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     enc — Encapsulating Interface


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

           device enc

     Alternatively, to load the driver as a module at boot time, place the following line in



     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”

     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 headers.

     Note that packets are captured by BPF before firewall processing.  The special value 0x4 can
     be configured in the ipsec_bpf_mask and packets will be also captured after firewall


     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)