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 routing table suggests to route it to faith interface. In this case, the packet will be accepted by the router, regardless of list of IPv6 interface addresses assigned to the router. The packet will be captured by an IPv6 TCP socket, if it has IN6P_FAITH flag turned on and it has matching address/port pairs. In 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 IPv6 TCP socket can be set by using setsockopt(2), with level equals to IPPROTO_IPV6 and optname equals to IPv6_FAITH.
To handle error reports by ICMPv6, some of ICMPv6 packets routed to faith interface will be delivered to IPv6 TCP, as well.
To understand how faith can be used, take a look at source code of faithd(8).
As faith interface implements potentially dangerous operation, great care must be taken when configuring faith interface. To avoid possible misuse, sysctl(8) variable net.inet6.ip6.keepfaith must be set to 1 prior to the use of the interface. When net.inet6.ip6.keepfaith is 0, no packet will be captured by faith interface.
faith interface is intended to be used on routers, not on hosts.
inet(4), inet6(4), faithd(8)
.Rs "An IPv6-to-IPv4 transport relay translator"