Hello, I had a routerboard and i was using it as VPN server with several ipsec vpns with other routers. My wan has static public IP but all the vpns we used mikrtotik cloud (sn.mynetname.net domain)

Now we have to replace this specific routerboard with a CCR router and the mikrotik cloud has changed.

Due to having static public IP, our old mikrotik ddns is still resolving and VPNs works. My question is does the mynetname expires after a while? It is very difficult to replace all these mikrotik cloud domains at all my VPN customers.

Manual of Cloud / DDNS IMO explains it nicely. Almost everything. If the feature is enabled, then router will contact cloud server(s) every 60 seconds. Which is TTL of DNS record as well. What is missing is how long do cloud servers keep DNS record if router doesn't refresh it (for any reason). And perhaps you want to ask that directly MT via support channels (e.g. by sending email to support@mikrotik.com ). If you get any informative answer, we'll surely appreciate if you forward it here.

A way around (but kinda too late after router has been already replaced) is to have alias (CNAME) in public DNS pointing at DDNS record. And then use that alias in configuration of remote devices. If router is changed (and thus serial number and DDNS entry), then it's enough to change target of alias in DNS server.

For the transition period until you change the domain name setting on the clients according to @mkx's advice with the cname record, you can put the original router to the LAN of the CCR with the cloud service enanled, so it will keep updating its sn.mynetname.net record with the public IP address of the CCR. But once it dies, you'll be in trouble quite fast.

Currently wiki says that the last IP address will stick forever:

Note: When the service is enable, a DNS name will be stored on the MikroTik's Cloud server permanently and this DNS name will resolve to the last IP that your IP has sent to the MikroTik's Cloud server.

theoretically if you have only one static public IP, even if you remove the router and replace it you can continue to point to the old DDNS always resolving the same address. The problem will arise when you change the public IP. Another solution could be to leave the old rb active and make a cname as already indicated

If you replace old router with a new one and the public IP address is the same, then you'll end up with two A records: <old_SN>.sn.mynetname.net and <new_SN>.sn.mynetname.net ... both pointing at same address. I don't see how this is a problem, if you know <new SN>, then old record won't make any harm.

If your public address changes, then next cloud call from your router will update <new SN>.sn.mynetname.net to the new IP address ... which obviously overwrites old record with same <new SN>.

If both things happen at the same time, i.e. you replace the router and after starting it receives another AP address, then you'll end up with two distinct A records: <old SN>.sn.mynetname.net pointing at old IP address and <new SN>.sn.mynetname.net pointing at new (current) IP address.

Both in first and last case, if you remember to start using <new SN>.sn.mynetname.net, the (lingering) <old SN> record isn't important and hence it doesn't matter to which IP address it points (the old or the new one).

The only harm of record not expiring is on Mikrotik ... as they have records in their DNS which are not needed any more.