Steven Allen
e8e86c94e7
Previously, enabling nssmdns would enable the full mdns module by default, as well as the minimal one. Unfortunately, the full mdns module introduces a 5 second delay whenever it fails to perform a reverse hostname lookup (`gethostbyaddr`). For example, this adds a 5 second delay to ping as follows: 1. Ping resolves the hostname (example.com) to an IP address. This query is serviced by the user's DNS resolver, so it returns quickly. 2. Ping attempts to resolve the IP address back into a hostname via `gethostbyaddr`. 3. If the upstream DNS server fails to resolve the IP address back into a hostname (many domain-names lack reverse DNS records), NSS falls back on querying via Avahi (via the `mdns` module). 4. mDNS is a broadcast protocol, so Avahi has to wait the full 5 second timeout before it can return a failure. This patch fixes this by disabling the full `mdns` NSS module unless the new disabled-by-default `nssmdnsFull` option is enabled. The still-enabled `mdns_minimal` module only performs reverse IP lookups for `169.254.0.0/16` (link-local IP addresses) so this 5 second delay will only apply to link-local pings by domain-name. In practice, this change shouldn't negatively affect the vast majority of users. The only affected users are those who: 1. Are relying on Avahi to resolve IP addresses into hostnames on their local network via `gethostbyaddr`. 2. Are using mDNS hostnames that don't end in `.local`. The minimal module is sufficient for, e.g., users who just want to resolve hostnames for local printers and network shares. Note: This patch not fix this by adding [!UNAVAIL=return] after `dns` as discussed in (#291108) as that would treat mDNS as a fallback for DNS when DNS is unavailable which, IMO, is incorrect. Whether or not we use mDNS shouldn't depend on whether or not our DNS server is available. fixes #291108
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