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Category: Tcl

Shorter display of OSPF database

Recently I had to explore the behavior of Cisco IOS OSPF implementation and had to inspect OSPF database on routers in various areas. If you're only interested in the contents of the database (not in low-level troubleshooting), variety of LSA fields (including LS Age, Options, Checksum, Length ...) are just cluttering the printout, so I fine-tuned the show filter to exclude all the non-relevant fields, ending with show ip ospf database parameters | exclude LS|Options|Check|Len|(MTID:[ 0-9]+$) (the MTID field appears in IOS release 12.2SRC).
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Simple CLI extensions: handling special characters

Last week I've described how you can extend the exec-mode CLI commands with almost no knowledge of Tcl. A bit more work is required if your commands include Tcl special characters (quotes, braces or backslashes).

For example, to display all routes advertised by customers of AS X, you'd use the following show command: show ip bgp regexp _X_([0-9]+)(_\1)*$ (the regular expression is explained in the AS-path based filter of customer BGP routes post). This command cannot be entered as a Tcl string with variable substitution; Tcl would interpret the [ and \ characters. You could enter the whole command in curly braces, but then there would be no variable substitution that we need to insert command line parameters. To make Tcl happy, use the following Tcl commands:
  1. set cmd {first-part-of-command} stores the command prefix into the cmd variable;
  2. append cmd $argv appends the command line arguments to the command;
  3. append cmd {rest-of-command} appends the rest of the IOS exec command;
  4. puts [exec $cmd] executes the command and prints the results.

For example, the following code will display the customers of a BGP AS specified in the command line (after being stored in a flash file and defined in an alias, of course):

set cmd {show ip bgp regexp _}
append cmd $argv
append cmd {_([0-9]+)(_\1)*$}
puts [exec $cmd]

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Simple extensions to exec-mode CLI

The various show filters available in Cisco IOS are a great tool to minimize the amount of printout you have to analyze, their only problem (from my perspective) is that you cannot make an alias out of them, as you usually have to supply one or more parameters to the show command and these parameters have to be inserted before the filter (and the alias command does not support replaceable parameters). You could solve the problem with Tcl shell, but I'm not sure many networking engineers are fluent Tcl programmers. Fortunately, the code you need is so simple anyone can create a working solution.
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Change the source IP address of an EEM SMTP session

I've got the following question from Levi:
I have a Tcl script that is used in conjunction with EEM to send email whenever the amount of CRC errors on a particular interface increases above a certain threshold. My problem is that the router uses the IP of the outgoing interface as the source IP when it communicates with the SMTP server. This particular interface happens to have a private IP. There's another interface with a public IP and I wanted to know how to get the router to use the public IP on the other interface when it's sending email generated by the TCL script.
There are several ways to solve this problem. If you use Tcl, you could write your own SMTP client and use the -myaddr parameter in the socket call to specify the source IP address. Those of us who prefer EEM applets are not so lucky, you have to use NAT to change the source IP address before the packet is sent toward the SMTP server.

This article is part of You've asked for it series.

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Create structured e-mails from EEM applets

A few weeks ago I've described how to use the append show filter and more command to send e-mails containing multiple printouts from an EEM applet. A few hours after I've published the post, David Houser sent me a great EEM applet that used texts stored in flash: files to generate headings between various show commands. While his solution works perfectly (and gives you all the flexibility you want), it's a bit verbose and requires lots of small files that clutter your flash: memory. I've thus decided to write a small Tcl script that executes the Cisco IOS command specified in the command line and appends the command results together with a heading in an output file.
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Tcl-based IOS backdoor

Andy Davis from the Information Risk Management has written an interesting "application note": how to create a backdoor to Cisco IOS using Tclsh (I've tested it and it works quite nicely). His backdoor implementation relies on a bug in Cisco IOS that allows a "hung" (or never-ending) Tclsh script to continue executing even after the user session has been disconnected (the only means of stopping such a script is with the clear line command). That bug has been fixed in recent IOS versions (I've tested that as well, the Tclsh script is killed as soon as the Telnet session is disconnected in IOS 12.4(15)T), but you can still use the same technique (although it might be a bit less convenient) if you convert the Tclsh script into an EEM policy and trigger it periodically with a timer event.
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Predefine your own Tcl functions

If you want to have your own Tcl functions available when you start tclsh, you could use the scripting tcl init file configuration command that I've briefly mentioned in one of the previous posts. This command specifies a source file that is executed every time you start Tcl shell. The source file can contain function definitions, package declarations or any other Tcl code.

If you need to, you can specify multiple initialization files.

For example, if you'd like to implement a comfortable Tcl-based pinger (similar to the one Ethan Banks found in the Sadikhov forums, store the following Tcl code into the file flash:pinger.tcl

proc pinger { iplist } {
  foreach ip $iplist {
    if { [regexp "(!!!)" [exec "ping $ip timeout 1" ]] } {
      puts "$ip"
    } else { puts "$ip **** failed ***" }
… and configure scripting tcl init flash:pinger.tcl. Now you can ping a number of hosts in a single operation:
R1(tcl)#pinger { } **** failed *** **** failed ***

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Display the names of the configured route-maps

I'm probably getting old … I keep forgetting the exact names (and capitalization) of route-maps I've configured on the router. The show route-maps command is way too verbose when I'm simply looking for the exact name of the route-map I want to use, so I wrote a Tcl script that displays the names of the route-maps configured on the router. If you add a -d switch, it also displays their descriptions (to be more precise, the first description configured in the route-map).

When using the -d switch, the script executes the show running command and might take a while to complete.

To use the script, download the routeMaps.tcl file (available from my web site) into the router's flash and follow the installation instructions in the source.

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Phase 2: Upload text files through a Telnet session

In a previous post, I've described how you can use Tcl shell to upload text content into router's flash if the router has no connectivity to a suitable file server (or you don't have FTP or TFTP server handy).

The trick works flawlessly, but typing the same obscure Tcl commands gets tedious after a while, so the first time I had to use this solution to develop a Tcl script, I've quickly written another script that takes file name as the parameter and hides all the other murky details.

To use it, transfer the contents of storeFile.tcl (available from my web site) to the router's flash (using the previously described trick), follow the installation instructions in the source and you're ready to go.

Note: You can adapt the Tcl script to your needs; for example, you could add instructions to re-register EEM Tcl policy every time you upload the new code.

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Implement “wc -l” in Cisco IOS

Sometimes it would be nice to have the full complement of Unix utilities available on Cisco IOS. That's not going to happen for a while, but we can use Tcl to make our life simpler in the meantime. Xavier Brouckaert, a regular contributor to my blog, has sent me the Tcl implementation of line counting utility (equivalent to wc -l on Unix).

First you have to define the wc Tcl procedure:
proc wc { cmd } { llength [split [exec $cmd] "\n" ] }
You can define the procedure interactively in Tclsh (but then you have to do it every time you start Tclsh) or you could store the code in a flash file and execute the file every time the Tclsh is started with the scripting tcl init filename global configuration command.

Once the wc procedure is defined, execute wc { IOS command } in Tclsh and you'll get the line count. For example, to get the number of directly connected routes use
wc { show ip route ¦ include ^C }

The include ^C filter includes all lines that start with letter C; in our case all directly connected routes

Obviously you could turn this idea into a full-blown Tclsh script that would accept CLI arguments … but I'll leave this as an exercise for the readers (you can probably tell I've been reading some academic literature lately :). However, if you find the time to write a more complete wc implementation on IOS, please do post the URL here.

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Generate terminal escape sequences from Tcl

One of my readers (who unfortunately prefered to stay anonymous, so I cannot give credit where it's due) figured out how to output escape sequences from IOS Tclsh: you have execute terminal international command first.

For example, to clear the screen and display red heading text, use the following commands:
exec terminal international;
puts "\033\[2J\033\[H\033\[1;31mHeader text\033\[m"
Obviously, you could easily use this functionality to build a nice full-screen menu system.
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