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.\"     $Id: faith.4,v 1.1.1.1 1999/08/08 23:30:37 itojun Exp $
.\"     $FreeBSD$
.\"
.Dd April 10, 1999
.Dt FAITH 4
.Os
.Sh NAME
.Nm faith
.Nd
.Tn IPv6-to-IPv4 TCP relay capturing interface
.Sh SYNOPSIS
.Cd "pseudo-device faith 1"
.Sh DESCRIPTION
The
.Nm
interface captures IPv6 TCP traffic
for implementing userland IPv6-to-IPv4 TCP relays
like
.Xr faithd 8 .
.Pp
Special action will be taken when IPv6 TCP traffic is seen on a router
and the routing table suggests to route 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 will be captured by an IPv6 TCP socket if it has the
.Dv IN6P_FAITH
flag turned on and it has matching address/port pairs.
As a result,
.Nm
will let you divert 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 an IPv6 TCP socket can be set by using
.Xr setsockopt 2 ,
with
.Fa level
set to
.Dv IPPROTO_IPV6
and
.Fa optname
set to
.Dv IPv6_FAITH .
.Pp
To handle error reports by ICMPv6 some of the ICMPv6 packets routed to the
.Nm
interface will need 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 potentially dangerous operations,
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 the use of the interface.
When
.Li net.inet6.ip6.keepfaith
is set to
.Li 0 ,
no packets will be 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 faithd 8
.\" .Rs
.\" .%A	Jun-ichiro itojun Hagino
.\" .%A	Kazu Yamamoto
.\" .%T	``FAITH'' IPv6-to-IPv4 TCP relay translator
.\" .%D	July 1999
.\" .Re
.\"
.Sh HISTORY
The FAITH IPv6-to-IPv4 TCP relay translator first appeared in
WIDE hydrangea IPv6 stack.