Why ip_forward_use_pmtu added in the result of sysctl in linux server

Question

So I did an OS version-up in a linux server, and was seeing if any setting has been changed. And when I typed "sysctl -a | grep "net.ipv4.ip_forward" The following line was added,

net.ipv4.ip_forward_use_pmtu = 0

I know that this is because this parameter is in /proc/sys. But I think if the result of sysctl before upload did not show this line, it was not in /proc/sys before as well, right ? I know that 0 means " this setting is not applied...So basically it does not do anything. But why this line is added.

The question is

Is there any possible reason that can add this line?

Thank you, ahead.

Answer 1

Even the question itself "added in the result of sysctl in linux server" is wrong here.

sysctl in the way you invoked it, lists all the entries. grep which you used to filter those entries "selects" matching texts, if you'd run grep foo against the list:

• foo
• foobar

both items would be matched. That's exactly what you see but the only difference is instead of "foo" you have "net.ipv4.ip_forward".

Using --color shows that clearly:

Pay attention to the use of fgrep instead of grep because people tend to forget that grep interprets some characters as regular expressions, and the dot . means any character, which might also lead to unexpected matches.