Command Section
ENC(4)                 FreeBSD Kernel Interfaces Manual                 ENC(4)

     enc - Encapsulating Interface

     To compile this driver into the kernel, place the following line in your
     kernel configuration file:

           device enc

     The enc interface is a software loopback mechanism that allows hosts or
     firewalls to filter ipsec(4) traffic using any firewall package that
     hooks in via the pfil(9) framework.

     The enc interface allows an administrator to see incoming and outgoing
     packets before and after they will be or have been processed by ipsec(4)
     via tcpdump(1).

     The ``enc0'' interface inherits all IPsec traffic.  Thus all IPsec
     traffic can be filtered based on ``enc0'', and all IPsec traffic could be
     seen by invoking tcpdump(1) on the ``enc0'' interface.

     What can be seen with tcpdump(1) and what will be passed on to the
     firewalls via the pfil(9) framework can be independently controlled using
     the following sysctl(8) variables:

     Name                             Defaults      Suggested
     net.enc.out.ipsec_bpf_mask       0x00000003    0x00000001
     net.enc.out.ipsec_filter_mask    0x00000001    0x00000001        0x00000001    0x00000002     0x00000001    0x00000002

     For the incoming path a value of 0x1 means ``before stripping off the
     outer header'' and 0x2 means ``after stripping off the outer header''.
     For the outgoing path 0x1 means ``with only the inner header'' and 0x2
     means ``with outer and inner headers''.

     incoming path                                          |------|
     ---- IPsec processing ---- (before) ---- (after) ----> |      |
                                                            | Host |
     <--- IPsec processing ---- (after) ----- (before) ---- |      |
     outgoing path                                          |------|

     Most people will want to run with the suggested defaults for
     ipsec_filter_mask and rely on the security policy database for the outer

     To see the packets the processed via ipsec(4), adjust the sysctl(8)
     variables according to your need and run:

           tcpdump -i enc0

     tcpdump(1), bpf(4), ipf(4), ipfw(4), ipsec(4), pf(4)

FreeBSD 11.1-RELEASE-p4        November 28, 2007       FreeBSD 11.1-RELEASE-p4
Command Section