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ocat(1) - OnionCat creates a transparent IPv6 layer on top of Tor's or I2P hidden

ocat, 2023-10-25



ocat -i onion_id                      (1st form)
ocat -o IPv6_address                  (2nd form)
ocat [OPTION] onion_id                (3rd form)
ocat -R [OPTION]                      (4th form)
ocat -I [OPTION] i2p_id                  (5th form)


OnionCat creates a transparent IPv6 layer on top of Tor's hidden services or I2P's tunnels. It transmits any kind of IP-based data transparently through the Tor/I2P network on a location hidden basis. You can think of it as a peer-to-peer VPN between hidden services.

OnionCat is a stand-alone application which runs in userland and is a connector between Tor/I2P and the local OS. Any protocol which is based on IP can be transmitted. Of course, UDP and TCP (and probably ICMP) are the most important ones but all other protocols can also be forwarded through it.

OnionCat opens a TUN device and assigns an IPv6 address to it. All packets forwarded to the TUN device by the kernel are forwarded by OnionCat to other OnionCats listening on Tor's hidden service ports or I2P's server tunnels. The IPv6 address depends on the onion_id or the i2p_id, respectively. The onion_id is the hostname of the locally configured hidden service (see tor(8)). Depending on the configuration of Tor the onion_id usually can be found at /var/lib/tor/hidden_service/hostname or similar location. The i2p_id is the 80 bit long Base32 encoded hostname of the I2P server tunnel.

OnionCat has two implemented loopback responders for easier network debugging. Ping responses from loopback address fd87:d87e:eb43::dead:beef indicate correct network setup between the kernel and OnionCat. Ping responses from fd87:d87e:eb43::feed:beef indicate that connection between OnionCat and Tor itself works, hidden service correctly configured and enabled.


In the following is a description of all options. Typically you won't need any of them except specifying your OnionCat's .onion hostname. Depending on your setup you may use options -g and -A.

  • -2
    This option is here only for simplicity. With this option OnionCat behaves like OnionCat3 (version < 0.4.0). Actually it is a short form for using the options -D -H -S.

  • -4
    Enable IPv4 forwarding. See
    IPv4-througth-IPv6 tunnel through OnionCat is recommended instead of using native IPv4 forwarding.

  • -5 'socks5'|'direct'
    This option has a mandatory argument which is literally either socks5 or direct.
    By default OnionCat uses SOCKS4A (version 4a) to connect to the anonymization network proxy (e.g. Tor). With this option set to socks5, OnionCat uses SOCKS5 (version 5 as specified in RFC1928), currently with no authentication mechanism. As of today it actually makes no difference but it might be desireable in future.
    If direct is used, OnionCat does not connect through the SOCKS server but instead it connects directly to the remote peers using the hosts lookup mechanism (see option -H). This feature is experimental and turns OnionCat into a distributed virtual switch based on regular Internet transport instead of Tor. It is a useful feature for a lab setup.
    This option also disables the remote hostname validation (option -J).
    Please note that OnionCat does not implement any encryption technique! It is a plain tunneling through TCP sessions.

  • -a
    OnionCat creates a log file at $HOME/.ocat/connect_log. All incoming connects are logged to that file. $HOME is determined from the user under which OnionCat runs (see option -u).

  • -A [ipv6/]hostname
    This option lets you add an IP address hostname pair to the internal hosts db. Typically it is enough to specify just the .onion hostname because the IPv6 address is derived from the hostname. Adding an IP address is either redundant or may be used in special scenarios where the IPv6 address differes from its original address. The latter will only work in a closed environment and is not interoperable with other OnionCats in the wild because by default the names and IPs are verified to be correct. This option may be specified multple times. Alternatively you could put the IP/hostname pairs into the OnionCat hosts file (see option -g).

  • -b
    Run OnionCat in background. This is default. OnionCat will detach from a running shell and close standard IO if no log file is given with option -L.

  • -B
    Run OnionCat in foreground. OnionCat will log to stderr by default.

  • -C
    Disable the local controller interface. The controller interfaces listens on localhost ( and ::1 port 8066) for incoming connections. It's currently used for debugging purpose and not thread-safe and does not have any kind of authentication or authorization mechanism. Hence, it should not be used in production environments.

  • -d n
    Set debug level to n. Default = 7 which is maximum. Debug output will only be created if OnionCat was compiled with option DEBUG (i.e. configure was run with option --enable-debug).

  • -D
    Disable OnionCat's DNS lookup. If OnionCat has to make an outgoing connection but no suitable entry is found in the internal hosts db, OnionCat will do a DNS query at the hosts found in the internal hosts db. This option deactivates this feature.
    Hostname lookups are enabled by default since OnionCat4.

  • -e ifup
    Execute script ifup to bring up the tunnel interface.
    OnionCat will create a new tunnel interface and execute ifup immediatly after opening the network interface. This is intended as a universial interface for configuring the tunnel device and do additinal tasks when starting OnionCat. The script is executed with the same privilege as OnionCat is started, i.e. before dropping privileges. This typically is root. The script is run only once at startup.

    See below in section EXAMPLES for a typical Linux ifup shell script.

    OnionCat executes the file ifup with a call to execlp(3) and will pass the following environment variables:

    This variable contains the name of the network interface, e.g. "tun0".

    This variable contains the IPv6 address which is associated with this instance of OnionCat and its hidden service address.

    This variable contains the prefix length of the IPv6 prefix which typically is 48.

    This variable contains the IPv6 prefix, i.e. the network. This typically is fd87:d87e:eb43:: in OnionCat (Tor) mode and fd60:db4d:ddb5:: in GarliCat (I2P) mode.

    This variable contains the short onion URL without domain appendix. It is a 16 character long string.

    This variable contains the long onion URL if available which is always true for I2P and in case of Tor it is used together with the HSv3 variant.

    This variable contains the TLD appendix which is .onion for Tor and .b32.i2p for I2P.

  • -E s
    This option sets the expiry time in secondes for remote entries in the internal hosts db.
    If the TTL of an entry expires, OnionCat will try to renew the entry by connecting to the remote OnionCat to retrieve a new keepalive. If it is unreachable it will retry after some time again. If it was unreachable for more than s seconds, the entry will be removed from the internal hosts db.
    The default expiry time is 604800 seconds which is 7 days.

  • -f config file
    Read initial configuration from config file.

  • -g hosts_path
    Set the path to the hosts file. This option automatically enables option -H (see there). If -g is not set, the path defaults to SYSCONFDIR/tor/onioncat.hosts where SYSCONFDIR typically is /etc or /usr/local/etc.
    On exit, OnionCat saves all collected hosts entries to DATADIR/onioncat/hosts.cached. This file is pulled in automatically at the next startup again. The entries are also save regularly every 5 minutes. This is only done if the internal hosts db was modified, i.e. if new entries where collected during the period of the last save to prevent unnecessary storage interaction. Please note that if you manually delete the file on the command line, it will not be recreated if no new entries where collected afterwards.

  • -H
    This option disables the hosts reverse lookup in the internal hosts db. Host lookups are required for Tor's hidden services V3 as well as for I2P. Thus, disabling the lookup function by using this options does only make sense when using Tor's hidden services V2.
    Reverse lookups are enabled by default since OnionCat4.

  • -h
    Display short usage message and shows options.

  • -i
    Convert onion_id to IPv6 address and exit.

  • -I
    Run OnionCat in GarliCat (I2P) mode.

  • -J
    Disable remote hostname validation. OnionCat is able to receive remote hostnames from keepalive messages and DNS queries. OnionCat validates if these names "make sense", i.e. it checks if the name is a valid onion name, and it checks if the name translates to the right IP.
    Hostname validation is enabled by default.
    This is a security feature. Rogue OnionCats could send special crafted keepalives or DNS answers which may trick OnionCat into connecting somewhere else instead outside of the Tor network or to a fake hidden service.

  • -l [ip:]port
    Bind OnionCat to specific _ip _ and/or port number for incoming connections. It defaults to This option could be set multiple times. IPv6 addresses must be given in square brackets.
    The parameter "none" deactivates the listener completely. This is for special purpose only and shall not be used in regular operation.

  • -L log_file
    Log output to log_file. If option is omitted, OnionCat logs to syslog if running in background or to stderr if running in foreground. If syslogging is desired while running in foreground, specify the special file name "syslog" as log file.

  • -o IPv6 address
    Convert IPv6 address to onion_id and exit program.

  • -p
    Use TAP device instead of TUN device. There are a few differences. See TAP DEVICE later.

  • -P [pid file]
    Create pid file at pid_file. If the option parameter is omitted OnionCat will create a pid file at /var/run/ In the latter case it must not be the last option in the list of options or the options list is terminated with a "--".

  • -r
    Run OnionCat as root and do not change user id (see option -u).

  • -R
    Use this option only if you really know what you do! OnionCat generates a random local onion_id. With this option it is not necessary to add a hidden service to the Tor configuration file torrc. One might use OnionCat services within Tor as usually but it is NOT possible to receive incoming connections. If you plan to also receive connections (e.g. because you provide a service or you use software which opens sockets for incoming connections like Bitorrent) you MUST configure a hidden service and supply its hostname to OnionCat on the command line. Please note that this option does only work if the remote OnionCat does NOT run in unidirectional mode which is default since SVN version 555 (see option -U). So usually you will not use this option.

  • -S
    OnionCat runs a lightweight DNS services to respond to DNS queries from other OnionCats (see also option -D). This option disables this DNS service. It responds only to reverse lookups within the Tor (FD87:D87E:EB43::/48) or I2P (FD60:DB4D:DDB5::/48) prefix.
    The name service is enable by default.

  • -s port
    Set OnionCat's virtual hidden service port to port. This should usually not be changed.

  • -t (IP|[IP:]port)
    Set Tor SOCKS IP and/or port. If no IP is specified will be used, if no port is specified, 9050 will be used as default. If compiled on Windows with Cygwin 9150 will be used because this is the default for the Tor browser bundle. In GarliCat mode it defaults to 9051. IPv6 addresses must be escaped by square brackets.
    The special parameter "none" disables OnionCat from making outbound connections. This shall be used only in special test scenarios.

  • -T tun_dev
    TUN device file to open for creation of TUN interface. It defaults to /dev/net/tun on Linux and /dev/tun0 on most other OSes, or /dev/tap0 if TAP mode is in use. Setup of a TUN device needs root permissions. OnionCat automatically changes its uid and gid after the TUN device is set up correctly.

  • -U
    Deactivate unidirectional mode. Before SVN version 555 OnionCat ran only in bidirectional mode. This is that a connection to another OC was used for outgoing and incoming packets. Since this could be a security risk under certain conditions, unidirectional mode was implemented in SVN r555 and set to default. With this option bidirectional mode can be enabled again. Please note that the unidirectional mode does not interoperate with option -R if the remote OC is working in unidirectional mode. If option -R is not used (which is the regular case), unidirectional und bidirectional OnionCats can be mixed. Please note that the only advantage of bidirectional mode is that it has a lower setup time since it needs only one Tor circuit. Unidirectional mode needs two circuits, one for each direction.

  • -u username
    username under which OnionCat should run. The uid is changed as soon as possible after the tun device setup. If -u is omitted, on OpenBSD and FreeBSD it tries to use the uid of the user "_tor" which is by default used for Tor. On all other systems it tries to get the uid for the user "tor". If it does not exists (it calls getpwnam(3)) it defaults to the uid 65534.


Usually OnionCat opens a TUN device which is a layer 3 interface. With option -p OnionCat opens a TAP device instead which is a virtual ethernet (layer 2) interface.


A typical ifup script for OnionCat for a modern Linux distribution using the `ip` command for configuring network related stuff could look like the following:

.in +3n #!/bin/sh

ip link set $OCAT_IFNAME up 

Onioncat and V3 Hidden Services

For a detailed explaination about the interaction between OnionCat4 and HSv3 have a look at the document doc/INTRO_TO_ONIONCAT4.txt found in the source folder or on GIthub at .

Originially Tor's v2 hidden service addresses had a binary length of 80 bits. This made it possible to let OnionCat map hidden service addresses to IPv6 addresses and vice versa. The development of OnionCat started in 2008, and this held for a very long time until recently Tor came up with version 3 of hidden services. To comply with ongoing development in the field of cryptography the new hidden service addresses of Tor (since version 0.3.2) are much bigger, meaning 336 bits. This obviously does not fit into an IPv6 address, hence, OnionCat is not able any more to translate back and forth between IPv6 and v3 onion addresses.

As a solution OnionCat offers the possibility to do an external hostname lookup within /etc/hosts instead. Please note that for security reasons, OnionCat does not use the system resolver, it definitely just reads the local hosts file. The big drawback for OnionCat is that with v3 hidden services OnionCat does not work out of the box any more. It requires that the destionations are configured manually beforehand.

To connect to a v3 hidden service, on the client side add a line to your /etc/hosts with the IPv6 address and the v3 hostname and run OnionCat with the additional option -H. The hosts entry could look like this (in one line!):

fd87:d87e:eb43:45g6:3bbb:9fxf:5877:4319 tulqpcvf7Oeuxzjod6odrpO77ryujc7o0g7kw6c76q9cbnbi7rqskxid.onion

If this client also has a v3 hidden service, you have to enter its IPv6/hostname pair to the hosts file on the opposite site as well, except you use -U option.

Please note that you could pick any IPv6 address in this case, although I suggest to truncate the long hostname just to the last 16 characters for use with OnionCat, e.g. truncate "tulqpcvf7Oeuxzjod6odrpO77ryujc7o0g7kw6c76q9cbnbi7rqskxid.onion" to "6q9cbnbi7rqskxid.onion" and use it as parameter for OnionCat.


In the versions of OnionCat up to 0.3.7 a symlink named gcat was created. OnionCat internally handled this as GarliCat which is equal to running ocat with the option -I. The symlink was removed due to a name conflict with a different binary (see BSD coreutils).
The default settings changed since OnionCat4 (versions >= 0.4.0). Actually hosts-lookup is now on by default and the meaning of option -H was inverted. This is because OnionCat4 is specifically configured to better match the necessities for Tor's hidden services V3.




Concepts, software, and man page written by Bernhard R. Fischer <>.


Credits go to Ferdinand Haselbacher, Daniel Haslinger, Wim Gaethofs, Marshalbanana, all package maintainers of several Linux and BSD distros, and many others who have contributed and reported bugs.

See Also

Onioncat source code

Further docs and howtos are found at

OnionCat source packages are also found at

Tor project homepage

I2P project homepage


Copyright 2008-2023 Bernhard R. Fischer.

This file is part of OnionCat.

OnionCat is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, version 3 of the License.

OnionCat is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with OnionCat. If not, see <>.