DNS Zone Transfers
DNS Zone Transfers
DNS zone transfers using the AXFR protocol are the simplest mechanism to replicate DNS records across DNS servers. To avoid the need to edit information on multiple DNS servers, you can edit information on one server and use AXFR to copy information to other servers. However, if you do not protect your servers, malicious parties may use AXFR to get information about all your hosts.
How DNS Works
DNS (Domain Name System) is like an Internet phonebook. It is responsible for resolving human-readable hostnames into machine-readable IP addresses. The system includes authoritative DNS servers that provide information and DNS caches that store that information temporarily for client lookups. A typical DNS query is very simple: a client provides a human-readable hostname and in response receives an IP address. However, the system assumes that the querying client knows the hostname.
DNS servers host zones. A DNS zone is a portion of the domain name space that is served by a DNS server. For example, example.com with all its subdomains may be a zone. However, second.example.com may also be a separate zone.
Why DNS Zone Transfer is Needed
DNS is a critical service. If a DNS server for a zone is not working and cached information has expired, the domain is inaccessible to all services (web, mail, and more). Therefore, each zone should have at least two DNS servers. For more critical zones, there may be even more.
However, a zone may be large and may require frequent changes. If you manually edit zone data on each server separately, it takes a lot of time and there is a a lot of potential for a mistake. This is why DNS zone transfer is needed.
You can use different mechanisms for DNS zone transfer but the simplest one is AXFR (technically speaking, AXFR refers to the protocol used during a DNS zone transfer). It is a client-initiated request. Therefore, you can edit information on the primary DNS server and then use AXFR from the secondary DNS server to download the entire zone.
How to Initiate a DNS Zone Transfer
Initiating an AXFR zone-transfer request from a secondary server is as simple as using the following dig commands, where zonetransfer.me is the domain that we want to initiate a zone transfer for. First, we need to get the list of DNS servers for the domain:
dig +short ns zonetransfer.me
Now, we can initiate an AXFR request to get a copy of the zone from the primary server:
dig axfr zonetransfer.me @nsztm1.digi.ninja
; <<>> DiG 9.16.24 <<>> axfr zonetransfer.me @nsztm1.digi.ninja
;; global options: +cmd
zonetransfer.me. 7200 IN SOA nsztm1.digi.ninja. robin.digi.ninja. 2019100801 172800 900 1209600 3600
zonetransfer.me. 300 IN HINFO "Casio fx-700G" "Windows XP"
zonetransfer.me. 301 IN TXT "google-site-verification=tyP28J7JAUHA9fw2sHXMgcCC0I6XBmmoVi04VlMewxA"
zonetransfer.me. 7200 IN MX 0 ASPMX.L.GOOGLE.COM.
zonetransfer.me. 7200 IN MX 10 ALT1.ASPMX.L.GOOGLE.COM.
zonetransfer.me. 7200 IN MX 10 ALT2.ASPMX.L.GOOGLE.COM.
zonetransfer.me. 7200 IN MX 20 ASPMX2.GOOGLEMAIL.COM.
zonetransfer.me. 7200 IN MX 20 ASPMX3.GOOGLEMAIL.COM.
zonetransfer.me. 7200 IN MX 20 ASPMX4.GOOGLEMAIL.COM.
zonetransfer.me. 7200 IN MX 20 ASPMX5.GOOGLEMAIL.COM.
zonetransfer.me. 7200 IN A 5.196.105.14
zonetransfer.me. 7200 IN NS nsztm1.digi.ninja.
zonetransfer.me. 7200 IN NS nsztm2.digi.ninja.
_acme-challenge.zonetransfer.me. 301 IN TXT "6Oa05hbUJ9xSsvYy7pApQvwCUSSGgxvrbdizjePEsZI"
_sip._tcp.zonetransfer.me. 14000 IN SRV 0 0 5060 www.zonetransfer.me.
14.105.196.5.IN-ADDR.ARPA.zonetransfer.me. 7200 IN PTR www.zonetransfer.me.
asfdbauthdns.zonetransfer.me. 7900 IN AFSDB 1 asfdbbox.zonetransfer.me.
asfdbbox.zonetransfer.me. 7200 IN A 127.0.0.1
asfdbvolume.zonetransfer.me. 7800 IN AFSDB 1 asfdbbox.zonetransfer.me.
canberra-office.zonetransfer.me. 7200 IN A 202.14.81.230
cmdexec.zonetransfer.me. 300 IN TXT "; ls"
contact.zonetransfer.me. 2592000 IN TXT "Remember to call or email Pippa on +44 123 4567890 or pippa@zonetransfer.me when making DNS changes"
dc-office.zonetransfer.me. 7200 IN A 143.228.181.132
deadbeef.zonetransfer.me. 7201 IN AAAA dead:beaf::
dr.zonetransfer.me. 300 IN LOC 53 20 56.558 N 1 38 33.526 W 0.00m 1m 10000m 10m
DZC.zonetransfer.me. 7200 IN TXT "AbCdEfG"
email.zonetransfer.me. 2222 IN NAPTR 1 1 "P" "E2U+email" "" email.zonetransfer.me.zonetransfer.me.
email.zonetransfer.me. 7200 IN A 74.125.206.26
Hello.zonetransfer.me. 7200 IN TXT "Hi to Josh and all his class"
home.zonetransfer.me. 7200 IN A 127.0.0.1
Info.zonetransfer.me. 7200 IN TXT "ZoneTransfer.me service provided by Robin Wood - robin@digi.ninja. See http://digi.ninja/projects/zonetransferme.php for more information."
internal.zonetransfer.me. 300 IN NS intns1.zonetransfer.me.
internal.zonetransfer.me. 300 IN NS intns2.zonetransfer.me.
intns1.zonetransfer.me. 300 IN A 81.4.108.41
intns2.zonetransfer.me. 300 IN A 167.88.42.94
office.zonetransfer.me. 7200 IN A 4.23.39.254
ipv6actnow.org.zonetransfer.me. 7200 IN AAAA 2001:67c:2e8:11::c100:1332
owa.zonetransfer.me. 7200 IN A 207.46.197.32
robinwood.zonetransfer.me. 302 IN TXT "Robin Wood"
rp.zonetransfer.me. 321 IN RP robin.zonetransfer.me. robinwood.zonetransfer.me.
sip.zonetransfer.me. 3333 IN NAPTR 2 3 "P" "E2U+sip" "!^.*$!sip:customer-service@zonetransfer.me!" .
sqli.zonetransfer.me. 300 IN TXT "' or 1=1 --"
sshock.zonetransfer.me. 7200 IN TXT "() { :]}; echo ShellShocked"
staging.zonetransfer.me. 7200 IN CNAME www.sydneyoperahouse.com.
alltcpportsopen.firewall.test.zonetransfer.me. 301 IN A 127.0.0.1
testing.zonetransfer.me. 301 IN CNAME www.zonetransfer.me.
vpn.zonetransfer.me. 4000 IN A 174.36.59.154
www.zonetransfer.me. 7200 IN A 5.196.105.14
xss.zonetransfer.me. 300 IN TXT "'><script>alert('Boo')</script>"
zonetransfer.me. 7200 IN SOA nsztm1.digi.ninja. robin.digi.ninja. 2019100801 172800 900 1209600 3600
;; Query time: 120 msec
;; SERVER: 81.4.108.41#53(81.4.108.41)
;; WHEN: Sat Feb 05 21:45:59 EST 2022
;; XFR size: 50 records (messages 1, bytes 1994)
AXFR Vulnerability and Prevention
AXFR offers no authentication, so any client can ask a DNS server for a copy of the entire zone. This means that unless some kind of protection is introduced, an attacker can get a list of all hosts for a domain, which gives them a lot of potential attack vectors.
In order to prevent this vulnerability from occurring, the DNS server should be configured to only allow zone transfers from trusted IP addresses. The following is an example of how this can be accomplished in the BIND DNS server.
# /etc/named.conf
acl trusted-nameservers {
192.168.0.10; //ns2
192.168.1.20; //ns3
};
zone zonetransfer.me {
type master;
file "zones/zonetransfer.me";
allow-transfer { trusted-nameservers; };
};
Additionally, it’s also recommended to use transaction signatures (TSIG) for zone transfers to prevent IP spoofing attempts.