dnstweak

Quick local DNS spoofing tool

dnstweak is a program that allows you to run something like:

$ sudo ./dnstweak foo.example.com=10.0.0.1,10.0.0.2

So that foo.example.com appears to resolve to 10.0.0.1 and 10.0.0.2.

It runs a DNS server that proxies all other lookups to the previous resolver from /etc/resolv.conf, and it inserts itself as the new resolver in /etc/resolv.conf.

Lookups for foo.example.com will get the 2 given A records (10.0.0.1 and 10.0.0.2) in a random order. It also logs requests and responses to stdout. Once you stop it with Ctrl-C it will put the previous /etc/resolv.conf contents back in place.

See the Releases page for Linux binaries, or build it yourself with go build.

Features

dnstweak can:

• forge responses to select DNS requests configured on the command line
• automatically take over all system DNS queries by inserting itself into /etc/resolv.conf
• lookup client addresses in procfs to find out which process a request came from

Why?

Sometimes you want to test how a piece of software responds to different DNS records without actually changing the real DNS records.

Example session

Open 2 terminal windows. We'll run dnstweak in the first and ping in the second.

First terminal:

$ sudo ./dnstweak example.com=127.0.0.1
2023/07/11 20:41:35 dnstweak starts
2023/07/11 20:41:35 listening on 127.0.0.1:53
2023/07/11 20:41:35 using 127.0.0.53:53 as upstream resolver

dnstweak stays running. It has inserted itself as the local resolver by modifying /etc/resolv.conf.

If you run dnstweak as a non-root user, it will still work as a DNS server, but it won't be able to listen on port 53 or insert itself into /etc/resolv.conf.

In the other terminal, start a ping to example.com:

$ ping example.com

Now we get some log output from dnstweak in our first terminal:

2023/07/11 20:41:37 127.0.0.1:48439 (ping/9975): A example.com: 127.0.0.1 (overridden)

A request came from 127.0.0.1:48439, which is a process called ping with PID 9975. It was an A lookup for example.com, and we returned 127.0.0.1. Back in the other terminal:

PING example.com (127.0.0.1) 56(84) bytes of data.
64 bytes from localhost (127.0.0.1): icmp_seq=1 ttl=64 time=0.020 ms
64 bytes from localhost (127.0.0.1): icmp_seq=2 ttl=64 time=0.037 ms

Great success.

Installation

Either get a binary from the Releases page, and:

$ sudo cp dnstweak.x86_64 /usr/bin/dnstweak
$ sudo chmod +x /usr/bin/dnstweak

Or build it yourself (see below).

Usage

dnstweak v0.2

usage: dnstweak [options] SPEC...

options:
  -listen string
        listen address (IP:PORT or just PORT) (default: see below)
  -no-proc
        disable discovering the client process by looking in /proc
  -no-resolvconf
        disable automatic update of /etc/resolv.conf
  -upstream string
        upstream DNS server (IP:PORT or just IP) (default: see below)

In the absence of -listen, dnstweak will first try to listen on any loopback IP
address (127.0.0.0/24) on port 53, and failing that use a random port number on
127.0.0.1.

In the absence of -upstream, dnstweak will take the first nameserver configured
in /etc/resolv.conf.

Each SPEC is a hostname, followed by an "=" sign, followed by a
comma-separated list of 1 or more IP addresses (for example
"example.com=127.0.0.1").

dnstweak is a program by James Stanley. You can email me at
james@incoherency.co.uk or read my blog at https://incoherency.co.uk/

Build

dnstweak is written in go. You can build it with:

$ go build

And then run:

$ ./dnstweak -help

Run the tests with:

$ go test

Future

In the future, maybe dnstweak will gain options to:

• listen on IPv6
• populate the DNS override map from /etc/hosts
• override responses to AAAA, CNAME, PTR, SRV requests
• make fake NXDOMAIN responses
• drop requests
• take a zonefile in a BIND-ish format instead of the made-up command-line format

What happens to resolv.conf if dnstweak crashes?

It will look something like this:

# created by dnstweak
nameserver 127.0.0.1
search lan
#dnstweak## This file is managed by man:systemd-resolved(8). Do not edit.
#dnstweak##
    [...]
#dnstweak#
#dnstweak#nameserver 127.0.0.53
#dnstweak#options edns0 trust-ad
#dnstweak#search lan

The lines prefixed with #dnstweak# are the lines that were there before dnstweak took over. Delete the uncommented lines at the top, and remove the "#dnstweak#" prefix from the rest, and you'll have your old resolv.conf back.

Other tools like this

https://github.com/iphelix/dnschef

https://github.com/marekjelen/dnshack

Contact

James Stanley

https://incoherency.co.uk

james@incoherency.co.uk