Secure DNS

Introduction

The DNS Security Extensions, also known as DNSSEC, make it possible verify the authenticity and integrity of data obtained from the domain name system. To enable DNSSEC operation modified resolvers are needed that check data received from DNS servers. On the part of the authoritative name servers no modifications are necessary except for additional DNS records containing the signature data.

Quick How-To

There is an excellent howto available from RIPE at [1].

What You Need

BIND 9.3

For example available in Debian Testing or Unstable. To get it into Debian Stable put the testing-sources into /etc/apt/sources.list, e.g.

 deb http://ftp.de.debian.org/debian etch main

deb http://security.debian.org/ etch/updates main and into /etc/apt/apt.conf:

 APT::Default-Release "3.1*"

You can then install bind 9.3 with

apt-get install bind9/testing libisccc0/testing dnsutils/testing

Key-Management

It's best to generate two keypairs for each zone: a key-signing key (KSK) and a zone-signing key (ZSK).

Key-signing key

The key-signing key will only be used to sign the zone-signing key and will be the key that is transmitted to the parent zone as a designed signer or to the clients as a secure entry-point. If this key is rolled over all those parties will have to be informed.

Zone-signing key

This key is used to actually sign the zone data. If this key is rolled over it will only have to be resigned by the key-signing key with no external notification necessary.

The keys are generated with

dnssec-keygen -a RSASHA1 -b 1024 -n ZONE snug.local

for the zone-signing key and

dnssec-keygen -f KSK -a RSASHA1 -b 2048 -n ZONE snug.local

for the key-signing key (where snug.local is the name of the zone). Of course you can choose keysize and algorithm to your heart's content (dnssec-keygen --help gives a list of possible combinations), but only DSA and RSASHA1 are required to be implemented. Note also that it would be wise to choose a stronger key for the KSK as it can't be changed easily. For the same reason it should be removed from the computer after use and brought to some safe storage.

After each key generation the base name of the key files is printed, e.g.

Ksnug.local.+005+40668

With this base name two files are generated: the private key file (Ksnug.local.+005+40668.private) and the public key file (Ksnug.local.+005+40668.key). For obvious reasons the private key should be kept secret.

The .key with the public key will have a DNSKEY suitable for including in a zone file, e.g. like the following (key material omitted for clarity):

snug.local. IN DNSKEY 257 3 5 AQO...USjD

In BIND the 257 in the fourth column indicates a key-signing key, a zone-signing key would have a 256 there.

Signing Zones

Before signing the zone there must be two modifications made to the zone:

1. The serial number has to be incremented (as we're going to change the zone data)
2. Both key files have to be included, e.g. like

$include Ksnug.local.+005+19919.key ; ZSK
$include Ksnug.local.+005+40668.key ; KSK

After that the zone can be signed, e.g.

dnssec-signzone -o snug.local \
                -k Ksnug.local.+005+40668  \
                db.snug.local \
                Ksnug.local.+005+19919.key

where snug.local is the name of the zone, Ksnug.local.+005+40668 is the base name of the key-signing key, db.snug.local is the name of the zonefile and Ksnug.local.+005+19919.key is the public part of the zone-signing key.

The dnssec-signzone tool will then create an output zone file db.snug.local.signed which should be used instead of db.snug.local in the BIND configuration