Oracle Linux Intermittently Stops to Send SYN/ACK in Response to SYN
Last updated on FEBRUARY 01, 2018
Applies to:Linux OS - Version Oracle Linux 5.0 and later
Information in this document applies to any platform.
This problem lies in the TCP Layer, so you might see "connection time out" error in various different TCP applications when you first encounter this issue.
One known cause of this issue is a customer running with a custom Java application running on a Windows server uploading files to vsftpd server running on Oracle Linux 6.8. In this scenario the custom Java application intermittently failed with "connection time out" errors.
By analyzing a network dump file captured with tcpdump, you can see that OL6.8 FTP server did receive 3 SYN packets from FTP client machine in 9 seconds during which the 2nd SYN is received in 3 seconds and the 3rd SYN is received in another 6 seconds. However vsftp server just did not respond. By checking vsftp log file and vsftpd process strace file, you don't see anything wrong. There are also quite a few successful file uploads in which Oracle Linux did respond with SYN/ACK and TCP handshake completed without any issue.
Customer may report they have no problem with similar setup on a physical FTP server and they are not aware of any changes to the current Oracle Linux which could be a Oracle VM Virtual Machine.
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