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.\"     $FreeBSD: src/share/man/man4/faith.4,v 1.6.2.3 2001/07/03 11:01:24 ume Exp $
.\"
.Dd April 10, 1999
.Dt FAITH 4
.Os
.Sh NAME
.Nm faith
.Nd IPv6-to-IPv4 TCP relay capturing interface
.Sh SYNOPSIS
.Cd "pseudo-device faith" Op Ar count
.Sh DESCRIPTION
The
.Nm
interface captures IPv6 TCP traffic,
for implementing userland IPv6-to-IPv4 TCP relay
like
.Xr faithd 8 .
.Pp
Special action will be taken when IPv6 TCP traffic is seen on a router,
and routing table suggests to route it to
.Nm
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
.Dv IN6P_FAITH
flag turned on and it has matching address/port pairs.
In result,
.Nm
will let you capture IPv6 TCP traffic to some specific destination addresses.
Userland programs, such as
.Xr 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
.Xr 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.
.Pp
The
.Dv IN6P_FAITH
flag on IPv6 TCP socket can be set by using
.Xr setsockopt 2 ,
with level equals to
.Dv IPPROTO_IPV6
and optname equals to
.Dv IPv6_FAITH .
.Pp
To handle error reports by ICMPv6, some of ICMPv6 packets routed to
.Nm
interface will be delivered to IPv6 TCP, as well.
.Pp
To understand how
.Nm
can be used, take a look at source code of
.Xr faithd 8 .
.Pp
As
.Nm
interface implements potentially dangerous operation,
great care must be taken when configuring
.Nm
interface.
To avoid possible misuse,
.Xr sysctl 8
variable
.Li net.inet6.ip6.keepfaith
must be set to
.Li 1
prior to the use of the interface.
When
.Li net.inet6.ip6.keepfaith
is
.Li 0 ,
no packet will be captured by
.Nm
interface.
.Pp
.Nm
interface is intended to be used on routers, not on hosts.
.\"
.Sh SEE ALSO
.Xr inet 4 ,
.Xr inet6 4 ,
.Xr faithd 8
.Rs
.%A Jun-ichiro itojun Hagino
.%A Kazu Yamamoto
.%T "An IPv6-to-IPv4 transport relay translator"
.%R internet draft
.%N draft-ietf-ngtrans-tcpudp-relay-04.txt
.%O work in progress material
.Re
.Sh HISTORY
The FAITH IPv6-to-IPv4 TCP relay translator was first appeared in
WIDE hydrangea IPv6 stack.