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nfnl_cthelper: User-space infrastructure for connection tracking helpers

Author: Pablo Neira Ayuso <[email protected]>


Connection tracking helpers allows you to filter multi-flow protocols
that usually separate control and data traffic into different flows.
This is the case of application protocols like FTP, SIP and H.323 that
are already supported by Netfilter. These helpers are implemented in

There are good reasons to implement helpers in user-space instead:

  • Rapid connection tracking helper development, as developing code in user-space is usually faster.
  • Reliability: A buggy helper does not crash the kernel. Moreover, we can monitor the helper process and restart it in case of problems.
  • Security: Avoid complex string matching and mangling in kernel-space running in unprivileged mode. Going further, we can even think about running user-space helpers as a non-root process.
  • It allows the development of very specific helpers (most likely non-standard proprietary protocols) that are very likely to be rejected for mainline inclusion in the form of kernel-space connection tracking helpers.

Basic operation

In a few steps:

  1. Register user-space helper

./nfct-helper-add test 0

This adds a helper `test' that uses the queue number 0.

  1. Add rules to enable the `test' user-space helper

For locally generated packets:

iptables -I OUTPUT -t raw -p tcp -j CT --helper test

For non-locally generated packets:

iptables -I PREROUTING -t raw -p tcp -j CT --helper test

  1. Run the test libnetfilter_queue program


  1. Generate traffic, if everything is OK, then `nfqnl_test' program displays lines like this:

    pkt received hw_protocol=0x0800 hook=4 id=4 outdev=3 payload_len=60 entering callback [...]

This means that the cthelper infrastructure is passing traffic to

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