Scripts (Network Troubleshooting Tools)

6.4. Scripts

Since most networks have evolved over time, they are frequently odd collections of equipment for which no single tool may be ideal. And even when the same tool can be used, differences in equipment may necessitate minor differences in how the tool is used. Since many of the tasks may need to be done on a regular basis, it should come as no surprise that scripting languages are a popular way to automate these tasks. Getting started can be labor intensive, but if your current approach is already labor intensive, it can be justified.

You will want to use a scripting language with extensions that support the collection of network data. To give an idea of this approach, Tcl and its extensions are briefly described here. Even if you don't really want to write your own tools, you may want to consider one of the tools based on Tcl that are freely available, most notably tkined.

Tcl was selected because it is provides a natural introduction to tkined. Of course, there are other scripting languages that you may want to consider. Perl is an obvious choice. Several packages and extensions are available for system and network administration. For example, you may want to look at spidermap. This is a set of Perl scripts that do network scans. For SNMP-based management, you'll probably want to get Simon Leinen's SNMP extensions SNMP_Session.pm and BER.pm. (Other tools you might also look at include mon and nocol.)

6.4.1. Tcl/Tk and scotty

Tool Command Language, or Tcl (pronounced "tickle"), is a scripting language that is well suited for network administration. Tcl was developed in the late 1980s by John Ousterhout, then a faculty member at UC Berkeley. Tcl was designed to be a generic, embeddable, and extensible interpreted language. Users frequently cite studies showing Tcl requires one-tenth the development time required by C/C++. Its major weakness is that it is not well suited for computationally intensive tasks, but that shouldn't pose much of a problem for network management. You can also write applets or tclets (pronounced "tik-lets") in Tcl.

Tcl can be invoked interactively using the shell tclsh (pronounced "ticklish") or with scripts. You may need to include a version number as part of the name. Here is an example:

bsd2# tclsh8.2
%

This really is a shell. You can change directories, print the working directory, copy files, remove files, and so forth, using the usual Unix commands. You can use the exit command to leave the program.

One thing that makes Tcl interesting is the number and variety of extensions that are available. Tk is a set of extensions that provides the ability to create GUIs in an X Window environment. These extensions make it easy to develop graphical interfaces for tools. Tk can be invoked interactively using the windowing shell wish. Both Tcl and Tk are implemented as C library packages that can be included in programs if you prefer.

scotty, primarily the work of Jürgen Schönwälder, adds network management extensions to Tcl/Tk. The tnm portion of scotty adds network administration support. The tkined portion of scotty, described in the next section, is a graphical network administration program. What tnm adds is a number of network management commands. These include support for a number of protocols including ICMP, UDP, DNS, HTTP, Sun's RPC, NTP, and, most significantly, SNMP. In addition, there are several sets of commands that simplify writing network applications. The netdb command gives access to local network databases such as /etc/hosts, the syslog command supports sending messages to the system logging facilities, and the job command simplifies scheduling tasks. A few examples should give an idea of how these commands could be used.

You can invoke the scotty interpreter directly as shown here. In this example, the netdb command is used to list the /etc/host table on a computer:

bsd4# scotty
% netdb hosts
{localhost.lander.edu 1.0.0.127} {bsd4.lander.edu 239.63.153.205} {bsd4.lander.e
du. 239.63.153.205} {bsd1.lander.edu 231.60.153.205} {sol1.lander.edu 233.60.153
.205} {lnx1.lander.edu 234.60.153.205}
% exit

The results are returned with each entry reduced to the canonical name and IP address in brackets. Here is the host table for the same system:

bsd4# cat /etc/hosts
127.0.0.1               localhost.lander.edu localhost
205.153.63.239          bsd4.lander.edu bsd4
205.153.63.239          bsd4.lander.edu.
205.153.60.231          bsd1.lander.edu bsd1
205.153.60.233          sol1.lander.edu sol1
205.153.60.234          lnx1.lander.edu lnx1

Note that there is not a separate entry for the alias bsd4.

Here are a few examples of other commands. In the first example, the name of the protocol with a value of 1 is looked up in /etc/protocols using the netdb command:

% netdb protocols name 1
icmp

In the second example, a reverse DNS lookup is done for the host at 205.153.63.30:

% dns name 205.153.63.30
sloan.lander.edu

Finally, an ICMP ECHO_REQUEST is sent to www.cisco.com:

% icmp echo www.cisco.com
{www.cisco.com 321}

The response took 321 ms. Other commands, such as snmp, require multiple steps to first establish a session and then access information. (Examples are given in Chapter 7, "Device Monitoring with SNMP".) If you are interested in using these tools in this manner, you will first want to learn Tcl. You can then consult the manpages for these extensions. A number of books and articles describe Tcl, some of them listed in Appendix B, "Resources and References". The source is freely available for all these tools.


6.3. Device Identification		6.5. Mapping or Diagramming