---
name: "faith"
section: "4"
platform: "linux"
package: "freebsd-manpages"
version: "6.0-5"
release: "Debian 4.0 (Etch)"
platforms: ["linux", "macos"]
url: "https://new.carta.tech/faith"
---

# faith(4)

IPv6-to-IPv4 TCP relay capturing interface

## NAME

**faith** – IPv6-to-IPv4 TCP relay capturing interface

## SYNOPSIS

**device faith**

## DESCRIPTION

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)](/ifconfig/8.md) **create** command or using the *cloned\_interfaces* variable in [rc.conf(5)](/rc.conf/5.md).

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)](/getsockname/2.md) 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)](/setsockopt/2.md), 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)](/sysctl/8.md) 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.

## SEE ALSO

[inet(4)](/inet/4.md), [inet6(4)](/inet6/4.md), faithd(8) Jun-ichiro itojun Hagino and Kazu Yamamoto, *An IPv6-to-IPv4 transport relay translator*, RFC3142.

## HISTORY

The FAITH IPv6-to-IPv4 TCP relay translator was first appeared in WIDE hydrangea IPv6 stack.