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                       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        
 dummy      Unused                                                             
 dummy      Unused                                                             
 interval   jiffies delay between kupdate flushes
 age_buffer Time for normal buffer to age before we flush it                   
 nfract_sync Percentage of buffer cache dirty to activate bdflush synchronously
 nfract_stop_bdflush Percetange of buffer cache dirty to stop bdflush
 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.

interval
--------

The interval between two kupdate runs. The value is expressed in
jiffies (clockticks),  the  number of jiffies per second is 100.

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.

age_buffer
----------

Finally, the age_buffer parameter 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.

nfract_sync
-----------

nfract_stop_bdflush
-------------------

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 how many ICMP destination unreachable to 
send  from  the  host  in question. ICMP destination unreachable messages are 
sent  when  we can not reach the next hop, while trying to transmit a packet. 
It  will also print some error messages to kernel logs if someone is ignoring 
our   ICMP  redirects.  The  higher  the  error_cost  factor  is,  the  fewer 
destination  unreachable  and error messages will be let through. Error_burst 
controls  when  destination  unreachable  messages and error messages will be
dropped. The default settings limit warning messages to five every second.

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.
------------------------------------------------------------------------------