a tool to enumerate the resource records of a DNS zone using its DNSSEC NSEC or NSEC3 chain
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n3map limit the number of zone type detection attempts Jun 15, 2017
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README.md

nsec3map - DNSSEC Zone Enumerator

nsec3map is a tool that can enumerate DNS zone entries based on DNSSEC NSEC or NSEC3 record chains. It can be used to discover hosts in a DNS zone quickly and with a minimum amount of queries if said zone is DNSSEC-enabled.

nsec3map was written primarily to show that NSEC3 does not offer meaningful protection against zone enumeration. Although originally only intended as a PoC and written in Python, it is actually quite fast and able to enumerate even large zones (with a million or more entries) in a short time given adequate hardware.

It also includes a simple John the Ripper plugin that can be used to crack the obtained NSEC3 hashes.

Usage Examples

Some typical usage examples are shown below. For a more detailed documentation, refer to the man pages or the output of n3map --help.

NSEC Zone Walking

The most basic example is to enumerate a particular zone (e.g. example.com) and store the retrieved NSEC/NSEC3 records in a file example.com.zone:

$ n3map -v -o example.com.zone example.com
looking up nameservers for zone example.com.
using nameserver: 199.43.133.53:53 (b.iana-servers.net.)
using nameserver: 199.43.132.53:53 (a.iana-servers.net.)
checking SOA...
detecting zone type...
zone uses NSEC records
starting enumeration in mixed query mode...
discovered owner: example.com.	A NS SOA TXT AAAA RRSIG NSEC DNSKEY
discovered owner: www.example.com.	A TXT AAAA RRSIG NSEC
;; walking example.com.: records =   2; queries =   4; ............. q/s = 11 ;;
finished mapping of example.com. in 0:00:00.196471

The -v switch is only used for more verbosity and not generally needed. With no further arguments, nsec3map detects automatically whether the zone uses NSEC or NSEC3 and uses the corresponding enumeration method. It also looks up the zone's nameservers by itself.

Some nameservers do not accept NSEC queries. In such a case, --query-mode A (short -A) can be used instead. For example, to enumerate the root zone, one could run the command:

n3map -v --query-mode A --output root.zone  .

NSEC3 Zone Enumeration

The following example shows the enumeration of a NSEC3 chain at example.com using a nameserver at 192.168.1.37. It also shows the NSEC3 zone size prediction and progress indicator (enabled using the -p switch).

$ n3map -3po example.com.zone 192.168.1.37 example.com
;; mapping example.com.: 79% [===========================================================================                   ] ;;
;; records = 797; queries = 802; hashes = 3840; predicted zone size = 1003; ............... q/s = 513; coverage =  95.677595% ;;

received SIGINT, terminating

Note that the enumeration will proceed slower towards the end as it becomes harder to find domain names that are not covered by any retrieved NSEC3 records. Therefore, finishing the enumeration of a large zone can take quite some time and computing resources. It is advisable to manually cancel the enumeration once the query rate drops under a certain limit.

You should also make use of the --limit-rate option to reduce stress on the nameservers. If you think the enumeration is too slow because of a high round-trip time to the nameservers, you can also use a more aggressive mode which sends multiple queries simultaneously (--aggressive option). The following example shows how to use these options:

n3map -3pvo example.com.zone --aggressive 16 --limit-rate 100/s example.com

This will cause nsec3map to send a maximum of 16 queries in parallel while at the same time keeping the query rate at or below roughly 100 queries per second.

It is also possible to continue the enumeration from a partially obtained NSEC3 (or NSEC) chain, as long as the zone's NSEC3 parameters (salt, iteration count) have not been changed:

n3map -3pv --input example.com.partial --output example.com.zone --ignore-overlapping example.com

This will first read the NSEC3 records from example.com.partial and then continue the enumeration, saving the NSEC3 chain to example.com.zone. The --ignore-overlapping option should be used for large zones, or if it is otherwise likely that changes are made to the zone during the enumeration. If specified, nsec3map will not abort the enumeration when it receives an NSEC3 record which overlaps with another record that was received earlier. Note however that you will not get a completely consistent view of the NSEC3 chain if you use this option.

Cracking NSEC3 Hashes

Once you obtained some NSEC3 records from a particular zone, you can (try to) crack them using John the Ripper and the supplied NSEC3 patch (see John the Ripper Plugin below on how to install it).

First, the NSEC3 records need to be converted to a different format used by the JtR patch:

n3map-johnify example.com.zone example.com.john

The records can then be cracked simply by running john on the resulting file:

john example.com.john

Refer to the JtR documentation for more information on how to make use of john's different cracking modes, wordlist rules and so on. It is probably a good idea to adapt the wordlist and mangling rules to the kind of zone you are trying to map.

You can also try to crack NSEC3 records using hashcat, using hashes converted to a slightly different format:

n3map-hashcatify example.com.zone example.com.hashcat

The records can then be cracked simply by running hashcat on the resulting file:

hashcat -m 8300 example.com.hashcat

Installation

Dependencies:

  • Python 2.6.x or 2.7.x
  • dnspython >= 1.8
  • openssl >= 0.9.8
  • Optionally numpy and scipy for zone size prediction (recommended)

On a Debian system, just run

sudo apt-get install python python-dev python-dnspython libssl libssl-dev python-numpy python-scipy

Installation:

After unpacking the tarball, run:

sudo python setup.py install

This will compile the extension modules(s) and install the scripts, python modules as well as the man pages.

Alternatively, you can also run nsec3map directly from the source directory without installing it system-wide. If you want to use OpenSSL accelerated hashing however, you still need to build the extension module:

python setup.py build_ext

This should compile a shared object nsec3hash.so in the build/ directory. You can then copy this file to the n3map/ directory.

John the Ripper Plugin

Update: The latest version of JtR-Jumbo includes the NSEC3 cracking patch from this project. There is no need to install it separately, just follow the build instructions for JtR-Jumbo. Using the latest source version is recommended.

Alternatively, you can also use hashcat.

Limitations

  • Many DNS errors are not handled correctly
  • No automatic parallelization of NSEC walking (though it is possible to do this manually by partitioning the namespace)
  • High memory usage (mostly as a result of using CPython)
  • ...

(remember that nsec3map is still mostly a PoC tool...)