The
faith
interface captures IPv6 TCP traffic,
for implementing userland IPv6-to-IPv4 TCP relay
similar to
faithd(8).
A
faith
interface can be created at runtime using the
ifconfigfaith
command or by setting up a
hostname.if(5)
configuration file for
netstart(8).
Special action will be taken when IPv6 TCP traffic is seen on a router,
and the routing table suggests routing 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 is captured by an IPv6 TCP socket, if it has the
IN6P_FAITH
flag turned on and it has matching address/port pairs.
Thus,
faith
allows captured IPv6 TCP traffic to be relayed 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 an IPv6 TCP socket can be set by using
setsockopt(2),
with level set to
IPPROTO_IPV6
and optname set to
IPv6_FAITH.
To handle error reports by ICMPv6, some of the ICMPv6 packets routed to the
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
faithd(8).
As the
faith
interface implements a potentially dangerous operation,
great care must be taken when configuring the
faith
interface.
To avoid possible misuse, the
sysctl(8)
variable
net.inet6.ip6.keepfaith
must be set to
1
prior to use of the interface.
When
net.inet6.ip6.keepfaith
is
0,
no packet is captured by the
faith
interface.
The
faith
interface is intended to be used on routers, not on hosts.