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Topic: [SOLVED] Sysctl net.ipv4.tcp_sack=0 selfresets to =1 (Read 469 times) previous topic - next topic
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[SOLVED] Sysctl net.ipv4.tcp_sack=0 selfresets to =1

 Hello guys made a custom sysctl.conf so the kernel suppose to read from there when the system starts but by some mysterious
 switch it changes the value back to 1 even if in my sysctl.conf is set to be 0.

 The parameter in question is 
Code: [Select]
net.ipv4.tcp_sack = 0

 Is there any other thing that overwrites that value switching it back to 1? I'm usually booting the hardened kernel.

 I remember when I tried to modify some icmp reply to be disabled and ufw wrote back the value as it wanted so had to edit some ufw
 file.

 What's the trick? Think this is gonna be a hard one, tnx for any tip you guys may have.

 ☮️ and have a nice day

Re: Sysctl net.ipv4.tcp_sack=0 selfresets to =1

Reply #2 –
 
  I guess I've set it right with.:
 
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# sysctl -w net.ipv4.tcp_sack=0
  Tried sysctl -p but fails cos it doesn't read from etc/sysctl.conf but from etc/sysct.d/sysctl.conf on Artix
  Anyway after a restart those params should be read from etc/sysctl.d/sysctl.conf like all the others that works as they should.
  Even if I don't 
Code: [Select]
sysctl -w net.ipv4.tcp_sack=0

 After restart it gets back to the value of 1  :o

Re: Sysctl net.ipv4.tcp_sack=0 selfresets to =1

Reply #3 –

 It does the same fallback to net.ipv4.tcp_sack=1 with the linux normal kernel. I suspect some other system component
 has the final word about that parameter and it's definitely not sysctl.conf but which one? Dhcpcd? Could be chrony that I use
 instead of classic ntp? Or maybe apparmor? Idk. Or one of ufw firewall default rule?

Re: Sysctl net.ipv4.tcp_sack=0 selfresets to =1

Reply #4 –
 
 Think I've got it. The last word passed to the kernel in the case of net.ipv4.tcp_sack comes from ufw like I assumed but didn't know
 for sure. So the kernel reads in that case from /etc/ufw/sysctl.conf and overwrites the etc/sysctl.d/sysctl.conf.  Looks like it's a tweak
 recommended by the BSD guys to change the value to 0

 To make ufw read all net parameters from the main sysctl.conf there is a file in etc/default/ufw where can be specified IPT_SYSCTL.
 But I preferred to edit like I said that ufw sysctl.conf to avoid any other headaches.

 Thanks

Re: Sysctl net.ipv4.tcp_sack=0 selfresets to =1

Reply #5 –
 
 Also for the command
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sysctl -p
to work  you can't cd into /etc/sysctl.d/ you have to do it like this
 
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# sysctl -p /etc/sysctl.d/sysctl.conf
and will work