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.
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
As of version 2016.4, the configured entries are:
- 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.
Expanding the use of DNS to multiplayer resulted from a discussion at fsweekend 2016 followed by a discussion on the mailing list. 
Multiplayer discovery via DNS is an experimental feature found in FlightGear branch next. It will be released with version 2017.1.1
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 (usually 5001). The port number is 0 (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