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FAITHD (8) | System administration commands and daemons | Unix Manual Pages | :man


faithd - FAITH IPv6/v4 translator daemon


Daemon mode
Access control
Exit Status
Daemon mode samples
Access control samples
See Also
Security Considerations


faithd [-dp] [-f configfile] service [serverpath [serverargs]]


The faithd utility provides IPv6-to-IPv4 TCP relay. It must be used on an IPv4/v6 dual stack router.

When faithd receives TCPv6 traffic, faithd will relay the TCPv6 traffic to TCPv4. Destination for relayed TCPv4 connection will be determined by the last 4 octets of the original IPv6 destination. For example, if 3ffe:0501:4819:ffff:: is reserved for faithd, and the TCPv6 destination address is 3ffe:0501:4819:ffff::0a01:0101, the traffic will be relayed to IPv4 destination

To use faithd translation service, an IPv6 address prefix must be reserved for mapping IPv4 addresses into. Kernel must be properly configured to route all the TCP connection toward the reserved IPv6 address prefix into the faith(4) pseudo interface, by using route(8) command. Also, sysctl(8) should be used to configure net.inet6.ip6.keepfaith to 1.

The router must be configured to capture all the TCP traffic toward reserved IPv6 address prefix, by using route(8) and sysctl(8) commands.

The faithd utility needs a special name-to-address translation logic, so that hostnames gets resolved into special IPv6 address prefix. For small-scale installation, use hosts(5). For large-scale installation, it is useful to have a DNS server with special address translation support. An implementation called totd is available at http://www.vermicelli.pasta.cs.uit.no/ipv6/software.html. Make sure you do not propagate translated DNS records to normal DNS cloud, it is highly harmful.

Daemon mode

When faithd is invoked as a standalone program, faithd will daemonize itself. The faithd utility will listen to TCPv6 port service. If TCPv6 traffic to port service is found, it relays the connection.

Since faithd listens to TCP port service, it is not possible to run local TCP daemons for port service on the router, using inetd(8) or other standard mechanisms. By specifying serverpath to faithd, you can run local daemons on the router. The faithd utility will invoke local daemon at serverpath if the destination address is local interface address, and will perform translation to IPv4 TCP in other cases. You can also specify serverargs for the arguments for the local daemon.

The following options are available:

-d Debugging information will be generated using syslog(3).
-f configfile
Specify a configuration file for access control. See below.
-p Use privileged TCP port number as source port, for IPv4 TCP connection toward final destination. For relaying ftp(1), this flag is not necessary as special program code is supplied.

The faithd utility will relay both normal and out-of-band TCP data. It is capable of emulating TCP half close as well. The faithd utility includes special support for protocols used by ftp(1). When translating FTP protocol, faithd translates network level addresses in PORT/LPRT/EPRT and PASV/LPSV/EPSV commands.

Inactive sessions will be disconnected in 30 minutes, to avoid stale sessions from chewing up resources. This may be inappropriate for some of the services (should this be configurable?).

inetd mode

When faithd is invoked via inetd(8), faithd will handle connection passed from standard input. If the connection endpoint is in the reserved IPv6 address prefix, faithd will relay the connection. Otherwise, faithd will invoke service-specific daemon like telnetd(8), by using the command argument passed from inetd(8).

The faithd utility determines operation mode by the local TCP port number, and enables special protocol handling whenever necessary/possible. For example, if faithd is invoked via inetd(8) on FTP port, it will operate as a FTP relay.

The operation mode requires special support for faithd in inetd(8).

Access control

To prevent malicious accesses, faithd implements a simple address-based access control. With /etc/faithd.conf (or configfile specified by -f ), faithd will avoid relaying unwanted traffic. The faithd.conf contains directives with the following format:
  • src / slen deny dst / dlen

    If the source address of a query matches src / slen, and the translated destination address matches dst / dlen, deny the connection.

  • src / slen permit dst / dlen

    If the source address of a query matches src / slen, and the translated destination address matches dst / dlen, permit the connection.

The directives are evaluated in sequence, and the first matching entry will be effective. If there is no match (if we reach the end of the ruleset) the traffic will be denied.

With inetd mode, traffic may be filtered by using access control functionality in inetd(8).


The faithd utility exits with EXIT_SUCCESS (0) on success, and EXIT_FAILURE (1) on error.


Before invoking faithd, faith(4) interface has to be configured properly.
# sysctl net.inet6.ip6.accept_rtadv=0
# sysctl net.inet6.ip6.forwarding=1
# sysctl net.inet6.ip6.keepfaith=1
# ifconfig faith0 up
# route add -inet6 3ffe:501:4819:ffff:: -prefixlen 96 ::1
# route change -inet6 3ffe:501:4819:ffff:: -prefixlen 96 -ifp faith0

Daemon mode samples

To translate telnet service, and provide no local telnet service, invoke faithd as follows:
# faithd telnet

If you would like to provide local telnet service via telnetd(8) on /usr/libexec/telnetd, use the following command line:
# faithd telnet /usr/libexec/telnetd telnetd

If you would like to pass extra arguments to the local daemon:
# faithd ftp /usr/libexec/ftpd ftpd -l

Here are some other examples. You may need -p if the service checks the source port range.
# faithd ssh
# faithd telnet /usr/libexec/telnetd telnetd

inetd mode samples

Add the following lines into inetd.conf(5). Syntax may vary depending upon your operating system.
telnet stream tcp6/faith nowait root faithd telnetd
ftpstream tcp6/faith nowait root faithd ftpd -l
sshstream tcp6/faith nowait root faithd /usr/sbin/sshd -i

inetd(8) will open listening sockets with enabling kernel TCP relay support. Whenever connection comes in, faithd will be invoked by inetd(8). If it the connection endpoint is in the reserved IPv6 address prefix. The faithd utility will relay the connection. Otherwise, faithd will invoke service-specific daemon like telnetd(8).

Access control samples

The following illustrates a simple faithd.conf setting.
# permit anyone from 3ffe:501:ffff::/48 to use the translator,
# to connect to the following IPv4 destinations:
# - any location except and
# Permit no other connections.
3ffe:501:ffff::/48 deny
3ffe:501:ffff::/48 deny
3ffe:501:ffff::/48 permit


faith(4), route(8), sysctl(8)
.Rs "An IPv6-to-IPv4 transport relay translator"



faith(4), route(8).

Created by Blin Media, 2008-2013