.\"	$OpenBSD: faith.4,v 1.16 2004/08/17 23:58:36 jaredy Exp $
.\"	$KAME: faith.4,v 1.10 2001/06/30 00:42:48 itojun Exp $
.\"
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.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
similar to
.Xr faithd 8 .
.Pp
A
.Nm
interface can be created at runtime using the
.Ic ifconfig faith Ns Ar N Ic create
command or by setting up a
.Xr hostname.if 5
configuration file for
.Xr netstart 8 .
.Pp
Special action will be taken when IPv6 TCP traffic is seen on a router,
and the routing table suggests routing it to the
.Nm
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
.Dv IN6P_FAITH
flag turned on and it has matching address/port pairs.
Thus,
.Nm
allows captured IPv6 TCP traffic to be relayed 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 an IPv6 TCP socket can be set by using
.Xr setsockopt 2 ,
with level set to
.Dv IPPROTO_IPV6
and optname set to
.Dv IPv6_FAITH .
.Pp
To handle error reports by ICMPv6, some of the ICMPv6 packets routed to the
.Nm
interface will be delivered to IPv6 TCP, as well.
.Pp
To understand how
.Nm
can be used, take a look at the source code of
.Xr faithd 8 .
.Pp
As the
.Nm
interface implements a potentially dangerous operation,
great care must be taken when configuring the
.Nm
interface.
To avoid possible misuse, the
.Xr sysctl 8
variable
.Li net.inet6.ip6.keepfaith
must be set to
.Li 1
prior to use of the interface.
When
.Li net.inet6.ip6.keepfaith
is
.Li 0 ,
no packet is captured by the
.Nm
interface.
.Pp
The
.Nm
interface is intended to be used on routers, not on hosts.
.\"
.Sh SEE ALSO
.Xr inet 4 ,
.Xr inet6 4 ,
.Xr hostname.if 5 ,
.Xr faithd 8 ,
.Xr netstart 8
.Rs
.%A Jun-ichiro itojun Hagino
.%A Kazu Yamamoto
.%T "An IPv6-to-IPv4 transport relay translator"
.%B RFC 3142
.%O ftp://ftp.isi.edu/in-notes/rfc3142.txt
.%D June 2001
.Re
.Sh HISTORY
The FAITH IPv6-to-IPv4 TCP relay translator first appeared in
WIDE hydrangea IPv6 stack.