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https://issues.apache.org/jira/browse/FTPSERVER-136?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12603564#action_12603564
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Amichai Rothman commented on FTPSERVER-136:
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I had the ftp server set up on a dyndns host, connected via FileZilla client
using passive mode, with one control channel and one data channel (simplest
firewall configuration for a single user and infrequent use). I would connect
to it once in a few days. this worked sometimes, but often enough the initial
connection (listing, etc.) worked fine, however the data connection would then
timeout, and would work properly only after the ftp service was restarted.
I read up on the FTP protocol to try and investigate the issue, and noticed
that the IP address sent to the client for opening the data channel was
incorrect (it's the PASV command I think) - it was not updated to reflect the
server's new IP address when it changed, while following a restart the correct
IP was used (at least until it changed again).
Currently I have a static IP so I can't verify, but the steps to recreate
should be:
1. start ftpserver on a host with dyndns.
2. connect (passive mode) to the server's domain name and transfer a file -
everything should be fine.
3. disconnect the host from the network and then reconnect it (leaving the ftp
service running), until the dyndns IP address changes.
4. connect (passive mode) to the server's domain name and transfer a file - the
PASV command's IP address will not be updated, and the transfer will fail.
btw this might be a problem in the dns library cache, or something of the sort
(just guessing)...
> incorrent IP used in opening data channel
> -----------------------------------------
>
> Key: FTPSERVER-136
> URL: https://issues.apache.org/jira/browse/FTPSERVER-136
> Project: FtpServer
> Issue Type: Bug
> Environment: Windows XP
> Reporter: Amichai Rothman
> Priority: Minor
>
> The IP used in opening the data channel (DATA command) appears to be
> determined when the ftp server starts, and never updated again. On systems
> where the IP address might change (such as any dynamic dns host) this causes
> all data connections to fail, and requires a full restart of the service
> whenever the IP address changes (which makes the availability of the ftp
> server unreliable for practical use).
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