A TCP client is not able to connect to a RHEL server: SYN is answered by chalange ACK and RST is ignored.

Solution Verified - Updated -


A customer application is from time to time not able to connect to RHEL hosted IMAP server. Sometimes an SYN and all its retransmissions are answered by a challenge ACK and RSTs are ignored. Connection is then timing out from the application point of view and failed connection attempt can be seen in the application log.


  • Red Hat Enterprise Linux (RHEL)
    • RHEL-8.4.z (kernel-4.18.0-305.76.1)
    • RHEL-8.7 GA (kernel-4.18.0-425)
    • RHEL-9.1 GA (kernel-5.14.0-162)

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