" ITREALMS: Observing packet spoofing by ISPs (9)


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Thursday, March 20, 2008

Observing packet spoofing by ISPs (9)

Market Intelligence:

However, the use of checksum offloading makes packet captures inaccurate because it prevents the local operating system from seeing what was actually transmitted. This may cause a discrepancy since one end mistakenly thinks it sent something slightly different from what the other end correctly received; the resulting mismatch of TCP or UDP checksum values could be misinterpreted as tampering by an ISP, since ISPs are not supposed to alter these checksums. Checksum offloading should, if possible, be disabled at both ends before beginning the experiment. If you know that your Ethernet card or network driver does not perform checksum offloading, you do not need to disable it. It may also be possible to get valid results when checksum offloading is enabled; workarounds for this purpose are described in a later section. Here are typical means of disabling checksum offloading on several popular operating systems:

On Linux (as root):

ethtool -K eth0 rx off tx off (choose correct network interface if not eth0)

On FreeBSD (as root):

ifconfig em0 -rcxsum -tcxsum (choose correct network interface if not em0)

On MacOS (as root):

sysctl -w net.link.ether.inet.apple_hwcksum_tx=0

sysctl -w net.link.ether.inet.apple_hwcksum_rx=0 (Note that this may cause some local applications to work incorrectly!)

On Windows

Right-click My Computer, then select Device Manager /Network Adapters/(select device) / Properties /Advanced; then disable checksum offloading, if the option is available.

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