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DNS

DNS is the phone book of the internet — and a misconfigured DNS server will happily read you every entry, including the ones meant to stay internal. Zone transfers, version disclosure, and subdomain brute-forcing turn a name server into a map of the target’s infrastructure.
Protocol: UDP (sometimes TCP) · Port: 53

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What Is DNS?

DNS (Domain Name System) resolves computer names into IP addresses. There’s no central database — it’s a massive distributed system, like a library full of phone books where each server is responsible for its own section and refers you elsewhere for the rest. DNS is mostly unencrypted, which means queries can be intercepted and spied on. That privacy gap drove the creation of encrypted variants you’ll occasionally encounter: DNS over TLS (DoT), DNS over HTTPS (DoH), and DNSCrypt.

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Default Configuration

All DNS servers work with three types of configuration files:
  • Local DNS configuration files
  • Zone files
  • Reverse name resolution files
On Linux, the most common DNS server is BIND9. Its local configuration files are usually:
  • named.conf.local
  • named.conf.options
  • named.conf.log

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Local DNS Configuration

The named.conf files define the server’s zones and behavior. A typical local zone definition looks like this: 
# /etc/bind/named.conf.local
zone "inlanefreight.htb" {
    type master;
    file "/etc/bind/db.inlanefreight.htb";
    allow-update { none; };
};
Zones are divided into individual files, each generally intended for one domain. A zone file is a text file describing a DNS zone in the BIND format.

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Zone Files

A zone file describes a zone completely. It must contain exactly one SOA (Start of Authority) record and at least one NS (Name Server) record. The SOA record sits at the top: 
$TTL 3600
@   IN  SOA     ns1.inlanefreight.htb. admin.inlanefreight.htb. (
                2024061301  ; Serial
                3600        ; Refresh
                1800        ; Retry
                604800      ; Expire
                86400 )     ; Minimum TTL
@       IN  NS      ns1.inlanefreight.htb.
@       IN  A       10.129.14.128
ns1     IN  A       10.129.14.128
www     IN  A       10.129.14.128

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Reverse Name Resolution Zone Files

For an IP address to be resolved back to its Fully Qualified Domain Name (FQDN), the DNS server needs a reverse lookup file. These use PTR records mapping addresses back to names: 
128     IN  PTR     www.inlanefreight.htb.

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Dangerous Settings

A DNS server can be attacked in many ways. Known vulnerabilities for BIND9 are catalogued at CVEdetails, and SecurityTrails maintains a list of the most popular DNS attack types. The settings most relevant to a pentester are the ones that control who is allowed to query, transfer, and update zones:
SettingRisk
allow-queryDefines who can send queries. Set too broadly, it exposes records to anyone
allow-recursionAllows recursive queries — an open resolver can be abused for amplification attacks
allow-transferThe big one — defines who can request a full zone transfer (AXFR). Set to any or a wide subnet, it hands the entire zone to an attacker
allow-updateDefines who can dynamically modify zone records
allow-transfer { any; }; is the classic misconfiguration. It lets anyone pull the complete zone file — every hostname, internal IP, and subdomain the server knows about — in a single request.

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Footprinting the Service

Footprinting DNS is all about the requests you send and what the server is willing to disclose in response. The workhorse tool is dig.

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NS Query — Find the Name Servers

Start by asking the server which name servers it knows about. The NS record reveals them, and the @ character specifies which DNS server to query: 
dig NS inlanefreight.htb @10.129.14.128

; <<>> DiG 9.16.1-Ubuntu <<>> NS inlanefreight.htb @10.129.14.128
;; ANSWER SECTION:
inlanefreight.htb.   604800  IN  NS  ns.inlanefreight.htb.

;; ADDITIONAL SECTION:
ns.inlanefreight.htb.  604800  IN  A  10.129.14.128
If other name servers exist, you can query them too.

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Version Query

Sometimes you can fish the server’s version out of a CHAOS class, TXT type query. The entry has to exist on the server for this to work, but when it does, it’s free version intel for CVE matching: 
dig CH TXT version.bind @10.129.14.128

;; ANSWER SECTION:
version.bind.   0   CH  TXT  "9.16.1-Ubuntu"

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ANY Query — Show Everything

The ANY option asks the server to return every record it’s willing to disclose. Note that not all entries from a zone will necessarily show up: 
dig ANY inlanefreight.htb @10.129.14.128

;; ANSWER SECTION:
inlanefreight.htb.   604800  IN  NS   ns.inlanefreight.htb.
inlanefreight.htb.   604800  IN  A    10.129.14.128
inlanefreight.htb.   604800  IN  MX   10 mail1.inlanefreight.htb.
inlanefreight.htb.   604800  IN  TXT  "v=spf1 include:inlanefreight.htb ..."

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AXFR Zone Transfer — The Jackpot

A zone transfer copies a zone from one server to another, normally over TCP port 53. The procedure is abbreviated AXFR (Asynchronous Full Transfer Zone). Servers normally authenticate each other with a shared secret (the rndc-key you’d see in the config), but if allow-transfer is misconfigured, anyone can request the whole zone: 
dig AXFR inlanefreight.htb @10.129.14.128

;; ANSWER SECTION:
inlanefreight.htb.       SOA    ns.inlanefreight.htb. ...
inlanefreight.htb.       NS     ns.inlanefreight.htb.
inlanefreight.htb.       A      10.129.14.128
ns.inlanefreight.htb.    A      10.129.14.128
www.inlanefreight.htb.   A      10.129.14.128
mail1.inlanefreight.htb. A      10.129.18.201
dev.inlanefreight.htb.   A      10.129.18.230
internal.inlanefreight.htb. A   10.129.1.6
That single command just handed you every hostname and IP the server knows — including internal ones.

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AXFR Across Subdomains

If the administrator set allow-transfer to a subnet or to any, you can query the entire zone file — and pivot to other zones, which may expose internal IP addresses and hostnames you wouldn’t otherwise see: 
dig AXFR internal.inlanefreight.htb @10.129.14.128

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Subdomain Brute-Forcing

When zone transfer is locked down, you can still discover individual A records by brute-forcing hostnames against a wordlist. SecLists provides good ones. A simple Bash for-loop sends a query per candidate and keeps the hits: 
for sub in $(cat /opt/useful/SecLists/Discovery/DNS/subdomains-top1million-110000.txt); do
  dig $sub.inlanefreight.htb @10.129.14.128 | grep -v ';\|SOA' | sed -r '/^\s*$/d' | grep $sub | tee -a subdomains.txt
done
Most dedicated tools work the same way under the hood. DNSenum is a common one: 
dnsenum --dnsserver 10.129.14.128 --enum -p 0 -s 0 -o subdomains.txt \
  -f /opt/useful/SecLists/Discovery/DNS/subdomains-top1million-110000.txt inlanefreight.htb

dnsenum VERSION:1.2.6
-----   inlanefreight.htb   -----
Brute forcing with /opt/useful/SecLists/Discovery/DNS/subdomains-top1million-110000.txt:
__________________________________________________________________
ns.inlanefreight.htb.        604800  IN  A  10.129.14.128
mail1.inlanefreight.htb.     604800  IN  A  10.129.18.201
www.inlanefreight.htb.       604800  IN  A  10.129.14.128
dev.inlanefreight.htb.       604800  IN  A  10.129.18.230

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Quick Reference

CommandPurpose
dig NS <domain> @<server>List the zone’s name servers
dig CH TXT version.bind @<server>Disclose the BIND version
dig ANY <domain> @<server>Show all disclosable records
dig AXFR <domain> @<server>Attempt a full zone transfer
dnsenum --dnsserver <server> -f <wordlist> <domain>Brute-force subdomains
The footprinting flow: NS query → version query → ANY query → attempt AXFR → fall back to subdomain brute-forcing if AXFR is refused.
Next: FTP — anonymous logins, banner grabbing, and pulling files off misconfigured file servers.