Apache Commons FTPClient Hanging

StackOverflow https://stackoverflow.com//questions/9706968

Question

We are using the following Apache Commons Net FTP code to connect to an FTP server, poll some directories for files, and if files are found, to retrieve them to the local machine:

try {
logger.trace("Attempting to connect to server...");

// Connect to server
FTPClient ftpClient = new FTPClient();
ftpClient.setConnectTimeout(20000);
ftpClient.connect("my-server-host-name");
ftpClient.login("myUser", "myPswd");
ftpClient.changeWorkingDirectory("/loadables/");

// Check for failed connection
if(!FTPReply.isPositiveCompletion(ftpClient.getReplyCode()))
{
    ftpClient.disconnect();
    throw new FTPConnectionClosedException("Unable to connect to FTP server.");
}

// Log success msg
logger.trace("...connection was successful.");

// Change to the loadables/ directory where we poll for files
ftpClient.changeWorkingDirectory("/loadables/");    

// Indicate we're about to poll
logger.trace("About to check loadables/ for files...");

// Poll for files.
FTPFile[] filesList = oFTP.listFiles();
for(FTPFile tmpFile : filesList)
{
    if(tmpFile.isDirectory())
        continue;

    FileOutputStream fileOut = new FileOutputStream(new File("tmp"));
    ftpClient.retrieveFile(tmpFile.getName(), fileOut);
    // ... Doing a bunch of things with output stream
    // to copy the contents of the file down to the local
    // machine. Ommitted for brevity but I assure you this
    // works (except when the WAR decides to hang).
    //
    // This was used because FTPClient doesn't appear to GET
    // whole copies of the files, only FTPFiles which seem like
    // file metadata...
}

// Indicate file fetch completed.
logger.trace("File fetch completed.");

// Disconnect and finish.
if(ftpClient.isConnected())
    ftpClient.disconnect();

logger.trace("Poll completed.");
} catch(Throwable t) {
    logger.trace("Error: " + t.getMessage());
}

We have this scheduled to run every minute, on the minute. When deployed to Tomcat (7.0.19) this code loads up perfectly fine and begins working without a hitch. Every time though, at some point or another, it seems to just hang. By that I mean:

  • No heap dumps exist
  • Tomcat is still running (I can see its pid and can log into the web manager app)
  • Inside the manager app, I can see my WAR is still running/started
  • catalina.out and my application-specific log show no signs of any exceptions being thrown

So the JVM is still running. Tomcat is still running, and my deployed WAR is still running, but its just hanging. Sometimes it runs for 2 hours and then hangs; other times it runs for days and then hangs. But when it does hang, it does so between the line that reads About to check loadables/ for files... (which I do see in the logs) and the line that reads File fetch completed. (which I don't see).

This tells me the hang occurs during the actual polling/fetching of the files, which kind of points me in the same direction as this question that I was able to find which concerns itself with FTPClient deadlocking. This has me wondering if these are the same issues (if they are, I'll happily delete this question!). However I don't think believe they're the same (I don't see the same exceptions in my logs).

A co-worker mentioned it might be a "Passive" vs. "Active" FTP thing. Not really knowing the difference, I am a little confused by the FTPClient fields ACTIVE_REMOTE_DATA_CONNECTION_MODE, PASSIVE_REMOTE_DATA_CONNECTION_MODE, etc. and didn't know what SO thought about that as being a potential issue.

Since I'm catching Throwables as a last resort here, I would have expected to see something in the logs if something is going wrong. Ergo, I feel like this is a definite hang issue.

Any ideas? Unfortunately I don't know enough about FTP internals here to make a firm diagnosis. Could this be something server-side? Related to the FTP server?

Was it helpful?

Solution

This could be a number of things, but your friend's suggestion would be worthwhile.

Try ftpClient.enterLocalPassiveMode(); to see if it helps.

I would also suggest to put the disconnect in the finally block so that it never leaves a connection out there.

OTHER TIPS

Yesterday, I didn't sleep but I think I solved the problem.

You can increase the buffer size with FTPClient.setBufferSize();

   /**
 * Download encrypted and configuration files.
 * 
 * @throws SocketException
 * @throws IOException
 */
public void downloadDataFiles(String destDir) throws SocketException,
        IOException {

    String filename;
    this.ftpClient.connect(ftpServer);
    this.ftpClient.login(ftpUser, ftpPass);

    /* CHECK NEXT 4 Methods (included the commented) 
    *  they were very useful for me!
    *  and icreases the buffer apparently solve the problem!!
    */
    //  ftpClient.addProtocolCommandListener(new PrintCommandListener(new PrintWriter(System.out), true));
    log.debug("Buffer Size:" + ftpClient.getBufferSize());
    this.ftpClient.setBufferSize(1024 * 1024);
    log.debug("Buffer Size:" + ftpClient.getBufferSize());


    /*  
     *  get Files to download
     */
    this.ftpClient.enterLocalPassiveMode();
    this.ftpClient.setAutodetectUTF8(true);
            //this.ftpClient.setFileType(FTP.BINARY_FILE_TYPE);
    this.ftpClient.enterLocalPassiveMode();
    FTPFile[] ftpFiles = ftpClient
            .listFiles(DefaultValuesGenerator.LINPAC_ENC_DIRPATH);

    /*
     * Download files
     */
    for (FTPFile ftpFile : ftpFiles) {

        // Check if FTPFile is a regular file           
        if (ftpFile.getType() == FTPFile.FILE_TYPE) {
            try{

            filename = ftpFile.getName();

            // Download file from FTP server and save
            fos = new FileOutputStream(destDir + filename);

            //I don't know what useful are these methods in this step
            // I just put it for try
            this.ftpClient.enterLocalPassiveMode();
            this.ftpClient.setFileType(FTP.BINARY_FILE_TYPE);
            this.ftpClient.setAutodetectUTF8(true);
            this.ftpClient.enterLocalPassiveMode();

            ftpClient.retrieveFile(
                    DefaultValuesGenerator.LINPAC_ENC_DIRPATH + filename,
                    fos
                    );

            }finally{
                fos.flush();
                fos.close();                }
        }
    }
    if (fos != null) {
        fos.close();
    }
}

I hope that this code could be usefull for someone!

I had to include the following after login in order to call s.listFiles and transfer without it 'hanging' and eventually failing:

s.login(username, password);
s.execPBSZ(0);
s.execPROT("P");

I had this same issue when trying to perform a listfiles from a Linux machine to a IIS server. The code worked great from my developer workstation, but would hang when running on the server specifically due to a firewall gumming up the mix.

Must do these things in order and will require you to extend FTPSClient 3.5

  1. connect (implicit = true, SSLContext = TLS)
  2. check isPositiveCompletion
  3. authenticate (of course)
  4. execPBSZ(0)
  5. execPROT("P")
  6. set boolean to indicate Skip Passive IP (custom FTPSClient class)
  7. set the save connection IP address (custom FTPSClient class)
  8. setUseEPSVwithIPv4(false)
  9. enterLocalPassiveMode() or enterRemotePassiveMode()
  10. initiateListParsing() or any list command a.) At this point the openDataConnection will be executed, be sure to save the port being used here b.) The PASV command is executed c.) The _parsePassiveModeReply is executed, here you will open the socket with the IP address you used for connecting and the saved port.
  11. disconnect (always)

More INFO: My issue is specific to a firewall between the Linux machine and IIS server.
The root of my problem is that in passive mode the IP address used to open the socket when doing a data connection is different that the one used to do the initial connection. So due to two issues (see below) with APACHE commons-net 3.5 it was incredibly difficult to figure out. My solution: Extend FTPSClient so that I could override methods _parsePassiveModeReply & openDataConnection. My parsePassiveModeReply is really just saving the port from the reply since the reply indicates what port is being used. My openDataConnection method is using the saved port and the original IP used during connection.

Problems with APACHE FTPCLient 3.5

  1. Data Connection does not time out (hangs) so its is not apparent what the problem is.
  2. The FTPSClient class does not skip passive IP addresses. Setting passiveNatWorkaround to true doesn't work as I expected or maybe it doesn't skip the IP at all.

Things to pay attention to:

  • When going through a firewall you must have access to the port range defined by IIS (see configuring Microsoft IIS firewall).
  • You should also ensure you have any appropriate certificates in your keystore or the cert specified at runtime.

  • Add the following to your class, this is so very helpful to know what FTP commands are being executed.

       ftpClient.addProtocolCommandListener(new PrintCommandListener(new PrintWriter(System.out), true));
  • Check the FTP server logs as it will tell you what is being performed and possible why you are having problems. You should always see a data channel opened before executing a list. Compare the results of your application to that of what a successful curl command performs.
  • Reply codes as they will indicate where a problem is occurring.

  • Use the curl command to verify you have connectivity, The following is a good start and if all is well will list the contents in the root directory. 

    curl -3 ftps://[user id]:[password][ftp server ip]:990/ -1 -v --disable-epsv --ftp-skip-pasv-ip --ftp-ssl --insecure

FTPSClient extended (SAMPLE CODE)

import java.io.IOException;
import java.net.Inet6Address;
import java.net.InetSocketAddress;
import java.net.Socket;

import javax.net.ssl.SSLContext;

import org.apache.commons.net.MalformedServerReplyException;
import org.apache.commons.net.ftp.FTPReply;
import org.apache.commons.net.ftp.FTPSClient;

/**
 * TODO Document Me!
 */
public class PassiveFTPSClient extends FTPSClient {
    private String passiveSkipToHost;
    private int passiveSkipToPort;
    private boolean skipPassiveIP;


    /** Pattern for PASV mode responses. Groups: (n,n,n,n),(n),(n) */
    private static final java.util.regex.Pattern PARMS_PAT;    
    static {
    PARMS_PAT = java.util.regex.Pattern.compile(
            "(\\d{1,3},\\d{1,3},\\d{1,3},\\d{1,3}),(\\d{1,3}),(\\d{1,3})");
       }
    /**
     * @param b
     * @param sslContext
     */
    public PassiveFTPSClient(boolean b, SSLContext sslContext) {
    super(b, sslContext);
    }

    protected void _parsePassiveModeReply(String reply) throws MalformedServerReplyException {
    if (isSkipPassiveIP()) {
        System.out.println( "================> _parsePassiveModeReply"  + getPassiveSkipToHost());
        java.util.regex.Matcher m = PARMS_PAT.matcher(reply);
        if (!m.find()) {
        throw new MalformedServerReplyException(
            "Could not parse passive host information.\nServer Reply: " + reply);
        }
        try {
        int oct1 = Integer.parseInt(m.group(2));
        int oct2 = Integer.parseInt(m.group(3));
        passiveSkipToPort = (oct1 << 8) | oct2;
        }
        catch (NumberFormatException e) {
        throw new MalformedServerReplyException(
            "Could not parse passive port information.\nServer Reply: " + reply);
        }            
        //do nothing
    } else {
        super._parsePassiveModeReply(reply);
    }
    }

    protected Socket _openDataConnection_(String command, String arg) throws IOException {
    System.out.println( "================> _openDataConnection_"  + getPassiveSkipToHost());
    System.out.println( "================> _openDataConnection_ isSkipPassiveIP: " + isSkipPassiveIP());        
    if (!isSkipPassiveIP()) {
        return super._openDataConnection_(command, arg);
    }
    System.out.println( "================> getDataConnectionMode: " + getDataConnectionMode());
    if (getDataConnectionMode() != ACTIVE_LOCAL_DATA_CONNECTION_MODE &&
        getDataConnectionMode() != PASSIVE_LOCAL_DATA_CONNECTION_MODE) {
        return null;
    }

    final boolean isInet6Address = getRemoteAddress() instanceof Inet6Address;

    Socket socket;
    if (getDataConnectionMode() == ACTIVE_LOCAL_DATA_CONNECTION_MODE) {
        return super._openDataConnection_(command, arg);

    }
    else
    { // We must be in PASSIVE_LOCAL_DATA_CONNECTION_MODE

        // Try EPSV command first on IPv6 - and IPv4 if enabled.
        // When using IPv4 with NAT it has the advantage
        // to work with more rare configurations.
        // E.g. if FTP server has a static PASV address (external network)
        // and the client is coming from another internal network.
        // In that case the data connection after PASV command would fail,
        // while EPSV would make the client succeed by taking just the port.
        boolean attemptEPSV = isUseEPSVwithIPv4() || isInet6Address;
        if (attemptEPSV && epsv() == FTPReply.ENTERING_EPSV_MODE)
        {

        System.out.println( "================> _parseExtendedPassiveModeReply a: ");                
        _parseExtendedPassiveModeReply(_replyLines.get(0));
        }
        else
        {
        if (isInet6Address) {
            return null; // Must use EPSV for IPV6
        }
        // If EPSV failed on IPV4, revert to PASV
        if (pasv() != FTPReply.ENTERING_PASSIVE_MODE) {
            return null;
        }
        System.out.println( "================> _parseExtendedPassiveModeReply b: ");
        _parsePassiveModeReply(_replyLines.get(0));
        }
        // hardcode fore testing
        //__passiveHost = "10.180.255.181";
        socket = _socketFactory_.createSocket();
        if (getReceiveDataSocketBufferSize() > 0) {
        socket.setReceiveBufferSize(getReceiveDataSocketBufferSize());
        }
        if (getSendDataSocketBufferSize()  > 0) {
        socket.setSendBufferSize(getSendDataSocketBufferSize() );
        }
        if (getPassiveLocalIPAddress() != null) {
        System.out.println( "================> socket.bind: " + getPassiveSkipToHost());
        socket.bind(new InetSocketAddress(getPassiveSkipToHost(), 0));
        }

        // For now, let's just use the data timeout value for waiting for
        // the data connection.  It may be desirable to let this be a
        // separately configurable value.  In any case, we really want
        // to allow preventing the accept from blocking indefinitely.
        //     if (__dataTimeout >= 0) {
        //         socket.setSoTimeout(__dataTimeout);
        //     }

        System.out.println( "================> socket connect: " + getPassiveSkipToHost() + ":" + passiveSkipToPort);
        socket.connect(new InetSocketAddress(getPassiveSkipToHost(), passiveSkipToPort), connectTimeout);
        if ((getRestartOffset() > 0) && !restart(getRestartOffset()))
        {
        socket.close();
        return null;
        }

        if (!FTPReply.isPositivePreliminary(sendCommand(command, arg)))
        {
        socket.close();
        return null;
        }
    }

    if (isRemoteVerificationEnabled() && !verifyRemote(socket))
    {
        socket.close();

        throw new IOException(
            "Host attempting data connection " + socket.getInetAddress().getHostAddress() +
            " is not same as server " + getRemoteAddress().getHostAddress());
    }

    return socket;
        }

    /**
    * Enable or disable passive mode NAT workaround.
    * If enabled, a site-local PASV mode reply address will be replaced with the
    * remote host address to which the PASV mode request was sent
    * (unless that is also a site local address).
    * This gets around the problem that some NAT boxes may change the
    * reply.
    *
    * The default is true, i.e. site-local replies are replaced.
    * @param enabled true to enable replacing internal IP's in passive
    * mode.
    */
    public void setSkipPassiveIP(boolean enabled) {
    super.setPassiveNatWorkaround(enabled);
    this.skipPassiveIP = enabled;
    System.out.println( "================> skipPassiveIP: " + skipPassiveIP);
    }
    /**
     * Return the skipPassiveIP.
     * @return the skipPassiveIP
     */
    public boolean isSkipPassiveIP() {
    return skipPassiveIP;
    }
    /**
     * Return the passiveSkipToHost.
     * @return the passiveSkipToHost
     */
    public String getPassiveSkipToHost() {
    return passiveSkipToHost;
    }

    /**
     * Set the passiveSkipToHost.
     * @param passiveSkipToHost the passiveSkipToHost to set
     */
    public void setPassiveSkipToHost(String passiveSkipToHost) {
    this.passiveSkipToHost = passiveSkipToHost;
    System.out.println( "================> setPassiveSkipToHost: " + passiveSkipToHost);
    }

}
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