forward traffic to ftp server using iptables

I have set up 3 VMware machines like this

machine A(client): machine B : : machine C(ftp) :

I want A to access C through B A->B->C

here are the commands I alr used on B: sysctl net.ipv4.ip_forward=1 iptables -t nat -A PREROUTING -p tcp --dport 21 -j DNAT --to-destination iptables -t nat -A POSTROUTING -j MASQUERADE

I am able to access the FTP using A, but I need to route the traffic from C back to A. Also open port 20, so that they can transfer data

when I try to use command “dir”: 500 illegal port command ftp: bind: address already in use

what are the other rules i should add in. beside that do i need modprobe ip_conntrack_ftp & modprobe ip_nat_ftp in this case?


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