Phantun is a project that obfuscated UDP packets into TCP connections. It aims to achieve maximum performance with minimum processing and encapsulation overhead.

It is commonly used in environments where UDP is blocked/throttled but TCP is allowed through.

Phantun simply converts a stream of UDP packets into obfuscated TCP stream packets. The TCP stack used by Phantun is designed to pass through most L3/L4 stateful/stateless firewalls/NAT devices. It will not be able to pass through L7 proxies. However, the advantage of this approach is that none of the common UDP over TCP performance killer such as retransmissions and flow control will occur. The underlying UDP properties such as out-of-order delivery are fully preserved even if the connection ends up looking like a TCP connection from the perspective of firewalls/NAT devices.

Phantun means Phantom TUN, as it is an obfuscator for UDP traffic that does just enough work to make it pass through stateful firewall/NATs as TCP packets.

Usage

For the example below, it is assumed that Phantun Server listens for incoming Phantun Client connections at port 4567 (the --local option for server), and it forwards UDP packets to UDP server at 127.0.0.1:1234 (the --remote option for server).

It is also assumed that Phantun Client listens for incoming UDP packets at 127.0.0.1:1234 (the --local option for client) and connects to Phantun Server at 10.0.0.1:4567 (the --remote option for client).

Phantun creates TUN interface for both the Client and Server. For Client, Phantun assigns itself the IP address 192.168.200.2 by default and for Server, it assigns 192.168.201.2 by default. Therefore, your Kernel must have net.ipv4.ip_forward enabled and setup appropriate iptables rules for NAT between your physical NIC address and Phantun’s TUN interface address.

You may customize the name of Tun interface created by Phantun and the assigned addresses. Please run the executable with -h options to see how to change them.

Another way to help understand this network topology (please see the diagram above for an illustration of this topology):

Phantun Client is like a machine with private IP address (192.168.200.2) behind a router. In order for it to reach the Internet, you will need to SNAT the private IP address before it’s traffic leaves the NIC.

Phantun Server is like a server with private IP address (192.168.201.2) behind a router. In order to access it from the Internet, you need to DNAT it’s listening port on the router and change the destination IP address to where the server is listening for incoming connections.

In those cases, the machine/iptables running Phantun acts as the “router” that allows Phantun to communicate with outside using it’s private IP addresses.

As of Phantun v0.2.2, IPv6 support for UDP endpoints has been added, however Fake TCP IPv6 support has not been finished yet. To specify an IPv6 address, use the following format: [::1]:1234 with the command line options.

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1. Enable Kernel IP forwarding

Edit /etc/sysctl.conf, add net.ipv4.ip_forward=1 and run sudo sysctl -p /etc/sysctl.conf.

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2. Add required firewall rules

Client

Client simply need SNAT enabled on the physical interface to translate Phantun’s address into one that can be used on the physical network. This can be done simply with masquerade.

Note: change eth0 to whatever actual physical interface name is

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Using nftables

table inet nat {
    chain postrouting {
        type nat hook postrouting priority srcnat; policy accept;
        iifname tun0 oif eth0 masquerade
    }
}

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Using iptables

iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE

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Server

Server needs to DNAT the TCP listening port to Phantun’s TUN interface address.

Note: change eth0 to whatever actual physical interface name is and 4567 to actual TCP port number used by Phantun server

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Using nftables

table ip nat {
    chain prerouting {
        type nat hook prerouting priority dstnat; policy accept;
        iif eth0 tcp dport 4567 dnat to 192.168.201.2
    }
}

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Using iptables

iptables -t nat -A PREROUTING -p tcp -i eth0 --dport 4567 -j DNAT --to-destination 192.168.201.2

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3. Run Phantun binaries as non-root (Optional)

It is ill-advised to run network facing applications as root user. Phantun can be run fully as non-root user with the cap_net_admin capability.

sudo setcap cap_net_admin=+pe phantun_server
sudo setcap cap_net_admin=+pe phantun_client

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4. Start Phantun daemon

Note: Run Phantun executable with -h option to see full detailed options.

Server

Note: 4567 is the TCP port Phantun should listen on and must corresponds to the DNAT rule specified above. 127.0.0.1:1234 is the UDP Server to connect to for new connections.

RUST_LOG=info /usr/local/bin/phantun_server --local 4567 --remote 127.0.0.1:1234

Or use host name with --remote:

RUST_LOG=info /usr/local/bin/phantun_server --local 4567 --remote example.com:1234

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Client

Note: 127.0.0.1:1234 is the UDP address and port Phantun should listen on. 10.0.0.1:4567 is the Phantun Server to connect.

RUST_LOG=info /usr/local/bin/phantun_client --local 127.0.0.1:1234 --remote 10.0.0.1:4567

Or use host name with --remote:

RUST_LOG=info /usr/local/bin/phantun_client --local 127.0.0.1:1234 --remote example.com:4567

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MTU overhead

Phantun aims to keep tunneling overhead to the minimum. The overhead compared to a plain UDP packet is the following:

Standard UDP packet: 20 byte IP header + 8 byte UDP header = 28 bytes

Phantun obfuscated UDP packet: 20 byte IP header + 20 byte TCP header = 40 bytes

Note that Phantun does not add any additional header other than IP and TCP headers in order to pass through stateful packet inspection!

Phantun’s additional overhead: 12 bytes. I other words, when using Phantun, the usable payload for UDP packet is reduced by 12 bytes. This is the minimum overhead possible when doing such kind of obfuscation.

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MTU calculation for WireGuard

For people who use Phantun to tunnel WireGuard® UDP packets, here are some guidelines on figuring out the correct MTU to use for your WireGuard interface.

WireGuard MTU = Interface MTU – IP header (20 bytes) – TCP header (20 bytes) – WireGuard overhead (32 bytes)

For example, for a Ethernet interface with 1500 bytes MTU, the WireGuard interface MTU should be set as:

1500 – 20 – 20 – 32 = 1428 bytes

The resulted Phantun TCP data packet will be 1500 bytes which does not exceed the interface MTU of 1500.

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Version compatibility

While the TCP stack is fairly stable, the general expectation is that you should run same minor versions of Server/Client of Phantun on both ends to ensure maximum compatibility.

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Performance

Performance was tested on AWS t3.xlarge instance with 4 vCPUs and 5 Gb/s NIC. WireGuard was used for tunneling TCP/UDP traffic between two test instances and MTU has been tuned to avoid fragmentation.

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Future plans

• IPv6 support for fake-tcp
• Load balancing a single UDP stream into multiple TCP streams
• Integration tests
• Auto insertion/removal of required firewall rules

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Compariation to udp2raw

udp2raw is another popular project by @wangyu- that is very similar to what Phantun can do. In fact I took inspirations of Phantun from udp2raw. The biggest reason for developing Phantun is because of lack of performance when running udp2raw (especially on multi-core systems such as Raspberry Pi). However, the goal is never to be as feature complete as udp2raw and only support the most common use cases. Most notably, UDP over ICMP and UDP over UDP mode are not supported and there is no anti-replay nor encryption support. The benefit of this is much better performance overall and less MTU overhead because lack of additional headers inside the TCP payload.

Here is a quick overview of comparison between those two to help you choose:

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License

Copyright 2021-2022 Datong Sun ([email protected])

Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or https://www.apache.org/licenses/LICENSE-2.0> or the MIT license <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your option. Files in the project may not be copied, modified, or distributed except according to those terms.

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