Userspace DNS tunnel with support for DoH and DoT https://www.bamsoftware.com/software/dnstt/ David Fifield [email protected] Public domain

dnstt is a DNS tunnel with these features:

• Works over DNS over HTTPS (DoH) and DNS over TLS (DoT) as well as plaintext UDP DNS.
• Embeds a sequencing and session protocol (KCP/smux), which means that the client does not have to wait for a response before sending more data, and any lost packets are automatically retransmitted.
• Encrypts the contents of the tunnel and authenticates the server by public key.

dnstt is an application-layer tunnel that runs in userspace. It doesn't provide a TUN/TAP interface; it only hooks up a local TCP port with a remote TCP port (like netcat or ssh -L) by way of a DNS resolver. It does not itself provide a SOCKS or HTTP proxy interface, but you can get the same effect by running a proxy on the tunnel server and having the tunnel terminate at the proxy.

.------.  |            .---------.             .------.
|tunnel|  |            | public  |             |tunnel|
|client|<---DoH/DoT--->|recursive|<--UDP DNS-->|server|
'------'  |c           |resolver |             '------'
   |      |e           '---------'                |
.------.  |n                                   .------.
|local |  |s                                   |remote|
| app  |  |o                                   | app  |
'------'  |r                                   '------'

DNS zone setup

Because the server side of the tunnel acts like an authoritative name server, you need to own a domain name and set up a subdomain for the tunnel. Let's say your domain name is example.com and your server's IP addresses are 203.0.113.2 and 2001:db8::2. Go to your name registrar and add three new records:

A	tns.example.com	points to 203.0.113.2
AAAA	tns.example.com	points to 2001:db8::2
NS	t.example.com	is managed by tns.example.com

The labels tns and t can be anything you want, but the tns label should not be a subdomain of the t label (that space is reserved for the contents of the tunnel), and the t label should be short (because there is limited space available in a DNS message, and the domain name takes up part of that space).

Now, when a recursive DNS resolver receives a query for a name like aaaa.t.example.com, it will forward the query to the tunnel server at 203.0.113.2 or 2001:db8::2.

Tunnel server setup

Compile the server:

tunnel-server$ cd dnstt-server
tunnel-server$ go build

First you need to generate the server keypair that will be used to authenticate the server and encrypt the tunnel.

tunnel-server$ ./dnstt-server -gen-key -privkey-file server.key -pubkey-file server.pub
privkey written to server.key
pubkey  written to server.pub

Run the server. You need to provide an address that will listen for UDP DNS packets (:5300), the private key file (server.key), the root of the DNS zone (t.example.com), and a TCP address to which incoming tunnel streams will be forwarded (127.0.0.1:8000).

tunnel-server$ ./dnstt-server -udp :5300 -privkey-file server.key t.example.com 127.0.0.1:8000

The tunnel server needs to be able to receive packets on an external port 53. You can have it listen on port 53 directly using -udp :53, but that requires the program to run as root. It is better to run the program as an ordinary user and have it listen on an unprivileged port (:5300 above), and port-forward port 53 to it. On Linux, use these commands to forward external port 53 to localhost port 5300:

tunnel-server$ sudo iptables -I INPUT -p udp --dport 5300 -j ACCEPT
tunnel-server$ sudo iptables -t nat -I PREROUTING -i eth0 -p udp --dport 53 -j REDIRECT --to-ports 5300
tunnel-server$ sudo ip6tables -I INPUT -p udp --dport 5300 -j ACCEPT
tunnel-server$ sudo ip6tables -t nat -I PREROUTING -i eth0 -p udp --dport 53 -j REDIRECT --to-ports 5300

You need to also run something for the tunnel server to connect to. It can be a proxy server or anything else. For testing, you can use an Ncat listener:

tunnel-server$ ncat -l -k -v 127.0.0.1 8000

Tunnel client setup

Compile the client:

tunnel-client$ cd dnstt-client
tunnel-client$ go build

Copy the server.pub file from the server to the client. You don't need server.key on the client; leave it on the server.

Choose a public DoH or DoT resolver. There is a list of DoH resolvers here:

• https://github.com/curl/curl/wiki/DNS-over-HTTPS#publicly-available-servers

And DoT resolvers here:

• https://dnsprivacy.org/wiki/display/DP/DNS+Privacy+Public+Resolvers#DNSPrivacyPublicResolvers-DNS-over-TLS%28DoT%29
• https://dnsencryption.info/imc19-doe.html

To run the tunnel client using DoH, you need to provide the URL of the DoH resolver (https://doh.example/dns-query), the server's public key files (server.pub), the root of the DNS zone (t.example.com), and the local TCP port that will receive connections and forward them through the tunnel (127.0.0.1:7000):

tunnel-client$ ./dnstt-client -doh https://doh.example/dns-query -pubkey-file server.pub t.example.com 127.0.0.1:7000

For DoT, it's the same, but use the -dot option instead:

tunnel-client$ ./dnstt-client -dot dot.example:853 -pubkey-file server.pub t.example.com 127.0.0.1:7000

Once the tunnel client is running, you can connect to the local end of the tunnel, type something, and see it appear at the remote end.

tunnel-client$ ncat -v 127.0.0.1 7000

The client also has a plaintext UDP mode that can work through a recursive resolver or directly to the tunnel server (-udp tns.example.com), but it does not provide any covertness for the tunnel and should only be used for testing.

How to make a proxy

dnstt is only a tunnel; it's up to you what you want to connect to it. You can make the tunnel work like an ordinary SOCKS or HTTP proxy by having the tunnel server forward to a standard proxy server. There are many ways to set it up; here are some examples.

Ncat HTTP proxy

Ncat has a simple built-in HTTP/HTTPS proxy, good for testing. Be aware that Ncat's proxy isn't intended for use by untrusted clients; it won't prevent them from connecting to localhost ports on the tunnel server, for example.

tunnel-server$ ncat -l -k --proxy-type http 127.0.0.1 8000
tunnel-server$ ./dnstt-server -udp :5300 -privkey-file server.key t.example.com 127.0.0.1:8000

On the client, have the tunnel client listen on 127.0.0.1:7000, and configure your applications to use 127.0.0.1:7000 as an HTTP proxy.

tunnel-client$ ./dnstt-client -doh https://doh.example/dns-query -pubkey-file server.pub t.example.com 127.0.0.1:7000
tunnel-client$ curl --proxy http://127.0.0.1:7000/ https://wtfismyip.com/text

SSH SOCKS proxy

OpenSSH has a built-in SOCKS proxy, which makes it easy to add a SOCKS proxy to a server that already has sshd installed.

On the server, make a localhost SSH connection, using the -D option to open a SOCKS listener at port 8000. Then configure the tunnel server to forward incoming connections to port 8000. Have the tunnel client listen on its own local port 7000.

tunnel-server$ ssh -N -D 127.0.0.1:8000 -o NoHostAuthenticationForLocalhost=yes 127.0.0.1
# Enter the password of the local user on tunnel-server
tunnel-server$ ./dnstt-server -udp :5300 -privkey-file server.key t.example.com 127.0.0.1:8000

tunnel-client$ ./dnstt-client -doh https://doh.example/dns-query -pubkey-file server.pub t.example.com 127.0.0.1:7000
tunnel-client$ curl --proxy socks5h://127.0.0.1:7000/ https://wtfismyip.com/text

The above configuration, by locating the SOCKS client port on the server, makes a SOCKS proxy that can be used by anyone with access to the DNS tunnel. Alternatively, you can make an SSH SOCKS proxy for your own private use, with the SSH connection going through the tunnel and the SOCKS client port being located at the client.

Let's assume you have the SSH details configured so that you can run ssh tunnel-server on the tunnel client. Make sure AllowTcpForwarding is set to yes (the default value) in sshd_config. Run the tunnel server and have it forward directly to the SSH port.

tunnel-server$ ./dnstt-server -udp :5300 -privkey-file server.key t.example.com 127.0.0.1:22

Run the tunnel client with the local listening port at 127.0.0.1:7000. The HostKeyAlias ssh option lets you connect to the SSH server as if it were located at 127.0.0.1:8000. Replace tunnel-server with the hostname or IP address of the SSH server.

tunnel-client$ ./dnstt-client -doh https://doh.example/dns-query -pubkey-file server.pub t.example.com 127.0.0.1:8000
tunnel-client$ ssh -N -D 127.0.0.1:7000 -o HostKeyAlias=tunnel-server -p 8000 127.0.0.1
tunnel-client$ curl --proxy socks5h://127.0.0.1:7000/ https://wtfismyip.com/text

Tor bridge

You can run a Tor bridge on the tunnel server and tunnel the connection to the bridge with dnstt, using dnstt as like a pluggable transport. The Tor client provides a SOCKS interface that other programs can use. Let's say your Tor bridge's ORPort is 9001.

tunnel-server$ ./dnstt-server -udp :5300 -privkey-file server.key t.example.com 127.0.0.1:9001

tunnel-client$ ./dnstt-client -doh https://doh.example/dns-query -pubkey-file server.pub t.example.com 127.0.0.1:7000

Add a Bridge line to /etc/tor/torrc, or paste it into Tor Browser. You can get FINGERPRINT from /var/lib/tor/fingerprint on the bridge.

Bridge 127.0.0.1:7000 FINGERPRINT

If you use a system tor, the client SOCKS port will be 127.0.0.1:9050. If you use Tor Browser, it will be 127.0.0.1:9150.

Covertness

Support for DoH and DoT is only to make it more difficult for a local observer to see that a DNS tunnel is being used, not for the overall security of the connection. There is a separate encryption layer inside the tunnel that protects the contents of the tunnel from the resolver itself.

The encryption of DoH or DoT prevents a network observer between the tunnel client and the resolver from seeing the remote destination of the tunnel. An observer can see that the tunnel client is connecting to a resolver, but cannot see where the resolver is forwarding its queries. An observer can probably infer, based on volume and other traffic characteristics, that a tunnel is being used, though it cannot tell where the remote end of the tunnel is, nor what the contents of the tunnel are. If the tunnel client is not using DoH or DoT but instead UDP (-udp option), then even an observer between the tunnel client and the resolver can see that a tunnel is being used and where the remote end of the tunnel is.

An observer between the resolver and the tunnel server (this includes the resolver itself) can easily tell that a tunnel is being used and where the remote end of the tunnel is, because there is no DoH or DoT encryption at that point. This kind of observer still cannot read the contents of the tunnel, because there is an additional layer of end-to-end encryption between the tunnel client and the tunnel server.

An observer who watches what leaves the tunnel server will be able to see anything that the tunnel server forwards to some other host (if the tunnel server is acting as a proxy, for example), unless that data has been separately encrypted before being sent through the tunnel.

The dnstt client does not do anything special to disguise its TLS fingerprint. It uses the crypto/tls package from Go, and its TLS fingerprint will depend on what version of Go it was compiled with. You should assume that the DNS tunnel client is identifiable by TLS fingerprint. A path to hiding the TLS fingerprint would be to integrate uTLS (https://github.com/refraction-networking/utls).

Encryption and authentication

The tunnel uses a Noise protocol (https://noiseprotocol.org/noise.html) for end-to-end security between the tunnel client and tunnel server. This protocol is independent of the DoH or DoT encryption between the tunnel client and resolver. The specific protocol is Noise_NK_25519_ChaChaPoly_BLAKE2s (https://noiseprotocol.org/noise.html#protocol-names-and-modifiers). The NK handshake pattern authenticates the server but not the client.

The Noise layer is sandwiched between two other protocol layers: KCP (https://github.com/xtaci/kcp-go) which creates a reliable stream on top of unreliable datagrams, and smux (https://github.com/xtaci/smux) which provides stream multiplexing and session features. An observer who can see DNS messages, such as the intermediary resolver, will be able to see the headers of the KCP layer, but not of the smux layer nor of the streams that are inside. The model is similar to what you would get with TLS or SSH over TCP: an observer can see TCP-level ACKs and sequence numbers, but cannot read the stream data.

application data
smux
Noise
KCP
DNS messages
DoH / DoT / UDP DNS

When you run dnstt-server -gen-key, you can save the private and public keys to a file using the -privkey-file and -pubkey-file options. You can then load the keys later using -privkey-file on the server and -pubkey-file on the client. Alternatively, you can deal with the keys as literal hexadecimal strings rather than files. If you run dnstt-server -gen-key without the -privkey-file and -pubkey-file options, it will display the keys rather than save them to files. You can then use the keys with -privkey on the server and -pubkey on the client.

$ ./dnstt-server -gen-key
privkey 0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef
pubkey  0000111122223333444455556666777788889999aaaabbbbccccddddeeeeffff
$ ./dnstt-server -udp :5300 -privkey 0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef t.example.com 127.0.0.1:8000
$ ./dnstt-client -dot dot.example:853 -pubkey 0000111122223333444455556666777788889999aaaabbbbccccddddeeeeffff t.example.com 127.0.0.1:7000

If you run the server without -privkey-file or -privkey, it will generate a temporary keypair and print the public key in the log. But the key will be different the next time you restart the server, and you will have to reconfigure clients.

Payload sizes

In the client, the available space for user data per query depends on the length of the domain name in use. Shorter domain names leave more space for user data.

In the server, the available space for user data per response depends on the maximum UDP payload size. The larger the UDP payload size, the more space there is for user data. You want to use as large a UDP payload size as possible, but not larger than what is supported by the resolver you are using. Values above 1452 may cause IP fragmentation which can reduce performance. You can control the maximum UDP payload size with the -mtu option on the server. The default is 1232 bytes; this ought to be supported by most resolvers that understand EDNS(0) (RFC 6891). For maximum compatibility, set the maximum to 512, but know that doing so will reduce downstream bandwidth.

$ ./dnstt-server -mtu 512 -doh https://doh.example/dns-query -pubkey-file server.pub t.example.com 127.0.0.1:7000

The client and server emit an "effective MTU" log line when starting up that shows how much space is available for user data in each query or response. For the server, there may be more space available in some responses and less in others (depending on the size of the corresponding query); the logged value is the minimum that is guaranteed to be supported in any response.