DNSSEC Part I: the Concepts

Like IPv6, DNSSEC is one of those great forward-looking protocols that unfortunately hasn't seen wide adoption yet. Before I implemented it myself, I could see why. Although some people think BIND itself is difficult to set up, DNSSEC adds an extra layer of keys, key management and a slew of additional DNS records. One day I decided to set up DNSSEC on a personal zone to familiarize myself with the concepts and process, and it turns out that the implementation isn't all that bad once you grasp a few concepts. In this article, I cover some of the general concepts, and in my next article, I'll describe the steps for using DNSSEC on your own zone.

How DNS Works

It can be difficult to understand how DNSSEC works if you don't completely grasp how DNS itself works. Really, I could spend a whole article just talking about how DNS works, but for the purposes of this article, here's a very high-level trace of a typical uncached DNS query that resolves a domain of mine: www.greenfly.org. When you type a URL in a Web browser and press Enter, behind the scenes, the OS starts a process to convert that hostname into an IP address. Although some people run DNS caching services on their personal computers, for the most part, you rely on an external DNS server you've either configured by hand or via DHCP. When your OS needs to convert a hostname into an IP, it sends a DNS query to a name server defined in /etc/resolv.conf (these days, if this file is managed by resolvconf, the real name servers can be trickier to track down). This starts what's known as a recursive query, as this remote DNS server acts on your behalf to talk to any other DNS servers it needs to contact to resolve your hostname to an IP.

In the case of resolving www.greenfly.org, the recursive DNS server starts by sending a query to one of the 13 root DNS servers on the Internet (192.33.4.12), asking for the IP for www.greenfly.org. The root name servers reply that they don't know that information, but the name servers for .org might know, and here are their names and IP addresses. Next, the recursive DNS server sends a query to one of the .org name servers (199.19.54.1), asking the same question. The .org name server replies that it also doesn't know the answer, but the name servers for greenfly.org might know, and here are their names and IP addresses. Finally, the recursive DNS server asks one of the greenfly.org name servers (75.101.46.232) and gets back the answer that www.greenfly.org is at 64.142.56.172.

If you are curious how this might work for a domain you own, just use the dig command with the +trace option. Here's example output for www.greenfly.org:


$ dig www.greenfly.org +trace

; <<>> DiG 9.8.1-P1 <<>> www.greenfly.org +trace
;; global options: +cmd
.                    498369  IN   NS    j.root-servers.net.
.                    498369  IN   NS    k.root-servers.net.
.                    498369  IN   NS    e.root-servers.net.
.                    498369  IN   NS    m.root-servers.net.
.                    498369  IN   NS    c.root-servers.net.
.                    498369  IN   NS    d.root-servers.net.
.                    498369  IN   NS    l.root-servers.net.
.                    498369  IN   NS    a.root-servers.net.
.                    498369  IN   NS    h.root-servers.net.
.                    498369  IN   NS    i.root-servers.net.
.                    498369  IN   NS    g.root-servers.net.
.                    498369  IN   NS    b.root-servers.net.
.                    498369  IN   NS    f.root-servers.net.
;; Received 436 bytes from 127.0.0.1#53(127.0.0.1) in 60 ms

org.                 172800  IN   NS    b2.org.afilias-nst.org.
org.                 172800  IN   NS    b0.org.afilias-nst.org.
org.                 172800  IN   NS    c0.org.afilias-nst.info.
org.                 172800  IN   NS    a0.org.afilias-nst.info.
org.                 172800  IN   NS    d0.org.afilias-nst.org.
org.                 172800  IN   NS    a2.org.afilias-nst.info.
;; Received 436 bytes from 192.33.4.12#53(192.33.4.12) in 129 ms

greenfly.org.        86400   IN   NS    ns2.greenfly.org.
greenfly.org.        86400   IN   NS    ns1.greenfly.org.
;; Received 102 bytes from 199.19.54.1#53(199.19.54.1) in 195 ms

www.greenfly.org.    900     IN   A     64.142.56.172
greenfly.org.        900     IN   NS    ns1.greenfly.org.
greenfly.org.        900     IN   NS    ns2.greenfly.org.
;; Received 118 bytes from 75.101.46.232#53(75.101.46.232) in 2 ms
]]>

Although this may seem like a lot of steps, in practice, name servers cache answers for a period of time known as the TTL, or Time To Live, that's assigned to every record. That way, a DNS resolver has to look up only any records that have expired.

DNS Security Issues

DNS has been around for quite a while, and it has had its share of security issues over time. DNS is designed to be an open, friendly service. Although some administrators might treat DNS records as secrets, generally speaking, a DNS record's purpose is to be looked up by anyone who requests it, so DNS records are not encrypted, and DNS queries generally occur over plain text. Here are a few DNS security issues facing us today:

  • Domain names sometimes look alike (google.com vs. googIe.com), which an attacker can take advantage of to encourage you to click on a legitimate-looking link.

  • Companies can't always register their name on all TLDs (.com vs. .biz vs. .net), so an attacker might register mybank.biz, which a victim may think is legitimate.

  • Many DNS servers (known as open resolvers) will perform recursive queries for anyone who asks.

  • Open resolvers commonly are used in modern DNS amplification DDOS attacks (an attack where a relatively small DNS query results in an orders of magnitude larger response that, due to DNS queries occurring over UDP, can be redirected to a different target than the host who initiated the request). With a DNS amplification attack, it takes much less bandwidth from an attacking machine to generate large amounts of traffic for a target.

  • DNS is subject to MitM (Man in the Middle) attacks where DNS records can be rewritten before they get back to the victim. This lets an attacker, for instance, change the IP of yourbank.com in a DNS request to point to the Web site the attacker controls instead.

  • DNS spoofing/cache poisoning attacks (this class of attacks was covered by a series of Paranoid Penguin columns in 2011) essentially allow an attacker to inject fake DNS records into a DNS resolver's cache to point victims, again, at an attacker's site instead of the site they intend to visit.

Of all of these different DNS security issues, DNSSEC attempts to address the last two, MitM attacks and DNS cache poisoning, by signing every DNS reply with a signature, much like a PGP signature in an e-mail. The DNSSEC signature verifies that the DNS result you see came from the authoritative DNS server for that domain and that it wasn't tampered with in any way in transit.

How DNSSEC Works

If you are somewhat familiar with the CA (Certificate Authority) system or with how public-key cryptography works with PGP-signed e-mails, understanding DNSSEC will be a bit simpler, as it has some similarities. With DNSSEC, a domain creates a set of public and private keys that it uses to sign every record set in its zone. The domain then publishes the public keys in the zone as a record of its own along with the signatures. With these public keys and signatures, anyone performing a DNS query against that domain can use the public key to validate the signature for a particular record. Because only someone with the private key could sign the record, you can be assured the result was signed by someone who controls the domain. If someone tampered with the record along the way, the signature no longer would match.

Like with PGP-signed e-mail, having cryptographic signatures attached to a document isn't a sufficient reason to trust the document. After all, attackers simply could generate a different key pair, change the record and attach their public key and updated signature instead. With DNSSEC, you need an outside mechanism to know you can trust that the public key you are getting truly came from the domain. With PGP-signed e-mail, you validate the public key with outside mechanisms, such as key-signing parties, with the hope that if you receive an e-mail from someone for which you don't immediately have a public key signature, someone you already trust does, and you can use that chain of trust to validate the signature. I don't know of any DNSSEC key-signing parties; instead, the chain of trust is built much like how it is with the CA system.

When you visit a site that's protected by HTTPS, the site will present you with a copy of its public key (here called a certificate), so you can establish a secure, encrypted communication channel with the site, but equally important, you also can validate that you are in fact communicating with, for instance, mail.google.com and not some attacker. Because you probably didn't go to a Google key-signing party either, how can you trust that certificate? It turns out that each certificate is signed by a CA like Verisign, Thawte or a large number of others. This signature is attached to the certificate you receive, and your browser itself has public keys for each of the CAs built in to it. The browser implicitly trusts these CA certificates, so if you receive a certificate from a site that has been signed by any of these CAs, you will trust it as valid. This trust, by the way, is why it is such a problem when a CA gets hacked. Attackers then can use the private keys from that CA to generate new certificates for any site they want to impersonate and browsers will trust them automatically.

DNSSEC signatures follow a similar chain of trust to PGP keys and CAs. In this case, the root DNS servers act as the trust anchor, and DNSSEC resolvers implicitly trust what the root DNS servers sign, much like browsers trust CAs. When a TLD (Top Level Domain) wants to implement DNSSEC, it submits a special DS record to the root DNS servers to sign. Those DS records contain a signature for the subdomain generated by the private key. The root DNS server hosts that DS record and signs it with its private key. In that way, because you trust root, you can trust that the signature for org has not been modified; therefore, you can trust it as well in the same way you would trust a certificate signed by a CA. Then if you want to enable DNSSEC for a .org domain, for instance, you would submit a DS record for each key through your registrar if it supports DNSSEC. Each DS record contains a key's signature for your domain that the org name servers then would sign and host.

In this model, the chain of trust basically follows the same order that a recursive DNS query like I outlined above would follow. A DNSSEC query adds an extra validation step to each part of the process. For instance, a query for www.isc.org starts at the root, uses the DS record for org to validate com signatures, then uses the DS record for isc.org to validate the isc.org signature attached to www.isc.org. You can add the +dnssec option to dig +trace to see the full transaction:


$ dig +trace +dnssec www.isc.org

; <<>> DiG 9.8.1-P1 <<>> +trace +dnssec www.isc.org
;; global options: +cmd
.                 492727  IN      NS      g.root-servers.net.
.                 492727  IN      NS      m.root-servers.net.
.                 492727  IN      NS      i.root-servers.net.
.                 492727  IN      NS      b.root-servers.net.
.                 492727  IN      NS      f.root-servers.net.
.                 492727  IN      NS      a.root-servers.net.
.                 492727  IN      NS      k.root-servers.net.
.                 492727  IN      NS      h.root-servers.net.
.                 492727  IN      NS      l.root-servers.net.
.                 492727  IN      NS      e.root-servers.net.
.                 492727  IN      NS      c.root-servers.net.
.                 492727  IN      NS      d.root-servers.net.
.                 492727  IN      NS      j.root-servers.net.
.                 518346  IN      RRSIG   NS 8 0 518400 20130517000000
20130509230000 20580 . M8pQTohc9iGqDHWfnACnBGDwPhFs7G/nqqOcZ4OobVxW8l
KIWa1Z3vho56IwomeVgYdj+LNX4Znp1hpb3up9Hif1bCASk+z3pUC4xMt7No179Ied
DsNz5iKfdNLJsMbG2PsKxv/C2fQTC5lRn6QwO4Ml09PAvktQ9F9z7IqS kUs=
;; Received 589 bytes from 127.0.0.1#53(127.0.0.1) in 31 ms

org.              172800  IN      NS      d0.org.afilias-nst.org.
org.              172800  IN      NS      b0.org.afilias-nst.org.
org.              172800  IN      NS      a2.org.afilias-nst.info.
org.              172800  IN      NS      b2.org.afilias-nst.org.
org.              172800  IN      NS      c0.org.afilias-nst.info.
org.              172800  IN      NS      a0.org.afilias-nst.info.
org.              86400   IN      DS      21366 7 1
E6C1716CFB6BDC84E84CE1AB5510DAC69173B5B2
org.              86400   IN      DS      21366 7 2
96EEB2FFD9B00CD4694E78278B5EFDAB0A80446567B69F634DA078F0 D90F01BA
org.              86400   IN      RRSIG   DS 8 1 86400 20130517000000
20130509230000 20580 . kirNDFgQeTmi0o5mxG4bduPm0y8LNo0YG9NgNgZIbYdz8
gdMK8tvSneJUGtJca5bIJyVGcOKxV3aqg/r5VThvz8its50tiF4l5lt+22n/AGnNRxv
onMl/NA5rt0K2vXtdskMbIRBLVUBoa5MprPDwEzwGg2xRSvJryxQEYcT 80Y=
;; Received 685 bytes from 192.203.230.10#53(192.203.230.10) in 362 ms
isc.org.          86400   IN      NS      ns.isc.afilias-nst.info.
isc.org.          86400   IN      NS      ams.sns-pb.isc.org.
isc.org.          86400   IN      NS      sfba.sns-pb.isc.org.
isc.org.          86400   IN      NS      ord.sns-pb.isc.org.
isc.org.          86400   IN      DS      12892 5 2
F1E184C0E1D615D20EB3C223ACED3B03C773DD952D5F0EB5C777586D E18DA6B5
isc.org.          86400   IN      DS      12892 5 1
982113D08B4C6A1D9F6AEE1E2237AEF69F3F9759
isc.org.          86400   IN      RRSIG   DS 7 2 86400 20130530155458
20130509145458 42353 org.
Qp7TVCt8qH74RyddE21a+OIBUhd6zyzAgSB1Qykl2NSkkebtJ1QeE5C5
R8eblh8XvmQXjqN7zwcj7sDaaHXBFXGZ2EeVT5nwJ1Iu4EGH2WK3L7To
BDjR+8wNofZqbd7kX/LOSvNu9jdikb4Brw9/qjkLk1XaOPgl/23WkIfp zn8=
;; Received 518 bytes from 199.19.56.1#53(199.19.56.1) in 400 ms

www.isc.org.      600     IN      A       149.20.64.42
www.isc.org.      600     IN      RRSIG   A 5 3 600 20130609211557
20130510211557 50012 isc.org.
tNE0KPAh/PUDWYumJ353BV6KmHl1nDdTEEDS7KuW8MVVMxJ6ZB+UTnUn
bzWC+kNZ/IbhYSD1mDhPeWvy5OGC5TNGpiaaKZ0/+OhFCSABmA3+Od3S
fTLSGt3p7HpdUZaC9qlwkTlKckDZ7OQPw5s0G7nFInfT0S+nKFUkZyuB OYA=
isc.org.          7200    IN      NS      ord.sns-pb.isc.org.
isc.org.          7200    IN      NS      sfba.sns-pb.isc.org.
isc.org.          7200    IN      NS      ns.isc.afilias-nst.info.
isc.org.          7200    IN      NS      ams.sns-pb.isc.org.
isc.org.          7200    IN      RRSIG   NS 5 2 7200 20130609211557
20130510211557 50012 isc.org.
SdMCLPfLXiyl8zrfbFpFDz22OiYQSPNXK18gsGRzTT2JgZkLZYZW9gyB
vPTzm8L+aunkMDInQwFmRPqvHcbO+5yS98IlW6FbQXZF0/D3Y9J2i0Hp
ylHzm306QNtquxM9vop1GOWvgLcc239Y2G5SaH6ojvx5ajKmr7QYHLrA 8l8=
;; Received 1623 bytes from 199.6.0.30#53(199.6.0.30) in 60 ms
]]>

You'll see a number of new record types in this response, but don't worry, I go over all of the new DNSSEC record types next.

DNSSEC Terminology

A lot of different acronyms and new terminology comes up when you read DNSSEC documentation, so here are a few common terms you'll want to be acquainted with as you use DNSSEC:

  • RR (Resource Record): this is the smallest unit of data in a zone, such as a single A record, NS record or MX record.

  • RRSET: a complete set of Resource Records. For instance, an RRSET might be all NS records or A records for a particular name.

  • KSK (Key-Signing Key): signs DNSKEY records in a zone.

  • ZSK (Zone-Signing Key): signs all of the other records in a zone.

  • SEP (Secure Entry Point): a flag set in a key to denote it as a KSK.

While best practice dictates a separate KSK and ZSK, it isn't an actual requirement. In my next article on DNSSEC implementation, I will discuss the main differences between the two key types and why separate keys is considered a best practice.

New DNSSEC Record Types

DNSSEC also has introduced a number of new DNS record types into the mix. These records are published with the zone along with the rest of your DNS records and are pulled down as needed by any DNSSEC-enabled query:

  • DNSKEY: this is a public key for the zone and can either be a KSK or ZSK.

  • RRSIG (Resource Record Signature): this record contains a signature for an RRSET created with a particular ZSK.

  • NSEC (Next Secure record): these records are used in "negative answers" to prove whether a name exists.

  • NSEC3 (Next Secure version 3): these records are like NSEC, but protect against "zone walking" where an outside user could use NSEC records to walk down the zone and discover all of the records in the zone (much like being able to perform a zone transfer).

  • DS (Delegation Signer): this record contains a KSK signature and is submitted to the zone's parent where it is signed and is used as part of a chain of trust.

  • DLV (DNSSEC Look-aside Validation): much like DS records, but are used when DS records are not supported by a zone, or as an alternate trust anchor if your registrar doesn't support DNSSEC.

DNSSEC Look-Aside Validation

DNSSEC has a sort of chicken-and-egg problem. If your TLD does not support DNSSEC, any outside resolvers won't have a complete chain of trust from root, through the TLD, to your zone. There also may be a case where your TLD does support DNSSEC, but your registrar doesn't provide a mechanism to upload a DS record to the TLD (most registrars sadly don't). In either case, DNSSEC Look-aside Validation (DLV) has been created to provide an alternate trust anchor.

You can find more details on DLV at http://dlv.isc.org (one of the main DLV providers), but essentially, instead of generating a DS record to submit to a TLD, you generate a special DLV record and submit it to a DLV server. As long as a DNS resolver is configured to trust, for instance, dlv.isc.org, it can use that to anchor the chain of trust and then trust your signed records.

It turns out DNSSEC has a lot of new concepts to understand before you even get to implementing it; however, the implementation isn't all that bad once you see the steps laid out, so be sure to follow up with my article next month when I talk about how to implement DNSSEC for a zone using BIND. I'll even cover how to set up DLV for the zone.

Resources

A Collection of Links to DNSSEC Information: http://dnssec.net

ISC's DLV Documentation: https://dlv.isc.org

DNSSEC Chain of Trust Visualizer: http://dnsviz.net

Kyle Rankin is a Tech Editor and columnist at Linux Journal and the Chief Security Officer at Purism. He is the author of Linux Hardening in Hostile Networks, DevOps Troubleshooting, The Official Ubuntu Server Book, Knoppix Hacks, Knoppix Pocket Reference, Linux Multimedia Hacks and Ubuntu Hacks, and also a contributor to a number of other O'Reilly books. Rankin speaks frequently on security and open-source software including at BsidesLV, O'Reilly Security Conference, OSCON, SCALE, CactusCon, Linux World Expo and Penguicon. You can follow him at @kylerankin.

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