# proc.txt

This appendix holds the file proc.txt, which explains the `proc` file system on Linux machines.

## The /proc Filesystem

This appendix contains a file from the documentation that comes with your Linux system, the `proc.txt` file. It explains in detail the contents of the `/proc` directory on your system. You'll find this directory very helpful for gathering system information.

    ------------------------------------------------------------------------------
                           T H E  /proc   F I L E S Y S T E M
    ------------------------------------------------------------------------------
    /proc/sys         Terrehon Bowden <terrehon@pacbell.net>        October 7 1999
                      Bodo Bauer <bb@ricochet.net>

    2.4.x update	  Jorge Nerin <comandante@zaralinux.com>      November 14 2000
    ------------------------------------------------------------------------------
    Version 1.3                                              Kernel version 2.2.12
    					      Kernel version 2.4.0-test11-pre4
    ------------------------------------------------------------------------------

    Table of Contents
    -----------------

      0     Preface
      0.1	Introduction/Credits
      0.2	Legal Stuff

      1	Collecting System Information
      1.1	Process-Specific Subdirectories
      1.2	Kernel data
      1.3	IDE devices in /proc/ide
      1.4	Networking info in /proc/net
      1.5	SCSI info
      1.6	Parallel port info in /proc/parport
      1.7	TTY info in /proc/tty

      2	Modifying System Parameters
      2.1	/proc/sys/fs - File system data
      2.2	/proc/sys/fs/binfmt_misc - Miscellaneous binary formats
      2.3	/proc/sys/kernel - general kernel parameters
      2.4	/proc/sys/vm - The virtual memory subsystem
      2.5	/proc/sys/dev - Device specific parameters
      2.6	/proc/sys/sunrpc - Remote procedure calls
      2.7	/proc/sys/net - Networking stuff
      2.8	/proc/sys/net/ipv4 - IPV4 settings
      2.9	Appletalk
      2.10	IPX

    ------------------------------------------------------------------------------
    Preface
    ------------------------------------------------------------------------------

    0.1 Introduction/Credits
    ------------------------

    This documentation is  part of a soon (or  so we hope) to be  released book on
    the SuSE  Linux distribution. As  there is  no complete documentation  for the
    /proc file system and we've used  many freely available sources to write these
    chapters, it  seems only fair  to give the work  back to the  Linux community.
    This work is  based on the 2.2.*  kernel version and the  upcoming 2.4.*. I'm
    afraid it's still far from complete, but we  hope it will be useful. As far as
    we know, it is the first 'all-in-one' document about the /proc file system. It
    is focused  on the Intel  x86 hardware,  so if you  are looking for  PPC, ARM,
    SPARC, APX, etc., features, you probably  won't find what you are looking for.
    It also only covers IPv4 networking, not IPv6 nor other protocols - sorry. But
    additions and patches  are welcome and will  be added to this  document if you
    mail them to Bodo.

    We'd like  to  thank Alan Cox, Rik van Riel, and Alexey Kuznetsov and a lot of
    other people for help compiling this documentation. We'd also like to extend a
    special thank  you to Andi Kleen for documentation, which we relied on heavily
    to create  this  document,  as well as the additional information he provided.
    Thanks to  everybody  else  who contributed source or docs to the Linux kernel
    and helped create a great piece of software... :)

    If you  have  any comments, corrections or additions, please don't hesitate to
    contact Bodo  Bauer  at  bb@ricochet.net.  We'll  be happy to add them to this
    document.

    The   latest   version    of   this   document   is    available   online   at
    http://skaro.nightcrawler.com/~bb/Docs/Proc as HTML version.

    If  the above  direction does  not works  for you,  ypu could  try the  kernel
    mailing  list  at  linux-kernel@vger.kernel.org  and/or try  to  reach  me  at
    comandante@zaralinux.com.

    0.2 Legal Stuff
    ---------------

    We don't  guarantee  the  correctness  of this document, and if you come to us
    complaining about  how  you  screwed  up  your  system  because  of  incorrect
    documentation, we won't feel responsible...

    ------------------------------------------------------------------------------
    CHAPTER 1: COLLECTING SYSTEM INFORMATION
    ------------------------------------------------------------------------------

    ------------------------------------------------------------------------------
    In This Chapter
    ------------------------------------------------------------------------------
    * Investigating  the  properties  of  the  pseudo  file  system  /proc and its
      ability to provide information on the running Linux system
    * Examining /proc's structure
    * Uncovering  various  information  about the kernel and the processes running
      on the system
    ------------------------------------------------------------------------------

    The proc  file  system acts as an interface to internal data structures in the
    kernel. It  can  be  used to obtain information about the system and to change
    certain kernel parameters at runtime (sysctl).

    First, we'll  take  a  look  at the read-only parts of /proc. In Chapter 2, we
    show you how you can use /proc/sys to change settings.

    1.1 Process-Specific Subdirectories
    -----------------------------------

    The directory  /proc  contains  (among other things) one subdirectory for each
    process running on the system, which is named after the process ID (PID).

    The link  self  points  to  the  process reading the file system. Each process
    subdirectory has the entries listed in Table 1-1.

    Table 1-1: Process specific entries in /proc
    ..............................................................................
     File    Content
     cmdline Command line arguments
     cpu	 Current and last cpu in wich it was executed		(2.4)(smp)
     cwd	 Link to the current working directory
     environ Values of environment variables
     exe	 Link to the executable of this process
     fd      Directory, which contains all file descriptors
     maps	 Memory maps to executables and library files		(2.4)
     mem     Memory held by this process
     root	 Link to the root directory of this process
     stat    Process status
     statm   Process memory status information
     status  Process status in human readable form
    ..............................................................................

    For example, to get the status information of a process, all you have to do is
    read the file /proc/PID/status:

      >cat /proc/self/status
      Name:   cat
      State:  R (running)
      Pid:    5452
      PPid:   743
      TracerPid:      0						(2.4)
      Uid:    501     501     501     501
      Gid:    100     100     100     100
      Groups: 100 14 16
      VmSize:     1112 kB
      VmLck:         0 kB
      VmRSS:       348 kB
      VmData:       24 kB
      VmStk:        12 kB
      VmExe:         8 kB
      VmLib:      1044 kB
      SigPnd: 0000000000000000
      SigBlk: 0000000000000000
      SigIgn: 0000000000000000
      SigCgt: 0000000000000000
      CapInh: 00000000fffffeff
      CapPrm: 0000000000000000
      CapEff: 0000000000000000

    This shows you nearly the same information you would get if you viewed it with
    the ps  command.  In  fact,  ps  uses  the  proc  file  system  to  obtain its
    information. The  statm  file  contains  more  detailed  information about the
    process memory usage. Its seven fields are explained in Table 1-2.

    Table 1-2: Contents of the statm files
    ..............................................................................
     File     Content
     size     total program size
     resident size of memory portions
     shared   number of pages that are shared
     trs      number of pages that are 'code'
     drs      number of pages of data/stack
     lrs      number of pages of library
     dt       number of dirty pages
    ..............................................................................

    1.2 Kernel data
    ---------------

    Similar to  the  process entries, the kernel data files give information about
    the running kernel. The files used to obtain this information are contained in
    /proc and  are  listed  in Table 1-3. Not all of these will be present in your
    system. It  depends  on the kernel configuration and the loaded modules, which
    files are there, and which are missing.

    Table 1-3: Kernel info in /proc
    ..............................................................................
     File        Content
     apm         Advanced power management info
     bus         Directory containing bus specific information
     cmdline     Kernel command line
     cpuinfo     Info about the CPU
     devices     Available devices (block and character)
     dma         Used DMS channels
     filesystems Supported filesystems
     driver	     Various drivers grouped here, currently rtc	(2.4)
     execdomains Execdomains, related to security			(2.4)
     fb	     Frame Buffer devices				(2.4)
     fs	     File system parameters, currently nfs/exports	(2.4)
     ide         Directory containing info about the IDE subsystem
     interrupts  Interrupt usage
     iomem	     Memory map						(2.4)
     ioports     I/O port usage
     irq	     Masks for irq to cpu affinity			(2.4)(smp?)
     isapnp	     ISA PnP (Plug&Play) Info				(2.4)
     kcore       Kernel core image (can be ELF or A.OUT(deprecated in 2.4))
     kmsg        Kernel messages
     ksyms       Kernel symbol table
     loadavg     Load average of last 1, 5 & 15 minutes
     locks       Kernel locks
     meminfo     Memory info
     misc        Miscellaneous
     modules     List of loaded modules
     mounts      Mounted filesystems
     net         Networking info (see text)
     partitions  Table of partitions known to the system
     pci	     Depreciated info of PCI bus (new way -> /proc/bus/pci/,
                 decoupled by lspci					(2.4)
     rtc         Real time clock
     scsi        SCSI info (see text)
     slabinfo    Slab pool info
     stat        Overall statistics
     swaps       Swap space utilization
     sys         See chapter 2
     sysvipc     Info of SysVIPC Resources (msg, sem, shm)		(2.4)
     tty	     Info of tty drivers
     uptime      System uptime
     version     Kernel version
     video	     bttv info of video resources			(2.4)
    ..............................................................................

    You can,  for  example,  check  which interrupts are currently in use and what
    they are used for by looking in the file /proc/interrupts:

      > cat /proc/interrupts
                 CPU0
        0:    8728810          XT-PIC  timer
        1:        895          XT-PIC  keyboard
        2:          0          XT-PIC  cascade
        3:     531695          XT-PIC  aha152x
        4:    2014133          XT-PIC  serial
        5:      44401          XT-PIC  pcnet_cs
        8:          2          XT-PIC  rtc
       11:          8          XT-PIC  i82365
       12:     182918          XT-PIC  PS/2 Mouse
       13:          1          XT-PIC  fpu
       14:    1232265          XT-PIC  ide0
       15:          7          XT-PIC  ide1
      NMI:          0

    In 2.4.* a couple of lines where added to this file LOC & ERR (this time is the
    output of a SMP machine):

      > cat /proc/interrupts

                 CPU0       CPU1
        0:    1243498    1214548    IO-APIC-edge  timer
        1:       8949       8958    IO-APIC-edge  keyboard
        2:          0          0          XT-PIC  cascade
        5:      11286      10161    IO-APIC-edge  soundblaster
        8:          1          0    IO-APIC-edge  rtc
        9:      27422      27407    IO-APIC-edge  3c503
       12:     113645     113873    IO-APIC-edge  PS/2 Mouse
       13:          0          0          XT-PIC  fpu
       14:      22491      24012    IO-APIC-edge  ide0
       15:       2183       2415    IO-APIC-edge  ide1
       17:      30564      30414   IO-APIC-level  eth0
       18:        177        164   IO-APIC-level  bttv
      NMI:    2457961    2457959
      LOC:    2457882    2457881
      ERR:       2155

    NMI is incremented in this case because every timer interrupt generates a NMI
    (Non Maskable Interrupt) which is used by the NMI Watchdog to detect lookups.

    LOC is the local interrupt counter of the internal APIC of every CPU.

    ERR is incremented in the case of errors in the IO-APIC bus (the bus that
    connects the CPUs in a SMP system. This means that an error has been detected,
    the IO-APIC automatically retry the transmission, so it should not be a big
    problem, but you should read the SMP-FAQ.

    In this context it could be interesting to note the new irq directory in 2.4.
    It could be used to set IRQ to CPU affinity, this means that you can "hook" an
    IRQ to only one CPU, or to exclude a CPU of handling IRQs. The contents of the
    irq subdir is one subdir for each IRQ, and one file; prof_cpu_mask

    For example
      > ls /proc/irq/
      0  10  12  14  16  18  2  4  6  8  prof_cpu_mask
      1  11  13  15  17  19  3  5  7  9
      > ls /proc/irq/0/
      smp_affinity

    The contents of the prof_cpu_mask file and each smp_affinity file for each IRQ
    is the same by default:

      > cat /proc/irq/0/smp_affinity
      ffffffff

    It's a bitmask, in wich you can specify wich CPUs can handle the IRQ, you can
    set it by doing:

      > echo 1 > /proc/irq/prof_cpu_mask

    This means that only the first CPU will handle the IRQ, but you can also echo 5
    wich means that only the first and fourth CPU can handle the IRQ.

    The way IRQs are routed is handled by the IO-APIC, and it's Round Robin
    between all the CPUs which are allowed to handle it. As usual the kernel has
    more info than you and does a better job than you, so the defaults are the
    best choice for almost everyone.

    There are  three  more  important subdirectories in /proc: net, scsi, and sys.
    The general  rule  is  that  the  contents,  or  even  the  existence of these
    directories, depend  on your kernel configuration. If SCSI is not enabled, the
    directory scsi  may  not  exist. The same is true with the net, which is there
    only when networking support is present in the running kernel.

    The slabinfo  file  gives  information  about  memory usage at the slab level.
    Linux uses  slab  pools for memory management above page level in version 2.2.
    Commonly used  objects  have  their  own  slab  pool (such as network buffers,
    directory cache, and so on).

    1.3 IDE devices in /proc/ide
    ----------------------------

    The subdirectory /proc/ide contains information about all IDE devices of which
    the kernel  is  aware.  There is one subdirectory for each IDE controller, the
    file drivers  and a link for each IDE device, pointing to the device directory
    in the controller specific subtree.

    The file  drivers  contains general information about the drivers used for the
    IDE devices:

      > cat /proc/ide/drivers
      ide-cdrom version 4.53
      ide-disk version 1.08

    More detailed  information  can  be  found  in  the  controller  specific
    subdirectories. These  are  named  ide0,  ide1  and  so  on.  Each  of  these
    directories contains the files shown in table 1-4.

    Table 1-4: IDE controller info in  /proc/ide/ide?
    ..............................................................................
     File    Content
     channel IDE channel (0 or 1)
     config  Configuration (only for PCI/IDE bridge)
     mate    Mate name
     model   Type/Chipset of IDE controller
    ..............................................................................

    Each device  connected  to  a  controller  has  a separate subdirectory in the
    controllers directory.  The  files  listed in table 1-5 are contained in these
    directories.

    Table 1-5: IDE device information
    ..............................................................................
     File             Content
     cache            The cache
     capacity         Capacity of the medium (in 512Byte blocks)
     driver           driver and version
     geometry         physical and logical geometry
     identify         device identify block
     media            media type
     model            device identifier
     settings         device setup
     smart_thresholds IDE disk management thresholds
     smart_values     IDE disk management values
    ..............................................................................

    The most  interesting  file is settings. This file contains a nice overview of
    the drive parameters:

      # cat /proc/ide/ide0/hda/settings
      name                    value           min             max             mode
      ----                    -----           ---             ---             ----
      bios_cyl                526             0               65535           rw
      bios_head               255             0               255             rw
      bios_sect               63              0               63              rw
      breada_readahead        4               0               127             rw
      bswap                   0               0               1               r
      file_readahead          72              0               2097151         rw
      io_32bit                0               0               3               rw
      keepsettings            0               0               1               rw
      max_kb_per_request      122             1               127             rw
      multcount               0               0               8               rw
      nice1                   1               0               1               rw
      nowerr                  0               0               1               rw
      pio_mode                write-only      0               255             w
      slow                    0               0               1               rw
      unmaskirq               0               0               1               rw
      using_dma               0               0               1               rw

    1.4 Networking info in /proc/net
    --------------------------------

    The subdirectory  /proc/net  follows  the  usual  pattern. Table 1-6 shows the
    additional values  you  get  for  IP  version 6 if you configure the kernel to
    support this. Table 1-7 lists the files and their meaning.

    Table 1-6: IPv6 info in /proc/net
    ..............................................................................
     File       Content
     udp6       UDP sockets (IPv6)
     tcp6       TCP sockets (IPv6)
     raw6       Raw device statistics (IPv6)
     igmp6      IP multicast addresses, which this host joined (IPv6)
     if_inet6   List of IPv6 interface addresses
     ipv6_route Kernel routing table for IPv6
     rt6_stats  Global IPv6 routing tables statistics
     sockstat6  Socket statistics (IPv6)
     snmp6      Snmp data (IPv6)
    ..............................................................................

    Table 1-7: Network info in /proc/net
    ..............................................................................
     File          Content
     arp           Kernel  ARP table
     dev           network devices with statistics
     dev_mcast     the Layer2 multicast groups a device is listening too
                   (interface index, label, number of references, number of bound
                   addresses).
     dev_stat      network device status
     ip_fwchains   Firewall chain linkage
     ip_fwnames    Firewall chain names
     ip_masq       Directory containing the masquerading tables
     ip_masquerade Major masquerading table
     netstat       Network statistics
     raw           raw device statistics
     route         Kernel routing table
     rpc           Directory containing rpc info
     rt_cache      Routing cache
     snmp          SNMP data
     sockstat      Socket statistics
     tcp           TCP  sockets
     tr_rif        Token ring RIF routing table
     udp           UDP sockets
     unix          UNIX domain sockets
     wireless      Wireless interface data (Wavelan etc)
     igmp          IP multicast addresses, which this host joined
     psched        Global packet scheduler parameters.
     netlink       List of PF_NETLINK sockets
     ip_mr_vifs    List of multicast virtual interfaces
     ip_mr_cache   List of multicast routing cache
    ..............................................................................

    You can  use  this  information  to see which network devices are available in
    your system and how much traffic was routed over those devices:

      > cat /proc/net/dev
      Inter-|Receive                                                   |[...
       face |bytes    packets errs drop fifo frame compressed multicast|[...
          lo:  908188   5596     0    0    0     0          0         0 [...
        ppp0:15475140  20721   410    0    0   410          0         0 [...
        eth0:  614530   7085     0    0    0     0          0         1 [...

      ...] Transmit
      ...] bytes    packets errs drop fifo colls carrier compressed
      ...]  908188     5596    0    0    0     0       0          0
      ...] 1375103    17405    0    0    0     0       0          0
      ...] 1703981     5535    0    0    0     3       0          0

    In addition, each Channel Bond interface has it's own directory.  For
    example, the bond0 device will have a directory called /proc/net/bond0/.
    It will contain information that is specific to that bond, such as the
    current slaves of the bond, the link status of the slaves, and how
    many times the slaves link has failed.

    1.5 SCSI info
    -------------

    If you  have  a  SCSI  host adapter in your system, you'll find a subdirectory
    named after  the driver for this adapter in /proc/scsi. You'll also see a list
    of all recognized SCSI devices in /proc/scsi:

      >cat /proc/scsi/scsi
      Attached devices:
      Host: scsi0 Channel: 00 Id: 00 Lun: 00
        Vendor: IBM      Model: DGHS09U          Rev: 03E0
        Type:   Direct-Access                    ANSI SCSI revision: 03
      Host: scsi0 Channel: 00 Id: 06 Lun: 00
        Vendor: PIONEER  Model: CD-ROM DR-U06S   Rev: 1.04
        Type:   CD-ROM                           ANSI SCSI revision: 02

    The directory  named  after  the driver has one file for each adapter found in
    the system.  These  files  contain information about the controller, including
    the used  IRQ  and  the  IO  address range. The amount of information shown is
    dependent on  the adapter you use. The example shows the output for an Adaptec
    AHA-2940 SCSI adapter:

      > cat /proc/scsi/aic7xxx/0

      Adaptec AIC7xxx driver version: 5.1.19/3.2.4
      Compile Options:
        TCQ Enabled By Default : Disabled
        AIC7XXX_PROC_STATS     : Disabled
        AIC7XXX_RESET_DELAY    : 5
      Adapter Configuration:
                 SCSI Adapter: Adaptec AHA-294X Ultra SCSI host adapter
                                 Ultra Wide Controller
          PCI MMAPed I/O Base: 0xeb001000
       Adapter SEEPROM Config: SEEPROM found and used.
            Adaptec SCSI BIOS: Enabled
                          IRQ: 10
                         SCBs: Active 0, Max Active 2,
                               Allocated 15, HW 16, Page 255
                   Interrupts: 160328
            BIOS Control Word: 0x18b6
         Adapter Control Word: 0x005b
         Extended Translation: Enabled
      Disconnect Enable Flags: 0xffff
           Ultra Enable Flags: 0x0001
       Tag Queue Enable Flags: 0x0000
      Ordered Queue Tag Flags: 0x0000
      Default Tag Queue Depth: 8
          Tagged Queue By Device array for aic7xxx host instance 0:
            {255,255,255,255,255,255,255,255,255,255,255,255,255,255,255,255}
          Actual queue depth per device for aic7xxx host instance 0:
            {1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1}
      Statistics:
      (scsi0:0:0:0)
        Device using Wide/Sync transfers at 40.0 MByte/sec, offset 8
        Transinfo settings: current(12/8/1/0), goal(12/8/1/0), user(12/15/1/0)
        Total transfers 160151 (74577 reads and 85574 writes)
      (scsi0:0:6:0)
        Device using Narrow/Sync transfers at 5.0 MByte/sec, offset 15
        Transinfo settings: current(50/15/0/0), goal(50/15/0/0), user(50/15/0/0)
        Total transfers 0 (0 reads and 0 writes)

    1.6 Parallel port info in /proc/parport
    ---------------------------------------

    The directory  /proc/parport  contains information about the parallel ports of
    your system.  It  has  one  subdirectory  for  each port, named after the port
    number (0,1,2,...).

    These directories contain the four files shown in Table 1-8.

    Table 1-8: Files in /proc/parport
    ..............................................................................
     File      Content
     autoprobe Any IEEE-1284 device ID information that has been acquired.
     devices   list of the device drivers using that port. A + will appear by the
               name of the device currently using the port (it might not appear
               against any).
     hardware  Parallel port's base address, IRQ line and DMA channel.
     irq       IRQ that parport is using for that port. This is in a separate
               file to allow you to alter it by writing a new value in (IRQ
               number or none).
    ..............................................................................

    1.7 TTY info in /proc/tty
    -------------------------

    Information about  the  available  and actually used tty's can be found in the
    directory /proc/tty.You'll  find  entries  for drivers and line disciplines in
    this directory, as shown in Table 1-9.

    Table 1-9: Files in /proc/tty
    ..............................................................................
     File          Content
     drivers       list of drivers and their usage
     ldiscs        registered line disciplines
     driver/serial usage statistic and status of single tty lines
    ..............................................................................

    To see  which  tty's  are  currently in use, you can simply look into the file
    /proc/tty/drivers:

      > cat /proc/tty/drivers
      pty_slave            /dev/pts      136   0-255 pty:slave
      pty_master           /dev/ptm      128   0-255 pty:master
      pty_slave            /dev/ttyp       3   0-255 pty:slave
      pty_master           /dev/pty        2   0-255 pty:master
      serial               /dev/cua        5   64-67 serial:callout
      serial               /dev/ttyS       4   64-67 serial
      /dev/tty0            /dev/tty0       4       0 system:vtmaster
      /dev/ptmx            /dev/ptmx       5       2 system
      /dev/console         /dev/console    5       1 system:console
      /dev/tty             /dev/tty        5       0 system:/dev/tty
      unknown              /dev/tty        4    1-63 console

    ------------------------------------------------------------------------------
    Summary
    ------------------------------------------------------------------------------
    The /proc file system serves information about the running system. It not only
    allows access to process data but also allows you to request the kernel status
    by reading files in the hierarchy.

    The directory  structure  of /proc reflects the types of information and makes
    it easy, if not obvious, where to look for specific data.
    ------------------------------------------------------------------------------

    ------------------------------------------------------------------------------
    CHAPTER 2: MODIFYING SYSTEM PARAMETERS
    ------------------------------------------------------------------------------

    ------------------------------------------------------------------------------
    In This Chapter
    ------------------------------------------------------------------------------
    * Modifying kernel parameters by writing into files found in /proc/sys
    * Exploring the files which modify certain parameters
    * Review of the /proc/sys file tree
    ------------------------------------------------------------------------------

    A very  interesting part of /proc is the directory /proc/sys. This is not only
    a source  of  information,  it also allows you to change parameters within the
    kernel. Be  very  careful  when attempting this. You can optimize your system,
    but you  can  also  cause  it  to  crash.  Never  alter kernel parameters on a
    production system.  Set  up  a  development machine and test to make sure that
    everything works  the  way  you want it to. You may have no alternative but to
    reboot the machine once an error has been made.

    To change  a  value,  simply  echo  the new value into the file. An example is
    given below  in the section on the file system data. You need to be root to do
    this. You  can  create  your  own  boot script to perform this every time your
    system boots.

    The files  in /proc/sys can be used to fine tune and monitor miscellaneous and
    general things  in  the operation of the Linux kernel. Since some of the files
    can inadvertently  disrupt  your  system,  it  is  advisable  to  read  both
    documentation and  source  before actually making adjustments. In any case, be
    very careful  when  writing  to  any  of these files. The entries in /proc may
    change slightly between the 2.1.* and the 2.2 kernel, so if there is any doubt
    review the kernel documentation in the directory /usr/src/linux/Documentation.
    This chapter  is  heavily  based  on the documentation included in the pre 2.2
    kernels, and became part of it in version 2.2.1 of the Linux kernel.

    2.1 /proc/sys/fs - File system data
    -----------------------------------

    This subdirectory  contains  specific  file system, file handle, inode, dentry
    and quota information.

    Currently, these files are in /proc/sys/fs:

    dentry-state
    ------------

    Status of  the  directory  cache.  Since  directory  entries  are  dynamically
    allocated and  deallocated,  this  file indicates the current status. It holds
    six values, in which the last two are not used and are always zero. The others
    are listed in table 2-1.

    Table 2-1: Status files of the directory cache
    ..............................................................................
     File       Content
     nr_dentry  Almost always zero
     nr_unused  Number of unused cache entries
     age_limit
                in seconds after the entry may be reclaimed, when memory is short
     want_pages internally
    ..............................................................................

    dquot-nr and dquot-max
    ----------------------

    The file dquot-max shows the maximum number of cached disk quota entries.

    The file  dquot-nr  shows  the  number of allocated disk quota entries and the
    number of free disk quota entries.

    If the number of available cached disk quotas is very low and you have a large
    number of simultaneous system users, you might want to raise the limit.

    file-nr and file-max
    --------------------

    The kernel  allocates file handles dynamically, but doesn't free them again at
    this time.

    The value  in  file-max  denotes  the  maximum number of file handles that the
    Linux kernel will allocate. When you get a lot of error messages about running
    out of  file handles, you might want to raise this limit. The default value is
    4096. To change it, just write the new number into the file:

      # cat /proc/sys/fs/file-max
      4096
      # echo 8192 > /proc/sys/fs/file-max
      # cat /proc/sys/fs/file-max
      8192

    This method  of  revision  is  useful  for  all customizable parameters of the
    kernel - simply echo the new value to the corresponding file.

    The three  values  in file-nr denote the number of allocated file handles, the
    number of  used file handles, and the maximum number of file handles. When the
    allocated file  handles  come close to the maximum, but the number of actually
    used ones  is  far  behind,  you've  encountered  a peak in your usage of file
    handles and you don't need to increase the maximum.

    inode-state and inode-nr
    ------------------------

    The file inode-nr contains the first two items from inode-state, so we'll skip
    to that file...

    inode-state contains  two  actual numbers and five dummy values. The numbers
    are nr_inodes and nr_free_inodes (in order of appearance).

    nr_inodes
    ~~~~~~~~~

    Denotes the  number  of  inodes the system has allocated. This number will
    grow and shrink dynamically.

    nr_free_inodes
    --------------

    Represents the  number of free inodes. Ie. The number of inuse inodes is
    (nr_inodes - nr_free_inodes).

    super-nr and super-max
    ----------------------

    Again, super  block structures are allocated by the kernel, but not freed. The
    file super-max  contains  the  maximum  number  of super block handlers, where
    super-nr shows the number of currently allocated ones.

    Every mounted file system needs a super block, so if you plan to mount lots of
    file systems, you may want to increase these numbers.

    2.2 /proc/sys/fs/binfmt_misc - Miscellaneous binary formats
    -----------------------------------------------------------

    Besides these  files, there is the subdirectory /proc/sys/fs/binfmt_misc. This
    handles the kernel support for miscellaneous binary formats.

    Binfmt_misc provides  the ability to register additional binary formats to the
    Kernel without  compiling  an additional module/kernel. Therefore, binfmt_misc
    needs to  know magic numbers at the beginning or the filename extension of the
    binary.

    It works by maintaining a linked list of structs that contain a description of
    a binary  format,  including  a  magic  with size (or the filename extension),
    offset and  mask,  and  the  interpreter name. On request it invokes the given
    interpreter with  the  original  program  as  argument,  as  binfmt_java  and
    binfmt_em86 and  binfmt_mz  do.  Since binfmt_misc does not define any default
    binary-formats, you have to register an additional binary-format.

    There are two general files in binfmt_misc and one file per registered format.
    The two general files are register and status.

    Registering a new binary format
    -------------------------------

    To register a new binary format you have to issue the command

      echo :name:type:offset:magic:mask:interpreter: > /proc/sys/fs/binfmt_misc/register

    with appropriate  name (the name for the /proc-dir entry), offset (defaults to
    0, if  omitted),  magic, mask (which can be omitted, defaults to all 0xff) and
    last but  not  least,  the  interpreter that is to be invoked (for example and
    testing /bin/echo).  Type  can be M for usual magic matching or E for filename
    extension matching (give extension in place of magic).

    Check or reset the status of the binary format handler
    ------------------------------------------------------

    If you  do a cat on the file /proc/sys/fs/binfmt_misc/status, you will get the
    current status (enabled/disabled) of binfmt_misc. Change the status by echoing
    0 (disables)  or  1  (enables)  or  -1  (caution:  this  clears all previously
    registered binary  formats)  to status. For example echo 0 > status to disable
    binfmt_misc (temporarily).

    Status of a single handler
    --------------------------

    Each registered  handler has an entry in /proc/sys/fs/binfmt_misc. These files
    perform the  same function as status, but their scope is limited to the actual
    binary format.  By  cating this file, you also receive all related information
    about the interpreter/magic of the binfmt.

    Example usage of binfmt_misc (emulate binfmt_java)
    --------------------------------------------------

      cd /proc/sys/fs/binfmt_misc
      echo ':Java:M::\xca\xfe\xba\xbe::/usr/local/java/bin/javawrapper:' > register
      echo ':HTML:E::html::/usr/local/java/bin/appletviewer:' > register
      echo ':Applet:M::<applet::/usr/local/java/bin/appletviewer:' > register
      echo ':DEXE:M::\x0eDEX::/usr/bin/dosexec:' > register

    These four  lines  add  support  for  Java  executables and Java applets (like
    binfmt_java, additionally  recognizing the .html extension with no need to put
    <!--applet> to  every  applet  file).  You  have  to  install  the JDK and the
    shell-script /usr/local/java/bin/javawrapper  too.  It  works  around  the
    brokenness of  the Java filename handling. To add a Java binary, just create a
    link to the class-file somewhere in the path.

    2.3 /proc/sys/kernel - general kernel parameters
    ------------------------------------------------

    This directory  reflects  general  kernel  behaviors. As I've said before, the
    contents depend  on  your  configuration.  Here you'll find the most important
    files, along with descriptions of what they mean and how to use them.

    acct
    ----

    The file contains three values; highwater, lowwater, and frequency.

    It exists  only  when  BSD-style  process  accounting is enabled. These values
    control its behavior. If the free space on the file system where the log lives
    goes below  lowwater  percentage,  accounting  suspends.  If  it  goes  above
    highwater percentage,  accounting  resumes. Frequency determines how often you
    check the amount of free space (value is in seconds). Default settings are: 4,
    2, and  30.  That is, suspend accounting if there is less than 2 percent free;
    resume it  if we have a value of 3 or more percent; consider information about
    the amount of free space valid for 30 seconds

    ctrl-alt-del
    ------------

    When the value in this file is 0, ctrl-alt-del is trapped and sent to the init
    program to  handle a graceful restart. However, when the value is greater that
    zero, Linux's  reaction  to  this key combination will be an immediate reboot,
    without syncing its dirty buffers.

    [NOTE]
        When a  program  (like  dosemu)  has  the  keyboard  in  raw  mode,  the
        ctrl-alt-del is  intercepted  by  the  program  before it ever reaches the
        kernel tty  layer,  and  it is up to the program to decide what to do with
        it.

    domainname and hostname
    -----------------------

    These files  can  be controlled to set the NIS domainname and hostname of your
    box. For the classic darkstar.frop.org a simple:

      # echo "darkstar" > /proc/sys/kernel/hostname
      # echo "frop.org" > /proc/sys/kernel/domainname

    would suffice to set your hostname and NIS domainname.

    osrelease, ostype and version
    -----------------------------

    The names make it pretty obvious what these fields contain:

      > cat /proc/sys/kernel/osrelease
      2.2.12

      > cat /proc/sys/kernel/ostype
      Linux

      > cat /proc/sys/kernel/version
      #4 Fri Oct 1 12:41:14 PDT 1999

    The files  osrelease and ostype should be clear enough. Version needs a little
    more clarification.  The  #4 means that this is the 4th kernel built from this
    source base and the date after it indicates the time the kernel was built. The
    only way to tune these values is to rebuild the kernel.

    panic
    -----

    The value  in  this  file  represents  the  number of seconds the kernel waits
    before rebooting  on  a  panic.  When  you  use  the  software  watchdog,  the
    recommended setting  is  60. If set to 0, the auto reboot after a kernel panic
    is disabled, which is the default setting.

    printk
    ------

    The four values in printk denote
    * console_loglevel,
    * default_message_loglevel,
    * minimum_console_level and
    * default_console_loglevel
    respectively.

    These values  influence  printk()  behavior  when  printing  or  logging error
    messages, which  come  from  inside  the  kernel.  See  syslog(2)  for  more
    information on the different log levels.

    console_loglevel
    ----------------

    Messages with a higher priority than this will be printed to the console.

    default_message_level
    ---------------------

    Messages without an explicit priority will be printed with this priority.

    minimum_console_loglevel
    ------------------------

    Minimum (highest) value to which the console_loglevel can be set.

    default_console_loglevel
    ------------------------

    Default value for console_loglevel.

    sg-big-buff
    -----------

    This file  shows  the size of the generic SCSI (sg) buffer. At this point, you
    can't tune  it  yet,  but  you  can  change  it  at  compile  time  by editing
    include/scsi/sg.h and changing the value of SG_BIG_BUFF.

    If you use a scanner with SANE (Scanner Access Now Easy) you might want to set
    this to a higher value. Refer to the SANE documentation on this issue.

    modprobe
    --------

    The location  where  the  modprobe  binary  is  located.  The kernel uses this
    program to load modules on demand.

    2.4 /proc/sys/vm - The virtual memory subsystem
    -----------------------------------------------

    The files  in  this directory can be used to tune the operation of the virtual
    memory (VM)  subsystem  of  the  Linux  kernel.  In addition, one of the files
    (bdflush) has some influence on disk usage.

    bdflush
    -------

    This file  controls  the  operation of the bdflush kernel daemon. It currently
    contains nine  integer  values,  six of which are actually used by the kernel.
    They are listed in table 2-2.

    Table 2-2: Parameters in /proc/sys/vm/bdflush
    ..............................................................................
     Value      Meaning
     nfract     Percentage of buffer cache dirty to  activate bdflush
     ndirty     Maximum number of dirty blocks to  write out per wake-cycle
     nrefill    Number of clean buffers to try to obtain  each time we call refill
     nref_dirt  buffer threshold for activating bdflush when trying to refill
                buffers.
     dummy      Unused
     age_buffer Time for normal buffer to age before we flush it
     age_super  Time for superblock to age before we flush it
     dummy      Unused
     dummy      Unused
    ..............................................................................

    nfract
    ------

    This parameter  governs  the  maximum  number  of  dirty buffers in the buffer
    cache. Dirty means that the contents of the buffer still have to be written to
    disk (as  opposed  to  a  clean  buffer,  which  can just be forgotten about).
    Setting this  to  a  higher value means that Linux can delay disk writes for a
    long time, but it also means that it will have to do a lot of I/O at once when
    memory becomes short. A lower value will spread out disk I/O more evenly.

    ndirty
    ------

    Ndirty gives the maximum number of dirty buffers that bdflush can write to the
    disk at  one  time.  A high value will mean delayed, bursty I/O, while a small
    value can lead to memory shortage when bdflush isn't woken up often enough.

    nrefill
    -------

    This is  the  number  of  buffers  that  bdflush  will add to the list of free
    buffers when  refill_freelist()  is  called.  It is necessary to allocate free
    buffers beforehand,  since  the  buffers  are  often  different sizes than the
    memory pages  and some bookkeeping needs to be done beforehand. The higher the
    number, the  more  memory  will be wasted and the less often refill_freelist()
    will need to run.

    nref_dirt
    ---------

    When refill_freelist() comes across more than nref_dirt dirty buffers, it will
    wake up bdflush.

    age_buffer and age_super
    ------------------------

    Finally, the age_buffer and age_super parameters govern the maximum time Linux
    waits before  writing  out  a  dirty buffer to disk. The value is expressed in
    jiffies (clockticks),  the  number of jiffies per second is 100. Age_buffer is
    the maximum age for data blocks, while age_super is for filesystems meta data.

    buffermem
    ---------

    The three  values  in  this  file  control  how much memory should be used for
    buffer memory.  The  percentage  is calculated as a percentage of total system
    memory.

    The values are:

    min_percent
    -----------

    This is  the  minimum  percentage  of  memory  that  should be spent on buffer
    memory.

    borrow_percent
    --------------

    When Linux is short on memory, and the buffer cache uses more than it has been
    allotted, the  memory  management  (MM)  subsystem will prune the buffer cache
    more heavily than other memory to compensate.

    max_percent
    -----------

    This is the maximum amount of memory that can be used for buffer memory.

    freepages
    ---------

    This file contains three values: min, low and high:

    min
    ---
    When the  number  of  free  pages  in the system reaches this number, only the
    kernel can allocate more memory.

    low
    ---
    If the number of free pages falls below this point, the kernel starts swapping
    aggressively.

    high
    ----
    The kernel  tries  to  keep  up to this amount of memory free; if memory falls
    below this point, the kernel starts gently swapping in the hopes that it never
    has to do really aggressive swapping.

    kswapd
    ------

    Kswapd is  the  kernel  swap  out daemon. That is, kswapd is that piece of the
    kernel that  frees  memory when it gets fragmented or full. Since every system
    is different, you'll probably want some control over this piece of the system.

    The file contains three numbers:

    tries_base
    ----------

    The maximum  number  of  pages kswapd tries to free in one round is calculated
    from this  number.  Usually  this  number  will  be  divided  by  4  or 8 (see
    mm/vmscan.c), so it isn't as big as it looks.

    When you  need to increase the bandwidth to/from swap, you'll want to increase
    this number.

    tries_min
    ---------

    This is  the  minimum number of times kswapd tries to free a page each time it
    is called. Basically it's just there to make sure that kswapd frees some pages
    even when it's being called with minimum priority.

    swap_cluster
    ------------

    This is probably the greatest influence on system performance.

    swap_cluster is  the  number  of  pages kswapd writes in one turn. You'll want
    this value  to  be  large  so that kswapd does its I/O in large chunks and the
    disk doesn't  have  to  seek  as  often, but you don't want it to be too large
    since that would flood the request queue.

    overcommit_memory
    -----------------

    This file  contains  one  value.  The following algorithm is used to decide if
    there's enough  memory:  if  the  value of overcommit_memory is positive, then
    there's always  enough  memory. This is a useful feature, since programs often
    malloc() huge  amounts  of  memory 'just in case', while they only use a small
    part of  it.  Leaving  this value at 0 will lead to the failure of such a huge
    malloc(), when in fact the system has enough memory for the program to run.

    On the  other  hand,  enabling this feature can cause you to run out of memory
    and thrash the system to death, so large and/or important servers will want to
    set this value to 0.

    pagecache
    ---------

    This file  does exactly the same job as buffermem, only this file controls the
    amount of memory allowed for memory mapping and generic caching of files.

    You don't  want  the  minimum level to be too low, otherwise your system might
    thrash when memory is tight or fragmentation is high.

    pagetable_cache
    ---------------

    The kernel  keeps a number of page tables in a per-processor cache (this helps
    a lot  on  SMP systems). The cache size for each processor will be between the
    low and the high value.

    On a  low-memory,  single  CPU system, you can safely set these values to 0 so
    you don't  waste  memory.  It  is  used  on SMP systems so that the system can
    perform fast  pagetable allocations without having to acquire the kernel memory
    lock.

    For large  systems,  the  settings  are probably fine. For normal systems they
    won't hurt  a  bit.  For  small  systems  (  less  than  16MB ram) it might be
    advantageous to set both values to 0.

    swapctl
    -------

    This file  contains  no less than 8 variables. All of these values are used by
    kswapd.

    The first four variables
    * sc_max_page_age,
    * sc_page_advance,
    * sc_page_decline and
    * sc_page_initial_age
    are used  to  keep  track  of  Linux's page aging. Page aging is a bookkeeping
    method to  track  which pages of memory are often used, and which pages can be
    swapped out without consequences.

    When a  page  is  swapped in, it starts at sc_page_initial_age (default 3) and
    when the  page  is  scanned  by  kswapd,  its age is adjusted according to the
    following scheme:

    * If  the  page  was used since the last time we scanned, its age is increased
      by sc_page_advance  (default  3).  Where  the  maximum  value  is  given  by
      sc_max_page_age (default 20).
    * Otherwise  (meaning  it wasn't used) its age is decreased by sc_page_decline
      (default 1).

    When a page reaches age 0, it's ready to be swapped out.

    The variables  sc_age_cluster_fract, sc_age_cluster_min, sc_pageout_weight and
    sc_bufferout_weight, can  be  used  to  control  kswapd's  aggressiveness  in
    swapping out pages.

    Sc_age_cluster_fract is used to calculate how many pages from a process are to
    be scanned by kswapd. The formula used is

    (sc_age_cluster_fract divided by 1024) times resident set size

    So if you want kswapd to scan the whole process, sc_age_cluster_fract needs to
    have a  value  of  1024.  The  minimum  number  of  pages  kswapd will scan is
    represented by sc_age_cluster_min, which is done so that kswapd will also scan
    small processes.

    The values  of  sc_pageout_weight  and sc_bufferout_weight are used to control
    how many  tries  kswapd  will make in order to swap out one page/buffer. These
    values can  be used to fine-tune the ratio between user pages and buffer/cache
    memory. When  you find that your Linux system is swapping out too many process
    pages in  order  to  satisfy  buffer  memory  demands,  you may want to either
    increase sc_bufferout_weight, or decrease the value of sc_pageout_weight.

    2.5 /proc/sys/dev - Device specific parameters
    ----------------------------------------------

    Currently there is only support for CDROM drives, and for those, there is only
    one read-only  file containing information about the CD-ROM drives attached to
    the system:

      >cat /proc/sys/dev/cdrom/info
      CD-ROM information, Id: cdrom.c 2.55 1999/04/25

      drive name:             sr0     hdb
      drive speed:            32      40
      drive # of slots:       1       0
      Can close tray:         1       1
      Can open tray:          1       1
      Can lock tray:          1       1
      Can change speed:       1       1
      Can select disk:        0       1
      Can read multisession:  1       1
      Can read MCN:           1       1
      Reports media changed:  1       1
      Can play audio:         1       1

    You see two drives, sr0 and hdb, along with a list of their features.

    2.6 /proc/sys/sunrpc - Remote procedure calls
    ---------------------------------------------

    This directory  contains four files, which enable or disable debugging for the
    RPC functions NFS, NFS-daemon, RPC and NLM. The default values are 0. They can
    be set to one to turn debugging on. (The default value is 0 for each)

    2.7 /proc/sys/net - Networking stuff
    ------------------------------------

    The interface  to  the  networking  parts  of  the  kernel  is  located  in
    /proc/sys/net. Table  2-3  shows all possible subdirectories. You may see only
    some of them, depending on your kernel's configuration.

    Table 2-3: Subdirectories in /proc/sys/net
    ..............................................................................
     Directory Content             Directory  Content
     core      General parameter   appletalk  Appletalk protocol
     unix      Unix domain sockets netrom     NET/ROM
     802       E802 protocol       ax25       AX25
     ethernet  Ethernet protocol   rose       X.25 PLP layer
     ipv4      IP version 4        x25        X.25 protocol
     ipx       IPX                 token-ring IBM token ring
     bridge    Bridging            decnet     DEC net
     ipv6      IP version 6
    ..............................................................................

    We will  concentrate  on IP networking here. Since AX15, X.25, and DEC Net are
    only minor players in the Linux world, we'll skip them in this chapter. You'll
    find some  short  info on Appletalk and IPX further on in this chapter. Review
    the online  documentation  and the kernel source to get a detailed view of the
    parameters for  those  protocols.  In  this  section  we'll  discuss  the
    subdirectories printed  in  bold letters in the table above. As default values
    are suitable for most needs, there is no need to change these values.

    /proc/sys/net/core - Network core options
    -----------------------------------------

    rmem_default
    ------------

    The default setting of the socket receive buffer in bytes.

    rmem_max
    --------

    The maximum receive socket buffer size in bytes.

    wmem_default
    ------------

    The default setting (in bytes) of the socket send buffer.

    wmem_max
    --------

    The maximum send socket buffer size in bytes.

    message_burst and message_cost
    ------------------------------

    These parameters  are used to limit the warning messages written to the kernel
    log from  the  networking  code.  They  enforce  a  rate  limit  to  make  a
    denial-of-service attack  impossible. A higher message_cost factor, results in
    fewer messages that will be written. Message_burst controls when messages will
    be dropped.  The  default  settings  limit  warning messages to one every five
    seconds.

    netdev_max_backlog
    ------------------

    Maximum number  of  packets,  queued  on  the  INPUT  side, when the interface
    receives packets faster than kernel can process them.

    optmem_max
    ----------

    Maximum ancillary buffer size allowed per socket. Ancillary data is a sequence
    of struct cmsghdr structures with appended data.

    /proc/sys/net/unix - Parameters for Unix domain sockets
    -------------------------------------------------------

    There are  only  two  files  in this subdirectory. They control the delays for
    deleting and destroying socket descriptors.

    2.8 /proc/sys/net/ipv4 - IPV4 settings
    --------------------------------------

    IP version  4  is  still the most used protocol in Unix networking. It will be
    replaced by  IP version 6 in the next couple of years, but for the moment it's
    the de  facto  standard  for  the  internet  and  is  used  in most networking
    environments around  the  world.  Because  of the importance of this protocol,
    we'll have a deeper look into the subtree controlling the behavior of the IPv4
    subsystem of the Linux kernel.

    Let's start with the entries in /proc/sys/net/ipv4.

    ICMP settings
    -------------

    icmp_echo_ignore_all and icmp_echo_ignore_broadcasts
    ----------------------------------------------------

    Turn on (1) or off (0), if the kernel should ignore all ICMP ECHO requests, or
    just those to broadcast and multicast addresses.

    Please note that if you accept ICMP echo requests with a broadcast/multi\-cast
    destination address  your  network  may  be  used as an exploder for denial of
    service packet flooding attacks to other hosts.

    icmp_destunreach_rate, icmp_echoreply_rate, icmp_paramprob_rate and icmp_timeexeed_rate
    ---------------------------------------------------------------------------------------

    Sets limits  for  sending  ICMP  packets  to specific targets. A value of zero
    disables all  limiting.  Any  positive  value sets the maximum package rate in
    hundredth of a second (on Intel systems).

    IP settings
    -----------

    ip_autoconfig
    -------------

    This file contains the number one if the host received its IP configuration by
    RARP, BOOTP, DHCP or a similar mechanism. Otherwise it is zero.

    ip_default_ttl
    --------------

    TTL (Time  To  Live) for IPv4 interfaces. This is simply the maximum number of
    hops a packet may travel.

    ip_dynaddr
    ----------

    Enable dynamic  socket  address rewriting on interface address change. This is
    useful for dialup interface with changing IP addresses.

    ip_forward
    ----------

    Enable or  disable forwarding of IP packages between interfaces. Changing this
    value resets  all other parameters to their default values. They differ if the
    kernel is configured as host or router.

    ip_local_port_range
    -------------------

    Range of  ports  used  by  TCP  and UDP to choose the local port. Contains two
    numbers, the  first  number  is the lowest port, the second number the highest
    local port.  Default  is  1024-4999.  Should  be  changed  to  32768-61000 for
    high-usage systems.

    ip_no_pmtu_disc
    ---------------

    Global switch  to  turn  path  MTU  discovery off. It can also be set on a per
    socket basis by the applications or on a per route basis.

    ip_masq_debug
    -------------

    Enable/disable debugging of IP masquerading.

    IP fragmentation settings
    -------------------------

    ipfrag_high_trash and ipfrag_low_trash
    --------------------------------------

    Maximum memory  used to reassemble IP fragments. When ipfrag_high_thresh bytes
    of memory  is  allocated  for  this  purpose,  the  fragment handler will toss
    packets until ipfrag_low_thresh is reached.

    ipfrag_time
    -----------

    Time in seconds to keep an IP fragment in memory.

    TCP settings
    ------------

    tcp_ecn
    -------

    This file controls the use of the ECN bit in the IPv4 headers, this is a new
    feature about Explicit Congestion Notification, but some routers and firewalls
    block trafic that has this bit set, so it could be necessary to echo 0 to
    /proc/sys/net/ipv4/tcp_ecn, if you want to talk to this sites. For more info
    you could read RFC2481.

    tcp_retrans_collapse
    --------------------

    Bug-to-bug compatibility with some broken printers. On retransmit, try to send
    larger packets to work around bugs in certain TCP stacks. Can be turned off by
    setting it to zero.

    tcp_keepalive_probes
    --------------------

    Number of  keep  alive  probes  TCP  sends  out,  until  it  decides  that the
    connection is broken.

    tcp_keepalive_time
    ------------------

    How often  TCP  sends out keep alive messages, when keep alive is enabled. The
    default is 2 hours.

    tcp_syn_retries
    ---------------

    Number of  times  initial  SYNs  for  a  TCP  connection  attempt  will  be
    retransmitted. Should  not  be  higher  than 255. This is only the timeout for
    outgoing connections,  for  incoming  connections the number of retransmits is
    defined by tcp_retries1.

    tcp_sack
    --------

    Enable select acknowledgments after RFC2018.

    tcp_timestamps
    --------------

    Enable timestamps as defined in RFC1323.

    tcp_stdurg
    ----------

    Enable the  strict  RFC793 interpretation of the TCP urgent pointer field. The
    default is  to  use  the  BSD  compatible interpretation of the urgent pointer
    pointing to the first byte after the urgent data. The RFC793 interpretation is
    to have  it  point  to  the last byte of urgent data. Enabling this option may
    lead to interoperatibility problems. Disabled by default.

    tcp_syncookies
    --------------

    Only valid  when  the  kernel  was  compiled  with CONFIG_SYNCOOKIES. Send out
    syncookies when  the  syn backlog queue of a socket overflows. This is to ward
    off the common 'syn flood attack'. Disabled by default.

    Note that  the  concept  of a socket backlog is abandoned. This means the peer
    may not  receive  reliable  error  messages  from  an  over loaded server with
    syncookies enabled.

    tcp_window_scaling
    ------------------

    Enable window scaling as defined in RFC1323.

    tcp_fin_timeout
    ---------------

    The length  of  time  in  seconds  it  takes to receive a final FIN before the
    socket is  always  closed.  This  is  strictly  a  violation  of  the  TCP
    specification, but required to prevent denial-of-service attacks.

    tcp_max_ka_probes
    -----------------

    Indicates how  many  keep alive probes are sent per slow timer run. Should not
    be set too high to prevent bursts.

    tcp_max_syn_backlog
    -------------------

    Length of  the per socket backlog queue. Since Linux 2.2 the backlog specified
    in listen(2)  only  specifies  the  length  of  the  backlog  queue of already
    established sockets. When more connection requests arrive Linux starts to drop
    packets. When  syncookies  are  enabled the packets are still answered and the
    maximum queue is effectively ignored.

    tcp_retries1
    ------------

    Defines how  often  an  answer  to  a  TCP connection request is retransmitted
    before giving up.

    tcp_retries2
    ------------

    Defines how often a TCP packet is retransmitted before giving up.

    Interface specific settings
    ---------------------------

    In the directory /proc/sys/net/ipv4/conf you'll find one subdirectory for each
    interface the  system  knows about and one directory calls all. Changes in the
    all subdirectory  affect  all  interfaces,  whereas  changes  in  the  other
    subdirectories affect  only  one  interface.  All  directories  have  the same
    entries:

    accept_redirects
    ----------------

    This switch  decides  if the kernel accepts ICMP redirect messages or not. The
    default is 'yes' if the kernel is configured for a regular host and 'no' for a
    router configuration.

    accept_source_route
    -------------------

    Should source  routed  packages  be  accepted  or  declined.  The  default  is
    dependent on  the  kernel  configuration.  It's 'yes' for routers and 'no' for
    hosts.

    bootp_relay
    ~~~~~~~~~~~

    Accept packets  with source address 0.b.c.d with destinations not to this host
    as local ones. It is supposed that a BOOTP relay daemon will catch and forward
    such packets.

    The default  is  0,  since this feature is not implemented yet (kernel version
    2.2.12).

    forwarding
    ----------

    Enable or disable IP forwarding on this interface.

    log_martians
    ------------

    Log packets with source addresses with no known route to kernel log.

    mc_forwarding
    -------------

    Do multicast routing. The kernel needs to be compiled with CONFIG_MROUTE and a
    multicast routing daemon is required.

    proxy_arp
    ---------

    Does (1) or does not (0) perform proxy ARP.

    rp_filter
    ---------

    Integer value determines if a source validation should be made. 1 means yes, 0
    means no.  Disabled by default, but local/broadcast address spoofing is always
    on.

    If you  set this to 1 on a router that is the only connection for a network to
    the net,  it  will  prevent  spoofing  attacks  against your internal networks
    (external addresses  can  still  be  spoofed), without the need for additional
    firewall rules.

    secure_redirects
    ----------------

    Accept ICMP  redirect  messages  only  for gateways, listed in default gateway
    list. Enabled by default.

    shared_media
    ------------

    If it  is  not  set  the kernel does not assume that different subnets on this
    device can communicate directly. Default setting is 'yes'.

    send_redirects
    --------------

    Determines whether to send ICMP redirects to other hosts.

    Routing settings
    ----------------

    The directory  /proc/sys/net/ipv4/route  contains  several  file  to  control
    routing issues.

    error_burst and error_cost
    --------------------------

    These parameters  are used to limit the warning messages written to the kernel
    log from  the  routing  code.  The  higher the error_cost factor is, the fewer
    messages will  be written. Error_burst controls when messages will be dropped.
    The default settings limit warning messages to one every five seconds.

    flush
    -----

    Writing to this file results in a flush of the routing cache.

    gc_elastic, gc_interval, gc_min_interval, gc_tresh, gc_timeout
    --------------------------------------------------------------

    Values to  control  the  frequency  and  behavior  of  the  garbage collection
    algorithm for the routing cache.

    max_size
    --------

    Maximum size  of  the routing cache. Old entries will be purged once the cache
    reached has this size.

    max_delay, min_delay
    --------------------

    Delays for flushing the routing cache.

    redirect_load, redirect_number
    ------------------------------

    Factors which  determine  if  more ICPM redirects should be sent to a specific
    host. No  redirects  will be sent once the load limit or the maximum number of
    redirects has been reached.

    redirect_silence
    ----------------

    Timeout for redirects. After this period redirects will be sent again, even if
    this has been stopped, because the load or number limit has been reached.

    Network Neighbor handling
    -------------------------

    Settings about how to handle connections with direct neighbors (nodes attached
    to the same link) can be found in the directory /proc/sys/net/ipv4/neigh.

    As we  saw  it  in  the  conf directory, there is a default subdirectory which
    holds the  default  values, and one directory for each interface. The contents
    of the  directories  are identical, with the single exception that the default
    settings contain additional options to set garbage collection parameters.

    In the interface directories you'll find the following entries:

    base_reachable_time
    -------------------

    A base  value  used for computing the random reachable time value as specified
    in RFC2461.

    retrans_time
    ------------

    The time,  expressed  in  jiffies  (1/100 sec), between retransmitted Neighbor
    Solicitation messages.  Used  for  address  resolution  and  to determine if a
    neighbor is unreachable.

    unres_qlen
    ----------

    Maximum queue  length  for a pending arp request - the number of packets which
    are accepted from other layers while the ARP address is still resolved.

    anycast_delay
    -------------

    Maximum for  random  delay  of  answers  to  neighbor solicitation messages in
    jiffies (1/100  sec). Not yet implemented (Linux does not have anycast support
    yet).

    ucast_solicit
    -------------

    Maximum number of retries for unicast solicitation.

    mcast_solicit
    -------------

    Maximum number of retries for multicast solicitation.

    delay_first_probe_time
    ----------------------

    Delay for  the  first  time  probe  if  the  neighbor  is  reachable.  (see
    gc_stale_time)

    locktime
    --------

    An ARP/neighbor  entry  is only replaced with a new one if the old is at least
    locktime old. This prevents ARP cache thrashing.

    proxy_delay
    -----------

    Maximum time  (real  time is random [0..proxytime]) before answering to an ARP
    request for  which  we have an proxy ARP entry. In some cases, this is used to
    prevent network flooding.

    proxy_qlen
    ----------

    Maximum queue length of the delayed proxy arp timer. (see proxy_delay).

    app_solcit
    ----------

    Determines the  number of requests to send to the user level ARP daemon. Use 0
    to turn off.

    gc_stale_time
    -------------

    Determines how  often  to  check  for stale ARP entries. After an ARP entry is
    stale it  will  be resolved again (which is useful when an IP address migrates
    to another  machine).  When  ucast_solicit is greater than 0 it first tries to
    send an  ARP  packet  directly  to  the  known  host  When  that  fails  and
    mcast_solicit is greater than 0, an ARP request is broadcasted.

    2.9 Appletalk
    -------------

    The /proc/sys/net/appletalk  directory  holds the Appletalk configuration data
    when Appletalk is loaded. The configurable parameters are:

    aarp-expiry-time
    ----------------

    The amount  of  time  we keep an ARP entry before expiring it. Used to age out
    old hosts.

    aarp-resolve-time
    -----------------

    The amount of time we will spend trying to resolve an Appletalk address.

    aarp-retransmit-limit
    ---------------------

    The number of times we will retransmit a query before giving up.

    aarp-tick-time
    --------------

    Controls the rate at which expires are checked.

    The directory  /proc/net/appletalk  holds the list of active Appletalk sockets
    on a machine.

    The fields  indicate  the DDP type, the local address (in network:node format)
    the remote  address,  the  size of the transmit pending queue, the size of the
    received queue  (bytes waiting for applications to read) the state and the uid
    owning the socket.

    /proc/net/atalk_iface lists  all  the  interfaces  configured for appletalk.It
    shows the  name  of the interface, its Appletalk address, the network range on
    that address  (or  network number for phase 1 networks), and the status of the
    interface.

    /proc/net/atalk_route lists  each  known  network  route.  It lists the target
    (network) that the route leads to, the router (may be directly connected), the
    route flags, and the device the route is using.

    2.10 IPX
    --------

    The IPX protocol has no tunable values in proc/sys/net.

    The IPX  protocol  does,  however,  provide  proc/net/ipx. This lists each IPX
    socket giving  the  local  and  remote  addresses  in  Novell  format (that is
    network:node:port). In  accordance  with  the  strange  Novell  tradition,
    everything but the port is in hex. Not_Connected is displayed for sockets that
    are not  tied to a specific remote address. The Tx and Rx queue sizes indicate
    the number  of  bytes  pending  for  transmission  and  reception.  The  state
    indicates the  state  the  socket  is  in and the uid is the owning uid of the
    socket.

    The /proc/net/ipx_interface  file lists all IPX interfaces. For each interface
    it gives  the network number, the node number, and indicates if the network is
    the primary  network.  It  also  indicates  which  device  it  is bound to (or
    Internal for  internal  networks)  and  the  Frame  Type if appropriate. Linux
    supports 802.3,  802.2,  802.2  SNAP  and DIX (Blue Book) ethernet framing for
    IPX.

    The /proc/net/ipx_route  table  holds  a list of IPX routes. For each route it
    gives the  destination  network, the router node (or Directly) and the network
    address of the router (or Connected) for internal networks.

    ------------------------------------------------------------------------------
    Summary
    ------------------------------------------------------------------------------
    Certain aspects  of  kernel  behavior  can be modified at runtime, without the
    need to  recompile  the kernel, or even to reboot the system. The files in the
    /proc/sys tree  can  not only be read, but also modified. You can use the echo
    command to write value into these files, thereby changing the default settings
    of the kernel.
    ------------------------------------------------------------------------------