The ONL Tutorial

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Overview

Plugins allow a user to perform custom packet processing. Below is a list of typical operations that a user may want to perform. There is a standard plugin that demonstrates some aspect of each operation. Each Web page in this section describes how one of these operations can be done with a plugin. And, in many cases, a standard plugin code can be used as a basis for your own custom processing.

The plugin code described in this section is located in the directory ~onl/stdPlugins. However, the code shown here has been simplified by omitting debug code and any empty code blocks resulting from the removal.

• Drop Packets: (stats:)
  • Remove the packet from the list of incoming packets
  • Free the buffer memory
• Modify UDP or TCP Packets: (stringSub:)
  • Recompute the UDP or TCP checksum
• Copy Packets (Avoid Reclassification): (multicast:)
  • Allocate memory for the packet buffer
  • Fill in the buffer
  • Modify the intraport shim
  • Forward the packet
• Delay Packets: (pdelay:)
  • Schedule a periodic callback to search for expired delays
  • Queue the packet
  • Dequeue the packet when its delay has expired
  • Forward the packet
• Create Packets and Reclassify: (udpdump:)
  • Allocate memory for the packet buffer
  • Build the packet in the buffer
  • Forward the packet

In most cases, the operations are confined to the handle_packet routine where the plugin is given the packet. In the case of the pdelay plugin, a callback function is called periodically so that the plugin can decide whether a packet's delay has expired and should be forwarded.

During the tour of these plugin snippets, you will encounter functions that access fields in a packet buffer or fields in an IP packet, UDP packet or TCP packet. These functions are summarized in Summary_Information => SPC_Macros_and_Functions.

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