept feature on the PIXs back in version 52 to limit the effectiveness of these kinds of attacks In the original implementation, you would define embryonic connection limits in the static and/or nat commands Once these limits were reached, the appliance would intercept the TCP SYNs and proxy the connection, sending back a SYN/ACK, pretending to be the destination The appliances would maintain this connection in their conn table If an ACK was not received in 30 seconds, the half-open connection was removed from the conn table If it was received within 30 seconds, the appliance would perform a three-way handshake to the real destination, bind the two connections source and destination and place the single bound connection in the conn table NOTE Besides specifying connection limits with the static and nat commands, you can also set up policies using the Cisco Modular Policy Framework (MPF) starting with version 70 The advantage of MPF is that it is more granular and will work with or without address translation MPF is discussed in 10
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