The audit log can be enabled for an individual traffic class in a policy map with the help of a global parameter-map object that defines the inspection parameters. To configure auditing of inspected sessions, use the configuration commands in Table For example, in the firewall setup from the preceding chapter, we might want to log all incoming SMTP sessions as shown in Figure To configure the audit trail for the SMTP session, we need to do the following:.
Because the new traffic class is a create a policy map as subset of the existing traffic class, we need to put it at the beginning of the policy map. There is no way to insert you would do with an class commands in a policy map they are always appended , so all class definitions have to be erased and reentered. Deleting a policy map silently Listing includes the step-by-step configuration process.
After this configuration change, the router generates a line in the syslog for every session establishment and termination event. Sample log messages are displayed in Listing The router constantly monitors the number of established and half-open connections and the connection creation rate and issues NOTE Half-open TCP connec- warnings or alerts when they exceed configured thresholds.
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The router also tries to cope with a DoS attack primarily tions are connections SYN attacks in which the intruder sends a large number of TCP SYN packets, also called SYN floods by aggressively where the SYN packet deleting half-open sessions when their number exceeds the threshold or when the session establishment rate exceeds the has been received, but the threshold.
It does not help preserve the service availability, because the legitimate sessions in the establishment phase also get dropped among the fake ones. TCP intercept is a much better mechanism to maintain service availability during an aggressive SYN flood. However, it has a big performance impact, which might prevent its use on a heavily loaded router. The zone-based policy firewall uses parameter maps that can be attached to every inspect statement in a policy map, resulting in a very granular solution that can be fine-tuned to the actual network environment and traffic patterns. The commands described in Table are used to configure the inspect thresholds in a parameter map.
The parameter- map syntax is described in Table Remember that all parameters apply to a single traffic class between a pair of zones. While the DoS protection is active, the firewall resets one half-open connection for every new connection attempt. The low number process starts after the high threshold has been exceeded and stops when the session establishment rate drops below the low threshold.
The offending host threshold [ block minutes ] could be optionally blocked for the specified duration. You can display the default values of all these parameters with the show parameter-map type inspect default command, which displays the values in Listing Advanced Zone- Based Policy Firewall LISTING Default Stateful Packet Inspection Thresholds Configuration show parameter-map type inspect default fw s parameter-map type inspect default values audit-trail off alert on max-incomplete low max-incomplete high one-minute low one-minute high udp idle-time 30 icmp idle-time 10 dns-timeout 5 tcp idle-time tcp finwait-time 5 tcp synwait-time 30 tcp max-incomplete host 50 block-time 0.
You would usually need to modify the inspect thresholds in high-volume environments when the router starts issuing warning and alert messages similar to Listing Before changing the thresholds, it might be beneficial to reduce the various timeouts which will by itself reduce the number of half-open sessions. For example, many host DNS resolvers time out in 2 seconds. Thus, having a 5-second DNS timeout is overkill. Similarly, a second synwait-time is useful only if there is a dialup connection in the outbound session establishment path; otherwise, you should use a timeout of a few seconds.
The second icmp idle- timeout can also be reduced unless you have very slow-speed congested links. Advanced Zone- Based Policy Firewall After you have adapted the timeouts to your environment, perform the following steps to fine-tune the half-open session Configuration thresholds:. Make sure your network is not infected with a worm or a target of a DoS attack. In both cases, tuning DoS protection will obviously not help. Step 2. Create a new parameter map that considerably increases the current thresholds for example, by a factor of Apply the new parameter map to the inspect statement in the policy map that triggers the DoS protec- tion.
The alert message indicates the policy map and the class within it. After a statistically significant time period at least a day , review the inspect counters see Listing Set the max-incomplete low parameter to the Maxever session counts half-open value. Set the max- Configuration incomplete high parameter to a value at least 25 percent above that. Increase the parameter to an even higher value if you expect a significant increase in network traffic for example, increased website traffic due to a marketing campaign.
NOTE Step 5. The Cisco IOS printout gives you no reliable information that would allow you to set the one-minute thresh- Server logs are an olds. As a starting point, set the one-minute low value to 3 times the Maxever session counts estab value and extremely valuable the one-minute high value to a 50 percent higher value. Computing one- Step 6. Repeat the process for every traffic class that needs adjusted DoS protection behavior. Perform ongoing firewall DoS protection monitoring, and log all related events to a syslog server. The logged server log is a simple events will help you perform additional fine-tuning of the parameters.
Identifying Masquerading Applications Some applications most notably, instant messaging and peer-to-peer applications are extremely aggressive in their attempts to establish connections with their servers. For example, Yahoo Messenger first tries to connect to Yahoo! To be fair, other instant messaging IM applications are no better, and Yahoo!
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The applications that can adapt their TCP or UDP port usage or use well-known ports reserved for other applications as a disguise are effectively trying to bypass firewall policies. However, they could still use IM applications, because the inside clients have unlimited access to outside web servers, giving the IM applications a TCP port they can use, as shown in Figure Inside Looks like web browsing, passed.
For example, all IM applications have to connect to well-known servers as they exchange the chat traffic through them. The only means of identifying these applications is by looking for their specific signatures in the data stream similar to the way virus scanners identify new viruses.
Cisco IOS can identify both types of applications starting in Release Identifying Peer-to-Peer Applications To identify the peer-to-peer applications by their signature, use the signature option in the match command of a class map. For example, to match all peer-to-peer applications in our sample firewall, you could use the traffic class definition in Listing LISTING Matching Peer-to-Peer Applications by Their Signature class-map type inspect match-any peer2peer match protocol gnutella signature match protocol edonkey signature match protocol kazaa2 signature match protocol fasttrack signature.
The signature-based classification bypasses the order of classes in a policy map, so the peer-to-peer sessions misusing ports from other applications are detected regardless of the order of class maps in a policy map. Advanced Zone- Based Policy Firewall Identifying Instant Messaging Applications Configuration Instant messaging applications and other applications that use unspecified ports to connect to their server are identified with the help of the protocol-info parameter-map using the syntax in Table NOTE The router allows you to attach a protocol-info parameter-map to any match protocol statement in a class map with the exception of peer-to- peer applications, where the signature keyword follows the protocol name , but it only works correctly for instant messaging applications in Cisco IOS Release The destination IP address match with the parameter map is done before the sequential scan of the policy map, so the IM applications are correctly identified regardless of their position in the policy map.
Updates: 0 failed, succeeded, next after 5. Updates: 0 failed, 17 succeeded, next after 0. Updates: 0 failed, 2 succeeded, next after 3. Application Layer Packet Inspection In addition to the transport layer stateful inspection which, for protocols with separate control and data sessions such as FTP and H. You can check whether the users are trying to use cleartext authorization. You can check maximum message length. You can block access to specific programs over the RPC mechanism. You can match and block transfer of files matching the specified regular expression. You can block the entire messaging service or just the text chat within it.
You can also identify applications misusing HTTP for tunneling, instant messaging, or peer-to-peer applications.
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Advanced Zone- Based Policy Firewall All application layer inspections are configured using application-specific class maps and policy maps that are then Configuration attached with the service-policy command as children of Layer 4 policy maps. Application-specific class maps are configured with the class-map type inspect protocol name configuration command.
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The list of protocols supported is displayed in Listing After can find a detailed list in the class maps have been defined, you can configure the application-specific policy maps with the policy-map type the Cisco IOS documen- inspect application name command. The set of applications is a bit shorter see Listing because you can use any tation.
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The extensive set instant-messaging class-map aol, msnmsgr, and ymsgr in im policy-map and similarly any peer-to-peer class map of HTTP match edonkey, fasttrack, gnutella in p2p policy-map. Application layer policies thus require stateful inspection at the parent level. This restriction might force you to implement more granular Layer 4 classes. If no action is specified in Configuration class-default, these packets are dropped. Packets not matched by an application layer policy map are not dropped NOTE but are handled according to the specifications of the parent policy map.
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Some of these issues could be solved with other mechanisms. To control the Internet access link utilization, the IT manager would like to prevent download of MP3 and IMAP could be files with peer-to-peer services and limit the size of e-mails the users can send. Furthermore, because the e-mail is hosted deployed instead of forcing the users to use on external servers, the users shall be forced to use non-clear-text authentication methods to prevent a potential intruder non-clear-text authentica- or an external wiretap from collecting the passwords Figure From there onward, use the remainder of the configuration from the beginning of Chapter 3.
LISTING Class Maps Matching Individual Deeply Inspected Application Protocols class-map type inspect match-any getMail match protocol pop3 class-map type inspect match-any sendMail match protocol smtp extended class-map type inspect match-any readMail match protocol imap class-map type inspect match-any peer2peer match protocol gnutella signature match protocol edonkey signature match protocol kazaa2 signature match protocol fasttrack signature.
Hypertext Transfer Protocol HTTP , the protocol used to exchange content between the web works only on non- server and its clients, is a notable exception. If the data exchange takes place identifying HTTP misuse for unauthorized traffic to arcane ones for example, the number of header fields in the over Secure Sockets request or response header.
You can use allow, reset, and log action commands within it.