Switching to VoIP

This is the Title of the Book, eMatter Edition

Copyright © 2007 O’Reilly & Associates, Inc. All rights reserved.

242

|

Chapter 10: Security and Monitoring

dmesg’s output is flat text that you can redirect to a file. Suppose you wanted to iso-

late the traffic prefixed with Chicago into a file by itself:

# dmesg | grep Chicago >chicagoVoIP.txt

Or better yet, email that log to somebody, perhaps so they can import it into a

spreadsheet for further analysis. In the following example, pressing Ctrl-C will stop

the dmesg application and an email will be sent containing the Chicago entries:

# dmesg | grep Chicago | mail [e-mail]

SNMP

Simple Network Management Protocol is a lightweight method of collecting traffic

and performance data from network devices such as servers and switches. Different

kinds of data use different parameter schemas, called management information

bases, or MIBs. MIBs define how SNMP refers to metafields specific to a certain kind

of data, such as Ethernet traffic or DNS-lookup statistics.

MIBs exist for SIP (http://www.iana.org), VOCAL (http://www.vovida.org), MEGACO

(http://www.ietf.org), and other VoIP technologies. There are some useful VoIP-related

SNMP monitoring tools (OpenNMS, Multirouter Traffic Grapher, etc.) that can be

customized to make use of these MIBs, too. There isn’t yet a fully integrated SNMP

MIB for Asterisk, though, which leaves Asterisk administrators only one performance

data-collection option: log reading. Fortunately, Asterisk is very flexible in the logging

department.

iptables rules often have a target specified by the -j TARGET option, which tells NetFilter

what to do with the packets matched by the rule. Some useful targets are:

LOG

Log the packet according to your system’s kernel-logging configuration.

REJECT

Don’t accept the matched packet, and send an error packet in response.

DROP

Don’t accept the matched packet, and don’t send any response.

DSCP

A target used with the mangle table to alter DSCP code points and classes in

packets.

TOS

A target used with the mangle table to alter IP Type of Service headers in packets.

There are dozens more options that can help you leverage iptables in Voice over IP.

Refer to the iptables manpage for more details.