sysctl(3c)

NAME
     sysctl - get or set system information

SYNOPSIS
     #include <sys/sysctl.h>
     #include <sys/socket.h>

     sysctl(int *name, u_int namelen, void *oldp, size_t *oldlenp,
          void *newp, size_t newlen)

DESCRIPTION
     The sysctl function retrieves system information.  The information
     available from sysctl consists of integers, strings, and tables, and is a
     small subset of that available from the function of the same name in
     4.4BSD-Lite.

     Unless explicitly noted below, sysctl returns a consistent snapshot of
     the data requested.  Consistency is obtained by locking the destination
     buffer into memory so that the data may be copied out without blocking.

     The state is described using a ``Management Information Base'' (MIB)
     style name, listed in name, which is a namelen length array of integers.

     The information is copied into the buffer specified by oldp.  The size of
     the buffer is given by the location specified by oldlenp before the call,
     and that location gives the amount of data copied after a successful
     call.  If the amount of data available is greater than the size of the
     buffer supplied, the call supplies as much data as fits in the buffer
     provided and returns with the error code ENOMEM.  If the old value is not
     desired, oldp and oldlenp should be set to NULL.

     The size of the available data can be determined by calling sysctl with a
     NULL parameter for oldp.  The size of the available data will be returned
     in the location pointed to by oldlenp.  For some operations, the amount
     of space may change often.  For these operations, the system attempts to
     round up so that the returned size is large enough for a call to return
     the data shortly thereafter.

     To set a new value, newp is set to point to a buffer of length newlen
     from which the requested value is to be taken.  If a new value is not to
     be set, newp should be set to NULL and newlen set to 0.

     The top level names are defined with a CTL_ prefix in <sys/sysctl.h>, and
     are as follows.  The next and subsequent levels down are found in the
     include files listed here, and described in separate sections below.


          Name      Next level names    Description
          CTL_NET   sys/socket.h        Networking

   CTL_NET
     The string and integer information available for the CTL_NET level is
     detailed below.  The changeable column shows whether a process with
     appropriate privilege may change the value.


          Second level name   Type
          PF_ROUTE       routing messages
          PF_INET        internet values

     PF_ROUTE
          Return the entire routing table or a subset of it.  The data is
          returned as a sequence of routing messages.  See route(7p) for the
          header file, format and meaning).  The length of each message is
          contained in the message header.

          The third level name is a protocol number, which is currently always
          0.  The fourth level name is an address family, which may be set to
          0 to select all address families.  The fifth and sixth level names
          are as follows:


          Fifth level name    Sixth level is:     Table
          NET_RT_FLAGS        rtflags        Routing
          NET_RT_DUMP         None      Routing (by flags)
          NET_RT_IFLIST       None      Interface list

     PF_INET
          Get or set various global information about the internet protocols.
          The third level name is the protocol.  The fourth level name is the
          variable name.  The currently defined protocols and names are:


          Protocol name  Variable name  Type
          ip             forwarding     integer
          ip             redirect  integer
          udp            checksum  integer

     The variables are as follows:

     ip.forwarding
          Returns 1 when IP forwarding is enabled for the host, meaning that
          the host is acting as a router.

     ip.redirect
          Returns 1 when ICMP redirects may be sent by the host.  This option
          is ignored unless the host is routing IP packets, and should
          normally be enabled on all systems.

     udp.checksum
          Returns 1 when UDP checksums are being computed and checked.
          Disabling UDP checksums is strongly discouraged.