WinPcap Do’s and Don’ts

Wednesday, June 17 th , 2009

Gianluca Varenni

Senior Software Engineer | CACE Technologies, Inc.

WinPcap Product Manager [email protected]

SHARK

FEST

'09

Stanford University June 15-18, 2009

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

• Do’s and Don’ts

Agenda

• Tips and tricks • Open discussion/questions

SHARKFEST '09 | Stanford University | June 15 –18, 2009 2

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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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!

Ethernet header IP header IP options L4 protocol 14 bytes 20 bytes 0 or more bytes

• 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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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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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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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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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.

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 // } Alternatively, put “nm” and “npf” as service dependencies using ChangeServiceConfig installing the service

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when

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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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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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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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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• •

Timestamps

They are generated in software after 1. The packet has been received by the NIC 2. 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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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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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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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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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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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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WinPcap and .NET

• You need to create your own wrapper, or use an existing one – No official wrappers – No support for 3 rd 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 wrapper

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Questions?

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