Hardware Security modules are used by domain publishers who wish to protect their key material by storing it in a specially-hardened hardware module, separate from BIND. Some organizations, managing top-level domains, or particularly security-conscious segments, may mandate use of HSMs for certification purposes.
PKCS#11 (Public Key Cryptography Standard #11) defines a platform-independent API for the control of hardware security modules (HSMs) and other cryptographic support devices.
BIND 9 is known to work with three HSMs: The AEP Keyper, which has been tested with Debian Linux, Solaris x86 and Windows Server 2003; the Thales nShield, tested with Debian Linux; and the Sun SCA 6000 cryptographic acceleration board, tested with Solaris x86. In addition, BIND can be used with all current versions of SoftHSM, a software-based HSM simulator library produced by the OpenDNSSEC project.
PKCS#11 makes use of a “provider library”: a dynamically loadable library which provides a low-level PKCS#11 interface to drive the HSM hardware. The PKCS#11 provider library comes from the HSM vendor, and it is specific to the HSM to be controlled.
There are two available mechanisms for PKCS#11 support in BIND 9: OpenSSL-based PKCS#11 and native PKCS#11. When using the first mechanism, BIND uses a modified version of OpenSSL, which loads the provider library and operates the HSM indirectly; any cryptographic operations not supported by the HSM can be carried out by OpenSSL instead. The second mechanism enables BIND to bypass OpenSSL completely; BIND loads the provider library itself, and uses the PKCS#11 API to drive the HSM directly.
For more details, read the Administrative Reference Manual on the topic of PKCS#11 support.