OPNsense WIDE-DHCPv6 client

WIDE-DHCPv6 is an open-source implementation of Dynamic Host
Configuration Protocol for IPv6 (DHCPv6) developed by the KAME
project.

The implementation mainly conforms to the following standards:

- RFC3315: Dynamic Host Configuration Protocol for IPv6 (DHCPv6)
- RFC3319: Dynamic Host Configuration Protocol (DHCPv6) Options for Session Initiation Protocol (SIP) Servers
- RFC3633: IPv6 Prefix Options for Dynamic Host Configuration Protocol (DHCP)
- RFC3646: DNS Configuration options for Dynamic Host Configuration Protocol for IPv6 (DHCPv6)
- RFC4075: Simple Network Time Protocol (SNTP) Configuration Option for DHCPv6
- RFC4242: Information Refresh Time Option for Dynamic Host Configuration Protocol for IPv6 (DHCPv6)

Supported message types are as follows:

Solicit, Advertise, Request, Renew, Rebind, Release, Reply and Information-request.

Note that the current implementation does not support IPv6
temporary address allocation by DHCPv6, and there is no plan to
implement that feature at the moment.

Improvements made over the years by OPNsense, Debian and FreeBSD:

- Increased log verbosity
- Fix socket leakage by setting FD_CLOEXEC
- Call a configuration script after addresses and prefixes are set on an interface
- Update ifid on interface restart, it may have changed
- "-n" flag to prevent address release from being sent to the DHCP server upon restart
- Reload the client configuration on SIGHUP
- Removed all unused binaries except dhcp6c
- Raw DHCP option configuration support using hex notation
- PDINFO delegated prefixes environment variable
- Accept interfaces from the configuration file
- Allow assigning the prefix to the WAN interface
- Several relevant patches from Debian
- Extended prefix-interface ifid options
- Added -t mode for configuration file testing
- Improved RELEASE, NOTONLINK and NOADDRSAVAIL behaviour
- Moved code to use arc4random_uniform() to avoid unsafe random()
- Timing parameter updates according to RFC 8415, Section 7.6
- Use the PID file to safely check for running process
- Use lease timings for prefixes so they can expire in the OS
- Support new_raw_option_nnnnn environment variables hex notation for unknown received options
- Support new_aftr_name environment variable
- NAINFO configured address environment variable

Bug Reports and patch submissions

Feel free to submit bug reports or patches on GitHub at:

https://github.com/opnsense/dhcp6c