SHARKFEST '09 | Stanford University | June 15–18, 2009

WinPcap Do’s and Don’tsWednesday, June 17th, 2009

Gianluca VarenniSenior Software Engineer | CACE Technologies, Inc.WinPcap Product Managergianluca.varenni@cacetech.com

SHARKFEST '09Stanford UniversityJune 15-18, 2009

SHARKFEST '09 | Stanford University | June 15–18, 2009

Agenda• Do’s and Don’ts

• Tips and tricks

• Open discussion/questions

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SHARKFEST '09 | Stanford University | June 15–18, 2009

Do’s and Don’ts

SHARKFEST '09 | Stanford University | June 15–18, 2009

Packet reception• Do NOT keep the packet pointers received from– pcap_next_ex– pcap_loop– pcap_dispatch– pcap_next

in your own data structures.• They are valid only up to the next call to pcap_next_ex.

• Copy the packets if needed.

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SHARKFEST '09 | Stanford University | June 15–18, 2009

Packet dissection• Packets can be truncated. – Be savvy when dissecting packets, check boundaries.– If you receive a 30 bytes IP packet, the IP header is

truncated!

• Do NOT assume that the headers have a fixed length!– The IP header is 20 bytes when there are no options– Compute the header length properly

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IP headerIP headerEthernet headerEthernet header IP options

14 bytes 20 bytes 0 or more bytes

L4 protocolL4 protocol

SHARKFEST '09 | Stanford University | June 15–18, 2009

Data link types• Do NOT assume that the link type is Ethernet

(DLT_EN10MB).

• Check the link type with pcap_datalink

• In case of wireless (AirPcap), three possible encapsulations– Bare 802.11 (no meta-information)– Per-Packet Information (PPI)– Radiotap

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SHARKFEST '09 | Stanford University | June 15–18, 2009

Packet API• Do NOT use it.– No longer documented (it was a mistake)– It can change between releases

• Do NOT access the npf.sys driver directly– IOCTLs change over time

• Use the pcap API

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SHARKFEST '09 | Stanford University | June 15–18, 2009

WinPcap installation• Do NOT create your custom WinPcap installer.– It works most of the times on Windows XP x86.

• What about Vista x64 or NT4?

– It corrupts any existing installation.– Debugging installation issues is a major pain.

• Solutions– Official WinPcap installer.– WinPcap Professional.

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SHARKFEST '09 | Stanford University | June 15–18, 2009

WinPcap and services• You can use WinPcap in a service.• You MUST call any WinPcap function after you have

notified the SCM that the service is started.

• Alternatively, put “nm” and “npf” as service dependencies using ChangeServiceConfig when installing the service

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VOID ServiceStart (DWORD dwArgc, LPTSTR *lpszArgv){ // // perform any initialization here // DO NOT CALL WINPCAP HERE // SetServiceStatus( ....SERVICE_RUNNING...);

// // Service is now running, perform work until shutdown // Start using WinPcap here //}

VOID ServiceStart (DWORD dwArgc, LPTSTR *lpszArgv){ // // perform any initialization here // DO NOT CALL WINPCAP HERE // SetServiceStatus( ....SERVICE_RUNNING...);

// // Service is now running, perform work until shutdown // Start using WinPcap here //}

SHARKFEST '09 | Stanford University | June 15–18, 2009

Kernel buffers• Do NOT use large kernel buffers.– It’s a cache for traffic spikes or app processing slowdowns.– Kernel memory is a precious resource.

• 4-8 MB is ok in most cases (even at 1Gbps).

• Optimize your processing code!

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SHARKFEST '09 | Stanford University | June 15–18, 2009

Tips and tricks

SHARKFEST '09 | Stanford University | June 15–18, 2009

Multiple devices support• You can open the same device multiple times– Within the same process.– From the same or different threads.– Each instance uses its own capture buffer and filter.– Packets are replicated among instances.

• Be careful with pcap_compile. It’s not thread safe (as of WinPcap 4.1beta5).– Future versions will fix the issue.– Use a critical section to protect the calls to pcap_compile.

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SHARKFEST '09 | Stanford University | June 15–18, 2009

Dumping to disk• Disks are generally slow.• Dumping all the packets to disk without losses

is not trivial on high speed links.

• Solutions– Dump just the first n bytes of a packet (snaplen).– Filter packets.– Dedicated disks (not partitions!).– RAID 0 (striping).

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SHARKFEST '09 | Stanford University | June 15–18, 2009

Use pcap_next_ex• Why? It’s much easier to use. Especially to stop

capture.

• Do not use pcap_loop/pcap_dispatch/pcap_next– They are less immediate to use.

• pcap_next_ex is blocking– It respects the timeout set in pcap_open_live

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SHARKFEST '09 | Stanford University | June 15–18, 2009

Timestamps• They are generated in software after

1. The packet has been received by the NIC2. The NIC has notified the OS about available packets

(interrupt coalescing)3. The NIC driver has processed the packet and notified

NDIS about the packet

• The precision is in the order of tens of microseconds in the best case.

• Do not rely on timestamps for critical measurements.

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SHARKFEST '09 | Stanford University | June 15–18, 2009

Responsiveness vs. performance

• Packets are received by the app when the timeout elapses or at least mintocopy bytes are in the kernel driver buffer (whatever comes first)

• Small read timeouts can affect performance

• Small mintocopy values can affect performance

• Do you really need to get the packets as soon as they arrive?

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SHARKFEST '09 | Stanford University | June 15–18, 2009

Devpack samples• Use them as a reference– Header files to include (or not)

– LIB files

– How to open/close an adapter or capture packets from it

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SHARKFEST '09 | Stanford University | June 15–18, 2009

GUI applications• The UI needs to be responsive while capturing.• Use a separate thread to capture (or inject)

packets.• Use messages for inter-thread communication.

– SendMessage– PostMessage

• Do NOT touch the UI in the capture thread!

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SHARKFEST '09 | Stanford University | June 15–18, 2009

Wireless capture • Most adapters (excluding AirPcap) do not support

promiscuous/monitor mode– It’s a limit of the hardware/NIC driver– It’s not a limit of WinPcap– Bug in WinPcap: it doesn’t detect lack of promiscuous support. Fixed in

4.1 betas.

• Ethernet “fake” frames. No management/control frames, no 802.11 headers.

• Vista native Wi-Fi drivers? Not really.

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SHARKFEST '09 | Stanford University | June 15–18, 2009

Privileges to run WinPcap• Pretty weak security model

• Admin privileges are needed to– Install WinPcap– Start the driver at each reboot

• Change the driver start type to SERVICE_AUTO_START to have the driver started at boot time

• Once the driver is running, a standard user can capture/inject packets

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SHARKFEST '09 | Stanford University | June 15–18, 2009

WinPcap and .NET• You need to create your own wrapper, or use an

existing one– No official wrappers– No support for 3rd party ones

• Marshalling packet contents (without copies) is not trivial

• Some APIs (e.g. pcap_findalldevs) are not .NET friendly

• Use managed C++ to create your wrapper21

SHARKFEST '09 | Stanford University | June 15–18, 2009

Questions?