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From sebb <seb...@gmail.com>
Subject Re: [NET] Errorhandling of "bad" FTP Servers
Date Sun, 22 Jan 2017 19:30:27 GMT
On 22 January 2017 at 16:37, Oliver Zemann <oliver.zemann@gmail.com> wrote:
>
>> Please prefix the subject line with the Commons Component in future (as
>> above)
>
> Sorry, next time i will do that.
>>
>>
>> On 22 January 2017 at 09:12, Oliver Zemann <oliver.zemann@gmail.com>
>> wrote:
>>>
>>> Hi, i would like to know how to handle FTP Servers that do not behave
>>> like
>>> they should (or behave different than most others).
>>> For example, currently i would like to connect to one of those servers
>>> which
>>> sends " MODE Z" directly after a login (USER) which is interpreted wrong
>>> by
>>> apache commons ftp, as its assumed that there should be \d\d\d \w* like
>>> "220
>>> all good my friend"
>>> But its simply: " MODE Z"
>>>
>>> Here is a log from filezilla which can handle this behaviour:
>>> 09:58:24    Trace:    CFtpControlSocket::SendNextCommand()
>>> 09:58:24    Befehl:    USER someUser
>>> 09:58:24    Trace:    CFtpControlSocket::OnReceive()
>>> 09:58:24    Antwort:    331  [33m ~wait~ user ok, password?
>>> 09:58:24    Trace:    CFtpControlSocket::SendNextCommand()
>>> 09:58:24    Befehl:    PASS *******
>>> 09:58:25    Trace:    CFtpControlSocket::OnReceive()
>>> 09:58:25    Antwort:    230  [32m.::WAIT::. welcome!.
>>
>> The login has completed here
>
> Yes, but apache commons sends first FEAT when i use autodetectUTF8(), which
> is imo wrong.
> The javadoc states that autodetectUTF8() has to be used before connect(),
> but i dont understand why.

setAutodetectUTF8 only sets a flag which is consulted during connect;
it does not send anything to the server.

> FileZilla first connects and does the login() and after that it uses the
> FEAT command to check if the server supports UTF8.
> The server behaves "bad" of that FEAT command, because it sends 211 END but
> then outputs MODE Z (which should before 211 END). And FEAT is issued
> because of autodetectUTF8()

That is perfectly reasonable behaviour for FileZilla and for Commons NET.

> Would it be possible to read from _controlInput_ until null is returned?
> Seems like someone else encountered something with another server, and
> therefore __ lenientCheck() exists?
>>>
>>> 09:58:25    Befehl:    FEAT
>>> 09:58:25    Trace:    CFtpControlSocket::OnReceive()
>>> 09:58:25    Antwort:    211-Extension supported
>>> 09:58:25    Antwort:     MDTM
>>> 09:58:25    Antwort:     MDTM
>>> 09:58:25    Antwort:     MDTM YYYYMMDDHHMMSS[+-TZ];filename
>>> 09:58:25    Antwort:     SIZE
>>> 09:58:25    Antwort:     SITE PSWD;EXEC;SET;INDEX;ZONE;CHMOD;MSG
>>> 09:58:25    Antwort:     XCRC filename;start;end
>>> 09:58:25    Antwort:    211 End
>>> 09:58:25    Status:    Der Server unterst├╝tzt keine Nicht-ASCII-Zeichen.
>>> 09:58:25    Status:    Angemeldet
>>> 09:58:25    Antwort:     MODE Z
>>> 09:58:25    Trace:    Unexpected reply, no reply was pending. <----
>>> something like this in apache commons ftp???
>>
>> Note that FileZilla detects this as an error, so clearly this is not
>> expected behaviour.
>>
>> Commons NET only looks for  a reply when one is expected, so it cannot
>> detect this.
>> AFAICS changing this would require a major rewrite.
>> Since the server appears to be broken, it's not worth doing this.
>
> That means, if a server answers a request wrong (like on that server with
> FEAT, because MODE Z should be before 211 END), all further communication
> with apache commons net ftp is impossible to that server because
> _controlInput_ is not "cleared"?
> Is there some kind of "clearControlInput()" method the user can invoke to
> "clear" all (possible) invalid data to make the ftp client more "stable"
> when it has to deal with such ftp servers? Because then i could simply call
> "clearControlInput()" each time before i send my commands.

No, there is no such method.

You could try calling getReply() and catching IO errors.
However if there is no data to read that will likely hang.
It so, try setting a timeout.

You could also try subclassing FTPClient and then you can access the
_controlInput_ field.
This is where the replies are read from.

However reading from the control channel may cause NET to break in subtle ways.

>>
>>> Some information: i dont have control over that server. I dont know what
>>> software the server is running.
>>
>> Try contacting the server maintainers to raise the issue of the
>> server's bad behaviour
>
> Unfortunately this is one of many thousands of servers, located somewhere...
> china, russia, usa etc.

AFAIK this is the first time this problem has been reported.
Which seems odd if there really are thousands of servers with this behaviour.

> I guess some of those servers are even not maintained anymore. So this is
> not a solution. I have to somehow handle those servers :(
>
>
>
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