MassDNS

A high-performance DNS stub resolver

MassDNS is a simple high-performance DNS stub resolver targeting those who seek to resolve a massive amount of domain names in the order of millions or even billions. Without special configuration, MassDNS is capable of resolving over 350,000 names per second using publicly available resolvers.

Contributors

• Quirin Scheitle, Technical University of Munich

Compilation

Clone the git repository and cd into the project root folder. Then run make to build from source. If you are not on Linux, run make nolinux. On Windows, the Cygwin packages gcc-core, git and make are required.

Usage

Usage: ./bin/massdns [options] [domainlist]
  -b  --bindto           Bind to IP address and port. (Default: 0.0.0.0:0)
      --busy-poll        Use busy-wait polling instead of epoll.
  -c  --resolve-count    Number of resolves for a name before giving up. (Default: 50)
      --drop-group       Group to drop privileges to when running as root. (Default: nogroup)
      --drop-user        User to drop privileges to when running as root. (Default: nobody)
      --extended-input   Input names are followed by a space-separated list of resolvers.
                         These are used before falling back to the resolvers file.
      --filter           Only output packets with the specified response code.
      --flush            Flush the output file whenever a response was received.
  -h  --help             Show this help.
      --ignore           Do not output packets with the specified response code.
  -i  --interval         Interval in milliseconds to wait between multiple resolves of the same
                         domain. (Default: 500)
  -l  --error-log        Error log file path. (Default: /dev/stderr)
      --norecurse        Use non-recursive queries. Useful for DNS cache snooping.
  -o  --output           Flags for output formatting.
      --predictable      Use resolvers incrementally. Useful for resolver tests.
      --processes        Number of processes to be used for resolving. (Default: 1)
  -q  --quiet            Quiet mode.
      --rand-src-ipv6    Use a random IPv6 address from the specified subnet for each query.
      --rcvbuf           Size of the receive buffer in bytes.
      --retry            Unacceptable DNS response codes.
                         (Default: All codes but NOERROR or NXDOMAIN)
  -r  --resolvers        Text file containing DNS resolvers.
      --root             Do not drop privileges when running as root. Not recommended.
  -s  --hashmap-size     Number of concurrent lookups. (Default: 10000)
      --sndbuf           Size of the send buffer in bytes.
      --status-format    Format for real-time status updates, json or ansi (Default: ansi)
      --sticky           Do not switch the resolver when retrying.
      --socket-count     Socket count per process. (Default: 1)
  -t  --type             Record type to be resolved. (Default: A)
      --verify-ip        Verify IP addresses of incoming replies.
  -w  --outfile          Write to the specified output file instead of standard output.

Output flags:
  L - domain list output
  S - simple text output
  F - full text output
  B - binary output
  J - ndjson output

Advanced flags for the domain list output mode:
  0 - Include NOERROR replies without answers.

Advanced flags for the simple output mode:
  d - Include records from the additional section.
  i - Indent any reply record.
  l - Separate replies using a line feed.
  m - Only output reply records that match the question name.
  n - Include records from the answer section.
  q - Print the question.
  r - Print the question with resolver IP address, Unix timestamp and return code prepended.
  s - Separate packet sections using a line feed.
  t - Include TTL and record class within the output.
  u - Include records from the authority section.

Advanced flags for the ndjson output mode:
  e - Write a record for each terminal query failure.

For a detailed description of the command line interface, please consult the man page using man ./doc/massdns.1.

Example

Resolve all AAAA records from domains within domains.txt using the resolvers within resolvers.txt in lists and store the results within results.txt:

$ ./bin/massdns -r lists/resolvers.txt -t AAAA domains.txt > results.txt

This is equivalent to:

$ ./bin/massdns -r lists/resolvers.txt -t AAAA -w results.txt domains.txt

Example output

By default, MassDNS will output response packets in text format which looks similar to the following:

;; Server: 77.41.229.2:53
;; Size: 93
;; Unix time: 1513458347
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 51298
;; flags: qr rd ra ; QUERY: 1, ANSWER: 1, AUTHORITY: 2, ADDITIONAL: 0

;; QUESTION SECTION:
example.com. IN A

;; ANSWER SECTION:
example.com. 45929 IN A 93.184.216.34

;; AUTHORITY SECTION:
example.com. 24852 IN NS b.iana-servers.net.
example.com. 24852 IN NS a.iana-servers.net.

The resolver IP address is included in order to make it easier for you to filter the output in case you detect that some resolvers produce bad results.

Resolving

The repository includes the file resolvers.txt consisting of a filtered subset of the resolvers provided by the subbrute project. Please note that the usage of MassDNS may cause a significant load on the used resolvers and result in abuse complaints being sent to your ISP. Also note that the provided resolvers are not guaranteed to be trustworthy. The resolver list is currently outdated with a large share of resolvers being dysfunctional.

MassDNS's custom, malloc-free DNS implementation currently only supports the most common records. You are welcome to help changing this by collaborating.

PTR records

MassDNS includes a Python script allowing you to resolve all IPv4 PTR records by printing their respective queries to the standard output.

$ ./scripts/ptr.py | ./bin/massdns -r lists/resolvers.txt -t PTR -w ptr.txt

Please note that the labels within in-addr.arpa are reversed. In order to resolve the domain name of 1.2.3.4, MassDNS expects 4.3.2.1.in-addr.arpa as input query name. As a consequence, the Python script does not resolve the records in an ascending order which is an advantage because sudden heavy spikes at the name servers of IPv4 subnets are avoided.

Reconnaissance by brute-forcing subdomains

Perform reconnaissance scans responsibly and adjust the -s parameter to not overwhelm authoritative name servers.

Similar to subbrute, MassDNS allows you to brute force subdomains using the included subbrute.py script:

$ ./scripts/subbrute.py example.com lists/names.txt | ./bin/massdns -r lists/resolvers.txt -t A -o S -w results.txt

As an additional method of reconnaissance, the ct.py script extracts subdomains from certificate transparency logs by scraping the data from crt.sh:

$ ./scripts/ct.py example.com | ./bin/massdns -r lists/resolvers.txt -t A -o S -w results.txt

The files names.txt and names_small.txt, which have been copied from the subbrute project, contain names of commonly used subdomains. Also consider using Jason Haddix' subdomain compilation with over 1,000,000 names or the Assetnote wordlist with over 9,000,000 million names.

MassDNS also includes a recon.py wrapper script (beta status) in the scripts folder, which performs subdomain enumeration against authoritative name servers directly and thus does not require third-party resolvers. The concurrency is determined automatically by MassDNS and supports hundreds of thousands of queries per second, while delivering reliable results. On a cheap dedicated server, the Assetnode wordlist can be enumerated in less than a minute. A current limitation is that zone delegation is only handled up to the delegation point. For example, if example.org is enumerated and sub.example.org is delegated to another name server, abc.sub.example.org will not be found by this script if abc.sub is contained in the word list. However, the script will report this fact as ?.sub.example.org in this case.

$ ./scripts/recon.py -d google.com -l lists/best-dns-wordlist.txt > google.txt

Screenshots

Security

MassDNS does not require root privileges and will therefore drop privileges to the user called "nobody" by default when being run as root. If the user "nobody" does not exist, MassDNS will refuse execution. In this case, it is recommended to run MassDNS as another non-privileged user. The privilege drop can be circumvented using the --root argument which is not recommended. Also note that other branches than master should not be used in production at all.

Practical considerations

Performance tuning

MassDNS is a simple single-threaded application designed for scenarios in which the network is the bottleneck. It is designed to be run on servers with high upload and download bandwidths. Internally, MassDNS makes use of a hash map which controls the concurrency of lookups. Setting the size parameter -s hence allows you to control the lookup rate. If you are experiencing performance issues, try adjusting the -s parameter in order to obtain a better success rate.

Rate limiting evasion

In case rate limiting by IPv6 resolvers is a problem, you can make use of --rand-src-ipv6 <your_ipv6_prefix>. MassDNS will then use a raw socket for sending and receiving DNS packets and randomly pick a source IPv6 address from the specified prefix for each query. This requires that MassDNS is run with CAP_NET_RAW privileges. When making use of this method, you should have iptables or nftables drop the DNS traffic received by MassDNS such that no ICMP Port unreachable responses are generated by the operating system, e.g. using ip6tables -p udp --sport 53 -I INPUT -j DROP. Note that this rule is just examplary and would drop all DNS traffic, including traffic for other applications. You might want to adapt the rule to be more fine-grained to fit your use case.

Result authenticity

If the authenticity of results is highly essential, you should not rely on the included resolver list. Instead, set up a local unbound resolver and supply MassDNS with its IP address. In case you are using MassDNS as a reconnaissance tool, you may wish to run it with the default resolver list first and re-run it on the found names with a list of trusted resolvers in order to eliminate false positives.

In case you are enumerating subdomains for a single name, e.g. for example.com, you may want to simply leave out third-party resolvers. In this case, you can directly probe the authoritative nameservers like so:

$ ./bin/massdns -r <(./scripts/auth-addrs.sh example.com) --norecurse -o Je example-com-subdomains.txt > results.txt

Todo

• Prevent flooding resolvers which are employing rate limits or refusing resolves after some time
• Implement bandwidth limits
• Employ cross-resolver checks to detect DNS poisoning and DNS spam (e.g. Level 3 DNS hijacking)
• Add wildcard detection for reconnaissance
• Improve reconnaissance reliability by adding a mode which re-resolves found domains through a list of trusted (local) resolvers in order to eliminate false positives
• Detect optimal concurrency automatically
• Parse the command line properly and allow the usage/combination of short options without spaces