Introduction to DNS and its vulnerabilities Olaf M

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enterprise Recursive DNS Mail server Internet http: //www. nlnetlabs. nl/ © 2011 Stichting NLnet

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How to Prot ect? 14

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DN Vu S S lne ys rab tem iliti es g ionin rovis ilities P narab Vul Registrars & Registrants Server vulnarability Man in the Middle Secondary DNS primary DNS Registry Secondary DNS 21 spoofing & Man in the Middle

Wha t can one prot do to ect. . . (skip ping DNS SEC ) 22

Taking Unbound as example. Othe r ser mak vers e oth migh t but a er cho ices ny m reso , oder lver n take s sim appr oach ilar es © 2006 -2012 NLnet Labs, Licensed under a Creative Commons Attribution 3. 0 Unported Licens

Security Choices in Unbound • In general, a modern paranoid resolver • DNSSEC support. • RFC 2181 support completely • Fine grained. Keeps track of where RRSets came from and won't upgrade them into answers. • Does not allow RRSets to be overridden by lower level rrsets © 2006 -2012 NLnet Labs, Licensed under a Creative Commons Attribution 3. 0 Unported Licens

Filtering • • Scrubber: Only in-bailiwick data is accepted in the answer • • The answer section must contain only answer CNAME, DNAME checked that chain is correct • CNAME cut off and only the first CNAME kept • • • Lookup rest yourself do not trust other server DNAME synthesize CNAME by unbound do not trust other server. Also cut off like above. DNAME from cache only used if DNSSEC-secure. © 2006 -2012 NLnet Labs, Licensed under a Creative Commons Attribution 3. 0 Unported Licens

Filtering II • No address records in authority, additional section unless relevant – i. e. mentioned in a NS record in the authority section. • Irrelevant data is removed • When the message only had preliminary parsing and has not yet been copied to the working region of memory © 2006 -2012 NLnet Labs, Licensed under a Creative Commons Attribution 3. 0 Unported Licens

Entropy • • • Randomness protects against spoof • • Arc 4 random() (Open. BSD): crypto strong. May not be perfectly random, but predicting it is a cryptographical breakin. Real entropy from OS as seed Query id – all 16 bits used. Port randomisation – uses all 16 bits there, goes out of its way to make sure every query gets a fresh port number © 2006 -2012 NLnet Labs, Licensed under a Creative Commons Attribution 3. 0 Unported Licens

Entropy II • Destination address, and ipv 4/ipv 6. RTT band of 400 msec (=everything). • • • Its not the timewindow but the randomness Query aggregation – same queries are not sent out – unless by different threads Qname strict match checked in reply 0 x 20 option Harden-referral-path (my draft) option Can use multiple source interfaces! • 4 outgoing IP address add +2 bits © 2006 -2012 NLnet Labs, Licensed under a Creative Commons Attribution 3. 0 Unported Licens

Other measures • Not for the wire itself • • Heap function pointer protection (whitelisted) Chroot() by default User privileges are dropped (lots of code!) ACL for recursion No detection of attacks – assume always under attack version. bind hostname. bind can be blocked or configured what to return (version hiding) Disprefer recursion lame servers – they have a cache that can be poisoned © 2006 -2012 NLnet Labs, Licensed under a Creative Commons Attribution 3. 0 Unported Licens

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Metaphor 31

Registrars & Registrants End t o End Secu rity Secondary DNS primary DNS Registry Secondary DNS 32

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Securing Host Communication © 2006 -2012 NLnet Labs, Licensed under a Creative Commons Attribution

Data flow through the DNS What should you protect. . . Registrars & Registrants HOST Security TSIG Secondary DNS primary DNS Registry TSIG (rarely) Secondary DNS © 2006 -2012 NLnet Labs, Licensed under a Creative Commons Attribution 3. 0 Unported Licens

Transaction Signature: TSIG • TSIG (RFC 2845) –Authorising dynamic updates and zone transfers –Authentication of caching forwarders –Independent from other features of DNSSEC • One-way hash function –DNS question or answer and timestamp • Traffic signed with “shared secret” key • Used in configuration, NOT in zone file © 2006 -2012 NLnet Labs, Licensed under a Creative Commons Attribution 3. 0 Unported Licens

TSIG Example Query: AXFR Sig: B 1@F 00 verification Slave Master KEY: $h@r 3 d. S 3 cr 3 t SOA … SOA SIG: FOOB@R verification Response: Zone © 2006 -2012 NLnet Labs, Licensed under a Creative Commons Attribution 3. 0 Unported Licens

TSIG for Zone Transfers 1. Generate secret 2. Communicate secret 3. Configure servers 4.

Importance of the Time Stamp • TSIG/SIG(0) signs a complete DNS request / response with time stamp – To prevent replay attacks – Currently hardcoded at five minutes • Operational problems when comparing times – Make sure your local time zone is properly defined – date -u will give UTC time, easy to compare between the two systems – Use NTP synchronisation! © 2006 -2012 NLnet Labs, Licensed under a Creative Commons Attribution 3. 0 Unported Licens

Authenticating Servers Using SIG(0) • Alternatively, it is possible to use SIG(0) –Not yet widely used –Works well in dynamic update environment • Public key algorithm –Authentication against a public key published in the DNS • SIG(0) specified in RFC 2931 © 2006 -2012 NLnet Labs, Licensed under a Creative Commons Attribution 3. 0 Unported Licens

Cool Application • Use TSIG-ed dynamic updates to configure your laptops name • My laptop is know by the name of aagje. secret-wg. org – http: //ops. ietf. org/dns/dynupd/secure-ddnshowto. html –Mac OS users: there is a bonjour based tool. • www. dns-sd. org © 2006 -2012 NLnet Labs, Licensed under a Creative Commons Attribution 3. 0 Unported Licens