My week old relay node got this flag today, I couldn't find any explanations about it figured I'd ask here. All I found out was that it was introduced in tor 4.x
StaleDesc was developed throught a proposal. Anyone can send in a proposal to improve the Tor protocol. The specific proposal has the number 293 with the title "Other ways for relays to know when to publish". The implementation was tracked in ticket #26770.
Authorities recognize that a relay descriptor is too old and assign this flag. A relay should upload its descriptor when it sees this flag.
So when your relay sees this flag it should update its descriptor. This happens automatically. However if your relay is not able to update it, you should investigate why this happens.
It has just happened to me and
tor gave me useful piece of information in logs:
Your server (xxx.xxx.xxx.xx:0) has not managed to confirm that its DirPort is reachable. Relays do not publish descriptors until their ORPort and DirPort are reachable. Please check your firewalls, ports, address, /etc/hosts file, etc.
What happened in my case was that I enabled DirPort on PRIVATE (RFC4193) address only together with NoAdvertise flag which obviously IS NOT reachable. Apparently it is not sufficient so the DirPort goes back to disabled state in my case.
Check that both OrPort and DirPort are reachable.