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T H E /proc F I L E S Y S T E M
/proc/sys Terrehon Bowden <> October 7 1999
Bodo Bauer <>
2.4.x update Jorge Nerin <> November 14 2000
Version 1.3 Kernel version 2.2.12
Kernel version 2.4.0-test11-pre4
Table of Contents
0 Preface
0.1 Introduction/Credits
0.2 Legal Stuff
1 Collecting System Information
1.1 Process-Specific Subdirectories
1.2 Kernel data
1.3 IDE devices in /proc/ide
1.4 Networking info in /proc/net
1.5 SCSI info
1.6 Parallel port info in /proc/parport
1.7 TTY info in /proc/tty
1.8 Miscellaneous kernel statistics in /proc/stat
2 Modifying System Parameters
2.1 /proc/sys/fs - File system data
2.2 /proc/sys/fs/binfmt_misc - Miscellaneous binary formats
2.3 /proc/sys/kernel - general kernel parameters
2.4 /proc/sys/vm - The virtual memory subsystem
2.5 /proc/sys/dev - Device specific parameters
2.6 /proc/sys/sunrpc - Remote procedure calls
2.7 /proc/sys/net - Networking stuff
2.8 /proc/sys/net/ipv4 - IPV4 settings
2.9 Appletalk
2.10 IPX
2.11 /proc/sys/fs/mqueue - POSIX message queues filesystem
0.1 Introduction/Credits
This documentation is part of a soon (or so we hope) to be released book on
the SuSE Linux distribution. As there is no complete documentation for the
/proc file system and we've used many freely available sources to write these
chapters, it seems only fair to give the work back to the Linux community.
This work is based on the 2.2.* kernel version and the upcoming 2.4.*. I'm
afraid it's still far from complete, but we hope it will be useful. As far as
we know, it is the first 'all-in-one' document about the /proc file system. It
is focused on the Intel x86 hardware, so if you are looking for PPC, ARM,
SPARC, AXP, etc., features, you probably won't find what you are looking for.
It also only covers IPv4 networking, not IPv6 nor other protocols - sorry. But
additions and patches are welcome and will be added to this document if you
mail them to Bodo.
We'd like to thank Alan Cox, Rik van Riel, and Alexey Kuznetsov and a lot of
other people for help compiling this documentation. We'd also like to extend a
special thank you to Andi Kleen for documentation, which we relied on heavily
to create this document, as well as the additional information he provided.
Thanks to everybody else who contributed source or docs to the Linux kernel
and helped create a great piece of software... :)
If you have any comments, corrections or additions, please don't hesitate to
contact Bodo Bauer at We'll be happy to add them to this
The latest version of this document is available online at as HTML version.
If the above direction does not works for you, ypu could try the kernel
mailing list at and/or try to reach me at
0.2 Legal Stuff
We don't guarantee the correctness of this document, and if you come to us
complaining about how you screwed up your system because of incorrect
documentation, we won't feel responsible...
In This Chapter
* Investigating the properties of the pseudo file system /proc and its
ability to provide information on the running Linux system
* Examining /proc's structure
* Uncovering various information about the kernel and the processes running
on the system
The proc file system acts as an interface to internal data structures in the
kernel. It can be used to obtain information about the system and to change
certain kernel parameters at runtime (sysctl).
First, we'll take a look at the read-only parts of /proc. In Chapter 2, we
show you how you can use /proc/sys to change settings.
1.1 Process-Specific Subdirectories
The directory /proc contains (among other things) one subdirectory for each
process running on the system, which is named after the process ID (PID).
The link self points to the process reading the file system. Each process
subdirectory has the entries listed in Table 1-1.
Table 1-1: Process specific entries in /proc
File Content
cmdline Command line arguments
cpu Current and last cpu in which it was executed (2.4)(smp)
cwd Link to the current working directory
environ Values of environment variables
exe Link to the executable of this process
fd Directory, which contains all file descriptors
maps Memory maps to executables and library files (2.4)
mem Memory held by this process
root Link to the root directory of this process
stat Process status
statm Process memory status information
status Process status in human readable form
wchan If CONFIG_KALLSYMS is set, a pre-decoded wchan
smaps Extension based on maps, presenting the rss size for each mapped file
For example, to get the status information of a process, all you have to do is
read the file /proc/PID/status:
>cat /proc/self/status
Name: cat
State: R (running)
Pid: 5452
PPid: 743
TracerPid: 0 (2.4)
Uid: 501 501 501 501
Gid: 100 100 100 100
Groups: 100 14 16
VmSize: 1112 kB
VmLck: 0 kB
VmRSS: 348 kB
VmData: 24 kB
VmStk: 12 kB
VmExe: 8 kB
VmLib: 1044 kB
SigPnd: 0000000000000000
SigBlk: 0000000000000000
SigIgn: 0000000000000000
SigCgt: 0000000000000000
CapInh: 00000000fffffeff
CapPrm: 0000000000000000
CapEff: 0000000000000000
This shows you nearly the same information you would get if you viewed it with
the ps command. In fact, ps uses the proc file system to obtain its
information. The statm file contains more detailed information about the
process memory usage. Its seven fields are explained in Table 1-2.
Table 1-2: Contents of the statm files (as of 2.6.8-rc3)
Field Content
size total program size (pages) (same as VmSize in status)
resident size of memory portions (pages) (same as VmRSS in status)
shared number of pages that are shared (i.e. backed by a file)
trs number of pages that are 'code' (not including libs; broken,
includes data segment)
lrs number of pages of library (always 0 on 2.6)
drs number of pages of data/stack (including libs; broken,
includes library text)
dt number of dirty pages (always 0 on 2.6)
1.2 Kernel data
Similar to the process entries, the kernel data files give information about
the running kernel. The files used to obtain this information are contained in
/proc and are listed in Table 1-3. Not all of these will be present in your
system. It depends on the kernel configuration and the loaded modules, which
files are there, and which are missing.
Table 1-3: Kernel info in /proc
File Content
apm Advanced power management info
buddyinfo Kernel memory allocator information (see text) (2.5)
bus Directory containing bus specific information
cmdline Kernel command line
cpuinfo Info about the CPU
devices Available devices (block and character)
dma Used DMS channels
filesystems Supported filesystems
driver Various drivers grouped here, currently rtc (2.4)
execdomains Execdomains, related to security (2.4)
fb Frame Buffer devices (2.4)
fs File system parameters, currently nfs/exports (2.4)
ide Directory containing info about the IDE subsystem
interrupts Interrupt usage
iomem Memory map (2.4)
ioports I/O port usage
irq Masks for irq to cpu affinity (2.4)(smp?)
isapnp ISA PnP (Plug&Play) Info (2.4)
kcore Kernel core image (can be ELF or A.OUT(deprecated in 2.4))
kmsg Kernel messages
ksyms Kernel symbol table
loadavg Load average of last 1, 5 & 15 minutes
locks Kernel locks
meminfo Memory info
misc Miscellaneous
modules List of loaded modules
mounts Mounted filesystems
net Networking info (see text)
partitions Table of partitions known to the system
pci Depreciated info of PCI bus (new way -> /proc/bus/pci/,
decoupled by lspci (2.4)
rtc Real time clock
scsi SCSI info (see text)
slabinfo Slab pool info
stat Overall statistics
swaps Swap space utilization
sys See chapter 2
sysvipc Info of SysVIPC Resources (msg, sem, shm) (2.4)
tty Info of tty drivers
uptime System uptime
version Kernel version
video bttv info of video resources (2.4)
You can, for example, check which interrupts are currently in use and what
they are used for by looking in the file /proc/interrupts:
> cat /proc/interrupts
0: 8728810 XT-PIC timer
1: 895 XT-PIC keyboard
2: 0 XT-PIC cascade
3: 531695 XT-PIC aha152x
4: 2014133 XT-PIC serial
5: 44401 XT-PIC pcnet_cs
8: 2 XT-PIC rtc
11: 8 XT-PIC i82365
12: 182918 XT-PIC PS/2 Mouse
13: 1 XT-PIC fpu
14: 1232265 XT-PIC ide0
15: 7 XT-PIC ide1
NMI: 0
In 2.4.* a couple of lines where added to this file LOC & ERR (this time is the
output of a SMP machine):
> cat /proc/interrupts
0: 1243498 1214548 IO-APIC-edge timer
1: 8949 8958 IO-APIC-edge keyboard
2: 0 0 XT-PIC cascade
5: 11286 10161 IO-APIC-edge soundblaster
8: 1 0 IO-APIC-edge rtc
9: 27422 27407 IO-APIC-edge 3c503
12: 113645 113873 IO-APIC-edge PS/2 Mouse
13: 0 0 XT-PIC fpu
14: 22491 24012 IO-APIC-edge ide0
15: 2183 2415 IO-APIC-edge ide1
17: 30564 30414 IO-APIC-level eth0
18: 177 164 IO-APIC-level bttv
NMI: 2457961 2457959
LOC: 2457882 2457881
ERR: 2155
NMI is incremented in this case because every timer interrupt generates a NMI
(Non Maskable Interrupt) which is used by the NMI Watchdog to detect lockups.
LOC is the local interrupt counter of the internal APIC of every CPU.
ERR is incremented in the case of errors in the IO-APIC bus (the bus that
connects the CPUs in a SMP system. This means that an error has been detected,
the IO-APIC automatically retry the transmission, so it should not be a big
problem, but you should read the SMP-FAQ.
In this context it could be interesting to note the new irq directory in 2.4.
It could be used to set IRQ to CPU affinity, this means that you can "hook" an
IRQ to only one CPU, or to exclude a CPU of handling IRQs. The contents of the
irq subdir is one subdir for each IRQ, and one file; prof_cpu_mask
For example
> ls /proc/irq/
0 10 12 14 16 18 2 4 6 8 prof_cpu_mask
1 11 13 15 17 19 3 5 7 9
> ls /proc/irq/0/
The contents of the prof_cpu_mask file and each smp_affinity file for each IRQ
is the same by default:
> cat /proc/irq/0/smp_affinity
It's a bitmask, in which you can specify which CPUs can handle the IRQ, you can
set it by doing:
> echo 1 > /proc/irq/prof_cpu_mask
This means that only the first CPU will handle the IRQ, but you can also echo 5
which means that only the first and fourth CPU can handle the IRQ.
The way IRQs are routed is handled by the IO-APIC, and it's Round Robin
between all the CPUs which are allowed to handle it. As usual the kernel has
more info than you and does a better job than you, so the defaults are the
best choice for almost everyone.
There are three more important subdirectories in /proc: net, scsi, and sys.
The general rule is that the contents, or even the existence of these
directories, depend on your kernel configuration. If SCSI is not enabled, the
directory scsi may not exist. The same is true with the net, which is there
only when networking support is present in the running kernel.
The slabinfo file gives information about memory usage at the slab level.
Linux uses slab pools for memory management above page level in version 2.2.
Commonly used objects have their own slab pool (such as network buffers,
directory cache, and so on).
> cat /proc/buddyinfo
Node 0, zone DMA 0 4 5 4 4 3 ...
Node 0, zone Normal 1 0 0 1 101 8 ...
Node 0, zone HighMem 2 0 0 1 1 0 ...
Memory fragmentation is a problem under some workloads, and buddyinfo is a
useful tool for helping diagnose these problems. Buddyinfo will give you a
clue as to how big an area you can safely allocate, or why a previous
allocation failed.
Each column represents the number of pages of a certain order which are
available. In this case, there are 0 chunks of 2^0*PAGE_SIZE available in
ZONE_DMA, 4 chunks of 2^1*PAGE_SIZE in ZONE_DMA, 101 chunks of 2^4*PAGE_SIZE
available in ZONE_NORMAL, etc...
Provides information about distribution and utilization of memory. This
varies by architecture and compile options. The following is from a
16GB PIII, which has highmem enabled. You may not have all of these fields.
> cat /proc/meminfo
MemTotal: 16344972 kB
MemFree: 13634064 kB
Buffers: 3656 kB
Cached: 1195708 kB
SwapCached: 0 kB
Active: 891636 kB
Inactive: 1077224 kB
HighTotal: 15597528 kB
HighFree: 13629632 kB
LowTotal: 747444 kB
LowFree: 4432 kB
SwapTotal: 0 kB
SwapFree: 0 kB
Dirty: 968 kB
Writeback: 0 kB
Mapped: 280372 kB
Slab: 684068 kB
CommitLimit: 7669796 kB
Committed_AS: 100056 kB
PageTables: 24448 kB
VmallocTotal: 112216 kB
VmallocUsed: 428 kB
VmallocChunk: 111088 kB
MemTotal: Total usable ram (i.e. physical ram minus a few reserved
bits and the kernel binary code)
MemFree: The sum of LowFree+HighFree
Buffers: Relatively temporary storage for raw disk blocks
shouldn't get tremendously large (20MB or so)
Cached: in-memory cache for files read from the disk (the
pagecache). Doesn't include SwapCached
SwapCached: Memory that once was swapped out, is swapped back in but
still also is in the swapfile (if memory is needed it
doesn't need to be swapped out AGAIN because it is already
in the swapfile. This saves I/O)
Active: Memory that has been used more recently and usually not
reclaimed unless absolutely necessary.
Inactive: Memory which has been less recently used. It is more
eligible to be reclaimed for other purposes
HighFree: Highmem is all memory above ~860MB of physical memory
Highmem areas are for use by userspace programs, or
for the pagecache. The kernel must use tricks to access
this memory, making it slower to access than lowmem.
LowFree: Lowmem is memory which can be used for everything that
highmem can be used for, but it is also availble for the
kernel's use for its own data structures. Among many
other things, it is where everything from the Slab is
allocated. Bad things happen when you're out of lowmem.
SwapTotal: total amount of swap space available
SwapFree: Memory which has been evicted from RAM, and is temporarily
on the disk
Dirty: Memory which is waiting to get written back to the disk
Writeback: Memory which is actively being written back to the disk
Mapped: files which have been mmaped, such as libraries
Slab: in-kernel data structures cache
CommitLimit: Based on the overcommit ratio ('vm.overcommit_ratio'),
this is the total amount of memory currently available to
be allocated on the system. This limit is only adhered to
if strict overcommit accounting is enabled (mode 2 in
The CommitLimit is calculated with the following formula:
CommitLimit = ('vm.overcommit_ratio' * Physical RAM) + Swap
For example, on a system with 1G of physical RAM and 7G
of swap with a `vm.overcommit_ratio` of 30 it would
yield a CommitLimit of 7.3G.
For more details, see the memory overcommit documentation
in vm/overcommit-accounting.
Committed_AS: The amount of memory presently allocated on the system.
The committed memory is a sum of all of the memory which
has been allocated by processes, even if it has not been
"used" by them as of yet. A process which malloc()'s 1G
of memory, but only touches 300M of it will only show up
as using 300M of memory even if it has the address space
allocated for the entire 1G. This 1G is memory which has
been "committed" to by the VM and can be used at any time
by the allocating application. With strict overcommit
enabled on the system (mode 2 in 'vm.overcommit_memory'),
allocations which would exceed the CommitLimit (detailed
above) will not be permitted. This is useful if one needs
to guarantee that processes will not fail due to lack of
memory once that memory has been successfully allocated.
PageTables: amount of memory dedicated to the lowest level of page
VmallocTotal: total size of vmalloc memory area
VmallocUsed: amount of vmalloc area which is used
VmallocChunk: largest contigious block of vmalloc area which is free
1.3 IDE devices in /proc/ide
The subdirectory /proc/ide contains information about all IDE devices of which
the kernel is aware. There is one subdirectory for each IDE controller, the
file drivers and a link for each IDE device, pointing to the device directory
in the controller specific subtree.
The file drivers contains general information about the drivers used for the
IDE devices:
> cat /proc/ide/drivers
ide-cdrom version 4.53
ide-disk version 1.08
More detailed information can be found in the controller specific
subdirectories. These are named ide0, ide1 and so on. Each of these
directories contains the files shown in table 1-4.
Table 1-4: IDE controller info in /proc/ide/ide?
File Content
channel IDE channel (0 or 1)
config Configuration (only for PCI/IDE bridge)
mate Mate name
model Type/Chipset of IDE controller
Each device connected to a controller has a separate subdirectory in the
controllers directory. The files listed in table 1-5 are contained in these
Table 1-5: IDE device information
File Content
cache The cache
capacity Capacity of the medium (in 512Byte blocks)
driver driver and version
geometry physical and logical geometry
identify device identify block
media media type
model device identifier
settings device setup
smart_thresholds IDE disk management thresholds
smart_values IDE disk management values
The most interesting file is settings. This file contains a nice overview of
the drive parameters:
# cat /proc/ide/ide0/hda/settings
name value min max mode
---- ----- --- --- ----
bios_cyl 526 0 65535 rw
bios_head 255 0 255 rw
bios_sect 63 0 63 rw
breada_readahead 4 0 127 rw
bswap 0 0 1 r
file_readahead 72 0 2097151 rw
io_32bit 0 0 3 rw
keepsettings 0 0 1 rw
max_kb_per_request 122 1 127 rw
multcount 0 0 8 rw
nice1 1 0 1 rw
nowerr 0 0 1 rw
pio_mode write-only 0 255 w
slow 0 0 1 rw
unmaskirq 0 0 1 rw
using_dma 0 0 1 rw
1.4 Networking info in /proc/net
The subdirectory /proc/net follows the usual pattern. Table 1-6 shows the
additional values you get for IP version 6 if you configure the kernel to
support this. Table 1-7 lists the files and their meaning.
Table 1-6: IPv6 info in /proc/net
File Content
udp6 UDP sockets (IPv6)
tcp6 TCP sockets (IPv6)
raw6 Raw device statistics (IPv6)
igmp6 IP multicast addresses, which this host joined (IPv6)
if_inet6 List of IPv6 interface addresses
ipv6_route Kernel routing table for IPv6
rt6_stats Global IPv6 routing tables statistics
sockstat6 Socket statistics (IPv6)
snmp6 Snmp data (IPv6)
Table 1-7: Network info in /proc/net
File Content
arp Kernel ARP table
dev network devices with statistics
dev_mcast the Layer2 multicast groups a device is listening too
(interface index, label, number of references, number of bound
dev_stat network device status
ip_fwchains Firewall chain linkage
ip_fwnames Firewall chain names
ip_masq Directory containing the masquerading tables
ip_masquerade Major masquerading table
netstat Network statistics
raw raw device statistics
route Kernel routing table
rpc Directory containing rpc info
rt_cache Routing cache
snmp SNMP data
sockstat Socket statistics
tcp TCP sockets
tr_rif Token ring RIF routing table
udp UDP sockets
unix UNIX domain sockets
wireless Wireless interface data (Wavelan etc)
igmp IP multicast addresses, which this host joined
psched Global packet scheduler parameters.
netlink List of PF_NETLINK sockets
ip_mr_vifs List of multicast virtual interfaces
ip_mr_cache List of multicast routing cache
You can use this information to see which network devices are available in
your system and how much traffic was routed over those devices:
> cat /proc/net/dev
Inter-|Receive |[...
face |bytes packets errs drop fifo frame compressed multicast|[...
lo: 908188 5596 0 0 0 0 0 0 [...
ppp0:15475140 20721 410 0 0 410 0 0 [...
eth0: 614530 7085 0 0 0 0 0 1 [...
...] Transmit
...] bytes packets errs drop fifo colls carrier compressed
...] 908188 5596 0 0 0 0 0 0
...] 1375103 17405 0 0 0 0 0 0
...] 1703981 5535 0 0 0 3 0 0
In addition, each Channel Bond interface has it's own directory. For
example, the bond0 device will have a directory called /proc/net/bond0/.
It will contain information that is specific to that bond, such as the
current slaves of the bond, the link status of the slaves, and how
many times the slaves link has failed.
1.5 SCSI info
If you have a SCSI host adapter in your system, you'll find a subdirectory
named after the driver for this adapter in /proc/scsi. You'll also see a list
of all recognized SCSI devices in /proc/scsi:
>cat /proc/scsi/scsi
Attached devices:
Host: scsi0 Channel: 00 Id: 00 Lun: 00
Vendor: IBM Model: DGHS09U Rev: 03E0
Type: Direct-Access ANSI SCSI revision: 03
Host: scsi0 Channel: 00 Id: 06 Lun: 00
Vendor: PIONEER Model: CD-ROM DR-U06S Rev: 1.04
Type: CD-ROM ANSI SCSI revision: 02
The directory named after the driver has one file for each adapter found in
the system. These files contain information about the controller, including
the used IRQ and the IO address range. The amount of information shown is
dependent on the adapter you use. The example shows the output for an Adaptec
AHA-2940 SCSI adapter:
> cat /proc/scsi/aic7xxx/0
Adaptec AIC7xxx driver version: 5.1.19/3.2.4
Compile Options:
TCQ Enabled By Default : Disabled
Adapter Configuration:
SCSI Adapter: Adaptec AHA-294X Ultra SCSI host adapter
Ultra Wide Controller
PCI MMAPed I/O Base: 0xeb001000
Adapter SEEPROM Config: SEEPROM found and used.
Adaptec SCSI BIOS: Enabled
IRQ: 10
SCBs: Active 0, Max Active 2,
Allocated 15, HW 16, Page 255
Interrupts: 160328
BIOS Control Word: 0x18b6
Adapter Control Word: 0x005b
Extended Translation: Enabled
Disconnect Enable Flags: 0xffff
Ultra Enable Flags: 0x0001
Tag Queue Enable Flags: 0x0000
Ordered Queue Tag Flags: 0x0000
Default Tag Queue Depth: 8
Tagged Queue By Device array for aic7xxx host instance 0:
Actual queue depth per device for aic7xxx host instance 0:
Device using Wide/Sync transfers at 40.0 MByte/sec, offset 8
Transinfo settings: current(12/8/1/0), goal(12/8/1/0), user(12/15/1/0)
Total transfers 160151 (74577 reads and 85574 writes)
Device using Narrow/Sync transfers at 5.0 MByte/sec, offset 15
Transinfo settings: current(50/15/0/0), goal(50/15/0/0), user(50/15/0/0)
Total transfers 0 (0 reads and 0 writes)
1.6 Parallel port info in /proc/parport
The directory /proc/parport contains information about the parallel ports of
your system. It has one subdirectory for each port, named after the port
number (0,1,2,...).
These directories contain the four files shown in Table 1-8.
Table 1-8: Files in /proc/parport
File Content
autoprobe Any IEEE-1284 device ID information that has been acquired.
devices list of the device drivers using that port. A + will appear by the
name of the device currently using the port (it might not appear
against any).
hardware Parallel port's base address, IRQ line and DMA channel.
irq IRQ that parport is using for that port. This is in a separate
file to allow you to alter it by writing a new value in (IRQ
number or none).
1.7 TTY info in /proc/tty
Information about the available and actually used tty's can be found in the
directory /proc/tty.You'll find entries for drivers and line disciplines in
this directory, as shown in Table 1-9.
Table 1-9: Files in /proc/tty
File Content
drivers list of drivers and their usage
ldiscs registered line disciplines
driver/serial usage statistic and status of single tty lines
To see which tty's are currently in use, you can simply look into the file
> 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
1.8 Miscellaneous kernel statistics in /proc/stat
Various pieces of information about kernel activity are available in the
/proc/stat file. All of the numbers reported in this file are aggregates
since the system first booted. For a quick look, simply cat the file:
> cat /proc/stat
cpu 2255 34 2290 22625563 6290 127 456
cpu0 1132 34 1441 11311718 3675 127 438
cpu1 1123 0 849 11313845 2614 0 18
intr 114930548 113199788 3 0 5 263 0 4 [... lots more numbers ...]
ctxt 1990473
btime 1062191376
processes 2915
procs_running 1
procs_blocked 0
The very first "cpu" line aggregates the numbers in all of the other "cpuN"
lines. These numbers identify the amount of time the CPU has spent performing
different kinds of work. Time units are in USER_HZ (typically hundredths of a
second). The meanings of the columns are as follows, from left to right:
- user: normal processes executing in user mode
- nice: niced processes executing in user mode
- system: processes executing in kernel mode
- idle: twiddling thumbs
- iowait: waiting for I/O to complete
- irq: servicing interrupts
- softirq: servicing softirqs
The "intr" line gives counts of interrupts serviced since boot time, for each
of the possible system interrupts. The first column is the total of all
interrupts serviced; each subsequent column is the total for that particular
The "ctxt" line gives the total number of context switches across all CPUs.
The "btime" line gives the time at which the system booted, in seconds since
the Unix epoch.
The "processes" line gives the number of processes and threads created, which
includes (but is not limited to) those created by calls to the fork() and
clone() system calls.
The "procs_running" line gives the number of processes currently running on
The "procs_blocked" line gives the number of processes currently blocked,
waiting for I/O to complete.
The /proc file system serves information about the running system. It not only
allows access to process data but also allows you to request the kernel status
by reading files in the hierarchy.
The directory structure of /proc reflects the types of information and makes
it easy, if not obvious, where to look for specific data.
In This Chapter
* Modifying kernel parameters by writing into files found in /proc/sys
* Exploring the files which modify certain parameters
* Review of the /proc/sys file tree
A very interesting part of /proc is the directory /proc/sys. This is not only
a source of information, it also allows you to change parameters within the
kernel. Be very careful when attempting this. You can optimize your system,
but you can also cause it to crash. Never alter kernel parameters on a
production system. Set up a development machine and test to make sure that
everything works the way you want it to. You may have no alternative but to
reboot the machine once an error has been made.
To change a value, simply echo the new value into the file. An example is
given below in the section on the file system data. You need to be root to do
this. You can create your own boot script to perform this every time your
system boots.
The files in /proc/sys can be used to fine tune and monitor miscellaneous and
general things in the operation of the Linux kernel. Since some of the files
can inadvertently disrupt your system, it is advisable to read both
documentation and source before actually making adjustments. In any case, be
very careful when writing to any of these files. The entries in /proc may
change slightly between the 2.1.* and the 2.2 kernel, so if there is any doubt
review the kernel documentation in the directory /usr/src/linux/Documentation.
This chapter is heavily based on the documentation included in the pre 2.2
kernels, and became part of it in version 2.2.1 of the Linux kernel.
2.1 /proc/sys/fs - File system data
This subdirectory contains specific file system, file handle, inode, dentry
and quota information.
Currently, these files are in /proc/sys/fs:
Status of the directory cache. Since directory entries are dynamically
allocated and deallocated, this file indicates the current status. It holds
six values, in which the last two are not used and are always zero. The others
are listed in table 2-1.
Table 2-1: Status files of the directory cache
File Content
nr_dentry Almost always zero
nr_unused Number of unused cache entries
in seconds after the entry may be reclaimed, when memory is short
want_pages internally
dquot-nr and dquot-max
The file dquot-max shows the maximum number of cached disk quota entries.
The file dquot-nr shows the number of allocated disk quota entries and the
number of free disk quota entries.
If the number of available cached disk quotas is very low and you have a large
number of simultaneous system users, you might want to raise the limit.
file-nr and file-max
The kernel allocates file handles dynamically, but doesn't free them again at
this time.
The value in file-max denotes the maximum number of file handles that the
Linux kernel will allocate. When you get a lot of error messages about running
out of file handles, you might want to raise this limit. The default value is
10% of RAM in kilobytes. To change it, just write the new number into the
# 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 file.
Historically, the three values in file-nr denoted the number of allocated file
handles, the number of allocated but unused file handles, and the maximum
number of file handles. Linux 2.6 always reports 0 as the number of free file
handles -- this is not an error, it just means that the number of allocated
file handles exactly matches the number of used file handles.
Attempts to allocate more file descriptors than file-max are reported with
printk, look for "VFS: file-max limit <number> reached".
inode-state and inode-nr
The file inode-nr contains the first two items from inode-state, so we'll skip
to that file...
inode-state contains two actual numbers and five dummy values. The numbers
are nr_inodes and nr_free_inodes (in order of appearance).
Denotes the number of inodes the system has allocated. This number will
grow and shrink dynamically.
Represents the number of free inodes. Ie. The number of inuse inodes is
(nr_inodes - nr_free_inodes).
aio-nr and aio-max-nr
aio-nr is the running total of the number of events specified on the
io_setup system call for all currently active aio contexts. If aio-nr
reaches aio-max-nr then io_setup will fail with EAGAIN. Note that
raising aio-max-nr does not result in the pre-allocation or re-sizing
of any kernel data structures.
2.2 /proc/sys/fs/binfmt_misc - Miscellaneous binary formats
Besides these files, there is the subdirectory /proc/sys/fs/binfmt_misc. This
handles the kernel support for miscellaneous binary formats.
Binfmt_misc provides the ability to register additional binary formats to the
Kernel without compiling an additional module/kernel. Therefore, binfmt_misc
needs to know magic numbers at the beginning or the filename extension of the
It works by maintaining a linked list of structs that contain a description of
a binary format, including a magic with size (or the filename extension),
offset and mask, and the interpreter name. On request it invokes the given
interpreter with the original program as argument, as binfmt_java and
binfmt_em86 and binfmt_mz do. Since binfmt_misc does not define any default
binary-formats, you have to register an additional binary-format.
There are two general files in binfmt_misc and one file per registered format.
The two general files are register and status.
Registering a new binary format
To register a new binary format you have to issue the command
echo :name:type:offset:magic:mask:interpreter: > /proc/sys/fs/binfmt_misc/register
with appropriate name (the name for the /proc-dir entry), offset (defaults to
0, if omitted), magic, mask (which can be omitted, defaults to all 0xff) and
last but not least, the interpreter that is to be invoked (for example and
testing /bin/echo). Type can be M for usual magic matching or E for filename
extension matching (give extension in place of magic).
Check or reset the status of the binary format handler
If you do a cat on the file /proc/sys/fs/binfmt_misc/status, you will get the
current status (enabled/disabled) of binfmt_misc. Change the status by echoing
0 (disables) or 1 (enables) or -1 (caution: this clears all previously
registered binary formats) to status. For example echo 0 > status to disable
binfmt_misc (temporarily).
Status of a single handler
Each registered handler has an entry in /proc/sys/fs/binfmt_misc. These files
perform the same function as status, but their scope is limited to the actual
binary format. By cating this file, you also receive all related information
about the interpreter/magic of the binfmt.
Example usage of binfmt_misc (emulate binfmt_java)
cd /proc/sys/fs/binfmt_misc
echo ':Java:M::\xca\xfe\xba\xbe::/usr/local/java/bin/javawrapper:' > register
echo ':HTML:E::html::/usr/local/java/bin/appletviewer:' > register
echo ':Applet:M::<!--applet::/usr/local/java/bin/appletviewer:' > register
echo ':DEXE:M::\x0eDEX::/usr/bin/dosexec:' > register
These four lines add support for Java executables and Java applets (like
binfmt_java, additionally recognizing the .html extension with no need to put
<!--applet> to every applet file). You have to install the JDK and the
shell-script /usr/local/java/bin/javawrapper too. It works around the
brokenness of the Java filename handling. To add a Java binary, just create a
link to the class-file somewhere in the path.
2.3 /proc/sys/kernel - general kernel parameters
This directory reflects general kernel behaviors. As I've said before, the
contents depend on your configuration. Here you'll find the most important
files, along with descriptions of what they mean and how to use them.
The file contains three values; highwater, lowwater, and frequency.
It exists only when BSD-style process accounting is enabled. These values
control its behavior. If the free space on the file system where the log lives
goes below lowwater percentage, accounting suspends. If it goes above
highwater percentage, accounting resumes. Frequency determines how often you
check the amount of free space (value is in seconds). Default settings are: 4,
2, and 30. That is, suspend accounting if there is less than 2 percent free;
resume it if we have a value of 3 or more percent; consider information about
the amount of free space valid for 30 seconds
When the value in this file is 0, ctrl-alt-del is trapped and sent to the init
program to handle a graceful restart. However, when the value is greater that
zero, Linux's reaction to this key combination will be an immediate reboot,
without syncing its dirty buffers.
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
domainname and hostname
These files can be controlled to set the NIS domainname and hostname of your
box. For the classic a simple:
# echo "darkstar" > /proc/sys/kernel/hostname
# echo "" > /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
#4 Fri Oct 1 12:41:14 PDT 1999
The files osrelease and ostype should be clear enough. Version needs a little
more clarification. The #4 means that this is the 4th kernel built from this
source base and the date after it indicates the time the kernel was built. The
only way to tune these values is to rebuild the kernel.
The value in this file represents the number of seconds the kernel waits
before rebooting on a panic. When you use the software watchdog, the
recommended setting is 60. If set to 0, the auto reboot after a kernel panic
is disabled, which is the default setting.
The four values in printk denote
* console_loglevel,
* default_message_loglevel,
* minimum_console_loglevel and
* default_console_loglevel
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 SG_BIG_BUFF.
If you use a scanner with SANE (Scanner Access Now Easy) you might want to set
this to a higher value. Refer to the SANE documentation on this issue.
The location where the modprobe binary is located. The kernel uses this
program to load modules on demand.
The value in this file affects behavior of handling NMI. When the value is
non-zero, unknown NMI is trapped and then panic occurs. At that time, kernel
debugging information is displayed on console.
NMI switch that most IA32 servers have fires unknown NMI up, for example.
If a system hangs up, try pressing the NMI switch.
This function and oprofile share a NMI callback. Therefore this function
cannot be enabled when oprofile is activated.
And NMI watchdog will be disabled when the value in this file is set to
2.4 /proc/sys/vm - The virtual memory subsystem
The files in this directory can be used to tune the operation of the virtual
memory (VM) subsystem of the Linux kernel.
Controls the tendency of the kernel to reclaim the memory which is used for
caching of directory and inode objects.
At the default value of vfs_cache_pressure=100 the kernel will attempt to
reclaim dentries and inodes at a "fair" rate with respect to pagecache and
swapcache reclaim. Decreasing vfs_cache_pressure causes the kernel to prefer
to retain dentry and inode caches. Increasing vfs_cache_pressure beyond 100
causes the kernel to prefer to reclaim dentries and inodes.
Contains, as a percentage of total system memory, the number of pages at which
the pdflush background writeback daemon will start writing out dirty data.
Contains, as a percentage of total system memory, the number of pages at which
a process which is generating disk writes will itself start writing out dirty
The pdflush writeback daemons will periodically wake up and write `old' data
out to disk. This tunable expresses the interval between those wakeups, in
100'ths of a second.
Setting this to zero disables periodic writeback altogether.
This tunable is used to define when dirty data is old enough to be eligible
for writeout by the pdflush daemons. It is expressed in 100'ths of a second.
Data which has been dirty in-memory for longer than this interval will be
written out next time a pdflush daemon wakes up.
If non-zero, this sysctl disables the new 32-bit mmap mmap layout - the kernel
will use the legacy (2.4) layout for all processes.
For some specialised workloads on highmem machines it is dangerous for
the kernel to allow process memory to be allocated from the "lowmem"
zone. This is because that memory could then be pinned via the mlock()
system call, or by unavailability of swapspace.
And on large highmem machines this lack of reclaimable lowmem memory
can be fatal.
So the Linux page allocator has a mechanism which prevents allocations
which _could_ use highmem from using too much lowmem. This means that
a certain amount of lowmem is defended from the possibility of being
captured into pinned user memory.
(The same argument applies to the old 16 megabyte ISA DMA region. This
mechanism will also defend that region from allocations which could use
highmem or lowmem).
The `lower_zone_protection' tunable determines how aggressive the kernel is
in defending these lower zones. The default value is zero - no
protection at all.
If you have a machine which uses highmem or ISA DMA and your
applications are using mlock(), or if you are running with no swap then
you probably should increase the lower_zone_protection setting.
The units of this tunable are fairly vague. It is approximately equal
to "megabytes". So setting lower_zone_protection=100 will protect around 100
megabytes of the lowmem zone from user allocations. It will also make
those 100 megabytes unavaliable for use by applications and by
pagecache, so there is a cost.
The effects of this tunable may be observed by monitoring
/proc/meminfo:LowFree. Write a single huge file and observe the point
at which LowFree ceases to fall.
A reasonable value for lower_zone_protection is 100.
page-cluster controls the number of pages which are written to swap in
a single attempt. The swap I/O size.
It is a logarithmic value - setting it to zero means "1 page", setting
it to 1 means "2 pages", setting it to 2 means "4 pages", etc.
The default value is three (eight pages at a time). There may be some
small benefits in tuning this to a different value if your workload is
Controls overcommit of system memory, possibly allowing processes
to allocate (but not use) more memory than is actually available.
0 - Heuristic overcommit handling. Obvious overcommits of
address space are refused. Used for a typical system. It
ensures a seriously wild allocation fails while allowing
overcommit to reduce swap usage. root is allowed to
allocate slighly more memory in this mode. This is the
1 - Always overcommit. Appropriate for some scientific
2 - Don't overcommit. The total address space commit
for the system is not permitted to exceed swap plus a
configurable percentage (default is 50) of physical RAM.
Depending on the percentage you use, in most situations
this means a process will not be killed while attempting
to use already-allocated memory but will receive errors
on memory allocation as appropriate.
Percentage of physical memory size to include in overcommit calculations
(see above.)
Memory allocation limit = swapspace + physmem * (overcommit_ratio / 100)
swapspace = total size of all swap areas
physmem = size of physical memory in system
nr_hugepages and hugetlb_shm_group
nr_hugepages configures number of hugetlb page reserved for the system.
hugetlb_shm_group contains group id that is allowed to create SysV shared
memory segment using hugetlb page.
laptop_mode is a knob that controls "laptop mode". All the things that are
controlled by this knob are discussed in Documentation/laptop-mode.txt.
block_dump enables block I/O debugging when set to a nonzero value. More
information on block I/O debugging is in Documentation/laptop-mode.txt.
This file contains valid hold time of swap out protection token. The Linux
VM has token based thrashing control mechanism and uses the token to prevent
unnecessary page faults in thrashing situation. The unit of the value is
second. The value would be useful to tune thrashing behavior.
Writing to this will cause the kernel to drop clean caches, dentries and
inodes from memory, causing that memory to become free.
To free pagecache:
echo 1 > /proc/sys/vm/drop_caches
To free dentries and inodes:
echo 2 > /proc/sys/vm/drop_caches
To free pagecache, dentries and inodes:
echo 3 > /proc/sys/vm/drop_caches
As this is a non-destructive operation and dirty objects are not freeable, the
user should run `sync' first.
2.5 /proc/sys/dev - Device specific parameters
Currently there is only support for CDROM drives, and for those, there is only
one read-only file containing information about the CD-ROM drives attached to
the system:
>cat /proc/sys/dev/cdrom/info
CD-ROM information, Id: cdrom.c 2.55 1999/04/25
drive name: sr0 hdb
drive speed: 32 40
drive # of slots: 1 0
Can close tray: 1 1
Can open tray: 1 1
Can lock tray: 1 1
Can change speed: 1 1
Can select disk: 0 1
Can read multisession: 1 1
Can read MCN: 1 1
Reports media changed: 1 1
Can play audio: 1 1
You see two drives, sr0 and hdb, along with a list of their features.
2.6 /proc/sys/sunrpc - Remote procedure calls
This directory contains four files, which enable or disable debugging for the
RPC functions NFS, NFS-daemon, RPC and NLM. The default values are 0. They can
be set to one to turn debugging on. (The default value is 0 for each)
2.7 /proc/sys/net - Networking stuff
The interface to the networking parts of the kernel is located in
/proc/sys/net. Table 2-3 shows all possible subdirectories. You may see only
some of them, depending on your kernel's configuration.
Table 2-3: Subdirectories in /proc/sys/net
Directory Content Directory Content
core General parameter appletalk Appletalk protocol
unix Unix domain sockets netrom NET/ROM
802 E802 protocol ax25 AX25
ethernet Ethernet protocol rose X.25 PLP layer
ipv4 IP version 4 x25 X.25 protocol
ipx IPX token-ring IBM token ring
bridge Bridging decnet DEC net
ipv6 IP version 6
We will concentrate on IP networking here. Since AX15, X.25, and DEC Net are
only minor players in the Linux world, we'll skip them in this chapter. You'll
find some short info on Appletalk and IPX further on in this chapter. Review
the online documentation and the kernel source to get a detailed view of the
parameters for those protocols. In this section we'll discuss the
subdirectories printed in bold letters in the table above. As default values
are suitable for most needs, there is no need to change these values.
/proc/sys/net/core - Network core options
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. A higher message_cost factor, results in
fewer messages that will be written. Message_burst controls when messages will
be dropped. The default settings limit warning messages to one every five
Maximum number of packets, queued on the 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.
2.8 /proc/sys/net/ipv4 - IPV4 settings
IP version 4 is still the most used protocol in Unix networking. It will be
replaced by IP version 6 in the next couple of years, but for the moment it's
the de facto standard for the internet and is used in most networking
environments around the world. Because of the importance of this protocol,
we'll have a deeper look into the subtree controlling the behavior of the IPv4
subsystem of the Linux kernel.
Let's start with the entries in /proc/sys/net/ipv4.
ICMP settings
icmp_echo_ignore_all and icmp_echo_ignore_broadcasts
Turn on (1) or off (0), if the kernel should ignore all ICMP ECHO requests, or
just those to broadcast and multicast addresses.
Please note that if you accept ICMP echo requests with a broadcast/multi\-cast
destination address your network may be used as an exploder for denial of
service packet flooding attacks to other hosts.
icmp_destunreach_rate, icmp_echoreply_rate, icmp_paramprob_rate and icmp_timeexeed_rate
Sets limits for sending ICMP packets to specific targets. A value of zero
disables all limiting. Any positive value sets the maximum package rate in
hundredth of a second (on Intel systems).
IP settings
This file contains the number one if the host received its IP configuration by
RARP, BOOTP, DHCP or a similar mechanism. Otherwise it is zero.
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. Changing 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 basis.
Enable/disable debugging of IP masquerading.
IP fragmentation settings
ipfrag_high_trash and ipfrag_low_trash
Maximum memory used to reassemble IP fragments. When ipfrag_high_thresh bytes
of memory is allocated for this purpose, the fragment handler will toss
packets until ipfrag_low_thresh is reached.
Time in seconds to keep an IP fragment in memory.
TCP settings
This file controls the use of the ECN bit in the IPv4 headers, this is a new
feature about Explicit Congestion Notification, but some routers and firewalls
block trafic that has this bit set, so it could be necessary to echo 0 to
/proc/sys/net/ipv4/tcp_ecn, if you want to talk to this sites. For more info
you could read RFC2481.
Bug-to-bug compatibility with some broken printers. On retransmit, try to send
larger packets to work around bugs in certain TCP stacks. Can be turned off by
setting it to zero.
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 ward
off the common 'syn flood attack'. Disabled by default.
Note that the concept of a socket backlog is abandoned. This means the peer
may not receive reliable error messages from an over loaded server with
syncookies enabled.
Enable window scaling as defined in RFC1323.
The length of time in seconds it takes to receive a final FIN before the
socket is always closed. This is strictly a violation of the TCP
specification, but required to prevent denial-of-service attacks.
Indicates how many keep alive probes are sent per slow timer run. Should not
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 ignored.
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, whereas changes in the other
subdirectories affect only one interface. All directories have the same
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
Accept packets with source address 0.b.c.d with destinations not to this host
as local ones. It is supposed that a BOOTP relay daemon will catch and forward
such packets.
The default is 0, since this feature is not implemented yet (kernel version
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.
Does (1) or does not (0) perform proxy ARP.
Integer value determines if a source validation should be made. 1 means yes, 0
means no. Disabled by default, but local/broadcast address spoofing is always
If you set this to 1 on a router that is the only connection for a network to
the net, it will prevent spoofing attacks against your internal networks
(external addresses can still be spoofed), without the need for additional
firewall rules.
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 whether to send ICMP redirects to other hosts.
Routing settings
The directory /proc/sys/net/ipv4/route contains several file to control
routing issues.
error_burst and error_cost
These parameters are used to limit how many ICMP destination unreachable to
send from the host in question. ICMP destination unreachable messages are
sent when we can not reach the next hop, while trying to transmit a packet.
It will also print some error messages to kernel logs if someone is ignoring
our ICMP redirects. The higher the error_cost factor is, the fewer
destination unreachable and error messages will be let through. Error_burst
controls when destination unreachable messages and error messages will be
dropped. The default settings limit warning messages to five every second.
Writing to this file results in a flush of the routing cache.
gc_elasticity, gc_interval, gc_min_interval_ms, gc_timeout, gc_thresh
Values to control the frequency and behavior of the garbage collection
algorithm for the routing cache. gc_min_interval is deprecated and replaced
by gc_min_interval_ms.
Maximum size of the routing cache. Old entries will be purged once the cache
reached has this size.
max_delay, min_delay
Delays for flushing the routing cache.
redirect_load, redirect_number
Factors which determine if more ICPM redirects should be sent to a specific
host. No redirects will be sent once the load limit or the maximum number of
redirects has been reached.
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, base_reachable_time_ms
A base value used for computing the random reachable time value as specified
in RFC2461.
Expression of base_reachable_time, which is deprecated, is in seconds.
Expression of base_reachable_time_ms is in milliseconds.
retrans_time, retrans_time_ms
The time between retransmitted Neighbor Solicitation messages.
Used for address resolution and to determine if a neighbor is
Expression of retrans_time, which is deprecated, is in 1/100 seconds (for
IPv4) or in jiffies (for IPv6).
Expression of retrans_time_ms is in milliseconds.
Maximum queue length for a pending arp request - the number of packets which
are accepted from other layers while the ARP address is still resolved.
Maximum for random delay of answers to neighbor solicitation messages in
jiffies (1/100 sec). Not yet implemented (Linux does not have anycast support
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 proxy_delay).
Determines the number of requests to send to the user level ARP daemon. Use 0
to turn off.
Determines how often to check for stale ARP entries. After an ARP entry is
stale it will be resolved again (which is useful when an IP address migrates
to another machine). When ucast_solicit is greater than 0 it first tries to
send an ARP packet directly to the known host When that fails and
mcast_solicit is greater than 0, an ARP request is broadcasted.
2.9 Appletalk
The /proc/sys/net/appletalk directory holds the Appletalk configuration data
when Appletalk is loaded. The configurable parameters are:
The amount of time we keep an ARP entry before expiring it. Used to age out
old hosts.
The amount of time we will spend trying to resolve an Appletalk address.
The number of times we will retransmit a query before giving up.
Controls the rate at which expires are checked.
The directory /proc/net/appletalk holds the list of active Appletalk sockets
on a machine.
The fields indicate the DDP type, the local address (in network:node format)
the remote address, the size of the transmit pending queue, the size of the
received queue (bytes waiting for applications to read) the state and the uid
owning the socket.
/proc/net/atalk_iface lists all the interfaces configured for appletalk.It
shows the name of the interface, its Appletalk address, the network range on
that address (or network number for phase 1 networks), and the status of the
/proc/net/atalk_route lists each known network route. It lists the target
(network) that the route leads to, the router (may be directly connected), the
route flags, and the device the route is using.
2.10 IPX
The IPX protocol has no tunable values in proc/sys/net.
The IPX protocol does, however, provide proc/net/ipx. This lists each IPX
socket giving the local and remote addresses in Novell format (that is
network:node:port). In accordance with the strange Novell tradition,
everything but the port is in hex. Not_Connected is displayed for sockets that
are not tied to a specific remote address. The Tx and Rx queue sizes indicate
the number of bytes pending for transmission and reception. The state
indicates the state the socket is in and the uid is the owning uid of the
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
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.
2.11 /proc/sys/fs/mqueue - POSIX message queues filesystem
The "mqueue" filesystem provides the necessary kernel features to enable the
creation of a user space library that implements the POSIX message queues
API (as noted by the MSG tag in the POSIX 1003.1-2001 version of the System
Interfaces specification.)
The "mqueue" filesystem contains values for determining/setting the amount of
resources used by the file system.
/proc/sys/fs/mqueue/queues_max is a read/write file for setting/getting the
maximum number of message queues allowed on the system.
/proc/sys/fs/mqueue/msg_max is a read/write file for setting/getting the
maximum number of messages in a queue value. In fact it is the limiting value
for another (user) limit which is set in mq_open invocation. This attribute of
a queue must be less or equal then msg_max.
/proc/sys/fs/mqueue/msgsize_max is a read/write file for setting/getting the
maximum message size value (it is every message queue's attribute set during
its creation).
Certain aspects of kernel behavior can be modified at runtime, without the
need to recompile the kernel, or even to reboot the system. The files in the
/proc/sys tree can not only be read, but also modified. You can use the echo
command to write value into these files, thereby changing the default settings
of the kernel.