Description of problem:

ssh -o ProxyCommand="ncat -v --proxy-type socks5 --proxy <ip>:<port> %h %p" <host>

Is not working with nmap-ncat-7.91-8.fc35.x86_64


Example:

create a sock5 proxy with ssh

ssh -D 4567 <host>

Try using the connection with:

ssh -o ProxyCommand="ncat -v --proxy-type socks5 --proxy 127.0.0.1:4567 %h %p" myhost

The conenction will fail with:

kex_exchange_identification: Connection closed by remote host
Connection closed by UNKNOWN port 65535

Using OpenBSD netcat will work:
ssh -o ProxyCommand="netcat -X 5 -x 127.0.0.1:4080 %h %p" myhost 

Or using the  ncat from the nmap page:
rpm -vhU https://nmap.org/dist/ncat-7.92-1.x86_64.rpm

So I guess something is broken with the ncat that fedora provides.