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Running a Google Compute Instance with an internal interface mapped to a temporary public IP... it seems impossible to establish an active ftp connection with an external ftp server. I saw another post without any solutions about active mode connection from google cloud. In my opinion there are some problem in the NAT / port forwarding rules between local and public IP.

ftp xxx.xxx.xxx.xxx
Connected to xxx.xxx.xxx.xxx.
220 Microsoft FTP Service
Name (xxx.xxx.xxx.xxx:user): username
331 Password required for username.
Password:
230-Welcome
230 User user logged in.
Remote system type is Windows_NT.
ftp> dir
500 Invalid PORT Command.
ftp: bind: Address already in use
ftp>

In the "google firewall" I opened all ports from xxx.xxx.xxx.xxx (ftp server address). The server accept only active mode connections (no passive).

There are no firewall rules in the local machine.

ifconfig
eth0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1460
        inet 10.132.0.3  netmask 255.255.255.255  broadcast 10.132.0.3
        ether 42:01:0a:84:00:03  txqueuelen 1000  (Ethernet)
        RX packets 19215527  bytes 28123647876 (26.1 GiB)
        RX errors 0  dropped 0  overruns 0  frame 1
        TX packets 10673733  bytes 814976332 (777.2 MiB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

lo: flags=73<UP,LOOPBACK,RUNNING>  mtu 65536
        inet 127.0.0.1  netmask 255.0.0.0
        inet6 ::1  prefixlen 128  scopeid 0x10<host>
        loop  txqueuelen 1  (Local Loopback)
        RX packets 162643  bytes 54080619 (51.5 MiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 162643  bytes 54080619 (51.5 MiB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

iptables -L
Chain INPUT (policy ACCEPT)
target     prot opt source               destination         

Chain FORWARD (policy ACCEPT)
target     prot opt source               destination         

Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination

Thanks in advance!

Shaphiro
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1 Answers1

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I found the solution! Under google cloud network is not possible to establish an active ftp connection with an external ftp server.

The big problem is that when a client program is using network address translation to hide behind a routing device on an internal network, when using PORT the client tells a server on the external network to connect to an address on the client's internal network.  I.e., from the example above:

Client: PORT 192,168,1,2,7,138

The Solution:

The solution depends from the network administrator of the client network to use high-quality network address translation software.  Devices can keep track of FTP data connections, and when a client on a private network uses "PORT" with an internal network address, the device should dynamically rewrite the packet containing the PORT and IP address and change the address so that it refers to the external IP address of the routing device.  The device would then have to route the connection incoming from the remote FTP server back to the internal network address of the client.  I.e., from the example above we had:

Client: PORT 192,168,1,2,7,138

When the packet containing this PORT reaches the routing device, it should be rewritten like this, assuming the external address is 17.254.0.26:

Client: PORT 17,254,0,26,7,138

The remote server would then attempt to connect to 17.254.0.26:1930.  The routing device in this example would then forward all traffic for this connection to and from the client address at 192.168.1.2:1930.

More details here

I don't know if in the google environment is possible this kind of solution.

The only solution found for google cloud (for now): to hack the ftp client (in my case it's simple: ruby) forcing the sending of the real external network address in the ftp PORT command.

Other better solutions are welcome!

Shaphiro
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