DNSSEC Part II: the Implementation

This article is the second in a series on DNSSEC. In the first one, I gave a general overview of DNSSEC concepts to lay the foundation for this article, which discusses how to enable DNSSEC for a zone using BIND. If you want to deploy DNSSEC but aren't sure what I mean when I say KSK, ZSK, DLV or DS record, you may want to go back to Part I to refresh yourself on the concepts, because in this article, I'm going to dive right in to implementation.

Adding DNSSEC to a zone using BIND involves a few extra steps on top of what you normally would do to configure BIND as a master for your zone. First, you will need to generate a Key-Signing Key (KSK) and Zone-Signing Key (ZSK), then update the zone's config and sign it with the keys. Finally, you will reconfigure BIND itself to support DNSSEC. After that, your zone should be ready, so if your registrar supports DNSSEC, you can update it or otherwise use DLV with a provider like dlv.isc.org. Now, let's look at the steps in more detail using my greenfly.org zone as an example.

Make the Keys

The first step is to generate the KSK and ZSK for your zone. As I mentioned in my previous article, the KSK is used only to sign ZSKs in the zone and to provide a signature for the zone's parent to sign, while ZSKs sign the records in each zone. Having separate keys also allows you to create a stronger KSK and have a weaker ZSK that you can rotate out each month. So first, let's create a KSK for greenfly.org using dnssec-keygen:

$ cd /etc/bind/
$ dnssec-keygen -a RSASHA1 -b 2048 -n ZONE -f KSK greenfly.org

By default, the dnssec-keygen command dumps the generated keys in the current directory, so change to the directory in which you store your BIND configuration. The -a and -b arguments set the algorithm (RSASHA1) and key size (2048 bit), while the -n option tells dnssec-keygen what kind of key it is creating (a ZONE key). You also can use dnssec-keygen to generate keys for DDNS and other BIND features, so you need to be sure to specify this is for a zone. I also added a -f KSK option that tells dnssec-keygen to set a bit that denotes this key as a KSK instead of a ZSK. Finally, I specified the name of the zone this key is for: greenfly.org. This command should create two files: a .key file, which is the public key published in the zone, and a .private file, which is the private key and should be treated like a secret. These files start with a K, then the name of the zone, and then a series of numbers (the latter of which is randomly generated), so in my case, it created two files: Kgreenfly.org.+005+10849.key and Kgreenfly.org.+005+10849.private.

Next I need to create the ZSK. The command is very similar to the command to create the KSK, except I lower the bit size to 1024 bits, and I remove the -f KSK argument:

$ dnssec-keygen -a RSASHA1 -b 1024 -n ZONE greenfly.org

This command creates two other key files: Kgreenfly.org.+005+58317.key and Kgreenfly.org.+005+58317.private. Now I'm ready to update and sign my zone.

Update the Zone File

Now that each key is created, I need to update my zone file for greenfly.org (the file that contains my SOA, NS, A and other records) to include the public KSK and ZSK. In BIND, you can achieve this by adding $INCLUDE lines to the end of your zone. In my case, I added these two lines:

$INCLUDE Kgreenfly.org.+005+10849.key ; KSK
$INCLUDE Kgreenfly.org.+005+58317.key ; ZSK

Sign the Zone

Once the keys are included in the zone file, you are ready to sign the zone itself. You will use the dnssec-signzone command to do this:

$ dnssec-signzone -o greenfly.org -k Kgreenfly.org.+005+10849 \
  db.greenfly.org Kgreenfly.org.+005+58317.key

In this example, the -o option specifies the zone origin, essentially the actual name of the zone to update (in my case, greenfly.org). The -k option is used to point to the name of the KSK to use to sign the zone. The last two arguments are the zone file itself (db.greenfly.org) and the name of the ZSK file to use.

If you are using DLV, you will add an extra -l option to specify the DLV server you are using:

$ dnssec-signzone -l dlv.isc.org -o greenfly.org -k \
  Kgreenfly.org.+005+10849 db.greenfly.org \

In either case, the command will create a new .signed zone file (in my case, db.greenfly.org.signed) that contains all of your zone information along with a lot of new DNSSEC-related records that list signatures for each RRSET in your zone. If you aren't using DLV, it also will create a dsset-zonename file that contains a DS record you will use to get your zone signed by the zone parent. If you are using DLV, you will get a dlvset-zonename file. Any time you make a change to the zone, simply update your regular zone file like you normally would, then run the dnssec-signzone command to create an updated .signed file. Some administrators recommend even putting the dnssec-signzone command in a cron job to run daily or weekly, as by default the key signatures will expire after a month if you don't run dnssec-signzone in that time.

Reconfigure Zone's BIND Config

Now that you have a new .signed zone file, you will need to update your zone's config in BIND so that it uses it instead of the plain-text file, which is pretty straightforward:

zone "greenfly.org" {
  type master;
  file "/etc/bind/db.greenfly.org.signed";
  allow-transfer { slaves; };

Enable DNSSEC Support in BIND

Next, update the options that are enabled in your main BIND configuration file (often found in named.conf or named.conf.options), so that DNSSEC is enabled, the server attempts to validate DNSSEC for any recursive queries and DLV (DNSSEC Lookaside Validation) is supported:

options {
  dnssec-enable yes;
  dnssec-validation yes;
  dnssec-lookaside auto;

When you set dnssec-lookaside to auto, BIND automatically will trust the DLV signature it has for dlv.isc.org as it's included with the BIND software. Alternatively, you can add a DLV key manually if you add an additional BIND option and trusted key:

options { dnssec-lookaside . trust-anchor dlv.isc.org.; };
trusted-keys {
        dlv.isc.org. 257 3 5
QKtUdvNXDrYJDSHZws3xiRXF1Rf+al9UmZfSav/4NWLKjHzpT59k/VSt TDN0YUuWrBNh";

Once you are done changing your BIND configuration files, reload or restart BIND, and your zone should be ready to reply to DNSSEC queries.


To test DNSSEC support for a zone, just add the +dnssec argument to dig. Here's an example query against www.greenfly.org:

$ dig +dnssec www.greenfly.org

; <<>> DiG 9.8.1-P1 <<>> +dnssec www.greenfly.org
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 13093
;; flags: qr aa rd ra; QUERY: 1, ANSWER: 2, AUTHORITY: 3, ADDITIONAL: 5

; EDNS: version: 0, flags: do; udp: 4096
;www.greenfly.org.              IN      A

www.greenfly.org.       900     IN      A
www.greenfly.org.       900     IN      RRSIG   A 5 3 900 20130523213855
20130423213855 58317 greenfly.org.
7LzmEumdAVM2MmrPVu+PKGfilPlfofjwJLbgVhyYqepbbD8xv3bmg0Np YnM=

greenfly.org.           900     IN      NS      ns2.greenfly.org.
greenfly.org.           900     IN      NS      ns1.greenfly.org.
greenfly.org.           900     IN      RRSIG   NS 5 2 900 20130523213855
20130423213855 58317 greenfly.org.
BqmNIHzROZkf6BOrx6Nqv65npSGoNLQBoEc90FvDFe/N5I27LBTIxCv4 3UQ=

ns1.greenfly.org.       900     IN      A
ns2.greenfly.org.       900     IN      A
ns1.greenfly.org.       900     IN      RRSIG   A 5 3 900 20130523213855
20130423213855 58317 greenfly.org.
Rp01qVkeBIZ7g+K7LY2XRU3DGSzbeFUKrViqtakbTQxZ9o3Oj6ZqL0Pv 0nQ=
ns2.greenfly.org.       900     IN      RRSIG   A 5 3 900 20130523213855
20130423213855 58317 greenfly.org.
UcSSD7wft9YO7UTIiQrc8LkItXZAKd72Gy1ZP4mhhLxwwOIhlHshQ9d2 uTY=

;; Query time: 196 msec
;; WHEN: Fri Apr 26 16:13:22 2013
;; MSG SIZE  rcvd: 817

Tell Your Parent

The final step once you have confirmed that DNSSEC is returning signed records for your zone is to go to your zone's parent (typically through the registrar you used to buy the domain to begin with) and provide them with the DS record (in that dsset-zonename file that dnssec-signzone generated) so they can sign it. Unfortunately, only a small number of registrars provide DNSSEC support today, and some charge extra for the service. In either case, you may want to use DLV instead via a service like dlv.isc.org. To do that, simply visit https://dlv.isc.org and follow the instructions to create an account and register your zone with them. They provide a simple interface that validates DNSSEC on your zone and even will send you alerts if you forget to update your zone's signatures after a month.

So, although enabling DNSSEC isn't as simple as a regular BIND configuration (and to many people even that is pretty complicated), it's also not all that difficult once you know the proper steps. Hopefully, this column has encouraged you to try out DNSSEC on your zones.

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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