Wait for Response Packets

• Overview
• Packet Sniffer Configuration
• Sniffing Response Packets

Overview

Packet Sniffers are a type of Passive Service. Rather than opening up a TCP port and actively listening for requests, the Packet Sniffer passively reads data packets off the network interface. The Sniffer assembles these Packets into complete messages that can then be passed into an associated policy.

Because the Packet Sniffer operates passively (i.e. does not listen on a TCP port) and, therefore, completely transparently to the client, it is most useful for monitoring and managing Web Services. For example, the Sniffer can be deployed on a machine running a Web Server acting as a container for Web Services. Assuming that the Web Server is listening on TCP port 80 for traffic, the Packet Sniffer can be configured to read all packets destined for port 80 (or any other port, if necessary). The packets can then be marshalled into complete HTTP/SOAP messages by the Sniffer and passed into a policy that logs the message to a database, for example.

Packet Sniffer Configuration

Because Packet Sniffers are mainly for use as passive monitoring agents, they are usually created within their own Service Group. For example, a new group can be created for this purpose by right-clicking the Process in the tree view of the Policy Studio, selecting the Add Service Group menu option, and the entering "Packet Sniffer Group" on the Add Service Group dialog.

We can then add a Relative Path Service to this Group by right-clicking on the "Packet Sniffer Group" and selecting the Add Relative Path menu option. Enter a path in the field provided and select the policy that you want to dispatch messages to when the Packet Sniffer detects a request for this path (after it assembles the packets). So, for example, if the Relative Path is configured as "/a", and the Packet Sniffer assembles packets into a request for this path, the request will be dispatched to the policy selected in the Relative Path Service.

Finally, we can add the Packet Sniffer itself by right-clicking on the "Packet Sniffer Group" node, selecting Packet Sniffer, and then the Add menu option. Complete the following fields on the Packet Sniffer dialog:

Device to Monitor:
Enter the name or identifier of the network interface that the Packet Sniffer will monitor. The default entry here is "any", but it is important to note that this is only valid on Linux. On UNIX-based systems, network interfaces are usually identified using names like "eth0", "eth1", and so on. On Windows, these names are more complicated, for example, "\Device\NPF_{00B756E0-518A-4144 ... }.

Filter:
The Packet Sniffer can be configured to only intercept certain types of packets. For example, it can ignore all UDP packets, only intercept packets destined for port 80 on the network interface, ignore packets from a certain IP address, listen for all packets on the network, and so on.

The Packet Sniffer uses the libpcap library filter language to achieve this. This language has a complicated but powerful syntax that allows you to filter what packets are intercepted and what packets are ignored. As a general rule, the syntax consists of one or more expressions combined with conjunctions, such as "and", "or", and "not". The following table lists a few examples of common filters and explains what they actually filter:

Filter Expression	What does it filter?
port 80	Capture only traffic for the HTTP Port (i.e. 80).
host 192.168.0.1	Capture traffic to and from IP address 192.168.0.1.
tcp	Capture only TCP traffic.
host 192.168.0.1 and port 80	Capture traffic to and from port 80 on IP address 192.168.0.1.
tcp portrange 8080-8090	Capture all TCP traffic destined for ports from 8080 through to 8090.
tcp port 8080 and not src host 192.168.0.1	Capture all TCP traffic destined for port 8080 but not from IP address 192.168.0.1.

The default filter of "tcp" simply captures all TCP packets arriving on the network interface. For more information on how to configure filter expressions like these, please refer to the man pages of tcpdump man page, which can be found here.

Promiscuous Mode:
When listening in "promiscuous mode", the Packet Sniffer will capture all packets on the same Ethernet network, regardless of whether or not the packets are addressed to the network interface that the Sniffer is monitoring.

Sniffing Response Packets

The Enterprise Gateway can capture both incoming and outgoing packets when it is listening passively (i.e. not opening any ports) on the network interface. Say, for example, a Web Service is deployed in a web server that listens on port 80. The Enterprise Gateway can be installed on the same machine as the web server. It is configured not to open any ports and to use a Packet Sniffer to capture all packets destined for TCP port 80.

When packets arrive on the network interface that are destined for this port, they are assembled by the Packet Sniffer into HTTP messages and passed into the configured policy. Typically this policy logs the message to an audit trail, and so usually consists of just a Log Message filter.

Assuming that we also want to log response messages passively, as is typically required for a complete audit trail, we can use the Wait for Response Packets filter to correlate response packets with their corresponding requests. The Wait for Response Packets filter assembles the response messages into HTTP messages and can then log them again using the Log Message Payload filter. The following circuit will log both request and response messages captured transparently by the Packet Sniffer:

You can see from the circuit that the first logging filter logs the request message. By this stage, the Packet Sniffer has assembled the request packets into a complete HTTP request, and this is what is passed to the "Log Request Message" filter.

The "Assemble response packets" filter is a Wait for Response Packets filter that assembles response packets into complete HTTP response messages and passes them to the "Log Response Message" filter, which logs the complete response message. More information on the Log Message Payload filter is available in the Log Message Payload help page.