I've recently got an intriguiging question: how can I change the color of the terminal emulator font from the CLI by sending the telnet program an escape sequence?

For those of you that haven't worked with escape sequences before - you can control a lot of parameters in you terminal emulation program by sending it a special sequence starting with [ ( begin character code 27). These sequences work even in the simplest telnet clients on Windows and Linux thanks to built-in operating system support or ANSI.SYS driver (on Windows); you can get an in-depth description and the list of all supported escape sequences from Wikipedia.

If you're using DHCP to get IP interface addresses on your router (using the ip address dhcp interface configuration command), the router will also inherit the DNS resolver settings included in the DHCP reply. Makes sense, but the implementation is "a bit" unexpected: if you configure the DNS name servers manually with the ip name-server address-list command, the ones matching the values in the DHCP reply packet are not included in the running configuration and thus not saved to NVRAM. Even worse, the statically-configured name-servers overwritten by a DHCP reply are lost if the DHCP-configured interface goes down.

To avoid total confusion, you thus have these options:

• Do not use DHCP to acquire IP interface addresses
• Make sure the DHCP server does not send DNS-related parameters (a bit hard if you're using DHCP with your ISP)
• Rely exclusively on DHCP to provide your router with the DNS name server addresses

If you want to execute IOS show commands periodically (for example, to monitor router status or take snapshots of routing tables), you can combine new output redirection features introduced in IOS release 12.2T in an Embedded Event Manager (EEM) applet. For example, to store the brief interface status into a file on an FTP server, use the following EEM applet:

event manager applet SaveInterfaceStatus
 event timer watchdog name SaveIfStat time 60
 action 1.0 cli command "show ip interface brief | redirect ftp://username@password:host/path"
 action 2.0 syslog msg "Interface status saved"

Notes:

In a comment to my post describing how to make a router into a primary DNS server, one of the readers noted that you could easily overload a router doing that ... and he's obviously right.

Apart from having too many valid DNS requests for the zone the router is responsible for, the observed behavior could be spam-related. Just a few days ago when I've discussed the router-based DNS server with my security engineers, they've pointed out that a lot of spammers perform regular DNS attacks trying to poison the DNS cache of unpatched open caching DNS servers.

Cisco has released security response acknowledging that the IPS software does not properly handle a rarely used Unicode encoding. Reading the security notice you might be left wondering what's going on. Here's the whole story.

Within an URI (web address), the ASCII characters can be encoded in one of three ways:

• Unless they are reserved, they can be included in the URI directly (for example, you can always use the letter a in an URI).
• You can always escape a character using its hexadecimal value. Letter a can thus be written as %61.
• Unicode character set includes full-width form of ASCII characters, where letter a can be encoded as a two-byte value 0xFF61 (or %ff%61 in an URI)

The IPS software (standalone or integrated in Cisco IOS) does not recognize the sequence %ff%61 as letter a. It's thus possible to evade some IPS triggers by replacing ASCII characters with their full-width Unicode encoding.

The Split DNS functionality introduced in IOS release 12.4(9)T has survived a single maintenance cycle before being broken. While you can still configure the DNS views in 12.4(11)T2 (and they still work), the view names are missing from the router-generated configuration (show running, for example), making the configuration syntactically incorrect. The router will thus reboot without DNS views after you've saved the running configuration to NVRAM.

One of my readers asked an interesting question: „why do the commands executed within a EEM Tcl policy fail with Command authorization fails message?“ The short answer is simple: If you use AAA command authorization (which you can only do if you're using a TACACS+ server), you have to specify the username under which the EEM will execute its CLI commands with the event manager session cli username user configuration command.

In a previous post, I've described how you could generate a (almost) continuous ping from a router using the extended ping command. While that approach is extremely simple to implement, it ties up a line (and if you're working from the console, it's highly impractical). You could get the same results (as Tom has already pointed out) using IP SLA feature of Cisco IOS. Configure the ping request with commands similar to these:

Whenever a router running CEF switching has LAN interfaces (or any other multi-access interfaces), you'll find cached adjacencies for active directly attached IP neighbors in its CEF table. These adjacencies ensure the smooth traffic flow toward the LAN-attached next-hops (preventing the initial packet drop symptom once the next-hop becomes active).

One of my readers made an interesting observation when faced with configuring zone-based firewall on Cisco IOS: „My main issue is a confusion between when to use self and when to use in/outside.“

The rules are simple: