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<body class='hmmessage'><div dir='ltr'>Greetings.<div><br></div><div>We am seeing the following situation and are not quite sure the proper way to handle it, so I thought I'd solicit the mailing list. Our application is an FTP server using OpenSSL. The peer is a non-OpenSSL FTP client in active mode.</div><div><br></div><div>The problem comes in with how the FTP client handles closing the data connection after doing a put and transferring a file from client to our server. Instead of sending a close_notify, it closes the connection with a TCP FIN. On our server, calls to SSL_read() and SSL_get_error() indicate an SSL_ERROR_SYSCALL, which we treat as a protocol violation and abort the connection. When the FTP implementation sees the abort, it tears down the connection and throws the file away.</div><div><br></div><div>Regarding SSL_ERROR_SYSCALL, the OpenSSL documentation says this:</div><div><br></div><div><span style="font-family: arial, sans-serif; font-size: 12pt; background-color: rgb(255, 255, 255);">"Some I/O error occurred. The OpenSSL error queue may contain more information on the error. If the error queue is empty (i.e. ERR_get_error() returns 0), </span><b style="font-family: arial, sans-serif; font-size: medium; background-color: rgb(255, 255, 255);">ret</b><span style="font-family: arial, sans-serif; font-size: 12pt; background-color: rgb(255, 255, 255);"> can be used to find out more about the error: If </span><b style="font-family: arial, sans-serif; font-size: medium; background-color: rgb(255, 255, 255);">ret == 0</b><span style="font-family: arial, sans-serif; font-size: 12pt; background-color: rgb(255, 255, 255);">, an EOF was observed that violates the protocol. If </span><b style="font-family: arial, sans-serif; font-size: medium; background-color: rgb(255, 255, 255);">ret == -1</b><span style="font-family: arial, sans-serif; font-size: 12pt; background-color: rgb(255, 255, 255);">, the underlying </span><b style="font-family: arial, sans-serif; font-size: medium; background-color: rgb(255, 255, 255);">BIO</b><span style="font-family: arial, sans-serif; font-size: 12pt; background-color: rgb(255, 255, 255);"> reported an I/O error (for socket I/O on Unix systems, consult </span><b style="font-family: arial, sans-serif; font-size: medium; background-color: rgb(255, 255, 255);">errno </b><span style="font-family: arial, sans-serif; font-size: 12pt; background-color: rgb(255, 255, 255);">for details)."</span></div><div><span style="font-family: arial, sans-serif; font-size: 12pt; background-color: rgb(255, 255, 255);"><br></span></div><div><span style="font-size: 12pt;">From this information, we determined we were seeing the case where ERR_get_error() returns 0, meaning an EOF was observed that violates the protocol; the FIN being sent without being preceded by a close_notify. If we change our server to look for this specific case, and indicate a close to the FTP application instead of an abort, the FTP transfer completes successfully.</span></div><div><span style="font-size: 12pt;"><br></span></div><div>This is where our questions arise. First of all, the FTP client's close without being preceded by a close_notify seems to be a violation of the protocol, and OpenSSL handles it as such. Does changing the way it's handled open up our application to truncation attacks?</div><div><br></div><div>We have also read that this particular behavior is not unheard of in SSL implementations, and treating the TCP FIN as a proper way to close the connection as described above is OK.</div><div><br></div><div>Given the conflicting information we have seen, what is the opinion of the OpenSSL team?</div><div><br></div><div>Thanks in advance.</div><div><br></div> </div></body>
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