Parse and Format Junos XML Protocol Server Responses in Perl Client Applications

The Junos XML protocol Perl distribution includes several sample Perl scripts that perform various functions on devices running Junos OS. The following code sample from the diagnose_bgp.pl and get_chassis_inventory.pl sample scripts uses XSLT to transform an operational response from the Junos XML protocol server into a more readable format. A detailed discussion of the functional subsections follows the complete code sample.

The first line of the preceding code sample illustrates how the scripts read the -o option from the command line to obtain the name of the file into which to write the results of the XSLT transformation:

The scripts use an XSLT file to transform the output. If the -x command-line option is included when the script is invoked, the script uses the XSLT file specified in the option. Otherwise, it uses the default XSLT file specified in the code. The script exits if the file does not exist. The following example is from the diagnose_bgp.pl script:

For examples of XSLT files, see the following directories in the Junos XML protocol Perl distribution:

• The examples/diagnose_bpg/xsl directory contains XSLT files for the diagnose_bpg.pl script: dhtml.xsl generates dynamic HTML, html.xsl generates HTML, and text.xsl generates ASCII text.

• The examples/get_chassis_inventory/xsl directory contains XSLT files for the get_chassis_inventory.pl script: chassis_inventory_csv.xsl generates a list of comma-separated values, chassis_inventory_html.xsl generates HTML, and chassis_inventory_xml.xsl generates XML.

The actual parsing operation begins by setting the variable $xmlfile to a filename of the form device-name.xml and invoking the printToFile function to write the Junos XML protocol server’s response into the file (the printToFile function is defined in the XML::DOM::Parser module):

The next line invokes the translateXSLtoRelease function (defined in the Junos::Response module) to alter one of the namespace definitions in the XSLT file. This is necessary because the XSLT 1.0 specification requires that every XSLT file define a specific value for each default namespace used in the data being transformed. The xmlns attribute in a Junos XML operational response tag element includes a code representing the Junos OS version, such as 20.4R1 for the initial version of Junos OS Release 20.4. Because the same XSLT file can be applied to operational response tag elements from devices running different versions of the Junos OS, the XSLT file cannot predefine an xmlns namespace value that matches all versions. The translateXSLtoRelease function alters the namespace definition in the XSLT file identified by the $xslfile variable to match the value in the Junos XML protocol server’s response. It assigns the resulting XSLT file to the $nm variable.

After verifying that the translateXSLtoRelease function succeeded, the invokes the format_by_xslt, which builds a command string and assigns it to the $command variable. The first part of the command string invokes the xsltproc command and specifies the names of the XSLT and configuration data files (stored in $nm and $deviceinfo{hostname}.xml):

If the $outputfile variable is defined (the file for storing the result of the XSLT transformation exists), the script appends a string to the $command variable to write the results of the xsltproc command to the file. (If the file does not exist, the script writes the results to standard out [stdout].) The script then invokes the system function to execute the command string and prints status messages to stdout.

If the translateXSLtoRelease function fails (the if ($nm) expression evaluates to "false"), the script prints an error:

The get_config.pl script in the examples\RDB directory uses the outconfig subroutine to write the configuration data obtained from the Junos XML protocol server to a file either as Junos XML tag elements or as formatted ASCII text.

The outconfig subroutine takes four parameters. Three must have defined values: the directory in which to store the output file, device hostname, and the XML DOM tree (the configuration data) returned by the Junos XML protocol server. The fourth parameter indicates whether to output the configuration as formatted ASCII text, and has a null value if the requested output is Junos XML tag elements. In the following code sample, the script obtains values for the four parameters and passes them to the outconfig subroutine. A detailed discussion of each line follows the complete code sample.

In the first lines of the preceding sample code, the get_config.pl script uses the following statements to obtain values for the four parameters to the outconfig subroutine:

• If the user provides the -t option on the command line, the getopts subroutine records it in the %opt hash. The value keyed to $opt{t} is passed as the fourth parameter to the outconfig subroutine. (For more information about reading options from the command line, see Collecting Parameters Interactively in Junos XML Protocol Perl Client Applications.)

• The following line reads the first element of the command line that is not an option preceded by a hyphen. It assigns the value to the $basepath variable, defining the name of the directory in which to store the file containing the output from the outconfig subroutine. The variable value is passed as the first parameter to the outconfig subroutine.

• The following line reads the next element on the command line. It assigns the value to the $hostname variable, defining the routing, switching, or security device hostname. The variable value is passed as the second parameter to the outconfig subroutine.

• The following line invokes the getconfig subroutine to obtain configuration data from the Junos XML protocol server on the specified device, assigning the resulting XML DOM tree to the $config variable. The variable value is passed as the third parameter to the outconfig subroutine.

The following code sample invokes and defines the outconfig subroutine. A detailed discussion of each functional subsection in the subroutine follows the complete code sample.

The first lines of the outconfig subroutine read in the four parameters passed in when the subroutine is invoked, assigning each to a local variable:

The subroutine constructs the name of the file to which to write the subroutine’s output and assigns the name to the $filename variable. The filename is constructed from the first two parameters (the directory name and hostname) and the $trailer variable, resulting in a name of the form directory-name/hostname.xmlconfig:

The subroutine checks that the first tag in the XML DOM tree correctly indicates the type of configuration data in the file. If the user included the -t option on the command line, the first tag should be <configuration-text> because the file contains formatted ASCII configuration statements; otherwise, the first tag should be <configuration> because the file contains Junos XML tag elements. The subroutine sets the $top_tag variable to the appropriate value depending on the value of the $text_mode variable (which takes its value from opt{t}, passed as the fourth parameter to the subroutine). The subroutine invokes the getTagName function (defined in the XML::DOM::Element module) to retrieve the name of the first tag in the input file, and compares the name to the value of the $top_tag variable. If the comparison succeeds, the XML DOM tree is assigned to the $config_node variable. Otherwise, the subroutine prints an error message because the XML DOM tree is not valid configuration data.

The subroutine then uses several nested if statements. The first if statement verifies that the XML DOM tree exists and contains data:

If the XML DOM tree contains data, the subroutine verifies that the output file can be opened for writing:

If the output file can be opened for writing, the script writes the configuration data into it. If the user requested Junos XML tag elements—the user did not include the -t option on the command line, so the $text_mode variable does not have a value—the script writes the string <?xml version=1.0?> as the first line in the output file, and then invokes the toString function (defined in the XML::DOM module) to write each Junos XML tag element in the XML DOM tree on a line in the output file:

If the user requested formatted ASCII text, the script invokes the getFirstChild and toString functions (defined in the XML::DOM module) to write the content of each tag on its own line in the output file. The script substitutes predefined entity references for disallowed characters (which are defined in the %escapes hash), writes the output to the output file, and closes the output file. (For information about defining the %escapes hash to contain the set of disallowed characters, see Converting Disallowed Characters in Junos XML Protocol Perl Client Applications.)