please tell me command for open port 28015
$ sudo ufw allow 28015/TCP
$ sudo ufw allow 28015/UDP
$sudo ufw reload
after I used port open test site
It was "cannt access port 28015"

Is your server behind a router?

I use VPS
windows server is able to open port
but KVM ubuntu is I cannt open port 

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this is

it will also fail if you do not have a service running on the port.......
no matter how "open" your firewall is..... if there is not a "responder" tied to the port, it will look like it is "blocked" since no reply can be obtained.
next check your NAT

??
Does that mean I have to start the Rust server?

you know..... that might have something to do with it.....

consider a "port" like a drinking straw... no matter how hard you suck ,
you aint gonna get that refrshemenrt if it's not in a liquid...

Thanks!
Ill try open port after build rust server

Merged post

rust needs port of 28015/udp 
right?

28015   udp & tcp  also
28016 tcp rcon
28082  tcp  rust+

one last tip......  
UDP is STATELESS , the shit just turns up .... or not..... and may or may not get a reply.
so unless what you are testing  it with sends the correct data to the port the "service"
WILL NOT give a reply , if at all.

TCP is STATEFUL so you build a connection  channel first, and as such you can detect the build of the channel.

II couldn't start rust server....
./rustserver start
./rustserver console

As soon as "assetwarm up" is over
"access console rustserver: Closing console"
Is displayed and the server goes down

that looks like  a "startup" issue  not a port issue, completely different kettle of fish.....

I will post in another matter

you will find a ton of info on this if you search the internet for something like:   rust startup server ubuntu fail

I tryed all

clearly not......
if it was me... but you are free to disagree.....

I'd throw away all the crap  and use "docker" in my ubuntu...
go look for "didstopia/rust-server"

it's FULLY contained... all you need is an external data directory  and your "rust.env" file in that directory