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With Tor itself, it is only possible to do TCP due to SOCKS5 limitations. OnionCat allows you to do more than TCP through Tor; how does this work?

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    title asks one question, then detail partially answers question and asks a second one
    – pmocek
    Sep 30, 2013 at 2:21

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OnionCat is a Tor-based decentralized peer-to-peer VPN. It acts like a single IPv6 subnet hidden inside Tor permitting a hidden service to be transformed into an IPv6 address on the subnet. In BSD land this shows up as a tun(4) device. One can do udp (dns, nfs, etc) or tcp or even IPSec between OnionCat instances (and only between OnionCat instances; not out to the clearnet).

As the OpenBSD ports tree states:

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

More info can be found here. OnionCat is not officially associated with the Tor Project.

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I try an alternative simplistic explanation.

OnionCat :

  • connects from hidden service to hidden service.
  • provides an IPv6 for you
  • provides an IPv6 for your partner

Now you can use IPv6, TCP and UDP with your partner.

What is OnionCat NOT:

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  • Tor does support IPv6 for most things, more are coming.
    – todd
    Sep 28, 2013 at 4:53
  • Could you run SIP through the setup explained above? ie lets say you and your partner setup a hidden service each, and created an OnionCat tunnel between you, could you now run SIP server such as FreeSWITCH or Asterisk and a SIP client such as Jitsi or Linphone between them for voice and video? That would mean running SIP/SRTP/ZRTP through the tunnel.
    – Zoe Dutot
    Jun 1, 2017 at 5:48
  • Yes, you can run any service through it, as long as it supports IPv6 (which is almost any service as of today).
    – Bernhard
    Jun 29, 2021 at 6:29

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