Exercise 24.1: System Tunables with sysctl

Hi,
Somehow I cannot deactivate the ping on my computer...
┌──(alu@nb)-[~] └─$ sudo sysctl net.ipv4.icmp_echo_ignore_all=1 net.ipv4.icmp_echo_ignore_all = 1 ┌──(alu@nb)-[~] └─$ ping -c 3 localhost PING localhost(localhost (::1)) 56 data bytes 64 bytes from localhost (::1): icmp_seq=1 ttl=64 time=0.013 ms 64 bytes from localhost (::1): icmp_seq=2 ttl=64 time=0.055 ms 64 bytes from localhost (::1): icmp_seq=3 ttl=64 time=0.041 ms --- localhost ping statistics --- 3 packets transmitted, 3 received, 0% packet loss, time 2048ms rtt min/avg/max/mdev = 0.013/0.036/0.055/0.017 ms
What could be the reason?
Thanks,
Urs
Answers
-
Hi Urs,
I just tried it on Ubuntu 24.04 and worked for me.
1.- What OS and version are you testing with?
2.- What's the kernel version?uname -r
Regards,
Luis.0 -
Hi Luis,
Thanks for your reply.
I'm on Debian:
┌──(alu@nb)-[~] └─$ cat /etc/os-release PRETTY_NAME="Debian GNU/Linux 12 (bookworm)" NAME="Debian GNU/Linux" VERSION_ID="12" VERSION="12 (bookworm)" VERSION_CODENAME=bookworm ID=debian HOME_URL="https://www.debian.org/" SUPPORT_URL="https://www.debian.org/support" BUG_REPORT_URL="https://bugs.debian.org/" ┌──(alu@nb)-[~] └─$ uname -r 6.1.0-25-amd64
Regards,
Urs0 -
Hi Urs,
Unfortunately I was able to reproduce the issue on Debian 12 and kernel version 6.1.0-26-amd64.
So, if I do "cat /proc/sys/net/ipv4/icmp_echo_ignore_all" it gives "1", which should deactivate ping localhost, but it isn't. Also, I got net.ipv4.icmp_echo_ignore_all=1 to /etc/sysctl.conf, restarted the associated service with "service procps force-reload", and it didn't work. That's bad, because that means that /etc/sysctl.conf is not working as expected. I also restarted the system and it didn't work either.Before all of that I stopped the default firewall, just in case.
You can check the Debian documentation about this item here:
https://www.debian.org/doc/manuals/securing-debian-manual/network-secure.en.html
So, at this point it looks to me as a bug. I recommend to try with any other distro. It worked for me with Ubuntu.
Regards,
Luis.0 -
Hi Urs,
On Debian you also need to do the following (block icmp for ipv6 as well):
echo "1" > /proc/sys/net/ipv6/icmp/echo_ignore_all
After that, pings to localhost will be ignored.
Regards,
Luis.0 -
Hi Luis,
Thank you very much.
Now I got it too. So it seams that, if icmp on ipv4 is disabled, Debian switches to icmp on ipv6 (if possible). In my case, as I use DHCP, apart form the ipv4 address, my computer was assigned an ipv6 address as well. Maybe that's the reason about this behavior.
In case it's not a bug, you may consider to update the according lab in the course.
Best wishes,
Urs0 -
Hi Urs, it's a pleasure!
Now I got it too. So it seams that, if icmp on ipv4 is disabled, Debian switches to icmp on ipv6 (if >possible). In my case, as I use DHCP, apart form the ipv4 address, my computer was assigned an ipv6 >address as well. Maybe that's the reason about this behavior.
Yes, it's a feature, not a bug, hehehe.
Regards,
Luis.0
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