Didier Stevens

Tuesday 27 December 2022

Combining dns-pydivert And dnsresolver

Filed under: Malware,My Software,Networking — Didier Stevens @ 0:00

I use my tools dns-pydivert and dnsresolver.py for dynamic analysis of software (malware and benign software).

On the virtual machine where I’m doing dynamic analysis, I disable IPv6 support.

I install dnslib and run dnsresolver.py with a command like this, for example:

dnsresolver.py "type=resolve,label=example.com,answer=. 1 IN A 127.0.0.1" "type=forwarder,server=8.8.8.8"

The first command is a resolve command: DNS A queries for example.com will be resolved to IPv4 address 127.0.0.1 with TTL 1 minute.

The second command is a forwarder command: all DNS requests not handled by other commands, are forwarded to 8.8.8.8. Make sure that the IPv4 address of the DNS server you forward requests to, is different from the VM’s default DNS server, otherwise this forwarding will be redirected by dns-pydivert too.

I don’t use this second resolver command if the VM is isolated from the Internet, I only use it when I want to allow some interaction with the Internet.

Then I install pydivert and run dns-pydivert.py as administrator.

You can’t run dns-pydivert.py properly without administrative permissions:

When dns-pydivert.py and dnsresolver.py are running, DNS traffic is altered according to our settings.

For example (picture above), when I issue a “ping google.com” command inside the VM, dns-pydivert sees this first DNS packet and configures itself with the addresses in this packet: 192.168.32.129 is the IPv4 address of the Windows VM and 192.168.32.2 is the IPv4 address of this Windows VM’s DNS server.

It alters this first request to be redirected to the VM itself (192.168.32.2 -> 192.168.32.129).

Then dnsresolver receives this packet, and forwards it to DNS server 8.8.8.8. It receives a reply from DNS server 8.8.8.8, and forwards it to the Windows VM (192.168.32.129).

Then dns-pydivert sees this reply, and changes its source from 192.168.32.129 to 192.168.32.2, so that it appears to come from the Windows VM’s default DNS server.

When I do the same (picture above) for example.com (ping example.com), the query is redirected to dnsresolver, which resolves this to 127.0.0.1 with a TTL of 1 minute (per resolve commands configuration).

Thus the ping command pings the localhost, instead of example.com’s web server.

And when I kill dns-pydivert (picture above) and issue a “ping example.com” again after waiting for 1 minute, the query is no longer redirected and example.com’s web server is pinged this time.

I used ping here to illustrate the process, but often it’s HTTP(S) traffic that I want to redirect, and then I also use my simple-listener.py tool to emulate simple web servers.

Remark that this will only redirect DNS traffic (per the configuration). This does not redirect traffic “directed” at IPv4 addresses (as opposed to hostnames).

This can be done too with pydivert, and I will probably release a tool for that too.

Leave a Comment »

No comments yet.

RSS feed for comments on this post. TrackBack URI

Leave a Reply (comments are moderated)

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.

Blog at WordPress.com.