11.4 The DNS Security Extensions

TSIG, which we described earlier in this chapter, is well suited to securing the communications between two name servers or between an updater and a name server. However, it won't protect you if one of your name servers is compromised: if someone breaks into the host that runs one of your name servers, he may also gain access to its TSIG keys. Moreover, because TSIG uses shared secrets, it isn't practical to configure TSIG among many name servers. You couldn't use TSIG to secure your name servers' communications with arbitrary name servers on the Internet because you can't distribute and manage that many keys.

The most common way to deal with key management problems like these is to use public key cryptography. The DNS Security Extensions, described in RFC 2535, use public key cryptography to enable zone administrators to digitally sign their zone data, thereby proving its authenticity.

Note: we'll describe the DNS Security Extensions, or DNSSEC, in their current form as described by RFC 2535. However, the IETF's DNSEXT working group is still working on DNSSEC and may change aspects of it before it becomes a standard.Another note: though BIND 8 provided preliminary support of DNSSEC as early as BIND 8.2,[5] DNSSEC wasn't really usable before BIND 9. Consequently, we'll use BIND 9 in our examples. If you want to use DNSSEC, you really shouldn't use anything older. [5] In particular, BIND 8 can't follow a chain of trust. It can verify SIG records only in zones it has trusted-keys statements for.

11.4.1 Public Key Cryptography and Digital Signatures

Public key cryptography solves the key distribution problem by using asymmetric cryptographic algorithms. In an asymmetric cryptographic algorithm, one key is used to decrypt data that another has encrypted. These two keys—a key pair—are generated at the same time using a mathematical formula. That's the only easy way to find two keys that have this special asymmetry (one decrypts what the other encrypts): it's very difficult to determine one key given the other. (In the most popular asymmetric cryptographic algorithm, RSA, that determination involves factoring very large numbers.)

In public key cryptography, an individual first generates a key pair. Then one key of the key pair is made public (e.g., published in a directory) while the other is kept private. Someone who wants to communicate securely with that individual can encrypt a message with the individual's public key and then send the encrypted message to the individual. (Or he could even post the message to a newsgroup or on a web site.) If the recipient has kept his private key private, only he can decrypt the message.

Conversely, the individual can encrypt a message with his private key and send it to someone. The recipient can verify that it came from the individual by attempting to decrypt it with the individual's public key. If the message decrypts to something reasonable (i.e., not gibberish) and the sender kept his private key to himself, then the individual must have encrypted it. Successful decryption also proves that the message wasn't modified in transit (e.g., while passing through a mail server), because if it had been, it wouldn't have decrypted correctly. So the recipient has authenticated the message.

Unfortunately, encrypting large amounts of data with asymmetric encryption algorithms tends to be slow—much slower than encryption using symmetric encryption algorithms. But when using public key encryption for authentication (and not for privacy), we don't have to encrypt the whole message. Instead, we run the message through a one-way hash function first. Then we can encrypt just the hash value, which represents the original data. We attach the encrypted hash value, now called a digital signature, to the message we want to authenticate. The recipient can still authenticate the message by decrypting the digital signature and running the message through her own copy of the one-way hash function. If the hash values match, the message is authentic. The process of signing and verifying a message is shown in Figure 11-7.

Figure 11-7. Signing and verifying a message

11.4.2 The KEY Record

In the DNS Security Extensions, or DNSSEC, each secure zone has a key pair associated with it. The zone's private key is stored somewhere safe, often in a file on the name server's filesystem. The zone's public key is advertised as a new type of record attached to the domain name of the zone, the KEY record.

The KEY record is actually a general-purpose record, as we'll see when we dissect one. You can use the KEY record to store different kinds of cryptographic keys, not just zones' public keys for use with DNSSEC. However, the only use we're going to explore in this book is to store a zone's public key.

A KEY record looks like this:

movie.edu. IN KEY 256 3 1 AQPdWbrGbVv1eDhNgRhpJMPonJfA3reyEo82ekwRnjbX7+uBxB11Bq
L7 LAB7/C+eb0vCtI53FwMhkkNkTmA6bI8B

The owner is the domain name of the zone that owns this public key. The first field after the type, 256, is the flags field. The flags field is two bytes long and encodes a set of one- and two-bit values:

0   1   2   3   4   5   6   7   8   9   0   1   2   3   4   5
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
   |  A/C  | Z | XT| Z | Z | NAMTYP| Z | Z | Z | Z |      SIG      |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+

If the value of the first bit is zero, the key can be used for authentication. Clearly, a key that we can't use for authentication isn't very useful in DNSSEC, so that bit is always zero.

If the value of the second bit is zero, the key can be used for confidentiality. DNSSEC doesn't make your zone data private, but also doesn't prohibit you from using your zone's public key for confidentiality, so this bit is always zero for a zone's public key.

A KEY record in which the first two bits are set to one is called a null key. Later on, we'll show you how null keys are used in parent zones.

The third bit is reserved for future use. For now, its value must be zero. The fourth bit is a "flag extension" bit. It's designed to provide future expandability. If it's set, the KEY record must include another two-byte field after the algorithm field (the third field after the type) and before the public key itself (normally the fourth field). The meanings of the bits in that additional two-byte field haven't been defined yet, so for now the fourth bit is always zero. Like the third bit, the fifth and sixth bits are reserved and must be zero.

The seventh and eighth bits encode the type of key:

00

This is a user's key. A mail user agent might use a user's key to encrypt email addressed to that user. This type of key isn't used in DNSSEC.

01

This is a zone's public key. All DNSSEC keys are this type of key.

10

This is a host's key. An IPSEC implementation might use a host's key to encrypt all IP packets sent to that host. DNSSEC doesn't use host keys.

11

Reserved for future use.

The ninth through twelfth bits are reserved and must be zero. The last four bits are the signatory field, which is now obsolete.

In the KEY record shown earlier, the flags field (the first field in the record after the type) says that this KEY is movie.edu's zone key and can be used for authentication and confidentiality.

The next field in the record, which in the example has the value 3, is called the protocol octet. Since you can use KEY records for different purposes, you have to specify which purpose a particular key is intended for. The following values are defined:

0

Reserved.

1

This key is used with Transport Layer Security (TLS), as described in RFC 2246.

2

This key is used in connection with email, e.g., an S/MIME key.

3

This key is used with DNSSEC. All DNSSEC keys, obviously, will have a protocol octet of 3.

4

This key is used with IPSEC.

255

This key is used with any protocol that can use a KEY record.

All of the values between 4 and 255 are available for future assignment.

The next (third) field in the KEY record, which here has the value 1, is the algorithm number. DNSSEC can work with a number of public key encryption algorithms, so you need to identify which algorithm a zone uses and which algorithm this key is used with here. The following values are defined:

0

Reserved.

1

RSA/MD5. RFC 2535 recommends, but doesn't require, the use of RSA/MD5. However, RSA is very popular and the patent covering the RSA algorithm recently ran out, so there has been some discussion of making the use of RSA mandatory.

2

Diffie-Hellman. RFC 2535 makes using Diffie-Hellman optional.

3

DSA. RFC 2535 makes support (not use) of DSA mandatory. However, as noted earlier, this may change soon.

4

Reserved for an elliptic curve-based public key algorithm.

We'll use RSA keys in our examples because we think RSA keys are likely to become the standard.

The final field in the KEY record is the public key itself, encoded in base 64. DNSSEC supports keys of many lengths, as we'll see shortly when we generate the movie.edupublic key. The longer the key, the harder it is to find the corresponding private key, but the longer it takes to sign zone data with the private key and verify it with the public key.

Null keys don't have a public key, though they do have a protocol octet and an algorithm number.

11.4.3 The SIG Record

If the KEY record stores a zone's public key, then there must be a new record to store the corresponding private key's signature, right? Sure enough, that's the SIG record. The SIG record stores the private key's digital signature on an RRset. An RRset is a group of resource records with the same owner, class, and type; for example, all of wormhole.movie.edu's address records make up an RRset. Likewise, all of movie.edu's MX records are another RRset.

Why sign RRsets rather than individual records? It saves time. There's no way to look up just one of wormhole.movie.edu's address records; a name server will always return them as a group. So why go to the trouble of signing each one individually when you can sign them together?

Here's the SIG record that "covers" wormhole.movie.edu's address records:

wormhole.movie.edu.             SIG     A 1 3 86400 20010102235426 (
                                20001203235426 27791 movie.edu.
                                1S/LuuxhSHs2LknPC7K/7v4+PNxESKZnjX6CtgGLZDWf
                                Rmovkw9VpW7htTNJYhzlFck/BO/k17tRj0fbQ6JWaA== )

The owner name is wormhole.movie.edu, the same as the owner of the records signed. The first field after the type, which holds the value A, is called the type covered. That tells us which of wormhole.movie.edu's records were signed; in this case, its address records. There would be a separate SIG record for each type of record wormhole.movie.edu might own.

The second field, which has the value 1, is the algorithm number. This is one of the same values used in the KEY record's algorithm number field, so 1 means RSA/MD5. If you generate an RSA key and use it to sign your zone, you'll get RSA/MD5 signatures, naturally. If you sign your zone with multiple types of keys, say an RSA key and a DSA key, you'll end up with two SIG records for each RRset, one with an algorithm number of 1 (RSA/MD5) and one with an algorithm number of 3 (DSA).^[6]

^[6] You might sign your zone with two different algorithms' keys so that people whose software supported only DSA could verify your data while people who preferred RSA could use RSA.

The third field is called the labels field. It indicates how many labels there are in the owner name of the records signed. wormhole.movie.eduobviously has three labels, so the labels field contains 3. When would the labels field ever differ from the number of labels in the SIG's owner? When the SIG record covered a wildcard record of some type. Unfortunately (or maybe fortunately, for our sanity's sake), BIND doesn't support wildcard records in secure zones.

The fourth field is the original TTL on the records in the RRset that was signed. (All the records in an RRset are supposed to have the same TTL.) The TTL needs to be stored here because a name server caching the RRset that this SIG record covers will decrement the TTLs on the cached records. Without the original TTL, it's impossible to feed the original address records through the one-way hash function in their original state to verify the digital signature.

The next two fields are the signature expiration and inception fields, respectively. They're both stored as an unsigned integer number of seconds since the Unix epoch, January 1, 1970, but in the SIG record's text representation, they're presented in the format YYYYMMDDHHMMSSfor convenience. (The signature expiration time for the SIG record we showed you earlier is just after 11:54 p.m. on January 2, 2001.) The signature inception time is usually the time you ran the program to sign your zone. You choose the signature expiration time when you run that program, too. After the signature's expiration, the SIG record is no longer valid and can't be used to verify the RRset. Bummer. This means that you have to re-sign your zone data periodically to keep the signatures valid. Fun. Thankfully, re-signing takes much less time than signing it for the first time.

The next (seventh) field in the SIG record, which in this record contains 27791, is the key tag field. The key tag is a fingerprint derived from the public key that corresponds to the private key that signed the zone. If the zone has more than one public key (and yours will when you're changing keys), DNSSEC verification software uses the key tag to determine which key to use to verify this signature.

The eighth field, which contains movie.edu, is the signer's name field. As you'd expect, it's the domain name of the public key that a verifier should use to check the signature. It, together with the key tag, identifies the KEY record to use. In most cases, the signer's name field is the domain name of the zone the signed records are in. In one case, however—which we'll cover soon—the signer's name is the domain name of the parent zone.

The final field is the signature field. This is the digital signature of the zone's private key on the signed records and the SIG record itself, minus this field. Like the key in the KEY record, this signature is encoded in base 64.

11.4.4 The NXT Record

DNSSEC introduces one more new record type: the NXT record. We'll explain what it's for.

What happens if you look up a domain name that doesn't exist in a secure zone? If the zone weren't secure, the name server would simply respond with the "no such domain name" response code. But how do you sign a response code? If you signed the whole response message, it would be difficult to cache.

The NXT record solves the problem of signing negative responses. It "spans" a gap between two consecutive domain names in a zone, telling you which domain name comes next after a given domain name—hence the name of the record.

But doesn't the notion of "consecutive domain names" imply a canonical order to the domain names in a zone? Why, yes, it does.

To order the domain names in a zone, you begin by sorting by the rightmost label in those domain names, then by the next label to the left, and so on. Labels are sorted case-insensitively and lexicographically (by dictionary order), with numbers coming before letters and nonexistent labels before numbers (in other words, movie.edu would come before 0.movie.edu). So the domain names in movie.edu would sort to the following:

movie.edu
bigt.movie.edu
carrie.movie.edu
cujo.movie.edu
dh.movie.edu
diehard.movie.edu
fx.movie.edu
bladerunner.fx.movie.edu
outland.fx.movie.edu
horror.movie.edu
localhost.movie.edu
misery.movie.edu
robocop.movie.edu
shining.movie.edu
terminator.movie.edu
wh.movie.edu
wh249.movie.edu
wh253.movie.edu
wormhole.movie.edu

Notice that just as movie.edu comes before bigt.movie.edu, fx.movie.edu precedes bladerunner.fx.movie.edu.

Once the zone is in canonical order, the NXT records make sense. Here's one NXT record (the first, in fact) from movie.edu:

movie.edu.                        NXT     bigt.movie.edu. ( NS SOA MX SIG NXT )

This record says that the next domain name in the zone after movie.edu is bigt.movie.edu, which we could see from our sorted list of domain names. It also says that movie.edu has NS records, an SOA record, MX records, a SIG record, and a NXT record.

The last NXT record in a zone is special. Since there's really no next domain name after the last one, the last NXT record "wraps around" to the first record in the zone:

wormhole.movie.edu.               NXT     movie.edu. ( A SIG NXT )

In other words, to indicate that wormhole.movie.edu is the last domain name in the zone, we say that the next domain name is movie.edu, the first domain name in the zone.

So how do NXT records provide authenticated negative responses? Well, if you looked up www.movie.edu internally, you'd get back the wormhole.movie.edu NXT record, telling you that there's no www.movie.edu because there are no domain names in the zone after wormhole.movie.edu. Similarly, if you tried to look up TXT records for movie.edu, you'd get the first NXT record we showed you, which tells you there are no TXT records for movie.edu, just NS, SOA, MX, SIG, and NXT records.

A SIG record covering the NXT record accompanies it in the response, authenticating the nonexistence of the domain name or type of data you asked for.

It's important that the NXT records, in toto, identify specifically what doesn't exist in the zone. A single catch-all record that simply says "That doesn't exist" could be sniffed off the wire and replayed to claim falsely that existing domain names or records don't actually exist.

For those of you worried about the prospects of adding of these new records to your zone and keeping them up to date manually—uh-oh, now that I've added a host, I've got to adjust my NXT records—take heart: BIND provides a tool to add NXT and SIG records for you automatically.

Some of you may also worry about the information NXT records reveal about your zone. A hacker could, for example, look up the NXT record attached to the domain name of your zone to find the lexicographically next domain name, then repeat the process to learn all the domain names in the zone. That, unfortunately, is an unavoidable side effect of securing your zone. Just repeat this mantra: "My zone data is secure, but public."

11.4.5 The Chain of Trust

There's one more aspect of DNSSEC theory that we should discuss: the chain of trust. (No, this isn't some touchy-feely team-building exercise.) So far, each RRset in our secure zone has a SIG record associated with it. To let others verify those SIG records, our zone advertises its public key to the world in a KEY record. But imagine that someone breaks into our primary master name server. What's to keep him from generating his own key pair? Then he could modify our zone data, re-sign our zone with his newly generated private key, and advertise his newly generated public key in a KEY record.

To combat this problem, our public key is "certified" by a higher authority. This higher authority attests to the fact that the movie.edu public key in our KEY record really belongs to the organization that owns and runs the zone, and not to some random yahoo. Before certifying us, this higher authority demanded some sort of proof that we were who we said we were and that we were the duly authorized administrators of movie.edu.

This higher authority is our parent zone, edu. When we generated our key pair and signed our zone, we also sent our public key to the administrators of edu, along with proof of our identity and of our positions as the Two True Administrators of movie.edu.^[7] They signed our KEY record with the eduzone's private key and sent it back to us so that we could add it to our zone. Here's our KEY record and its accompanying SIG record:

^[7] In fact, there aren't any top-level zones signing their child zone's KEY records yet, though some European registries will likely begin signing KEY records soon.

movie.edu               IN SIG  KEY 1 2 3600 20010104010141 (
                                20001205010141 65398 edu.
                                aE4sCZKgFtp5RuD1sib0+19dc3MF/y9S2Fr8+h66g+Y2
                                1bc31M4y0493cSoyRpapJrd7qfG+Cr7GK+uY+eLCRA== )
                        KEY     256 3 1 (
                                AQPdWbrGbVv1eDhNgRhpJMPonJfA3reyEo82ekwRnjbX
                                7+uBxB11BqL7LAB7/C+eb0vCtI53FwMhkkNkTmA6bI8B )

Note that the signer's name field of the SIG record is edu, not movie.edu, showing that our KEY record was signed by our parent zone's private key, not our own.

What if someone were to break into the edu zone's primary master name server? The edu zone's KEY record is signed by the root zone's private key. And the root zone? Well, the root zone's public key is very widely known and configured on every name server that supports DNSSEC.^[8]

^[8] This reminds us of the tale of the man who asks the priest what holds the Earth up. The priest tells him that the Earth rests on the back of a turtle, which holds it up. The man then asks what the turtle rests on. "On the back of an elephant," replies the priest. "But what," the man asks, "does the elephant rest on?" The frustrated priest snaps back, "It's elephants all the way down!"

That is, the root zone's public key will beconfigured on every name server once DNSSEC is widely implemented. Right now, neither the root zone nor the edu zone is signed, and neither has a key pair. Until DNSSEC is widely implemented, though, it's possible to use DNSSEC piecemeal.

11.4.5.1 Security roots

Let's say we want to begin using DNSSEC at Movie U. to improve the security of our zone data. We've signed the movie.edu zone but can't have edu sign our KEY record since they haven't secured their zone yet and don't have a key pair. How can other name servers on the Internet verify our zone data? How can our own name servers verify our zone data, for that matter?

BIND 9 name servers provide a mechanism for specifying the public key that corresponds to a particular zone in the named.conf file: the trusted-keys statement. Here's the trusted-keys statement for movie.edu:

trusted-keys {
	movie.edu. 256 3 1 "AQPdWbrGbVv1eDhNgRhpJMPonJfA3reyEo82ekwRnjbX7+uBxB11Bq
L7 LAB7/C+eb0vCtI53FwMhkkNkTmA6bI8B";
};

It's basically the KEY record without the class and type fields and with the key itself quoted. The domain name of the zone may be quoted, but it's not necessary. If movie.edu had more than one public key—say a DSA key—we could include it, too:

trusted-keys {
	movie.edu. 256 3 1 "AQPdWbrGbVv1eDhNgRhpJMPonJfA3reyEo82ekwRnjbX7+uBxB11Bq
L7 LAB7/C+eb0vCtI53FwMhkkNkTmA6bI8B";
	movie.edu. 256 3 3 "AMnD8GXACuJ5GVnfCJWmRydg2A6JptSm6tjH7QoL81SfBY/kcz1Nbe
Hh z4l9AT1GG2kAZjGLjH07BZHY+joz6iYMPRCDaPOIt9LO+SRfBNZg62P4 aSPT5zVQPahDIMZmTIvv
O7FV6IaTV+cQiKQl6noro8uTk4asCADrAHw0 iVjzjaYpoFF5AsB0cJU18fzDiCNBUb0VqE1mKFuRA/K
1KyxM2vJ3U7IS to0IgACiCfHkYK5r3qFbMvF1GrjyVwfwCC4NcMsqEXIT8IEI/YYIgFt4 Ennh";
};

This trusted-keys statement enables a BIND 9 name server to verify any records in the movie.edu zone. The name server can also verify any records in child zones like fx.movie.edu, assuming their KEY records are signed by movie.edu's private key and it can verify records in their child zones (movie.edu's grandchildren), assuming a valid chain of trust back to the movie.edu zone's public key. In other words, movie.edubecomes a security root, below which our name server can verify any secure zone data.

11.4.5.2 Null keys

A security root lets your name server verify signed records below a certain point in the namespace. Null keys do the opposite: they tell your name server that records below a certain point aren't secured. Let's say the administrators of fx.movie.edu haven't secured their zone yet. When we sign movie.edu, the BIND 9 signer software adds a special null key to the movie.eduzone for fx.movie.edu:

fx.movie.edu.           KEY     49408 3 3 (
                                 )

Note that there's no base 64 encoding of a public key in the record. If you look very closely (or get out your scientific calculator), you'll see that the flags field indicates that this key should not be used for either authentication or confidentiality; that is, it's a null key. DNSSEC-aware name servers interpret this to mean that the fx.movie.edu zone isn't secure and that they shouldn't expect signed data from it.

If BIND 9's signer software finds a file containing fx.movie.edu's KEY record when it runs, it omits the null key, implying that fx.movie.edu is secure.

11.4.6 How the Records Are Used

Let's go through what a DNSSEC-capable name server does to verify a record in movie.edu. In particular, let's see what happens when it looks up the address of wormhole.movie.edu. First, of course, the name server sends a query for the address:

% dig +dnssec +norec wormhole.movie.edu.

; <<>> DiG 9.1.0 <<>> +dnssec +norec wormhole.movie.edu.
;; global options:  printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 15766
;; flags: qr aa ra; QUERY: 1, ANSWER: 3, AUTHORITY: 4, ADDITIONAL: 6

;; OPT PSEUDOSECTION:
; EDNS: version:    0, udp=   4096
;; QUESTION SECTION:
;wormhole.movie.edu.            IN      A

;; ANSWER SECTION:
wormhole.movie.edu.     86400   IN      A       192.249.249.1
wormhole.movie.edu.     86400   IN      A       192.253.253.1
wormhole.movie.edu.     86400   IN      SIG     A 1 3 86400 20010215174848 
20010116174848 27791 movie.edu. 
cYKQvgVksHjwGedNz72iyIpjXBhtSOeUEQA6V0b6l8asG3mpV6hzrzNf 
YwTpLoh9FSjSf0kUzmXkW9aYJmd5Bw==

;; AUTHORITY SECTION:
movie.edu.              86400   IN      NS      outland.fx.movie.edu.
movie.edu.              86400   IN      NS      wormhole.movie.edu.
movie.edu.              86400   IN      NS      terminator.movie.edu.
movie.edu.              86400   IN      SIG     NS 1 2 86400 20010215174848 
20010116174848 27791 movie.edu. 
ZXRnlbJBWJa4XX3YTWgkYnoQjGLFDN+2JwoGpLpxTidwkJ0FT+N3gMSw anSxa22b+X/
7v4b99t2WMcxCtUIXvw==

;; ADDITIONAL SECTION:
outland.fx.movie.edu.   86400   IN      A       192.253.254.3
terminator.movie.edu.   86400   IN      A       192.249.249.3
terminator.movie.edu.   86400   IN      SIG     A 1 3 86400 20010215174848 
20010116174848 27791 movie.edu. 
GSnxseyN4w5sA2Fb9uK9zVNSRJRbbcvr0DaDRwLDO8X2m6ZBbkRssSHJ tZYwoO4ZIFERLKakB//
VTDMhYJmNvw==
movie.edu.              86400   IN      KEY     256 3 1 
AQPdWbrGbVv1eDhNgRhpJMPonJfA3reyEo82ekwRnjbX7+uBxB11BqL7 LAB7/
C+eb0vCtI53FwMhkkNkTmA6bI8B
movie.edu.              86400   IN      SIG     KEY 1 2 86400 20010215174837 
20010116174837 65398 edu. LW+nc2gmz618u/LjDtlKSorv9OkJOwC8wj/sa/CpzCJJqceB/55JhsWI 
t1ADlfQwb4h9hs6oMeN2sU9jHiYQmw==

;; Query time: 3 msec
;; SERVER: 206.168.194.122#53(206.168.194.122)
;; WHEN: Tue Jan 16 10:49:48 2001
;; MSG SIZE  rcvd: 671

Notice that we had to specify +dnssec on the command line. BIND 9.1.0 and later name servers include DNSSEC records (SIG, NXT, and KEY) in a response only if the querier indicates that it can handle DNSSEC. How do queriers do that? By setting a special flag in a "pseudosection" of the header. It's called a pseudosection because it's not actually part of the bits in the header. Instead, it's a new record type, OPT, that's carried in the DNS message. OPT records usually indicate the capabilities of the querier.

Also notice that the response includes four SIG records: one covering the records in the answer section, one covering the records in the authority section, one covering terminator.movie.edu's address record in the additional section, and one covering movie.edu's KEY record in the additional section. The additional section would have included a SIG record covering outland.fx.movie.edu, address records for wormhole.movie.edu, and a SIG record covering those addresses if they'd fit in a single UDP datagram, but there wasn't enough room.

To verify the SIG records, the name server must look at movie.edu's KEY record, included in the additional section. But before using the key, it must verify the SIG on that key. That requires at least one additional query: to one of the edu name servers for the edu zone's public key—unless, of course, the name server already knows the movie.edu public key from a trusted-keys statement.

11.4.7 DNSSEC and Performance

It should be evident from this dig output that DNSSEC increases the average size of a DNS message, that it requires substantially more computational horsepower from name servers verifying zone data, and that signing a zone increases its size substantially—current estimates are that signing multiplies the size of a zone by a factor of seven. Each of these effects has its own consequences, some of which are less obvious:

• Larger DNS messages means more truncated messages, which means more fallback to the use of TCP. Using TCP, of course, is more resource-intensive than using UDP.

• Verifying zone data takes time and slows the resolution process.

• Larger zones mean larger and harder-to-administer named processes.

In fact, DNSSEC's complexity meant that BIND 8's architecture couldn't support DNSSEC. DNSSEC also provided part of the impetus for developing BIND 9 and for designing it to run on multiprocessor hosts. If you're planning on signing your zones, make sure your authoritative name servers have enough memory to load the new, larger zones. If your name servers are resolving more records in secure zones, make sure they have enough processor power to verify all those digital signatures—and remember that BIND 9 can take advantage of any processors you can add to the host it runs on.

11.4.8 Signing a Zone

Okay, now you've got the theoretical background you need to actually sign your zone. We'll show you how we signed movie.edu. Remember, we used the BIND 9 tools—they're much easier to use than the BIND 8 tools, and of course BIND 9 supports DNSSEC much more completely than BIND 8 does.

11.4.8.1 Generating your key pair

First, we generated a key pair for movie.edu:

# cd /var/named
# dnssec-keygen -a RSA -b 512 -n ZONE movie.edu.
Kmovie.edu.+001+27791

We ran dnssec-keygen in our name server's working directory. That's mostly for convenience: the zone data files are in this directory, so we won't need to use full pathnames as arguments. If we want to use dynamic update with DNSSEC, though, we'd need the keys in the name server's working directory.

Recall dnssec-keygen's options from the TSIG section of this chapter (oh, so long ago):

-a

The cryptographic algorithm to use, in this case RSA. We could also have used DSA, but RSA is more efficient.

-b

The length of the keys to generate, in bits. RSA keys can be anywhere from 512 to 2000 bits long. DSA keys can be 512 to 1024 bits long, as long as the length is divisible by 64.

-n

The type of key. DNSSEC keys are always zone keys.

The only non-option argument is the domain name of the zone, movie.edu. The dnssec-keygen program prints the basename of the files it's written the keys to. The numbers at the end of the basename (001 and 27791), as we explained in the TSIG section, are the key's DNSSEC algorithm number as used in the KEY record (001 is RSA/MD5), and the key's fingerprint, used to distinguish one key from another when multiple keys are associated with the same zone.

The public key is written to the file basename.key (Kmovie.edu.+001+27791.key). The private key is written to the file basename.private (Kmovie.edu.+001+27791.private). Remember to protect the private key; anyone who knows the private key can forge signed zone data. dnssec-keygen does what it can to help you: it makes the .private file readable and writable only by the user who runs it.

11.4.8.2 Sending your keys to be signed

Next, we sent our KEY record to the administrator of our parent zone to sign. BIND 9 includes a nice little program to package up the key for transmission, dnssec-makekeyset:

# dnssec-makekeyset -t 172800 Kmovie.edu.+001+27791.key

dnssec-makekeyset created a file called keyset-movie.edu,^[9] which contains the following:

^[9] In versions of dnssec-makekeyset shipped with BIND 9.0.1 and earlier, the name of the file would have been movie.edu.keyset. However, putting the domain name of the zone first caused problems: the name of the root zone's keyset file was .keyset, which is a hidden file on a Unix filesystem.

$ORIGIN .
$TTL 172800      ; 2 days
movie.edu               IN SIG  KEY 1 2 86400 20010104034839 (
                                20001205034839 27791 movie.edu.
                                M7RDKMyc9w1djDc0mQAXQc1PJdmLRBg3nfaGEUZe9Fbi
                                mjiNVaQK33IWhzI95oD8AS0WqRDy5TusTXt4nx1/dQ== )
                        KEY     256 3 1 (
                                AQPdWbrGbVv1eDhNgRhpJMPonJfA3reyEo82ekwRnjbX
                                7+uBxB11BqL7LAB7/C+eb0vCtI53FwMhkkNkTmA6bI8B )

The -t option takes a TTL for the records to submit. This serves as a suggestion to your parent zone's administrator of the TTL (in seconds) you'd like for your record. They may ignore it, of course. The SIG record actually contains a signature covering your zone's KEY record, generated with your zone's private key. That proves you really have the private key that corresponds to the public key in the KEY record—you're not just submitting a KEY record you found on the street.

The signature expiration and inception fields default to "now" and "30 days from now," respectively. These serve as a suggestion to the signer of the signature lifetime you'd like. You can use the -s (start) and -e (end) options to adjust the signature expiration and inception times. Both options accept either an absolute time as an argument, in the form YYYYMMDDHHMMSS, or an offset. For -s, the offset is calculated from the current time. For -e, the offset is calculated from the start time.

You can also use the signature expiration and inception fields to bundle up several keysets and submit them all at once to your parent zone's administrator to sign. For example, you could submit keysets valid for January, February, and March to your parent zone's administrator all at once, and then put them into production one per month.

Then we sent our file off to our parent zone's administrators to sign. Since the message included proof of our identity,^[10] they signed it with the dnssec-signkey program:

^[10] Since top-level zones haven't started signing zones yet, there's still some question as to how they'll require us to authenticate ourselves. The use of cryptographically signed email messages is a possibility.

# dnssec-signkey keyset-movie.edu Kedu.+001+65398.private

and sent the resulting file, movie.edu.signedkey, back to us:

$ORIGIN .
$TTL 172800       ; 1 hour
movie.edu               IN SIG  KEY 1 2 3600 20010104010141 (
                                20001205010141 65398 edu.
                                aE4sCZKgFtp5RuD1sib0+19dc3MF/y9S2Fr8+h66g+Y2
                                1bc31M4y0493cSoyRpapJrd7qfG+Cr7GK+uY+eLCRA== )
                        KEY     256 3 1 (
                                AQPdWbrGbVv1eDhNgRhpJMPonJfA3reyEo82ekwRnjbX
                                7+uBxB11BqL7LAB7/C+eb0vCtI53FwMhkkNkTmA6bI8B )

If we didn't care about getting our KEY record signed, we could have skipped this step. But then only name servers with a trusted-keys entry for movie.edu could verify our data.

11.4.8.3 Signing your zone

Before signing our zone, we had to add the KEY record to our plain-Jane zone data file:

# cat "$INCLUDE Kmovie.edu.+001+27791.key" >> db.movie.edu

That gave the signer program the information it needed to know which key to use to sign the zone. It automatically finds and includes the contents of movie.edu.signedkey.

Then we signed the zone with dnssec-signzone :

# dnssec-signzone -o movie.edu. db.movie.edu

We used the -o option to specify the origin in the zone data file, because dnssec-signzone doesn't read named.conf to determine which zone the file describes. The only non-option argument is the name of the zone data file.

This produces a new zone data file, db.movie.edu.signed, which begins like this:

$ORIGIN .
$TTL 86400      ; 1 day
movie.edu               IN SOA  terminator.movie.edu. al.robocop.movie.edu. (
                                2000092603 ; serial
                                10800      ; refresh (3 hours)
                                3600       ; retry (1 hour)
                                604800     ; expire (1 week)
                                3600       ; minimum (1 hour)
                                )
                        SIG     SOA 1 2 86400 20010104041530 (
                                20001205041530 27791 movie.edu.
                                aO1eZKhGSm99GgC9PfLXfHjl3tAWN/Vn33msppmyhN7a
                                RlfvJMTpSoJ9XwQCdjqhz0lcnCnQiL+jZkqU3uUecg== )
                        NS      terminator.movie.edu.
                        NS      wormhole.movie.edu.
                        NS      outland.fx.movie.edu.
                        SIG     NS 1 2 86400 20010104041530 (
                                20001205041530 27791 movie.edu.
                                pkmZJHqFlNmZdNjyupBMMzDDGweGsf9TS1EGci9cwKe5
                                c0o9h/yncInn2e8QSakjxpwKB8aw9D9uiStxJ/sLvQ== )
                        SIG     MX 1 2 86400 20010104041530 (
                                20001205041530 27791 movie.edu.
                                ZcKKeT0XaNlw83eSzRxt74DaLXvQtPYCdgKGOfSiJmYQ
                                WxI5zZUEWA6ku3w48mo9jbVF+/7nF3QcpFTIiwV1ug== )
$TTL 3600       ; 1 hour
                        SIG     NXT 1 2 3600 20010104041530 (
                                20001205041530 27791 movie.edu.
                                upMjK21eD7OQkrHpxSWqkOPcRXbfL8WAgQVK1aGHcTPE
                                X3JtaLtCLuKLd3YFs7T8BuZoN7aJYRVREWSPVedYPw== )
                        NXT     bigt.movie.edu. ( NS SOA MX SIG KEY NXT )
$TTL 172800     ; 2 days
                        SIG     KEY 1 2 172800 20001205040220 (
                                20001205040219 65398 edu.
                                HIReZ98rieIuRI04XsoL+xLRLe8tCQbNKD8USlV35vb4
                                VsLUGCAEgBq7lLsHty7YCskbxhQu8ncysBKnR/muiA== )
                        KEY     256 3 1 (
                                AQPdWbrGbVv1eDhNgRhpJMPonJfA3reyEo82ekwRnjbX
                                7+uBxB11BqL7LAB7/C+eb0vCtI53FwMhkkNkTmA6bI8B )
$TTL 86400      ; 1 day
                        MX      10 postmanrings2x.movie.edu.

Believe it or not, those are just the records attached to the domain name movie.edu. The zone data file as a whole nearly quadrupled in length and quintupled in size. Oy!

Finally, we changed the zone statement in named.conf so that named would load the new zone data file:

zone "movie.edu" {
	type master;
	file "db.movie.edu.signed";
};

Then we reloaded the zone and checked syslog.

dnssec-signzone does take some options that we didn't use:

-s , -e

These options specify the signature inception and expiration times to use in SIG records; they have exactly the same syntax as in dnssec-makekeyset.

-i

Specifies as an option argument the cycle period for resigning records (which we'll cover in a minute). This was the -c option before BIND 9.1.0.

-f

Specifies as an option argument the name of the file to write the signed zone to. The default is the name of the zone data file with .signed concatenated.

You can also specify, as a second non-option argument, which private key to use to sign the zone. By default, dnssec-signzone signs the zone with each of the zone's private keys in the directory. If you specify the name of one or more files that contain the zone's private keys as arguments, it will sign using only those keys.

Remember, you'll need to re-sign the zone each time you change the zone data, though you certainly don't need to generate a new key pair or have your KEY record re-signed each time. You can re-sign the zone by running dnssec-signzone on the signed zone data:

# dnssec-signzone -o movie.edu -f db.movie.edu.signed.new db.movie.edu.signed
# mv db.movie.edu.signed db.movie.edu.signed.bak
# mv db.movie.edu.signed.new db.movie.edu.signed
# rndc reload movie.edu

The program is smart enough to recalculate NXT records, sign new records, and re-sign records whose signature expiration times are approaching. By default, dnssec-signzone re-signs records whose signatures expire within 7.5 days (a quarter of the difference between the default signature inception and expiration times). If you specify different inception and expiration times, dnssec-signzone adjusts the re-signing cycle time accordingly. Or you can simply specify a cycle time with the -i (formerly the -c) option.

11.4.9 DNSSEC and Dynamic Update

dnssec-signzone isn't the only way to sign zone data. The BIND 9 name server is capable of signing dynamically updated records on the fly.^[11] Color us impressed!

^[11] Yet another DNSSEC capability BIND 8 doesn't have.

As long as the private key for a secure zone is available in the name server's working directory (in the correctly named .private file), a BIND 9 name server signs any records that are added via dynamic update. If any records are added to or deleted from the zone, the name server adjusts (and re-signs) the neighboring NXT records, too.

Let's show you this in action. First, we'll look up a domain name that doesn't yet exist in movie.edu:

% dig +dnssec perfectstorm.movie.edu.

; <<>> DiG 9.1.0 <<>> +dnssec perfectstorm.movie.edu.
;; global options:  printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NXDOMAIN, id: 4705
;; flags: qr aa rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 4, ADDITIONAL: 3

;; OPT PSEUDOSECTION:
; EDNS: version:    0, udp=   4096
;; QUESTION SECTION:
;perfectstorm.movie.edu.                IN      A

;; AUTHORITY SECTION:
movie.edu.              3600    IN      SOA     terminator.movie.edu. al.robocop.
movie.edu. 2001011600 10800 3600 604800 3600
movie.edu.              3600    IN      SIG     SOA 1 2 86400 20010215174848 
20010116174848 27791 movie.edu. 
Ea0+xyEsj0Hy4JP115r0D0UFVpWfxqf0NQA8hpKwlLCsxJ3rA+sJBg2Q ZiCTEwfAcwGRfbNsRYu/CcuV/
VJTDA==
misery.movie.edu.       86400   IN      NXT     robocop.movie.edu. A SIG NXT
misery.movie.edu.       86400   IN      SIG     NXT 1 3 86400 20010215174848 
20010116174848 27791 movie.edu. ZVfV9KbPb8hKZdZirlpv+WnUxv72di8lUgZiot/
JaWDsZPfNoYqSnKPW ND4H92guwj7oR6CgrhsgLJ9dMDYSpg==

(We trimmed the output a little.) Notice misery.movie.edu's NXT record, indicating that the domain name doesn't exist. Now we'll use nsupdate to add an address record for perfectstorm.movie.edu:

% nsupdate
> update add perfectstorm.movie.edu. 3600 IN A 192.249.249.91
>

Now, let's look up perfectstorm.movie.edu again:

% dig +dnssec perfectstorm.movie.edu.

; <<>> DiG 9.1.0 <<>> +dnssec perfectstorm.movie.edu.
;; global options:  printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 11973
;; flags: qr aa rd ra; QUERY: 1, ANSWER: 2, AUTHORITY: 4, ADDITIONAL: 9

;; OPT PSEUDOSECTION:
; EDNS: version:    0, udp=   4096
;; QUESTION SECTION:
;perfectstorm.movie.edu.                IN      A

;; ANSWER SECTION:
perfectstorm.movie.edu. 3600    IN      A       192.249.249.91
perfectstorm.movie.edu. 3600    IN      SIG     A 1 3 3600 20010215195456 
20010116185456 27791 movie.edu. 
C/JXdCLUdugxN91v0DZuUDTusi2XNNttb4bdB2nBujLxjwwPAf/D5MJz //
cDtuZ3X+uYzhkN8MDROqOwUQuQSA==

;; AUTHORITY SECTION:
movie.edu.              86400   IN      NS      terminator.movie.edu.
movie.edu.              86400   IN      NS      outland.fx.movie.edu.
movie.edu.              86400   IN      NS      wormhole.movie.edu.
movie.edu.              86400   IN      SIG     NS 1 2 86400 20010215195301 
20010116195301 27791 movie.edu. 
1ZR592izM1AjMusJ26e4lvQ0V9lFiFvQh6hCluBxSv7FwNqF7TcJFImc 
W52XhXbHUEtiFOzDqYMHOzPV7j23nA==

(Again, we trimmed the output a little.) Now not only was an address record generated, but there is also a SIG record generated from movie.edu's private key. The signature expiration is set to 30 days from the update by default, but you can change it with the sig-validity-interval substatement, which takes a number of days as an argument:^[12]

^[12] Before BIND 9.1.0, sig-validity-interval interpreted its argument as seconds, not days.

options {
	sig-validity-interval 7;  // We want SIG on updated records to last a week
};

The signature inception is always set to one hour before the update to allow for verifiers with clocks that may be slightly skewed from ours.

If we look up perfectstorm2.movie.edu (though how there'd be a sequel to that movie I don't know), we find the following:

% dig +dnssec perfectstorm2.movie.edu.

; <<>> DiG 9.1.0 <<>> +dnssec perfectstorm2.movie.edu.
;; global options:  printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NXDOMAIN, id: 11232
;; flags: qr aa rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 4, ADDITIONAL: 3

;; OPT PSEUDOSECTION:
; EDNS: version:    0, udp=   4096
;; QUESTION SECTION:
;perfectstorm2.movie.edu.       IN      A

;; AUTHORITY SECTION:
movie.edu.              3600    IN      SOA     terminator.movie.edu. al.robocop.
movie.edu. 2001011601 10800 3600 604800 3600
movie.edu.              3600    IN      SIG     SOA 1 2 86400 20010215195456 
20010116185456 27791 movie.edu. c1RwtgBX2SO8Q7Hz7vJD0aJfNfA6lsrqH4txHJI/slRpx/
UFYbnz3Gje N0JspZEihdLw0ZYMEiN6hnwRAzB4ag==
perfectstorm.movie.edu. 3600    IN      NXT     robocop.movie.edu. A SIG NXT
perfectstorm.movie.edu. 3600    IN      SIG     NXT 1 3 3600 20010215195456 
20010116185456 27791 movie.edu. 
qsB9l5AmSrB+qKmv+cKa+htCw84zwaakTmPC2yl+shzSEparrKwIMSR6 x5N69w8cze/
AW+gyFIwQZZkfZInJZA==

Notice the NXT record: it was added automatically when we added perfectstorm.movie.edu's address record because perfectstorm.movie.eduwas a new domain name in the zone. Sweet!

As impressive as this is, you should be careful when allowing dynamic updates to secure zones. You should make sure that you use strong authentication (e.g., TSIG) to authenticate the updates, or you'll give a hacker an easy backdoor to use to modify your "secure" zone. And you should ensure you have enough horsepower for the task: normally, dynamic updates don't take much to process. But dynamic updates to a secure zone require NXT recalculation and, more significantly, asymmetric encryption (to calculate new SIG records), so you should expect your name server to take longer and need more resources to process them.

11.4.10 Changing Keys

Though we said you don't need to generate a new key each time you sign your zone, there are occasions when you'll need to create a new key, either because you've "used up" your private key or, worse, because your private key has been cracked.

After a certain amount of use, it becomes dangerous to continue signing records with your private key. The larger the pool of available data that's been encrypted using your private key, the easier it is for a hacker to determine your private key using cryptanalysis. While there's no simple rule to tell you when your private key's time is up, here are some guidelines:

• The larger your zone is, the more data your private key encrypts when it signs it. If your zone is very large, change keys more frequently.

• The longer your key, the harder it is to crack. Long keys don't need to be changed as often as short keys.

• The more frequently you update and sign your zone data, the more data encrypted by your private key is available. If you update your zone data frequently—and particularly if you dynamically update your secure zone—change keys frequently.

• The more valuable it would be for a hacker to spoof your zone data, the more time and money he will spend trying to crack your private key. If the integrity of your zone data is particularly crucial, change keys frequently.

Since we update movie.edu only about once a day and it's not particularly large, we change our key pair every six months. We're only a university, after all. If we were more concerned about our zone data, we would use longer keys or change keys more frequently.

Unfortunately, rolling over to a new key isn't as easy as just generating a new key and replacing the old one with it. If you did that, you'd leave name servers that had cached your zone's data with no way to retrieve your zone's KEY record and verify that data. So rolling over to a new key is a multistep process:

Let's go through the process. First, we generate a new key pair:

# dnssec-keygen -a RSA -b 512 -n ZONE movie.edu.
Kmovie.edu.+001+47703

Next, we make a new keyset that contains both KEY records and send it to our parent zone's administrator:

# dnssec-makekeyset -t 172800 Kmovie.edu.+001+27791.key Kmovie.edu.+001+47703.key
# mail -s "Sign my keys, please" hostmaster@nsiregistry.net < keyset-movie.edu
# mv keyset-movie.edu keyset-movie.edu.2key

Then we make a keyset that contains just the new KEY record and send it to our parent zone's administrator, too:

% dnssec-makekeyset -t 172800 Kmovie.edu.+001+47703.key
% mail -s "Sign my keys, please" hostmaster@nsiregistry.net < keyset-movie.edu

(Okay, it would take a lot more than just those messages to get anyone to sign our keys.)

The first keyset includes both KEY records and SIG records covering both:

$ORIGIN .
$TTL 172800     ; 2 days
movie.edu               IN SIG  KEY 1 2 172800 20010104060917 (
                                20001205060917 27791 movie.edu.
                                RyNYoZ/k0tHqnFhUiVs2yjJWPFNeP8BKZ/Jaw+7xO9Jl
                                ZwJN2ZYQjVNVGLk30rJlxQRjCCdaaYQSq8u81up3xw== )
                        IN SIG  KEY 1 2 172800 20010104060917 (
                                20001205060917 47703 movie.edu.
                                1JGNBQydq6U+qKfq1wxfu1nsu283Zf7mNDDmuBtuuB7o
                                lwaeBL96tzBKpMUAcDYXsM8zxiStF+wTY+I5wfgevA== )
                        KEY     256 3 1 (
                                AQPdWbrGbVv1eDhNgRhpJMPonJfA3reyEo82ekwRnjbX
                                7+uBxB11BqL7LAB7/C+eb0vCtI53FwMhkkNkTmA6bI8B )
                        KEY     256 3 1 (
                                AQPjAfGtDkx6PSgYFs6G2vY1bJEldAMudngn6ZpGJnyg
                                k+LeU5PYiYd48wFLimihjyzlTWMb+C+egtQGpDVQulez )

Note that one of the SIG records was generated by the key with key tag 27791 (the old private key) while the other was generated by the key with tag 47703 (the new private key). This proves we have both of the corresponding private keys.

Once we get a response back from our parent zone's administrator, we save it to /var/named as movie.edu.signedkey, the filename we $INCLUDEd into db.movie.edu.signed. Here's what movie.edu.signedkey looks like:

$ORIGIN .
$TTL 172800     ; 2 days
movie.edu               IN SIG  KEY 1 2 172800 20010104060917 (
                                20001205060917 65398 edu.
                                qzvmuTVv9yGZf963ZuN2jxk8brEX/VP3sI5pOM/g2mU/
                                EPa57fyhHDNo7ny8Q2Su5vXnAIoxaaKAR8VmognQ7A== )
                        KEY     256 3 1 (
                                AQPdWbrGbVv1eDhNgRhpJMPonJfA3reyEo82ekwRnjbX
                                7+uBxB11BqL7LAB7/C+eb0vCtI53FwMhkkNkTmA6bI8B )
                        KEY     256 3 1 (
                                AQPjAfGtDkx6PSgYFs6G2vY1bJEldAMudngn6ZpGJnyg
                                k+LeU5PYiYd48wFLimihjyzlTWMb+C+egtQGpDVQulez )

edu's SIG record covers both KEY records, so we can use either or both to sign our zone data.

Then we sign our zone data using only the new private key:

# dnssec-signzone -o movie.edu. db.movie.edu Kmovie.edu.+001+47703.private

dnssec-signzone doesn't like re-signing zones that were signed with another key, so we started over from an unsigned version of the movie.eduzone. Here's a snippet from the signed zone data file, db.movie.edu.signed:

$ORIGIN .
$TTL 86400      ; 1 day
movie.edu               IN SOA  terminator.movie.edu. al.robocop.movie.edu. (
                                2000092603 ; serial
                                10800      ; refresh (3 hours)
                                3600       ; retry (1 hour)
                                604800     ; expire (1 week)
                                3600       ; minimum (1 hour)
                                )
                        SIG     SOA 1 2 86400 20010104062430 (
                                20001205062430 47703 movie.edu.
                                LIsndGD5q2VPWb+Ha0ffFP54UE6RYPweqtTp1xhgw4B9
                                Pyb/7z54J8q8LC0NmzQ6SthnfecBQhDBpc72HfNeJQ== )
                        NS      terminator.movie.edu.
                        NS      wormhole.movie.edu.
                        NS      outland.fx.movie.edu.
                        SIG     NS 1 2 86400 20010104062430 (
                                20001205062430 47703 movie.edu.
                                Ktq2mYMzTrBfGjdSb2F7ghyh2nXaLc0iTPV4k8I64jl0
                                nJt/hsBZPpeyM2u+Zymvp3mJMWg66E4tirj0AvlGXw== )
                        SIG     MX 1 2 86400 20010104062430 (
                                20001205062430 47703 movie.edu.
                                20001205062430 47703 movie.edu.
                                l/XnJ+JWhmAZLp6YF27YQQl0yT7iZ0qGDXPw860P6U1H
                                NmgDkUKoHfD6CdYwpKz15NyxRKilVmx2ne3oB0TUEQ== )
$TTL 3600       ; 1 hour
                        SIG     NXT 1 2 3600 20010104062430 (
                                20001205062430 47703 movie.edu.
                                2sxN3rQXn/JklugmyGV+on1Io6tV1wEYP6m4oD1xHCP1
                                +NHPR+uT2IknW8SvGc3Kaj16kb2Ej+i3RvleWSI4Tg== )
                        NXT     bigt.movie.edu. ( NS SOA MX SIG KEY NXT )
$TTL 172800     ; 2 days
                        SIG     KEY 1 2 172800 20010104060917 (
                                20001205060917 65398 edu.
                                qzvmuTVv9yGZf963ZuN2jxk8brEX/VP3sI5pOM/g2mU/
                                EPa57fyhHDNo7ny8Q2Su5vXnAIoxaaKAR8VmognQ7A== )
                        KEY     256 3 1 (
                                AQPdWbrGbVv1eDhNgRhpJMPonJfA3reyEo82ekwRnjbX
                                7+uBxB11BqL7LAB7/C+eb0vCtI53FwMhkkNkTmA6bI8B )
                        KEY     256 3 1 (
                                AQPjAfGtDkx6PSgYFs6G2vY1bJEldAMudngn6ZpGJnyg
                                k+LeU5PYiYd48wFLimihjyzlTWMb+C+egtQGpDVQulez )
$TTL 86400      ; 1 day
                        MX      10 postmanrings2x.movie.edu.

Although the zone includes two KEY records and edu's SIG record, which covers both, the other records in the zone were signed only by the new private key, with key tag 47703.

We need the second signed keyset when we delete the old KEY record: at that point, the SIG record edu sent us that covers both KEYs is no good. If we use the keyset that includes just the new KEY record, we'll be fine.

We're guessing that after reading that, you'll probably decide to use the longest keys available just to avoid ever needing to roll your keys over.

11.4.11 What Was That All About?

We realize that DNSSEC is a bit, er, daunting. (We nearly fainted the first time we saw it.) But it's designed to do something very important: make DNS spoofing much, much harder. And as people do more and more business over the Internet, knowing you're really getting where you thought you were going becomes crucial.

That said, we realize that DNSSEC and the other security measures we've described in this chapter aren't for all of you. (Certainly they're not all for all of you.) You should balance your need for security against the cost of implementing it, in terms of the burden it places both on your infrastructure and on your productivity.