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                T H E  /proc   F I L E S Y S T E M

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/proc/sys      Terrehon Bowden <terrehon@wpi.com>       January 27 1999
               Bodo Bauer <bb@ricochet.net>                
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Version 1.1                                         Kernel version 2.2
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Contents

1   Introduction/Credits

1.1  Legal Issues

2   The /proc file system

2.1  Process specific subdirectories
2.2  Kernel data
2.3  IDE devices in /proc/ide
2.4  Networking info in /proc/net
2.5  SCSI info
2.6  Parallel port info in /proc/parport
2.7  TTY info in /proc/tty

3   Reading and modifying kernel parameters

3.1  /proc/sys/debug and /proc/sys/proc
3.2  /proc/fs - File system data
3.3  /proc/fs/binfmt_misc - Miscellaneous binary formats
3.4  /proc/sys/kernel - General kernel parameters
3.5  /proc/sys/vm - The virtual memory subsystem
3.6  /proc/sys/dev - Device specific parameters
3.7  /proc/sys/sunrpc - Remote procedure calls
3.8  /proc/sys/net - Networking stuff
3.9  /proc/sys/net/ipv4 - IPV4 settings
3.10 Appletalk
3.11 IPX

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1   Introduction/Credits

This documentation is part of a soon to be released book published by
IDG Books 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 this chapter, it seems only fair to give
the work back to the Linux community. This work is based on the
2.1.132 and 2.2.0-pre-kernel versions. 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.

We'd like to thank Alan Cox, Rik van Riel, and Alexey Kuznetsov.  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://www.suse.com/~bb/Docs/proc.html in HTML, ASCII, and as 
Postscript file. 

1.1  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...

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2   The /proc file 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.  It contains
(among other things) one subdirectory for each process running on the
system which is named after the process id (PID) of the process. The
link self points to the process reading the file system.

2.1  Process specific subdirectories

Each process subdirectory has the in table 1.1 listed entries.

      _________________________________________________
      cmdline Command line arguments
      environ Values of environment variables
      fd    Directory, which contains all file descriptors
      mem   Memory held by this process
      stat   Process status
      status  Process status in human readable form
      cwd   Link to the current working directory
      exe   Link to the executable of this process
      maps   Memory maps
      root   Link to the root directory of this process
      statm  Process memory status information
     _________________________________________________
      Table 1.1: Process specific entries in /proc

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:    5633
PPid:   5609
Uid:    501     501     501     501
Gid:    100     100     100     100
Groups: 100 16 
VmSize:      804 kB
VmLck:         0 kB
VmRSS:       344 kB
VmData:       68 kB
VmStk:        20 kB
VmExe:        12 kB
VmLib:       660 kB
SigPnd: 0000000000000000
SigBlk: 0000000000000000
SigIgn: 0000000000000000
SigCgt: 0000000000000000
CapInh: 00000000fffffeff
CapPrm: 0000000000000000
CapEff: 0000000000000000

This shows you almost the same information as 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. It contains seven values with the following meanings:

size       total program size
resident   size of in memory portions
shared     number of the 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

The ratio text/data/library is approximate only by heuristics.

2.2  Kernel data

Similar to the process entries, these are files which give information
about the running kernel. The files used to obtain this information
are contained in /proc and are listed in table 1.2. 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.

      ________________________________________________
      apm           Advanced power management info
      cmdline       Kernel command line
      cpuinfo       Info about the CPU
      devices       Available devices (block and character)
      dma           Used DMS channels
      filesystems   Supported filesystems
      interrupts    Interrupt usage
      ioports       I/O port usage
      kcore         Kernel core image
      kmsg          Kernel messages
      ksyms         Kernel symbol table
      loadavg       Load average
      locks         Kernel locks
      meminfo       Memory info
      misc          Miscellaneous
      modules       List of loaded modules
      mounts        Mounted filesystems
      partitions    Table of partitions known to the system
      rtc           Real time clock
      slabinfo      Slab pool info
      stat          Overall statistics
      swaps         Swap space utilization
      uptime        System uptime
      version       Kernel version
      ________________________________________________
           Table 1.2: Kernel info in /proc

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

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

The slabinfo file gives information about memory usage on 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 (like
network buffers, directory cache, etc.).

2.3  IDE devices in /proc/ide

This subdirectory contains information about all IDE devices that the
kernel is aware of.  There is one subdirectory for each device
(i.e. hard disk) containing the following files:

       cache             The cache
       capacity          Capacity of the medium
       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

2.4  Networking info in /proc/net

This directory follows the usual pattern. Table 1.3 lists the files
and their meaning.

     ____________________________________________________
     arp             Kernel ARP table
     dev             network devices with statistics
     dev_mcast       Lists the Layer2 multicast groups a
                     device is listening to (interface index,
                     label, number of references, number of
                     bound addresses).
     dev_stat        network device status
     ip_fwchains     Firewall chain linkage
     ip_fwnames      Firewall chains
     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.
     udp6            UDP sockets (IPv6)
     tcp6            TCP sockets (IPv6)
     raw6            Raw device statistics (IPv6)
     igmp6	     IP multicast addresses, which this host joineed (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 1.3: Network info in /proc/net

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

2.5  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: QUANTUM  Model: XP34550W         Rev: LXY4
  Type:   Direct-Access                    ANSI SCSI revision: 02
Host: scsi0 Channel: 00 Id: 01 Lun: 00
  Vendor: SEAGATE  Model: ST34501W         Rev: 0018
  Type:   Direct-Access                    ANSI SCSI revision: 02
Host: scsi0 Channel: 00 Id: 02 Lun: 00
  Vendor: SEAGATE  Model: ST34501W         Rev: 0017
  Type:   Direct-Access                    ANSI SCSI revision: 02
Host: scsi0 Channel: 00 Id: 04 Lun: 00
  Vendor: ARCHIVE  Model: Python 04106-XXX Rev: 703b
  Type:   Sequential-Access                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:

>cat /proc/scsi/ncr53c8xx/0
General information:
 Chip NCR53C875, device id 0xf, revision id 0x4
 IO port address 0xec00, IRQ number 11
 Synchronous period factor 12, max commands per lun 4

2.6  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,...).

This directory contains four files:

    autoprobe   Autoprobe results of this port
    devices     Connected device modules
    hardware    Hardware info (port type, io-port, DMA, IRQ, etc.)
    irq         Used interrupt, if any

2.7  TTY info in /proc/tty

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

     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

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3   Reading and modifying kernel parameters

A very interesting part of /proc is the directory /proc/sys. This not
only provides information, it also allows you to change parameters
within the kernel. Be very careful when trying this. You can optimize
your system, but you also can crash it. Never play around with 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 get this done
every time your system boots. 

The files in /proc/sys can be used to 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 review the kernel documentation if there is any
doubt. You'll find the documentation in the directory
/usr/src/linux/Documentation/sys. This chapter is heavily based on the
documentation included in the pre 2.2 kernels. Thanks to Rick van Riel
for providing this information.

3.1  /proc/sys/debug and /proc/sys/proc

These two subdirectories are empty.

3.2  /proc/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 gives information
   about the current status. It holds six values, in which the last
   two are not used and are always zero. The other four mean:

       nr_dentry   Seems to be zero all the time
       nr_unused   Number of unused cache entries
       age_limit   Age in seconds after the entry may be
                   reclaimed, when memory is short
       want_pages  internal

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 free 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 as yet
   doesn't free them again.

   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, inode-nr and inode-max
   As with file handles, the kernel allocates the inode structures
   dynamically, but can't free them yet.

   The value in inode-max denotes the maximum number of inode
   handlers. This value should be 3 to 4 times larger than the value
   in file-max, since stdin, stdout, and network sockets also need an
   inode struct to handle them. If you regularly run out of inodes,
   you should increase this value.

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

   inode-state contains three actual numbers and four dummy values. The 
   actual numbers are (in order of appearance) nr_inodes, nr_free_inodes,
   and preshrink.

   nr_inodes 
     Denotes the number of inodes the system has allocated. This can
     be slightly more than inode-max because Linux allocates them one
     pageful at a time.

   nr_free_inodes 
     Represents the number of free inodes and pre shrink is nonzero
     when the nr_inodes > inode-max and the system needs to prune the
     inode list instead of allocating more.

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.

3.3  /proc/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

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 and 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).

To 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 three 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.

3.4  /proc/sys/kernel - general kernel parameters

This directory reflects general kernel behaviors. As I've said before,
the contents are depend on your configuration. I'll list 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%, accounting suspends. If
   it goes above highwater%, 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 left <= 2% free; resume it if we have a value
   >=3%; 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(1) program to handle a graceful restart. However, when
   the value is > 0, 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
   2.1.131
   >cat /proc/sys/kernel/ostype
   Linux
   >cat /proc/sys/kernel/version
   #8 Mon Jan 25 19:45:02 PST 1999
 
   The files osrelease and ostype should be clear enough. Version
   needs a little more clarification however. The #8 means that this
   is the 8th kernel built from this source base and the date behind
   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, this 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. Look into 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.

3.5  /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 a little influence on disk usage.

bdflush
   This file controls the operation of the bdflush kernel daemon. It
   currently contains 9 integer values, 6 of which are actually used
   by the kernel:

    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   Dirty buffer threshold for activating bdflush
                when trying to refill buffers.
    dummy       unused
    age_buffer  Time for normal buffer to age before you flush it
    age_super   Time for superblock to age before you 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 high 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 low 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 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 mangement (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 gets below this point, the kernel
     starts swapping aggressively.

   high
     The kernel tries to keep up to this amount of memory free; if
     memory comes below this point, the kernel gently starts 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 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
   aquire the kernel memory lock.

   For large systems, the settings are probably fine. For normal
   systems they won't hurt a bit. For small systems (<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:

    o If the page was used since the last time we scanned, its age
      is increased by sc_page_advance (default 3) up to a
      maximum of sc_max_page_age (default 20).

    o Else (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 next four 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
           -------------------- * resident set size
              1024   

   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, this is done so 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 might want to either
   increase sc_bufferout_weight, or decrease the value of
   sc_pageout_weight.

3.6  /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

drive name:           sr0  hdc
drive speed:           0    6
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 hdc, and their lists of features.

3.7  /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)

3.8  /proc/sys/net - Networking stuff

The interface to the networking parts of the kernel is located in
/proc/sys/net. The table below shows all possible subdirectories. You
may see only some of them, depending on the configuration of your
kernel:

+-------------------------------------------------------------+
| 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. As 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 to Appletalk and IPX further down
in section 3.10 and 3.11. Please look in 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. The higher the
   message_cost factor is, the less messages 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
   Maximal number of packets, queued on 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.

3.9  /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 itself.

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/multicast 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 hundredths of a second (on Intel systems).

IP settings

ip_autoconfig
   This file contains one, if the host got 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. A
   change of 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

ip_always_defrag
   Replaces the former Kernel-Configuration option:
	CONFIG_IP_ALWAYS_DEFRAG
   All incoming fragments (parts of IP packets
   that arose when some host between origin and destination decided
   that the packets were too large and cut them into pieces) will be
   reassembled (defragmented) before being processed, even if they are
   about to be forwarded.

   Only say Y here if running either a firewall that is the sole link
   to your network or a transparent proxy; never ever say Y here for a
   normal router or host.

   This is automagically enabled when enabling masquerading.

ipfrag_high_tresh and ipfrag_low_tresh
   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_retrans_collapse
   Bug-to-bug compatibility with some broken printers. On retransmit
   try to send bigger 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 prevent against 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
   How many seconds to wait for 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
   How many keepalive probes are sent per slow timer run. Shouldn't 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, where
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 destined not to this
   host as local ones. It is supposed that BOOTP relay daemon will
   catch and forward such packets.

   The default is 'no', as this feature is not implemented yet
   (kernel version 2.2.0-pre?).

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
   Do (1) or don't (0) do proxy ARP.

rp_filter
   Integer value deciding if 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 evidently prevents 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.

hidden
   Hide addresses attached to this device from another devices.
   Such addresses will never be selected by source address autoselection
   mechanism, host does not answer broadcast ARP requests for them,
   does not announce it as source address of ARP requests,	but they
   are still reachable via IP. This flag is activated only if it is
   enabled both in specific device section and in "all" section.

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 if or if not 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 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 - how many packets
   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. 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 (useful when an IP address
   migrates to another machine). When ucast_solicit is > 0 it first
   tries to send an ARP packet directly to the known host, when that
   fails and mcast_solicit is > 0, an ARP request is broadcasted.

3.10  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 AARP 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 expiries 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 ad- dress (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 via.

3.11  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 transmit and receive. 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.