Default configuration for /etc/nsswitch.conf
--------------------------------------------

On first installation of libnss-mdns, /etc/nsswitch.conf is modified
for the upstream-recommended configuration, which usually looks like:

    hosts:          files mdns4_minimal [NOTFOUND=return] dns

If you also have systemd's libnss-resolve enabled, mDNS resolution will
be configured to occur before that (higher priority):

    hosts:          files mdns4_minimal [NOTFOUND=return] resolve [!UNAVAIL=return] dns

If any of the strings 'mdns', 'mdns_minimal', 'mdns4', 'mdns4_minimal',
'mdns6' or 'mdns6_minimal' appear on the hosts line, your
/etc/nsswitch.conf file will not be modified.


Enabling IPv6 name resolution
-----------------------------

Sites where IPv6 is extensively used might prefer to replace mdns4_minimal
with mdns_minimal, like this:

    hosts:          files mdns_minimal [NOTFOUND=return] dns

This works well for modern software that uses getaddrinfo() with address
family AF_UNSPEC. However, some programs implement IPv6 support by
making an IPv6-only name resolution attempt (with AF_INET6), then falling
back to IPv4-only (AF_INET) if that fails. Failed mDNS name resolution
attempts take several seconds, so this configuration will make those
programs slow when contacting IPv4-only hosts in the .local zone. See
<https://github.com/lathiat/nss-mdns/issues/62> for more information.


Falling back to mDNS for names outside .local
---------------------------------------------

In versions older than 0.10-4 we used this configuration:

    hosts:          files mdns4_minimal [NOTFOUND=return] dns mdns4

This looks in mDNS whenever a non-link-local address does not have a PTR
(reverse DNS) record in DNS. If the host with that address is local
and available in mDNS, this means it becomes reverse-resolvable, but if
the host with that address is not local, it causes a 5 second delay,
so this is no longer enabled by default. If a local administrator wishes
to reinstate mdns4, they can do so.


Sites with a .local DNS zone
----------------------------

Making nss-mdns authoritative for .local breaks resolution
on networks where .local is a real domain name. To work
around this, nss-mdns implements the heuristic described in
<https://support.apple.com/en-us/HT201275>: it looks up the SOA record
for the .local zone, and disables itself automatically if that record
is found.