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DNS enumeration

DNS​

  • Stands for "Domain Name System"
  • Hierarchical and decentralized naming system
  • Used for resources connected to the Internet including computers and services
  • Runs on TCP/UDP port 53

DNS records​

  • Database record used to map a URL to an IP address

  • Stored in zone files in DNS servers

    • A DNS server contains a "zone file" for each domain
    • Zone file is made up of "resource records" (RRs)

  • Helps users connect their websites to the outside world.

  • 📝 Common DNS records include

    • A

      • Points a domain to an IPv4 address, such as 11.22.33.44.

    • AAAA

      • Points a domain to an IPv6 address, such as FE80::0202:B3FF:FE1E:8329.

    • MX

    • NS

      • Used to delegate a domain or subdomain to a set of name servers

    • SOA

      • Contains data to control the zone transfer.

      • Includes serial number, timestamps, mail address of zone responsible..

      • E.g.


          $TTL 86400
          @   IN  SOA     ns.icann.org. noc.dns.icann.org. (
                  2020080302  ;Serial
                  7200        ;Refresh
                  3600        ;Retry
                  1209600     ;Expire
                  3600        ;Minimum TTL
          )

    • CNAME

      • Link a subdomain to a domain's existing A or AAAA record
      • E.g. www.cloudarchitecture.io to cloudarchitecture.io

    • PTR

      • Opposite of A, points an IP to domain
      • Commonly used for spam verification for e-mail programs

    • HINFO

      • System information including CPU and OS type.

DNS enumeration techniques​

  • Check all NS Records for zone transfers.
  • Enumerate general DNS records for a given domain.
  • Perform common SRV Record Enumeration.
    • Service records contain the hostname, port and priority of servers for a given service.
    • Enumerates e.g. • LDAP • Autodiscover for Exchange • Kerberos...
    • E.g. by nmap --script dns-srv-enum --script-args "dns-srv-enum.domain='google.com'"
  • Brute force subdomain and host A and AAAA records discovery with given top domain and wordlist.
  • DNS PTR lookup given a IP range CIDR range
    • Querying dns for PTR record of each IP in subnet
  • See also DNS interrogation

DNS cache snooping​

  • Checks a DNS server cached records. Done by performing non-recursive (or also known as iterative) DNS queries
    • Also known as iterative query
    • Server returns either its own record or another DNS server that may know the answer.
    • As opposed to recursive DNS lookup where servers communicates with other DNS servers.
  • Tools
    • Automated: dnsrecon
    • 📝 Manual:
      • dig with +norecurse flag
      • nslookup with -norecurse flag
      • host with -r flag

Zone transfers​

  • DNS server passes a copy of part of it's database ("zone") to another DNS server
  • There's one master DNS server, and one or more slave DNS servers
    • Slaves ask master for a copy of records
  • Uses TCP port 53
  • 📝 Uses AXFR (full) protocol or IXFR (incremental).
  • The secondary server request a new copy if the primary SOA serial number is higher.
    • The primary increments the serial number every time the SOA changes
    • If the secondary checks in and the primary’s copy has a higher serial number

DNS zone transfer attack​

  • Pretending to be a slave and ask for records

  • Allows an attacker to obtain sensitive information about internal DNS records (network).

  • 📝 Flow

    1. Get NS records (DNS servers that are responsible for resolving the queries)

      • Using dig: dig ns zonetransfer.me or dig +short ns zonetransfer.me
      • Using nslookup: nslookup zonetransfer.me

    2. Initiate AXFR request to get a copy of the zone from name server

      • Using dig: dig axfr @<DNS you are querying> <target>

        • E.g. dig axfr @nsztm1.digi.ninja zonetransfer.me

      • Using nslookup

        • nslookup -ls -d nsztm1.digi.ninja
          • -d: list all records for DNS domain
          • Sends AXFR query to the remote nameserver
          • Initiates zone transfer if and only if the remote nameserver is dumb enough to respond to unsolicited, unauthorized AXFRs originating from random machines on the Internet.
        • Or using interactive mode with specified a DNS server:
          $ nslookup
          > server <DNS you are querying>
          > set type=any
          > ls -d <target>

      • Or nslookup -query=AXFR <target> <DNS you are querying>

      • Using host: host -l nsztm1.digi.ninja

  • 🤗 In June 2017 the registrar responsible for Russian top-level-domains accidentally enabled DNS zone transfers via AXFR which led to 5.6 million records being accidentally exposed | source

Zone transfers countermeasures​

  • Do not allow or restrict zone transfers
  • Use split DNS

Split DNS​

  • Also known as split-horizon DNS, split-view DNS, split-brain DNS or split DNS
  • 📝 Separation of internal network (intranet) DNS and public network (Internet) DNS
  • Provides different answers to DNS queries based on the source address of the DNS request.
  • Can be accomplished with hardware or software solutions

DNS enumeration tools​

dnsrecon​

  • Open source python script
  • E.g. ./dnsrecon.py -d cloudarchitecture.io
  • Enumerates DNS records and more

nslookup​

  • Limited: Depends on existence of DNS reverse lookup zone.
  • Forward lookup (normal): Here's name give me IP
  • Reverse lookup: Here's IP give me back the name

dig​

  • *Nix tool for querying DNS
  • E.g. dig cloudarchitecture.io any
    • any argument (optional): all records it can find
  • dig axfr cloudarchitecture.io

host​

  • On Unix-like operating systems, the host command is a DNS lookup utility
  • Using e.g. host <target-domain> to see all records.
  • 📝 You can also set type -t to see specific records e.g.
    • host -t a <target-domain> to see A records
    • host -t ns <target-domain> to see NS records
    • ...