Unit Instance

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        "There are several details of the operation of DNSSEC that are worth being discussed. First, a server that supports DNSSEC must have a public-private key pair. The public key is distributed with the `DNSKEY` record. The private key is never distributed and it does not even need to be stored on the server that uses the public key. DNSSEC does not require the DNSSEC servers to perform any operation that requires a private key in real time. All the `RRSIG` records can be computed offline, possibly on a different server than the server that returns the DNSSEC replies. The initial motivation for this design choice was the CPU complexity of computing the `RRSIG` signatures for zones that contain millions of records. In the early days of DNSSEC, this was an operational constraint. Today, this is less an issue, but avoiding costly signature operations in real time has two important benefits. First, this reduces the risk of denial of service attacks since an attacker cannot force a DNSSEC server to perform computationally intensive signing operations. Second, the private key can be stored offline, which means that even if an attacker gains access to the DNSSEC server, it cannot retrieve its private key. Using offline signatures for the `RRSIG` records has some practical implications that are reflected in the content of this record. First, each `RRSIG` record contains the original TTL of the signed record. When DNS resolvers cache records, they change the value of the TTL of these cached records and then return the modified records to their clients. When a resolver receives a signed DNS record, it must replace the received TTL of the record with the original TTL (and check that the received TTL is smaller than the original one) before checking the signature. Second, the `RRSIG` records contain a validity period, i.e. a starting time and an ending time for the validity of the signature. This period is specified as two timestamps. This period is only the validity of the signature. It does not affect the TTL of the signed record and is independent from the TTL. In practice, the validity period is important to allow DNS server operators to update their public/private keys. When such a key is changed, e.g. because the private could have been compromised, there is some period of time during which records signed with the two keys coexist in the network. The validity period allows ensuring that old signatures do not remain in DNS caches for ever."
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