I am trying to fix latest security bug by doing:

sysctl net.ipv4.tcp_challenge_ack_limit=999999999

however this file is missing on CentOS 5, kernel 2.6.18-371.12.1.el5PAE

Where is net.ipv4.tcp_challenge_ack_limit for CentOS 5?


This parameter was introduced with RFC 5961 support in 3.6 kernel, so you probably are not concerned by this flaw; if you were, you would be able to read and write this parameter.


Officially the 2.6.x kernels do not have the tcp_challenge_ack_limit sysctl flag. If CentOS/RedHat released a version of the kernel with this code backported in it, then you probably have to update to a newer kernel.

Simply put, if you can't find a sysctl 'file' (it's a virtual file pointing to a kernel setting), then your kernel doesn't have that setting.

  • I can confirm that RHEL6, running 2.6.x, is affected by this. Can't speak for other 2.6.x distributions. Aug 12 '16 at 21:09
  • Yeah, redhat backports lots of things to older kernels. Perhaps OP has an update waiting for his kernel package
    – madeddie
    Aug 12 '16 at 21:13

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