Torsten created a branch branch with a first implementation - just in case somebody might want to have a look. It does contain full lookup of a server list by checking the SRV records and each server's properties via TXT records with name=value text and value being a base64-encoded JSON string. It does not /yet/ contain a "is-online" check.
The latter being considered to be implemented using UDP ‘ping’, i.e. because we can include some interesting info in it.
Torsten has meanwhile created the necessary DNS entries for the flightgear.org domain. The topics/mpdiscovery-via-dns branch should now be fully functional and discover the required multiplayer servers using DNS. It might take until Sat, 12th 00:00 UTC before the TTL of the old records have expired and your nameserver pulls in the changes.
Usage of DNS within FlightGear
The (DNS) is being used within FlightGear to look up resources providing several services. Its use started with release 2016.2.1 as an experimental feature to resolve the location of the TerraSync servers. This page documents the used services in detail.
Its creation was inspired by a discussion that took place on the FlightGear devel mailing list in 11/2016 to stop using a dedicated web service for retrieving a list of active multiplayer servers and instead use DNS , the idea of storing the information about multiplayer server in DNS instead of providing those through a web service. The motivation behind is to remove another single point of failure. If the current service goes away for some reason, the current implementation falls back to a hardcoded list of multiplayer servers. We consider DNS as an "always-on" service and already use it for looking up a mirror for terrasync
FlightGear systems using DNS
Currently, (as of version 2016.4.0) these systems use DNS for service discovery:
- The TerraSync client to find a TerraSync server
- The multiplayer client to list multiplayer servers and retrieve information about each server
- terrasync.flightgear.org. IN NAPTR 100 100 "U" "ws20" "!^.*$!http://flightgear.sourceforge.net/scenery!" .
- terrasync.flightgear.org. IN NAPTR 100 50 "U" "ws20" "!^.*$!http://fgfs.goneabitbursar.com/terrascenery!" .
- terrasync.flightgear.org. IN NAPTR 100 50 "U" "ws20" "!^.*$!http://mpserver16.flightgear.org/scenery!" .
At system start, FlightGear sends a query for the DNS name
terrasync.flightgear.org. The URL for scenery download is encoded in the regex field between the last two exclamation marks. This string MUST start with
!^.*$! and must end with
!, the regex itself is not interpreted by the TerraSync client, only the fixed string is taken.
The DNS to query is currently not configurable at system start by setting a property. The version number can be changed by setting property /sim/terrasync/scenery-version to a value other than the default "ws20".
Algorithm to pick a Server
- At startup, if TerraSync is enabled and if the property /sim/terrasync/http-server is set to "automatic" (the default), a DNS query for terrasync.flightgear.org is sent and the response filtered for entries with the flag field set to "U".
- All entries that have a "order" value higher than the lowest order are discarded. (currently the sourceforge entry)
- The remaining entries are filtered for those with the lowest preference. (both other entries survive this filter)
- a random entry from the remaining list is picked for the entire session of FlightGear.
This is implemented in
The property /sim/terrasync/http-server can be set to user defined URL pointing to the location of a custom TerraSync server e.g.
This will disable the above algorithm.
The SRV record for a multiplayer server looks like this:
_fgms._udp IN SRV 10 20 5001 mpserver01
All entries currently use the same values for priority and weight. The port numbers for active servers are set to the port number of the running fgms instance. The port number is zero for inactive servers.
The TXT record for each referenced multiplayer server looks like this:
mpserver01 IN TXT "flightgear-mpserver=eyJuYW1lIjoibXBzZXJ2ZXIwMSIsImxvY2F0aW9uIjoiRnJhbmtmdXJ0LEdlcm1hbnkifQo="
with the text data used as in RFC 1464 - Using the Domain Name System To Store Arbitrary String Attributes. The attribute name flightgear-mpserver marks this entry as "ours" while it's attribute value is a base64 encoded json string containing additional attributes for the server not available within the SRV record. For the given example, the base64 string of
Any FGMS changes will likely be quite small, but it would be good to ensure the ‘main network’ is relatively close to current versions. In a perfect would we would have an automated system to update the servers (any thoughts on ideas how to achieve that? Doesn’t need to be smart, could be as simple as git pull; make; make check -> if it passes make install; restart fgms) 
All our currently available mpserver seem to be running *nix, so indeed a cron'ed procedure [...] should be easy to implement