Hi,

if I modify sysctl.conf file, do I need to restart the box? How does it geet affected? Thanks in advance

You must restart your network with something like
or just reboot.
It will be affected so that whatever you have changed will be loaded on boot.
Please,edit your profile so that we can know what distro you are using,I belive it's Red hat or Fedora,so I based my answer on that.

No you don't. Sysctls are dynamic so running 'sysctl -p' (takes /etc/sysctl.conf as default or supply filename) will do fine. And you can check that by running 'sysctl some.value' or catting it. Sysctls are strictly kernel so the GNU/Linux distro used doesn't matter for answering this particular question.

Yes you do,try to change packet forwarding from disabled to enabled,it want work if you don't restart network or system(at least not on distros that I have tried to do it).Besides some sysctl variables can not be changed after reboot.
Maybe it will,but will not always take affect instantly.
Yes it does,in a matter of how one want to change something,for example the command I have provided in my first post works for some distros(if one wants to change network parameters without rebooting),and some distros don't even change some parameters with sysctl.conf,for example in an old Red hat that I have,packet forwarding one must change in /etc/sysconfig/network,and if OP is using BSD then read this from Wikipedia;

Could that perhaps be due to Something Completely Different? Like say routing and firewalling?


AFAIK there are only three types of sysctls that can't be changed: those set by the kernel directly (like random.boot_id, random.uuid), those that only display device info (like cdrom.info) and those that are related to LSM users (like GRSecurity). AFAIK they can't be set or changed, and regardless when (the exception being sealing GRSecurity at boot), so the argument of using 'sysctl' on unchangable sysctls doesn't compute anyway, does it?


Only example I can think of is the routing cache for which you need to write to the "flush" sysctl, but maybe you have a better example?


If you look under the hood you see /etc/rc.d/init.d/network uses 'sysctl' to set those variables. So wouldn't you say it's simpler to call 'sysctl -p' directly and achieve setting sysctls?


/etc/sysconfig/network is just a configuration file. What distro-agnostic utility do you think picks up those variables to set them?


No, this is Linux Newbie. BSD questions go in the Other *NIX forum :-]

No.

Well,you just can't be sure,right?

For the rest of this little discussion,I will just say,you're right and I'm right and since OP is not posting anything anymore I will leave it on that.