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

Interesting reference on the "proc" filesystem in Linux. Gives definintions and what kinds of statistics can be gathered from the files under /proc.Image -----------------------------------------------------------------------

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

/proc/sys Terrehon Bowden January 27 1999
Bodo Bauer -----------------------------------------------------------------------
Version 1.1 Kernel version 2.2

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=20
3.10 Appletalk
3.11 IPX


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 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...


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
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
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

> 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


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:

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

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

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

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.

However, there is still a per process limit of open files, which
unfortunatly can't be changed that easily. It is set to 1024 by
default. To change this you have to edit the files limits.h and
fs.h in the directory /usr/src/linux/include/linux. Change the
definition of NR_OPEN and recompile the kernel.

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.

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.

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

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

Example usage of binfmt_misc (emulate binfmt_java)

cd /proc/sys/fs/binfmt_misc
echo ':Java:M::xcaxfexbaxbe::/usr/local/java/bin/javawrapper:' > register
echo ':HTML:E::html::/usr/local/java/bin/appletviewer:' > register
echo ':Applet:M:: 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 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

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.

The file contains three values; highwater, lowwater, and

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 =3%; consider information about the amount of free space valid
for 30 seconds

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
>cat /proc/sys/kernel/ostype
>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.

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

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.

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

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

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

Default value for console_loglevel.

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

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.

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.

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

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 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.

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.

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.

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:

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

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

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

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

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

If the number of free pages gets below this point, the kernel
starts swapping aggressively.

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 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:

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.

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.

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.

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.

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

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 (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

| 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

The default setting of the socket receive buffer in bytes.

The maximum receive socket buffer size in bytes.

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

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.

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

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

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

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

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

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.

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.

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

Enable/disable debugging of IP masquerading.

IP fragmentation settings

Replaces the former Kernel-Configuration option:
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_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

Time in seconds to keep an IP fragment in memory.

TCP settings

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.

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

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

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.

Enable select acknowledgments after RFC2018.

Enable timestamps as defined in RFC1323.

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.

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.

Enable window scaling as defined in RFC1323.

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.

How many keepalive probes are sent per slow timer run. Shouldn't be
set too high to prevent bursts.

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

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

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:

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.

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.

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?).

Enable or disable IP forwarding on this interface.

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

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

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

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.

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

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

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.

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.

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.


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

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:

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

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.

Maximum queue length for a pending arp request - how many packets
are accepted from other layers while the arp address is still

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).

Maximum number of retries for unicast solicitation.

Maximum number of retries for multicast solicitation.

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

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

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.

Maximum queue length of the delayed proxy arp timer (see

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

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:

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

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

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

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

Last Updated ( Oct 01, 2007 at 02:52 PM )
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