DNS caching
===========

User experience may be greatly improved when dns is accelerated. Also, it
seems like a good idea to keep the number of packages being exchanged
between node and gateway as small as possible. In order to do this, a
DNS cache may be used on a node. The dnsmasq instance listening on port
53 on the node will be reconfigured to answer requests, use a list of
upstream servers and a specific cache size if the options listed below are
added to site.conf. Upstream servers are the DNS servers which are normally
used by the nodes to resolve hostnames (e.g. gateways/supernodes).

There are the following settings:
    servers
    cacheentries

To use the node's DNS server, both options should be set. The node will cache at
most 'cacheentries' many DNS records in RAM. The 'servers' list will be used to
resolve the received DNS queries if the request cannot be answered from
cache. Gateways should announce the "next node" address via DHCP and RDNSS (if
any). Note that not setting 'servers' here will lead to DNS not working: Once
the gateways all announce the "next node" address for DNS, there is no way for
nodes to automatically determine DNS servers. They have to be baked into the
firmware.

If these settings do not exist, the cache is not initialized and RAM usage will
not increase.

When next_node.name is set, an A record and an AAAA record for the
next-node IP address are placed in the dnsmasq configuration. This means that
the content of next_node.name may be resolved even without upstream connectivity.
It is suggested to use the same name as the DNS server provides:
e.g. nextnode.location.community.example.org (This way the name also works if a
client uses static DNS Servers). Hint: If next_node.name does not contain a dot
some browsers would open the searchpage instead.

::

  dns = {
    cacheentries = 5000,
    servers = { '2001:db8::1', },
  },

  next_node = {
    name = { 'nextnode.location.community.example.org', 'nextnode', 'nn' },
    ip6 = '2001:db8:8::1',
    ip4 = '198.51.100.1',
  }


Each cache entry will occupy about 90 bytes of RAM.