>Does someone know how to add a new sysctl parameter?
>
>I like to add a tcp-level sysctl parameter to control my own function.
>I have tried to modify /sys/netinet/tcp_var.h, tcp_subr.c and
>/usr/include/netinet/tcp_var.h, then re-compile sysctl command.
>
>But it seems not work, I can't see my new sysctl var with sysctl.

it should suffice for you to modify tcp_usrreq.c, recompile your
kernel, and reboot.  the sysctl bianry should not need to be rebuilt.

>Could you give me a guide?

see sysctl(9) and tcp_usrreq.c (among others).

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