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Category: logging

NAT translation logging

The ip nat log translations syslog command starts NAT logging: every NAT translation created on the router is logged in syslog (which can be sent to console, syslog host or internal buffer). You could use this command as a poor man’s reporting tool if you have to monitor the address translations on your edge routers (for example, due to security policy or auditing requirements). Obviously you should configure the no logging console first in a production environment; otherwise your router will hang a few moments after you’ve enabled NAT logging.

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Catching all syslog messages

If you use UDP-based syslog servers, you might have noticed that they miss a message or two generated during a router reload (particularly when the syslog server is on a directly connected LAN). The reason is simple - when the first message is sent to the syslog server, its MAC address is not yet in the router's ARP cache and the message is dropped. To prevent the message loss, you can use the logging server-arp configuration command (introduced in IOS release 12.3T), making sure that the router sends ARP request to the configured syslog server(s) before generating the first syslog messages.

Note: this issue does not occur when using TCP as the syslog transport mechanism.
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Cisco IOS Login Enhancements

Cisco has in IOS release 12.3(4)T (integrated into 12.4) finally introduced features (long available in Unix and Windows) that slow down dictionary attacks on a router. On top of logging of login failures, you can also slow down the login process by delaying the router response after a login failure with the login delay seconds command.

On top of that, the you can configure the router to enter quiet mode after several login failures have been detected in specified timeframe with the login block-for seconds attempts tries within seconds configuration command.
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Enhanced OSPF adjacency logging

The log-adjacency-changes OSPF configuration command got improved (IOS documentation claims it's happened in release 12.1) with the detail command that logs every step of OSPF adjacency establishment (sample printout below). Great troubleshooting tool :)

%OSPF-5-ADJCHG: Process 1, Nbr on Serial0/0/0.100 from DOWN to INIT, Received Hello
%OSPF-5-ADJCHG: Process 1, Nbr on Serial0/0/0.100 from INIT to 2WAY, 2-Way Received
%OSPF-5-ADJCHG: Process 1, Nbr on Serial0/0/0.100 from 2WAY to EXSTART, AdjOK?
%OSPF-5-ADJCHG: Process 1, Nbr on Serial0/0/0.100 from EXSTART to EXCHANGE, Negotiation Done
%OSPF-5-ADJCHG: Process 1, Nbr on Serial0/0/0.100 from EXCHANGE to LOADING, Exchange Done
%OSPF-5-ADJCHG: Process 1, Nbr on Serial0/0/0.100 from LOADING to FULL, Loading Done
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Disable console logging

Large amount of logging output (most often produced in a debugging process) sent to a router's console can significantly increase the router's CPU load and even stop the box from forwarding packets (high-end routers with distributed forwarding architecture are obviously an exception, but even they can lose routing adjacencies). The reason is very simple - console interrupt is one of the highest-priority interrupts on the router (otherwise you wouldn't be able to get a response to the BREAK key on a hung box).

Update January 9th 2007: The router does not check if a user is logged into the console port or a device (for example, a terminal) is attached to it; if console logging is enabled, messages are always sent to the console port (causing CPU load).

To stop the console logging, use the no logging console global configuration command (highly recommended for routers that are not usually accessed through the console port) or you might want to limit the amount of messages sent to the console with the logging console level configuration command (for example, logging console notifications).

Note: this article is part of You've asked for it series.
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Remove timestamps from syslog messages

The ability to replace router uptime with date and time in the logging messages with the service timestamps log datetime command was present in IOS for a long time, but I was always annoyed at timestamps when collecting syslog messages for demonstration purposes. The command to turn them off has also been available "forever", but was too obvious for me to try out ... the no service timestamps log command.
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Configuration Change Logging ignores the configuration downloads

The Configuration Change Notification and Logging feature is supposed to log changes to the router's configuration. While it does a great job of logging commands entered in the router configuration mode, it completely ignores configuration changes due to configuration download (for example, with configure network or copy tftp running-config command).
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Log terminal access to your router

In a previous post, I've shown how you can log the changes in interactive user's privilege level. With the Cisco IOS Login Enhancements (introduced in IOS release 12.3(4)T, integrated in 12.4), you can also log all login successes and failures, even when using local user database (a similar functionality was previously achievable only when using central TACACS+ or RADIUS server).

The configuration commands to enable terminal access logging are login on-success log and login on-failure log. You can also specify that you want send SNMP traps in these circumstances (with the trap option) or that you only want to log every Nth attempt with the every n option.
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Router Configuration Management … Too Good to be True?

In the Router Configuration Management … Too Good to be True?, the latest IP Corner article, I'm describing two of the router configuration management features introduced in Cisco IOS release 12.4: Configuration Change Notification and Contextual Configuration Diff utility. While the first one behaves as expected, the second one produced unexpected results under the stress test.
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