DOKK / manpages / debian 13 / clatd / clatd.8.en
clatd(8) clatd - CLAT, SIIT-DC and IPv6-only with many XLAT engines clatd(8)

clatd - a CLAT / SIIT-DC Edge Relay implementation for Linux

clatd implements the CLAT component of the 464XLAT network architecture specified in RFC 6877. It allows an IPv6-only host to have IPv4 connectivity that is translated to IPv6 before being routed to an upstream PLAT (which is typically a Stateful NAT64 operated by the ISP) and there translated back to IPv4 before being routed to the IPv4 internet. This is especially useful when local applications on the host requires actual IPv4 connectivity or cannot make use of DNS64 (for example because they use legacy AF_INET socket calls, or if they are simply not using DNS64).

clatd may also be used to implement an SIIT-DC Edge Relay as described in RFC 7756. In this scenario, the PLAT is in reality a SIIT-DC Border Relay (see RFC 7755) instead of a Stateful NAT64 (see RFC6146). When used as a SIIT-DC Edge Relay, you will probably want to manually configure the settings clat-v4-addr, clat-v6-addr, and plat-prefix to mirror the SIIT-DC Border Relay's configuration.

It relies either on the software package TAYGA by Nathan Lutchansky or on the kernel module nat46 by Andrew Yourtchenko for the actual translation of packets between IPv4 and IPv6 (RFC 6145) TAYGA may be downloaded from its home page at <http://www.litech.org/tayga/>, nat46 from its repository at <https://github.com/ayourtch/nat46>.

clatd [options]

Quiet mode; suppress normal output. This is the same as setting quiet=1. Warnings and errors are still outputted, to silence those too, repeat -q.
Enable debugging output. This is the same as setting debug=1. Repeat for even more debugging output, which is the equivalent of setting debug=2.
Read configuration settings from conf-file. See section CONFIGURATION below for more info.
Print a brief usage help and exit.
Set configuration key to value, overriding any setting found in the configuration file. Refer to the section CONFIGURATION below for more info.

clatd is meant to be run under a daemonising control process such as systemd or similar. It is further meant to be (re)started whenever a network interface goes up/down as this might mean a change in the PLAT availability or which prefixes/addresses needs to be used for the CLAT to work. It may also be run directly from the command line. It will run until killed with SIGINT (^C) or SIGTERM, at which point it will clean up after itself and exit gracefully.

See the scripts/ directory in the source distribution for some examples on how to invoke it it.

clatd is designed to be able to run without any user-supplied configuration in most cases. However, user-specified configuration settings may be added to the configuration file, the path to which may be given on the command line using the -c option, or if it is not, the default location /etc/clatd.conf is used. Configuration settings may also be given directly on the command line when starting clatd, which takes precedence over settings in the configuration file.

Settings are of the form key=value. A list of recognised keys and their possible values follow below:

Set this to 1 to suppress normal output from clatd. This is the same as providing the command line option -q. Set it to 2 to additionally suppress warnings and errors. Note that this does not suppress debugging output.
Set this to 1 to get debugging output from clatd, or 2 to get even more of the stuff. These are the equivalent of providing the command line option -d the specified number of times.
Specify a custom script to be run when clatd is starting up. The invocation of this script is the last thing that happens before TAYGA starts up, so all the preparations have been completed at that point (i.e., the clat-dev exists and has routing/addressing configured, forwarding has been enabled, and so on).

The script is run by the system shell, so you can do everything you could in an interactive shell: run multiple commands by separating them by semi-colon or double ampersands, use standard if/else statements, use variable substitutions, redirect output to files, set up command pipelines, and so on. However it must all be on one line, so if you want to do complex things or use some other programming language it's probably better to put the script itself in a separate executable file and just make script-up invoke that file instead.

If the script returns a nonzero exit status, this is considered a fatal error, and clatd will abort. This can be prevented by appending || true at the end of the script.

All of clatd's configuration settings are available as standard variables in the script's environment (hyphens are replaced with underscores).

Logging or debug messages from the script may simply be sent to stdout, where it will be picked up by the init system along with clatd's own output. The script may of course consult the $quiet and $debug environment variables in order to determine how much output is appropriate.

The script should not be enclosed in quotes in the configuration file (even though it contains whitespace). For example:

script-up=echo `date -Ins`: clatd started on $clat_dev | tee -a ~/clatd.log

If on the other hand you want to supply a script-up containing whitespace directly clatd's command line, quoting is required in order to prevent the shell from splitting it up and into multiple command line arguments. For example:

clatd 'script-up=ip route add 192.0.2.0/24 dev $clat_dev || true'

This works exactly the same as script-up, only that this script is run right after TAYGA has exited, before the clean-up process of restoring any settings that were changed.

An unsuccessful exit code from script-down will cause clatd to exit unsuccessfully too. Beyond that an unsuccessful exit won't change anything, because script-down is invoked at a point in time where the only thing left for clatd to do is to clean up after itself and exit anyway.

The name of the network device used by the CLAT. There should be no reason to change the default, unless you plan on running multiple instances of clatd simultaneously.
The IPv4 address that will be assigned to the CLAT device. Local applications will bind to this address when communicating with external IPv4 destinations. In a standard 464XLAT environment with a stateful NAT64 serving as the PLAT, there should be no need to change the default.

When using clatd as an SIIT-DC Edge Relay (RFC 7756), you will want to set this to the IPv4 Service Address configured in the SIIT-DC Border Relay. This way, local applications can correctly identify which public address they'll be using on the IPv4 internet, and will be able to provide fully functional references to it in application-level payload, and so on.

The default address is one from RFC 7335.

The IPv6 address of the CLAT. Traffic to/from the clat-v4-addr will be translated into this address. When using clatd as an SIIT-DC Edge Relay, you will want to set this to the same IPv6 address in the Explicit Address Mapping configured in the SIIT-DC Border Relay.

When set to shared (the default), clatd will re-use and share the primary IPv6 address used by the host OS when communicating with directly with the PLAT prefix. This allows clatd to operate without requiring a separate, dedicated IPv6 address assigned to the CLAT function. While this mode is the most compatible with various network deployments, it comes with certain trade-offs, see the LIMITATIONS section for more information.

When set to derived, clatd will attempt to figure out which network device will be used for traffic towards the PLAT, see if there is any SLAAC-based globally scoped addresses on it (i.e., a /64 with '0xfffe' in the middle of the Interface ID), and will if so substitute that '0xfffe' value with '0xc1a7' ("clat") to generate a CLAT IPv6 address.

If only a non-SLAAC global address is found on the PLAT-facing device, clatd will substitute its Interface ID with a random integer and use the result as the CLAT IPv6 address. It will only do so if the prefix length is /120 or smaller, as otherwise the risk of IID collisions is considered to be too high. Note that on most Perl platforms, the rand() function is limited to 48 bits, which means that for longer IIDs, the least significant bits will be all 0.

The derived mode is not guaranteed to result in a working configuration, see the LIMITATIONS section for more information.

If multiple addresses are found in either category, the one that shares the longest common prefix with the PLAT prefix will be preferred when deriving the CLAT IPv6 address according to the algorithm described above.

If set to a non-zero integer, nftables will be used to mark outgoing connections through the CLAT with this connection tracking mark, and the Linux kernel routing policy is set up so that only response traffic from the PLAT that matches this connection tracking mark is routed back to the CLAT.

This means that connections initiated from the external IPv4 network via the PLAT will not reach the CLAT, therefore the use of this feature is incompatible with using clatd as a SIIT-DC Edge Relay (RFC 7756).

Comma-separated list of DNS64 servers to use when discovering the PLAT prefix using the method described in RFC 7050. By default, clatd will first try to determine if systemd-networkd is aware of a PLAT prefix (learned from the PREF64 Router Advertisement option, cf. RFC 8781), falling back on using DNS64 discovery towards the system resolver if it isn't.

It might be useful to override this in case your network does not advertise the PREF64 RA option, your ISP doesn't provide you with a DNS64-enabled name server, and you want to test clatd using any of the public DNS64/NAT64 instances on the internet. The first PLAT prefix encountered will be used.

Path to the ip binary from the iproute2 package available at <https://www.kernel.org/pub/linux/utils/net/iproute2>. Required.
Path to the networkctl binary from systemd-networkd. Required in order to use any PLAT prefix discovered by systemd-networkd from the PREF64 Router Advertisement option (see RFC 8781 and the UsePREF64 option in systemd.network(5) for more information). The first prefix returned is used, any others are ignored.

To prevent PLAT prefix discovery via systemd-networkd from being attempted, set this to an empty string.

Path to the nft binary from the nftables package available at <https://nftables.org/projects/nftables/>. Required if ctmark is set.
Path to the tayga binary from the TAYGA package available at <http://www.litech.org/tayga>. Required.
Path to the ufw binary from the UFW local firewall framework available at <https://launchpad.net/ufw> commonly seen on Debian-based distributions. If this command is present on the system, and ufw status reports that the firewall is active, clatd will add firewall rules ensuring traffic between the CLAT and the PLAT prefix is allowed when it is starting up, and remove them when it is shutting down.

If you don't want clatd to add and remove UFW firewall rules, set this to an empty string.

Controls whether or not clatd should enable IPv6 forwarding if necessary. IPv6 forwarding is necessary for clatd to work correctly. It will also ensure that the accept_ra sysctl is to '2' for all devices have it set to '1', in order to prevent any connectivity loss as a result of enabling forwarding.

All sysctls that are modified will be restored to their original values when clatd is shutting down.

Which network device is facing the PLAT (NAT64). By default, this is auto-detected by performing a route table lookup towards the PLAT prefix. This setting is used when generating the CLAT IPv6 address, adding Proxy-ND entries, and nftables rules.
The IPv6 translation prefix into which the PLAT maps the IPv4 internet. See RFC 6052 for a closer description. By default, this is auto-detected from DNS64 answers using the method in RFC 7050.
The IPv6 translation prefix fallback. This is used if no plat-prefix is set or auto detected.
Controls whether or not clatd should add a Proxy-ND entry for the CLAT IPv6 address on the network device facing the PLAT. This is probably necessary on Ethernet networks (otherwise the upstream IPv6 router won't know where to send packets to the CLAT's IPv6 address), but likely not necessary on point-to-point links like PPP or 3GPP mobile broadband, as in those cases IPv6 ND isn't used. However it doesn't hurt to add Proxy-ND entries in that case, either.

If the CLAT is sharing an IPv6 address with the host OS, this is not necessary as the host OS will be handling NDP anyway, so Proxy-ND does get enabled by default when clat-v6-addr is set to its default value shared.

Any entries added wil be removed when clatd is shutting down.

The Linux kernel routing table used to hold the route that directs IPv6 packets from the PLAT to the CLAT. clatd will add a custom routing policy entry (using ip -6 rule add) so that this routing table is used instead of the default one.
Where to write the TAYGA configuration file. By default, a temporary file will be created (and also deleted when clatd is shutting down), but you may also specify an explicit configuration file here, which will not be deleted on shutdown.
The IPv4 address assigned to the TAYGA process. This is used for emitting ICMPv4 errors back to the host (i.e., it will show up as the first hop when tracerouting to IPv4 destinations), and you may also ping it to verify that the TAYGA process is still alive and well.

The default address is one from RFC 7335.

Whether or not to check if the system has IPv4 connectivity before starting the CLAT. If it does, then clatd will simply exit without doing anything. This is meant so that you can always enable clatd to the system startup scripts or network-up event scripts (such as NetworkManager's dispatcher scripts), but not have clatd interfering with native IPv4 connectivity when this is present.

If you want to always start the CLAT whenever possible, even though the system has IPv4 connectivity, disable this setting. You may instead use the v4-defaultroute-enable and v4-defaultroute-metric settings to prevent clatd from interfering with native IPv4 connectivity.

Note that enabling v4-defaultroute-replace will override v4-conncheck-enable and unconditionally disable IPv4 connectivity checking.

When performing an IPv4 connectivity check, wait this number of seconds before actually doing anything. This is to avoid a race condition where for example IPv6 SLAAC finshes and triggers a network-up event script to start clatd, while IPv4 DHCPv4 is still running in the background. This is at least a likely scenario when using NetworkManager, as it will start the dispatcher scripts as soon as either IPv4 or IPv6 has completed, and IPv6 SLAAC is typically faster than IPv4 DHCPv4.

Set it to 0 to perform the check immediately.

Whether or not to add an IPv4 default route pointing to the CLAT. In a typical 464XLAT environment, you want this. However when using clatd in an environment where native IPv4 connectivity is also present, you might want to disable this and instead control manually which IPv4 destinations is reached through the CLAT and which are not.
Instructs clatd to remove any pre-existing IPv4 default routes, replacing it with one pointing to the CLAT (assuming v4-defaultroute-enable is yes). The replacement is temporary, any pre-existing routes that were removed will be restored when clatd is shutting down.

Note that nothing prevents software like a connection manager or a DHCPv4 client daemon from re-adding any replaced routes while clatd is running.

If you enable v4-defaultroute-replace while at the same time disabling v4-defaultroute-enable, clatd will remove any pre-existing IPv4 default routes but not add any of its own.

Setting v4-defaultroute-replace to yes will disable the IPv4 connectivity check.

The metric of the IPv4 default route pointing to the CLAT. The default is chosen because it is higher than that of a native IPv4 default route added by NetworkManager, which makes it so that the native IPv4 connectivity is preferred if present.
The MTU of the default route pointing to the CLAT. The default is the default IPv6 MTU used by TAYGA (1280, which in turn comes from RFC 6145) minus 20 to compensate for the difference in header size between IPv4 and IPv6. This prevents outbound packets from having to be fragmented by TAYGA, and also makes local applications advertise a TCP MSS to their remote peers that prevent them from sending packets beck to us that would require fragmentation.

If you know that the IPv6 Path MTU between the host and the PLAT is larger than 1280, you may increase this, but then you should also recompile TAYGA with a larger ipv6_offlink_mtu setting in conffile.c.

The "advmss" value assigned to the the default route potining to the CLAT. This controls the advertised TCP MSS value for TCP connections made through the CLAT.

You should normally not need to set this. By default the value is calculated by taking the value of v4-defaultroute-mtu and substracting 40 (20 bytes for the IPv4 header + 20 bytes for the TCP header). If v4-defaultroute-mtu is unset or 0, there is no default.

When using clat-v6-addr=shared (or when clat-v6-addr is set to another address assigned to a local interface) and ctmark is set to 0, the host OS will not be able to communicate bi-directionally with IPv4 destinations directly through the PLAT (e.g., ping6 64:ff9b::192.0.2.1). This is because the response traffic will be routed back to the CLAT, and ultimately return to the Linux kernel as an IPv4 packet, which does not match the outgoing IPv6 socket. Such direct communication is normal when using DNS64 synthesis for all queries (as opposed to just ipv4only.arpa).

When using clat-v6-addr=derived and no IPv6 addresses on the PLAT-facing device are EUI-64-derived (e.g., when using SLAAC with RFC 4941 or RFC 7217 privacy addressing or static addresses), clatd will generate and use an CLAT IPv6 address using a random Interface ID from the same subnet prefix (if it is /120 or shorter). RFC 6877 suggests DHCPv6 IA_PD should be attempted in this case instead, but this isn't currently implemented.

clatd will not attempt to perform Duplicate Address Detection for the IPv6 address it generates when using clat-v6-addr=derived. This is a violation of RFC 6877.

There is no guarantee that the generated CLAT IPv6 address is in fact usable, as the network might block its use.

If the upstream network is using DHCPv6, clatd will not be able to generate a CLAT IPv6 address at all, due to the fact that DHCPv6-assigned addresses do not carry a prefix length.

clatd will not attempt to perform a connectivity check to a discovered PLAT prefix before setting up the CLAT, as RFC 7050 suggest it should.

If you are experiencing any bugs or have any feature requests, head over to <https://github.com/toreanderson/clatd/issues> and submit a new issue (if someone else hasn't already done so). Please make sure to include logs with full debugging output (using -d -d on the command line or debug=2 in the configuration file) when reporting a bug.

Copyright (c) 2014-2025 Tore Anderson <tore@fud.no>.

SPDX-License-Identifier: MIT

ip(8), nft(8), systemd.network(5), tayga(8), tayga.conf(5)

RFC 6052, RFC 6145, RFC 6146, RFC 6877, RFC 7050, RFC 7335 RFC 7755, RFC 7756, RFC 7757, RFC 8781

2025-04-16 perl v5.40.1