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faith — IPv6-to-IPv4 TCP relay capturing interface
The faith interface captures IPv6 TCP traffic, for implementing userland
IPv6-to-IPv4 TCP relay like faithd(8).
Each faith interface is created at runtime using interface cloning. This
is most easily done with the ifconfig(8) create command or using the
cloned_interfaces variable in rc.conf(5).
Special action will be taken when IPv6 TCP traffic is seen on a router,
and the routing table suggests to route it to the faith interface. In
this case, the packet will be accepted by the router, regardless of the
list of IPv6 interface addresses assigned to the router. The packet will
be captured by an IPv6 TCP socket, if it has the IN6P_FAITH flag turned
on and matching address/port pairs. As a result, faith will let you
capture IPv6 TCP traffic to some specific destination addresses.
Userland programs, such as faithd(8) can use this behavior to relay IPv6
TCP traffic to IPv4 TCP traffic. The program can accept some specific
IPv6 TCP traffic, perform getsockname(2) to get the IPv6 destination
address specified by the client, and perform application-specific address
mapping to relay IPv6 TCP to IPv4 TCP.
The IN6P_FAITH flag on a IPv6 TCP socket can be set by using
setsockopt(2), with level IPPROTO_IPV6 and optname IPv6_FAITH.
To handle error reports by ICMPv6, some ICMPv6 packets routed to an faith
interface will be delivered to IPv6 TCP, as well.
To understand how faith can be used, take a look at the source code of
As the faith interface implements potentially dangerous operations, great
care must be taken when configuring it. To avoid possible misuse, the
sysctl(8) variable net.inet6.ip6.keepfaith must be set to 1 prior to
using the interface. When net.inet6.ip6.keepfaith is 0, no packets will
be captured by the faith interface.
The faith interface is intended to be used on routers, not on hosts.
inet(4), inet6(4), faithd(8)
Jun-ichiro itojun Hagino and Kazu Yamamoto, An IPv6-to-IPv4 transport
relay translator, RFC3142.
The FAITH IPv6-to-IPv4 TCP relay translator first appeared in the WIDE
hydrangea IPv6 stack.