tnat64(8) Library for intercepting outgoing network connections and


Set LD_PRELOAD to load the library then use applications as normal

The syntax to force preload of the library for different shells is specified below:
  Bash, Ksh and Bourne shell -

export LD_PRELOAD=/usr/lib/tnat64/

C Shell -

setenv LD_PRELOAD=/usr/lib/tnat64/

This process can be automated (for Bash, Bourne and Korn shell users) for a single command or for all commands in a shell session by using the tnat64(1) script

You can also setup tnat64 in such a way that all processes automatically use it, a very useful configuration. For more information on this configuration see the CAVEATS section of this manual page.


tnat64 is a library to allow IPv4-only application to access IPv4 hosts on IPv6-only systems by using NAT64. It wraps the normal connect() function. When a connection is attempted, it consults the configuration file (which is defined at configure time but defaults to /etc/tnat64.conf) and determines if the IP address specified is local. If it is not, the library redirects the connection to an IPv6 address inside the NAT64 prefix that is specified in the configuration file. The redirection is performed only if the destination host is unreachable (when there's no route to host). Also, after the first successful redirection all the following connection attempts are redirected immediately without even trying to connect to the host via IPv4. When the destination host cannot be reached via IPv6, the redirection is disabled.

tnat64 avoids the need to recompile applications so they can use reach hosts even if they do not support IPv6 and there's no IPv4 connectivity.

tnat64 is heavily based on the source code of tsocks, a library that intercepts outgoing TCP connections and redirects them though SOCKS proxy. Many thanks for tsocks author, Shaun Clowes!


Most arguments to tnat64 are provided in the configuration file (the location of which is defined at configure time by the --with-conf=<file> argument but defaults to /etc/tnat64.conf). The structure of this file is documented in tnat64.conf(8)

Some configuration options can be specified at run time using environment variables as follows:

This environment variable overrides the default location of the tnat64 configuration file. This variable is not honored if the program tnat64 is embedded in is setuid. In addition this environment variable can be compiled out of tnat64 with the --disable-envconf argument to configure at build time

This environment variable sets the level of debug output that should be generated by tnat64 (debug output is generated in the form of output to standard error). If this variable is not present by default the logging level is set to 0 which indicates that only error messages should be output. Setting it to higher values will cause tnat64 to generate more messages describing what it is doing. If set to -1 tnat64 will output absolutely no error or debugging messages. This is only needed if tnat64 output interferes with a program it is embedded in. Message output can be permanently compiled out of tnat64 by specifying the --disable-debug option to configure at build time

This option can be used to redirect the tnat64 output (which would normally be sent to standard error) to a file. This variable is not honored if the program tnat64 is embedded in is setuid. For programs where tnat64 output interferes with normal operation this option is generally better than disabling messages (with TNAT64_DEBUG = -1)


tnat64 will generate error messages and print them to stderr when there are problems with the configuration file if the TNAT64_DEBUG environment variable is not set to -1 or and --disable-debug was not specified at compile time. This output may cause some problems with programs that redirect standard error.


tnat64 will not in the above configuration be able to provide proxying to setuid applications or applications that are not run from a shell. You can force all applications to LD_PRELOAD the library by placing the path to libtnat64 in /etc/ Please make sure you correctly enter the full path to the library in this file if you do this. If you get it wrong, you will be UNABLE TO DO ANYTHING with the machine and will have to boot it with a rescue disk and remove the file (or try the saveme program, see the INSTALL file for more info). THIS IS A ***WARNING***, please be careful. Also be sure the library is in the root filesystem as all hell will break loose if the directory it is in is not available at boot time.


tnat64 can only proxy outgoing TCP connections

tnat64 does NOT work correctly with asynchronous sockets (though it does work with non blocking sockets). This bug would be very difficult to fix and there appears to be no demand for it (I know of no major application that uses asynchronous sockets)

tnat64 uses ELF dynamic loader features to intercept dynamic function calls from programs in which it is embedded. As a result, it cannot trace the actions of statically linked executables, non-ELF executables, or executables that make system calls directly with the system call trap or through the syscall() routine.


/etc/tnat64.conf - default tnat64 configuration file


Andrew O. Shadura ([email protected]), Shaun Clowes ([email protected])


Copyright 2011 Andrew O. Shadura

Original tsocks manual page, copyright 2000 Shaun Clowes

tnat64 and its documentation may be freely copied under the terms and conditions of version 2 of the GNU General Public License, as published by the Free Software Foundation (Cambridge, Massachusetts, United States of America).

This documentation is heavily based on the documentation for tsocks, transparent SOCKSification library, whose documentation itself is based on the documentation for logwrites, another shared library interceptor. One line of code from it was used in tsocks and a lot of the documentation :) logwrites is by [email protected] (Adam J. Richter) and can be had from pub/dist/pkg