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2 T H E /proc F I L E S Y S T E M
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4 /proc/sys Terrehon Bowden <terrehon@pacbell.net> October 7 1999
5 Bodo Bauer <bb@ricochet.net>
7 2.4.x update Jorge Nerin <comandante@zaralinux.com> November 14 2000
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9 Version 1.3 Kernel version 2.2.12
10 Kernel version 2.4.0-test11-pre4
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17 0.1 Introduction/Credits
20 1 Collecting System Information
21 1.1 Process-Specific Subdirectories
23 1.3 IDE devices in /proc/ide
24 1.4 Networking info in /proc/net
26 1.6 Parallel port info in /proc/parport
27 1.7 TTY info in /proc/tty
28 1.8 Miscellaneous kernel statistics in /proc/stat
30 2 Modifying System Parameters
31 2.1 /proc/sys/fs - File system data
32 2.2 /proc/sys/fs/binfmt_misc - Miscellaneous binary formats
33 2.3 /proc/sys/kernel - general kernel parameters
34 2.4 /proc/sys/vm - The virtual memory subsystem
35 2.5 /proc/sys/dev - Device specific parameters
36 2.6 /proc/sys/sunrpc - Remote procedure calls
37 2.7 /proc/sys/net - Networking stuff
38 2.8 /proc/sys/net/ipv4 - IPV4 settings
41 2.11 /proc/sys/fs/mqueue - POSIX message queues filesystem
43 ------------------------------------------------------------------------------
45 ------------------------------------------------------------------------------
47 0.1 Introduction/Credits
48 ------------------------
50 This documentation is part of a soon (or so we hope) to be released book on
51 the SuSE Linux distribution. As there is no complete documentation for the
52 /proc file system and we've used many freely available sources to write these
53 chapters, it seems only fair to give the work back to the Linux community.
54 This work is based on the 2.2.* kernel version and the upcoming 2.4.*. I'm
55 afraid it's still far from complete, but we hope it will be useful. As far as
56 we know, it is the first 'all-in-one' document about the /proc file system. It
57 is focused on the Intel x86 hardware, so if you are looking for PPC, ARM,
58 SPARC, AXP, etc., features, you probably won't find what you are looking for.
59 It also only covers IPv4 networking, not IPv6 nor other protocols - sorry. But
60 additions and patches are welcome and will be added to this document if you
63 We'd like to thank Alan Cox, Rik van Riel, and Alexey Kuznetsov and a lot of
64 other people for help compiling this documentation. We'd also like to extend a
65 special thank you to Andi Kleen for documentation, which we relied on heavily
66 to create this document, as well as the additional information he provided.
67 Thanks to everybody else who contributed source or docs to the Linux kernel
68 and helped create a great piece of software... :)
70 If you have any comments, corrections or additions, please don't hesitate to
71 contact Bodo Bauer at bb@ricochet.net. We'll be happy to add them to this
74 The latest version of this document is available online at
75 http://skaro.nightcrawler.com/~bb/Docs/Proc as HTML version.
77 If the above direction does not works for you, ypu could try the kernel
78 mailing list at linux-kernel@vger.kernel.org and/or try to reach me at
79 comandante@zaralinux.com.
84 We don't guarantee the correctness of this document, and if you come to us
85 complaining about how you screwed up your system because of incorrect
86 documentation, we won't feel responsible...
88 ------------------------------------------------------------------------------
89 CHAPTER 1: COLLECTING SYSTEM INFORMATION
90 ------------------------------------------------------------------------------
92 ------------------------------------------------------------------------------
94 ------------------------------------------------------------------------------
95 * Investigating the properties of the pseudo file system /proc and its
96 ability to provide information on the running Linux system
97 * Examining /proc's structure
98 * Uncovering various information about the kernel and the processes running
100 ------------------------------------------------------------------------------
103 The proc file system acts as an interface to internal data structures in the
104 kernel. It can be used to obtain information about the system and to change
105 certain kernel parameters at runtime (sysctl).
107 First, we'll take a look at the read-only parts of /proc. In Chapter 2, we
108 show you how you can use /proc/sys to change settings.
110 1.1 Process-Specific Subdirectories
111 -----------------------------------
113 The directory /proc contains (among other things) one subdirectory for each
114 process running on the system, which is named after the process ID (PID).
116 The link self points to the process reading the file system. Each process
117 subdirectory has the entries listed in Table 1-1.
120 Table 1-1: Process specific entries in /proc
121 ..............................................................................
123 cmdline Command line arguments
124 cpu Current and last cpu in wich it was executed (2.4)(smp)
125 cwd Link to the current working directory
126 environ Values of environment variables
127 exe Link to the executable of this process
128 fd Directory, which contains all file descriptors
129 maps Memory maps to executables and library files (2.4)
130 mem Memory held by this process
131 root Link to the root directory of this process
133 statm Process memory status information
134 status Process status in human readable form
135 wchan If CONFIG_KALLSYMS is set, a pre-decoded wchan
136 ..............................................................................
138 For example, to get the status information of a process, all you have to do is
139 read the file /proc/PID/status:
141 >cat /proc/self/status
157 SigPnd: 0000000000000000
158 SigBlk: 0000000000000000
159 SigIgn: 0000000000000000
160 SigCgt: 0000000000000000
161 CapInh: 00000000fffffeff
162 CapPrm: 0000000000000000
163 CapEff: 0000000000000000
166 This shows you nearly the same information you would get if you viewed it with
167 the ps command. In fact, ps uses the proc file system to obtain its
168 information. The statm file contains more detailed information about the
169 process memory usage. Its seven fields are explained in Table 1-2.
172 Table 1-2: Contents of the statm files (as of 2.6.8-rc3)
173 ..............................................................................
175 size total program size (pages) (same as VmSize in status)
176 resident size of memory portions (pages) (same as VmRSS in status)
177 shared number of pages that are shared (i.e. backed by a file)
178 trs number of pages that are 'code' (not including libs; broken,
179 includes data segment)
180 lrs number of pages of library (always 0 on 2.6)
181 drs number of pages of data/stack (including libs; broken,
182 includes library text)
183 dt number of dirty pages (always 0 on 2.6)
184 ..............................................................................
189 Similar to the process entries, the kernel data files give information about
190 the running kernel. The files used to obtain this information are contained in
191 /proc and are listed in Table 1-3. Not all of these will be present in your
192 system. It depends on the kernel configuration and the loaded modules, which
193 files are there, and which are missing.
195 Table 1-3: Kernel info in /proc
196 ..............................................................................
198 apm Advanced power management info
199 buddyinfo Kernel memory allocator information (see text) (2.5)
200 bus Directory containing bus specific information
201 cmdline Kernel command line
202 cpuinfo Info about the CPU
203 devices Available devices (block and character)
204 dma Used DMS channels
205 filesystems Supported filesystems
206 driver Various drivers grouped here, currently rtc (2.4)
207 execdomains Execdomains, related to security (2.4)
208 fb Frame Buffer devices (2.4)
209 fs File system parameters, currently nfs/exports (2.4)
210 ide Directory containing info about the IDE subsystem
211 interrupts Interrupt usage
212 iomem Memory map (2.4)
213 ioports I/O port usage
214 irq Masks for irq to cpu affinity (2.4)(smp?)
215 isapnp ISA PnP (Plug&Play) Info (2.4)
216 kcore Kernel core image (can be ELF or A.OUT(deprecated in 2.4))
218 ksyms Kernel symbol table
219 loadavg Load average of last 1, 5 & 15 minutes
223 modules List of loaded modules
224 mounts Mounted filesystems
225 net Networking info (see text)
226 partitions Table of partitions known to the system
227 pci Depreciated info of PCI bus (new way -> /proc/bus/pci/,
228 decoupled by lspci (2.4)
230 scsi SCSI info (see text)
231 slabinfo Slab pool info
232 stat Overall statistics
233 swaps Swap space utilization
235 sysvipc Info of SysVIPC Resources (msg, sem, shm) (2.4)
236 tty Info of tty drivers
238 version Kernel version
239 video bttv info of video resources (2.4)
240 ..............................................................................
242 You can, for example, check which interrupts are currently in use and what
243 they are used for by looking in the file /proc/interrupts:
245 > cat /proc/interrupts
247 0: 8728810 XT-PIC timer
248 1: 895 XT-PIC keyboard
250 3: 531695 XT-PIC aha152x
251 4: 2014133 XT-PIC serial
252 5: 44401 XT-PIC pcnet_cs
255 12: 182918 XT-PIC PS/2 Mouse
257 14: 1232265 XT-PIC ide0
261 In 2.4.* a couple of lines where added to this file LOC & ERR (this time is the
262 output of a SMP machine):
264 > cat /proc/interrupts
267 0: 1243498 1214548 IO-APIC-edge timer
268 1: 8949 8958 IO-APIC-edge keyboard
269 2: 0 0 XT-PIC cascade
270 5: 11286 10161 IO-APIC-edge soundblaster
271 8: 1 0 IO-APIC-edge rtc
272 9: 27422 27407 IO-APIC-edge 3c503
273 12: 113645 113873 IO-APIC-edge PS/2 Mouse
275 14: 22491 24012 IO-APIC-edge ide0
276 15: 2183 2415 IO-APIC-edge ide1
277 17: 30564 30414 IO-APIC-level eth0
278 18: 177 164 IO-APIC-level bttv
283 NMI is incremented in this case because every timer interrupt generates a NMI
284 (Non Maskable Interrupt) which is used by the NMI Watchdog to detect lockups.
286 LOC is the local interrupt counter of the internal APIC of every CPU.
288 ERR is incremented in the case of errors in the IO-APIC bus (the bus that
289 connects the CPUs in a SMP system. This means that an error has been detected,
290 the IO-APIC automatically retry the transmission, so it should not be a big
291 problem, but you should read the SMP-FAQ.
293 In this context it could be interesting to note the new irq directory in 2.4.
294 It could be used to set IRQ to CPU affinity, this means that you can "hook" an
295 IRQ to only one CPU, or to exclude a CPU of handling IRQs. The contents of the
296 irq subdir is one subdir for each IRQ, and one file; prof_cpu_mask
300 0 10 12 14 16 18 2 4 6 8 prof_cpu_mask
301 1 11 13 15 17 19 3 5 7 9
305 The contents of the prof_cpu_mask file and each smp_affinity file for each IRQ
306 is the same by default:
308 > cat /proc/irq/0/smp_affinity
311 It's a bitmask, in wich you can specify wich CPUs can handle the IRQ, you can
314 > echo 1 > /proc/irq/prof_cpu_mask
316 This means that only the first CPU will handle the IRQ, but you can also echo 5
317 wich means that only the first and fourth CPU can handle the IRQ.
319 The way IRQs are routed is handled by the IO-APIC, and it's Round Robin
320 between all the CPUs which are allowed to handle it. As usual the kernel has
321 more info than you and does a better job than you, so the defaults are the
322 best choice for almost everyone.
324 There are three more important subdirectories in /proc: net, scsi, and sys.
325 The general rule is that the contents, or even the existence of these
326 directories, depend on your kernel configuration. If SCSI is not enabled, the
327 directory scsi may not exist. The same is true with the net, which is there
328 only when networking support is present in the running kernel.
330 The slabinfo file gives information about memory usage at the slab level.
331 Linux uses slab pools for memory management above page level in version 2.2.
332 Commonly used objects have their own slab pool (such as network buffers,
333 directory cache, and so on).
335 ..............................................................................
337 > cat /proc/buddyinfo
339 Node 0, zone DMA 0 4 5 4 4 3 ...
340 Node 0, zone Normal 1 0 0 1 101 8 ...
341 Node 0, zone HighMem 2 0 0 1 1 0 ...
343 Memory fragmentation is a problem under some workloads, and buddyinfo is a
344 useful tool for helping diagnose these problems. Buddyinfo will give you a
345 clue as to how big an area you can safely allocate, or why a previous
348 Each column represents the number of pages of a certain order which are
349 available. In this case, there are 0 chunks of 2^0*PAGE_SIZE available in
350 ZONE_DMA, 4 chunks of 2^1*PAGE_SIZE in ZONE_DMA, 101 chunks of 2^4*PAGE_SIZE
351 available in ZONE_NORMAL, etc...
353 ..............................................................................
357 Provides information about distribution and utilization of memory. This
358 varies by architecture and compile options. The following is from a
359 16GB PIII, which has highmem enabled. You may not have all of these fields.
364 MemTotal: 16344972 kB
371 HighTotal: 15597528 kB
372 HighFree: 13629632 kB
381 CommitLimit: 7669796 kB
382 Committed_AS: 100056 kB
384 VmallocTotal: 112216 kB
386 VmallocChunk: 111088 kB
388 MemTotal: Total usable ram (i.e. physical ram minus a few reserved
389 bits and the kernel binary code)
390 MemFree: The sum of LowFree+HighFree
391 Buffers: Relatively temporary storage for raw disk blocks
392 shouldn't get tremendously large (20MB or so)
393 Cached: in-memory cache for files read from the disk (the
394 pagecache). Doesn't include SwapCached
395 SwapCached: Memory that once was swapped out, is swapped back in but
396 still also is in the swapfile (if memory is needed it
397 doesn't need to be swapped out AGAIN because it is already
398 in the swapfile. This saves I/O)
399 Active: Memory that has been used more recently and usually not
400 reclaimed unless absolutely necessary.
401 Inactive: Memory which has been less recently used. It is more
402 eligible to be reclaimed for other purposes
404 HighFree: Highmem is all memory above ~860MB of physical memory
405 Highmem areas are for use by userspace programs, or
406 for the pagecache. The kernel must use tricks to access
407 this memory, making it slower to access than lowmem.
409 LowFree: Lowmem is memory which can be used for everything that
410 highmem can be used for, but it is also availble for the
411 kernel's use for its own data structures. Among many
412 other things, it is where everything from the Slab is
413 allocated. Bad things happen when you're out of lowmem.
414 SwapTotal: total amount of swap space available
415 SwapFree: Memory which has been evicted from RAM, and is temporarily
417 Dirty: Memory which is waiting to get written back to the disk
418 Writeback: Memory which is actively being written back to the disk
419 Mapped: files which have been mmaped, such as libraries
420 Slab: in-kernel data structures cache
421 CommitLimit: Based on the overcommit ratio ('vm.overcommit_ratio'),
422 this is the total amount of memory currently available to
423 be allocated on the system. This limit is only adhered to
424 if strict overcommit accounting is enabled (mode 2 in
425 'vm.overcommit_memory').
426 The CommitLimit is calculated with the following formula:
427 CommitLimit = ('vm.overcommit_ratio' * Physical RAM) + Swap
428 For example, on a system with 1G of physical RAM and 7G
429 of swap with a `vm.overcommit_ratio` of 30 it would
430 yield a CommitLimit of 7.3G.
431 For more details, see the memory overcommit documentation
432 in vm/overcommit-accounting.
433 Committed_AS: The amount of memory presently allocated on the system.
434 The committed memory is a sum of all of the memory which
435 has been allocated by processes, even if it has not been
436 "used" by them as of yet. A process which malloc()'s 1G
437 of memory, but only touches 300M of it will only show up
438 as using 300M of memory even if it has the address space
439 allocated for the entire 1G. This 1G is memory which has
440 been "committed" to by the VM and can be used at any time
441 by the allocating application. With strict overcommit
442 enabled on the system (mode 2 in 'vm.overcommit_memory'),
443 allocations which would exceed the CommitLimit (detailed
444 above) will not be permitted. This is useful if one needs
445 to guarantee that processes will not fail due to lack of
446 memory once that memory has been successfully allocated.
447 PageTables: amount of memory dedicated to the lowest level of page
449 VmallocTotal: total size of vmalloc memory area
450 VmallocUsed: amount of vmalloc area which is used
451 VmallocChunk: largest contigious block of vmalloc area which is free
454 1.3 IDE devices in /proc/ide
455 ----------------------------
457 The subdirectory /proc/ide contains information about all IDE devices of which
458 the kernel is aware. There is one subdirectory for each IDE controller, the
459 file drivers and a link for each IDE device, pointing to the device directory
460 in the controller specific subtree.
462 The file drivers contains general information about the drivers used for the
465 > cat /proc/ide/drivers
466 ide-cdrom version 4.53
467 ide-disk version 1.08
469 More detailed information can be found in the controller specific
470 subdirectories. These are named ide0, ide1 and so on. Each of these
471 directories contains the files shown in table 1-4.
474 Table 1-4: IDE controller info in /proc/ide/ide?
475 ..............................................................................
477 channel IDE channel (0 or 1)
478 config Configuration (only for PCI/IDE bridge)
480 model Type/Chipset of IDE controller
481 ..............................................................................
483 Each device connected to a controller has a separate subdirectory in the
484 controllers directory. The files listed in table 1-5 are contained in these
488 Table 1-5: IDE device information
489 ..............................................................................
492 capacity Capacity of the medium (in 512Byte blocks)
493 driver driver and version
494 geometry physical and logical geometry
495 identify device identify block
497 model device identifier
498 settings device setup
499 smart_thresholds IDE disk management thresholds
500 smart_values IDE disk management values
501 ..............................................................................
503 The most interesting file is settings. This file contains a nice overview of
504 the drive parameters:
506 # cat /proc/ide/ide0/hda/settings
507 name value min max mode
508 ---- ----- --- --- ----
509 bios_cyl 526 0 65535 rw
510 bios_head 255 0 255 rw
512 breada_readahead 4 0 127 rw
514 file_readahead 72 0 2097151 rw
516 keepsettings 0 0 1 rw
517 max_kb_per_request 122 1 127 rw
521 pio_mode write-only 0 255 w
527 1.4 Networking info in /proc/net
528 --------------------------------
530 The subdirectory /proc/net follows the usual pattern. Table 1-6 shows the
531 additional values you get for IP version 6 if you configure the kernel to
532 support this. Table 1-7 lists the files and their meaning.
535 Table 1-6: IPv6 info in /proc/net
536 ..............................................................................
538 udp6 UDP sockets (IPv6)
539 tcp6 TCP sockets (IPv6)
540 raw6 Raw device statistics (IPv6)
541 igmp6 IP multicast addresses, which this host joined (IPv6)
542 if_inet6 List of IPv6 interface addresses
543 ipv6_route Kernel routing table for IPv6
544 rt6_stats Global IPv6 routing tables statistics
545 sockstat6 Socket statistics (IPv6)
546 snmp6 Snmp data (IPv6)
547 ..............................................................................
550 Table 1-7: Network info in /proc/net
551 ..............................................................................
554 dev network devices with statistics
555 dev_mcast the Layer2 multicast groups a device is listening too
556 (interface index, label, number of references, number of bound
558 dev_stat network device status
559 ip_fwchains Firewall chain linkage
560 ip_fwnames Firewall chain names
561 ip_masq Directory containing the masquerading tables
562 ip_masquerade Major masquerading table
563 netstat Network statistics
564 raw raw device statistics
565 route Kernel routing table
566 rpc Directory containing rpc info
567 rt_cache Routing cache
569 sockstat Socket statistics
571 tr_rif Token ring RIF routing table
573 unix UNIX domain sockets
574 wireless Wireless interface data (Wavelan etc)
575 igmp IP multicast addresses, which this host joined
576 psched Global packet scheduler parameters.
577 netlink List of PF_NETLINK sockets
578 ip_mr_vifs List of multicast virtual interfaces
579 ip_mr_cache List of multicast routing cache
580 ..............................................................................
582 You can use this information to see which network devices are available in
583 your system and how much traffic was routed over those devices:
587 face |bytes packets errs drop fifo frame compressed multicast|[...
588 lo: 908188 5596 0 0 0 0 0 0 [...
589 ppp0:15475140 20721 410 0 0 410 0 0 [...
590 eth0: 614530 7085 0 0 0 0 0 1 [...
593 ...] bytes packets errs drop fifo colls carrier compressed
594 ...] 908188 5596 0 0 0 0 0 0
595 ...] 1375103 17405 0 0 0 0 0 0
596 ...] 1703981 5535 0 0 0 3 0 0
598 In addition, each Channel Bond interface has it's own directory. For
599 example, the bond0 device will have a directory called /proc/net/bond0/.
600 It will contain information that is specific to that bond, such as the
601 current slaves of the bond, the link status of the slaves, and how
602 many times the slaves link has failed.
607 If you have a SCSI host adapter in your system, you'll find a subdirectory
608 named after the driver for this adapter in /proc/scsi. You'll also see a list
609 of all recognized SCSI devices in /proc/scsi:
613 Host: scsi0 Channel: 00 Id: 00 Lun: 00
614 Vendor: IBM Model: DGHS09U Rev: 03E0
615 Type: Direct-Access ANSI SCSI revision: 03
616 Host: scsi0 Channel: 00 Id: 06 Lun: 00
617 Vendor: PIONEER Model: CD-ROM DR-U06S Rev: 1.04
618 Type: CD-ROM ANSI SCSI revision: 02
621 The directory named after the driver has one file for each adapter found in
622 the system. These files contain information about the controller, including
623 the used IRQ and the IO address range. The amount of information shown is
624 dependent on the adapter you use. The example shows the output for an Adaptec
625 AHA-2940 SCSI adapter:
627 > cat /proc/scsi/aic7xxx/0
629 Adaptec AIC7xxx driver version: 5.1.19/3.2.4
631 TCQ Enabled By Default : Disabled
632 AIC7XXX_PROC_STATS : Disabled
633 AIC7XXX_RESET_DELAY : 5
634 Adapter Configuration:
635 SCSI Adapter: Adaptec AHA-294X Ultra SCSI host adapter
636 Ultra Wide Controller
637 PCI MMAPed I/O Base: 0xeb001000
638 Adapter SEEPROM Config: SEEPROM found and used.
639 Adaptec SCSI BIOS: Enabled
641 SCBs: Active 0, Max Active 2,
642 Allocated 15, HW 16, Page 255
644 BIOS Control Word: 0x18b6
645 Adapter Control Word: 0x005b
646 Extended Translation: Enabled
647 Disconnect Enable Flags: 0xffff
648 Ultra Enable Flags: 0x0001
649 Tag Queue Enable Flags: 0x0000
650 Ordered Queue Tag Flags: 0x0000
651 Default Tag Queue Depth: 8
652 Tagged Queue By Device array for aic7xxx host instance 0:
653 {255,255,255,255,255,255,255,255,255,255,255,255,255,255,255,255}
654 Actual queue depth per device for aic7xxx host instance 0:
655 {1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1}
658 Device using Wide/Sync transfers at 40.0 MByte/sec, offset 8
659 Transinfo settings: current(12/8/1/0), goal(12/8/1/0), user(12/15/1/0)
660 Total transfers 160151 (74577 reads and 85574 writes)
662 Device using Narrow/Sync transfers at 5.0 MByte/sec, offset 15
663 Transinfo settings: current(50/15/0/0), goal(50/15/0/0), user(50/15/0/0)
664 Total transfers 0 (0 reads and 0 writes)
667 1.6 Parallel port info in /proc/parport
668 ---------------------------------------
670 The directory /proc/parport contains information about the parallel ports of
671 your system. It has one subdirectory for each port, named after the port
674 These directories contain the four files shown in Table 1-8.
677 Table 1-8: Files in /proc/parport
678 ..............................................................................
680 autoprobe Any IEEE-1284 device ID information that has been acquired.
681 devices list of the device drivers using that port. A + will appear by the
682 name of the device currently using the port (it might not appear
684 hardware Parallel port's base address, IRQ line and DMA channel.
685 irq IRQ that parport is using for that port. This is in a separate
686 file to allow you to alter it by writing a new value in (IRQ
688 ..............................................................................
690 1.7 TTY info in /proc/tty
691 -------------------------
693 Information about the available and actually used tty's can be found in the
694 directory /proc/tty.You'll find entries for drivers and line disciplines in
695 this directory, as shown in Table 1-9.
698 Table 1-9: Files in /proc/tty
699 ..............................................................................
701 drivers list of drivers and their usage
702 ldiscs registered line disciplines
703 driver/serial usage statistic and status of single tty lines
704 ..............................................................................
706 To see which tty's are currently in use, you can simply look into the file
709 > cat /proc/tty/drivers
710 pty_slave /dev/pts 136 0-255 pty:slave
711 pty_master /dev/ptm 128 0-255 pty:master
712 pty_slave /dev/ttyp 3 0-255 pty:slave
713 pty_master /dev/pty 2 0-255 pty:master
714 serial /dev/cua 5 64-67 serial:callout
715 serial /dev/ttyS 4 64-67 serial
716 /dev/tty0 /dev/tty0 4 0 system:vtmaster
717 /dev/ptmx /dev/ptmx 5 2 system
718 /dev/console /dev/console 5 1 system:console
719 /dev/tty /dev/tty 5 0 system:/dev/tty
720 unknown /dev/tty 4 1-63 console
723 1.8 Miscellaneous kernel statistics in /proc/stat
724 -------------------------------------------------
726 Various pieces of information about kernel activity are available in the
727 /proc/stat file. All of the numbers reported in this file are aggregates
728 since the system first booted. For a quick look, simply cat the file:
731 cpu 2255 34 2290 22625563 6290 127 456
732 cpu0 1132 34 1441 11311718 3675 127 438
733 cpu1 1123 0 849 11313845 2614 0 18
734 intr 114930548 113199788 3 0 5 263 0 4 [... lots more numbers ...]
741 The very first "cpu" line aggregates the numbers in all of the other "cpuN"
742 lines. These numbers identify the amount of time the CPU has spent performing
743 different kinds of work. Time units are in USER_HZ (typically hundredths of a
744 second). The meanings of the columns are as follows, from left to right:
746 - user: normal processes executing in user mode
747 - nice: niced processes executing in user mode
748 - system: processes executing in kernel mode
749 - idle: twiddling thumbs
750 - iowait: waiting for I/O to complete
751 - irq: servicing interrupts
752 - softirq: servicing softirqs
754 The "intr" line gives counts of interrupts serviced since boot time, for each
755 of the possible system interrupts. The first column is the total of all
756 interrupts serviced; each subsequent column is the total for that particular
759 The "ctxt" line gives the total number of context switches across all CPUs.
761 The "btime" line gives the time at which the system booted, in seconds since
764 The "processes" line gives the number of processes and threads created, which
765 includes (but is not limited to) those created by calls to the fork() and
766 clone() system calls.
768 The "procs_running" line gives the number of processes currently running on
771 The "procs_blocked" line gives the number of processes currently blocked,
772 waiting for I/O to complete.
775 ------------------------------------------------------------------------------
777 ------------------------------------------------------------------------------
778 The /proc file system serves information about the running system. It not only
779 allows access to process data but also allows you to request the kernel status
780 by reading files in the hierarchy.
782 The directory structure of /proc reflects the types of information and makes
783 it easy, if not obvious, where to look for specific data.
784 ------------------------------------------------------------------------------
786 ------------------------------------------------------------------------------
787 CHAPTER 2: MODIFYING SYSTEM PARAMETERS
788 ------------------------------------------------------------------------------
790 ------------------------------------------------------------------------------
792 ------------------------------------------------------------------------------
793 * Modifying kernel parameters by writing into files found in /proc/sys
794 * Exploring the files which modify certain parameters
795 * Review of the /proc/sys file tree
796 ------------------------------------------------------------------------------
799 A very interesting part of /proc is the directory /proc/sys. This is not only
800 a source of information, it also allows you to change parameters within the
801 kernel. Be very careful when attempting this. You can optimize your system,
802 but you can also cause it to crash. Never alter kernel parameters on a
803 production system. Set up a development machine and test to make sure that
804 everything works the way you want it to. You may have no alternative but to
805 reboot the machine once an error has been made.
807 To change a value, simply echo the new value into the file. An example is
808 given below in the section on the file system data. You need to be root to do
809 this. You can create your own boot script to perform this every time your
812 The files in /proc/sys can be used to fine tune and monitor miscellaneous and
813 general things in the operation of the Linux kernel. Since some of the files
814 can inadvertently disrupt your system, it is advisable to read both
815 documentation and source before actually making adjustments. In any case, be
816 very careful when writing to any of these files. The entries in /proc may
817 change slightly between the 2.1.* and the 2.2 kernel, so if there is any doubt
818 review the kernel documentation in the directory /usr/src/linux/Documentation.
819 This chapter is heavily based on the documentation included in the pre 2.2
820 kernels, and became part of it in version 2.2.1 of the Linux kernel.
822 2.1 /proc/sys/fs - File system data
823 -----------------------------------
825 This subdirectory contains specific file system, file handle, inode, dentry
826 and quota information.
828 Currently, these files are in /proc/sys/fs:
833 Status of the directory cache. Since directory entries are dynamically
834 allocated and deallocated, this file indicates the current status. It holds
835 six values, in which the last two are not used and are always zero. The others
836 are listed in table 2-1.
839 Table 2-1: Status files of the directory cache
840 ..............................................................................
842 nr_dentry Almost always zero
843 nr_unused Number of unused cache entries
845 in seconds after the entry may be reclaimed, when memory is short
846 want_pages internally
847 ..............................................................................
849 dquot-nr and dquot-max
850 ----------------------
852 The file dquot-max shows the maximum number of cached disk quota entries.
854 The file dquot-nr shows the number of allocated disk quota entries and the
855 number of free disk quota entries.
857 If the number of available cached disk quotas is very low and you have a large
858 number of simultaneous system users, you might want to raise the limit.
863 The kernel allocates file handles dynamically, but doesn't free them again at
866 The value in file-max denotes the maximum number of file handles that the
867 Linux kernel will allocate. When you get a lot of error messages about running
868 out of file handles, you might want to raise this limit. The default value is
869 10% of RAM in kilobytes. To change it, just write the new number into the
872 # cat /proc/sys/fs/file-max
874 # echo 8192 > /proc/sys/fs/file-max
875 # cat /proc/sys/fs/file-max
879 This method of revision is useful for all customizable parameters of the
880 kernel - simply echo the new value to the corresponding file.
882 Historically, the three values in file-nr denoted the number of allocated file
883 handles, the number of allocated but unused file handles, and the maximum
884 number of file handles. Linux 2.6 always reports 0 as the number of free file
885 handles -- this is not an error, it just means that the number of allocated
886 file handles exactly matches the number of used file handles.
888 Attempts to allocate more file descriptors than file-max are reported with
889 printk, look for "VFS: file-max limit <number> reached".
891 inode-state and inode-nr
892 ------------------------
894 The file inode-nr contains the first two items from inode-state, so we'll skip
897 inode-state contains two actual numbers and five dummy values. The numbers
898 are nr_inodes and nr_free_inodes (in order of appearance).
903 Denotes the number of inodes the system has allocated. This number will
904 grow and shrink dynamically.
909 Represents the number of free inodes. Ie. The number of inuse inodes is
910 (nr_inodes - nr_free_inodes).
912 super-nr and super-max
913 ----------------------
915 Again, super block structures are allocated by the kernel, but not freed. The
916 file super-max contains the maximum number of super block handlers, where
917 super-nr shows the number of currently allocated ones.
919 Every mounted file system needs a super block, so if you plan to mount lots of
920 file systems, you may want to increase these numbers.
922 aio-nr and aio-max-nr
923 ---------------------
925 aio-nr is the running total of the number of events specified on the
926 io_setup system call for all currently active aio contexts. If aio-nr
927 reaches aio-max-nr then io_setup will fail with EAGAIN. Note that
928 raising aio-max-nr does not result in the pre-allocation or re-sizing
929 of any kernel data structures.
931 2.2 /proc/sys/fs/binfmt_misc - Miscellaneous binary formats
932 -----------------------------------------------------------
934 Besides these files, there is the subdirectory /proc/sys/fs/binfmt_misc. This
935 handles the kernel support for miscellaneous binary formats.
937 Binfmt_misc provides the ability to register additional binary formats to the
938 Kernel without compiling an additional module/kernel. Therefore, binfmt_misc
939 needs to know magic numbers at the beginning or the filename extension of the
942 It works by maintaining a linked list of structs that contain a description of
943 a binary format, including a magic with size (or the filename extension),
944 offset and mask, and the interpreter name. On request it invokes the given
945 interpreter with the original program as argument, as binfmt_java and
946 binfmt_em86 and binfmt_mz do. Since binfmt_misc does not define any default
947 binary-formats, you have to register an additional binary-format.
949 There are two general files in binfmt_misc and one file per registered format.
950 The two general files are register and status.
952 Registering a new binary format
953 -------------------------------
955 To register a new binary format you have to issue the command
957 echo :name:type:offset:magic:mask:interpreter: > /proc/sys/fs/binfmt_misc/register
961 with appropriate name (the name for the /proc-dir entry), offset (defaults to
962 0, if omitted), magic, mask (which can be omitted, defaults to all 0xff) and
963 last but not least, the interpreter that is to be invoked (for example and
964 testing /bin/echo). Type can be M for usual magic matching or E for filename
965 extension matching (give extension in place of magic).
967 Check or reset the status of the binary format handler
968 ------------------------------------------------------
970 If you do a cat on the file /proc/sys/fs/binfmt_misc/status, you will get the
971 current status (enabled/disabled) of binfmt_misc. Change the status by echoing
972 0 (disables) or 1 (enables) or -1 (caution: this clears all previously
973 registered binary formats) to status. For example echo 0 > status to disable
974 binfmt_misc (temporarily).
976 Status of a single handler
977 --------------------------
979 Each registered handler has an entry in /proc/sys/fs/binfmt_misc. These files
980 perform the same function as status, but their scope is limited to the actual
981 binary format. By cating this file, you also receive all related information
982 about the interpreter/magic of the binfmt.
984 Example usage of binfmt_misc (emulate binfmt_java)
985 --------------------------------------------------
987 cd /proc/sys/fs/binfmt_misc
988 echo ':Java:M::\xca\xfe\xba\xbe::/usr/local/java/bin/javawrapper:' > register
989 echo ':HTML:E::html::/usr/local/java/bin/appletviewer:' > register
990 echo ':Applet:M::<!--applet::/usr/local/java/bin/appletviewer:' > register
991 echo ':DEXE:M::\x0eDEX::/usr/bin/dosexec:' > register
994 These four lines add support for Java executables and Java applets (like
995 binfmt_java, additionally recognizing the .html extension with no need to put
996 <!--applet> to every applet file). You have to install the JDK and the
997 shell-script /usr/local/java/bin/javawrapper too. It works around the
998 brokenness of the Java filename handling. To add a Java binary, just create a
999 link to the class-file somewhere in the path.
1001 2.3 /proc/sys/kernel - general kernel parameters
1002 ------------------------------------------------
1004 This directory reflects general kernel behaviors. As I've said before, the
1005 contents depend on your configuration. Here you'll find the most important
1006 files, along with descriptions of what they mean and how to use them.
1011 The file contains three values; highwater, lowwater, and frequency.
1013 It exists only when BSD-style process accounting is enabled. These values
1014 control its behavior. If the free space on the file system where the log lives
1015 goes below lowwater percentage, accounting suspends. If it goes above
1016 highwater percentage, accounting resumes. Frequency determines how often you
1017 check the amount of free space (value is in seconds). Default settings are: 4,
1018 2, and 30. That is, suspend accounting if there is less than 2 percent free;
1019 resume it if we have a value of 3 or more percent; consider information about
1020 the amount of free space valid for 30 seconds
1025 When the value in this file is 0, ctrl-alt-del is trapped and sent to the init
1026 program to handle a graceful restart. However, when the value is greater that
1027 zero, Linux's reaction to this key combination will be an immediate reboot,
1028 without syncing its dirty buffers.
1031 When a program (like dosemu) has the keyboard in raw mode, the
1032 ctrl-alt-del is intercepted by the program before it ever reaches the
1033 kernel tty layer, and it is up to the program to decide what to do with
1036 domainname and hostname
1037 -----------------------
1039 These files can be controlled to set the NIS domainname and hostname of your
1040 box. For the classic darkstar.frop.org a simple:
1042 # echo "darkstar" > /proc/sys/kernel/hostname
1043 # echo "frop.org" > /proc/sys/kernel/domainname
1046 would suffice to set your hostname and NIS domainname.
1048 osrelease, ostype and version
1049 -----------------------------
1051 The names make it pretty obvious what these fields contain:
1053 > cat /proc/sys/kernel/osrelease
1056 > cat /proc/sys/kernel/ostype
1059 > cat /proc/sys/kernel/version
1060 #4 Fri Oct 1 12:41:14 PDT 1999
1063 The files osrelease and ostype should be clear enough. Version needs a little
1064 more clarification. The #4 means that this is the 4th kernel built from this
1065 source base and the date after it indicates the time the kernel was built. The
1066 only way to tune these values is to rebuild the kernel.
1071 The value in this file represents the number of seconds the kernel waits
1072 before rebooting on a panic. When you use the software watchdog, the
1073 recommended setting is 60. If set to 0, the auto reboot after a kernel panic
1074 is disabled, which is the default setting.
1079 The four values in printk denote
1081 * default_message_loglevel,
1082 * minimum_console_loglevel and
1083 * default_console_loglevel
1086 These values influence printk() behavior when printing or logging error
1087 messages, which come from inside the kernel. See syslog(2) for more
1088 information on the different log levels.
1093 Messages with a higher priority than this will be printed to the console.
1095 default_message_level
1096 ---------------------
1098 Messages without an explicit priority will be printed with this priority.
1100 minimum_console_loglevel
1101 ------------------------
1103 Minimum (highest) value to which the console_loglevel can be set.
1105 default_console_loglevel
1106 ------------------------
1108 Default value for console_loglevel.
1113 This file shows the size of the generic SCSI (sg) buffer. At this point, you
1114 can't tune it yet, but you can change it at compile time by editing
1115 include/scsi/sg.h and changing the value of SG_BIG_BUFF.
1117 If you use a scanner with SANE (Scanner Access Now Easy) you might want to set
1118 this to a higher value. Refer to the SANE documentation on this issue.
1123 The location where the modprobe binary is located. The kernel uses this
1124 program to load modules on demand.
1129 The value in this file affects behavior of handling NMI. When the value is
1130 non-zero, unknown NMI is trapped and then panic occurs. At that time, kernel
1131 debugging information is displayed on console.
1133 NMI switch that most IA32 servers have fires unknown NMI up, for example.
1134 If a system hangs up, try pressing the NMI switch.
1137 This function and oprofile share a NMI callback. Therefore this function
1138 cannot be enabled when oprofile is activated.
1139 And NMI watchdog will be disabled when the value in this file is set to
1143 2.4 /proc/sys/vm - The virtual memory subsystem
1144 -----------------------------------------------
1146 The files in this directory can be used to tune the operation of the virtual
1147 memory (VM) subsystem of the Linux kernel.
1152 Controls the tendency of the kernel to reclaim the memory which is used for
1153 caching of directory and inode objects.
1155 At the default value of vfs_cache_pressure=100 the kernel will attempt to
1156 reclaim dentries and inodes at a "fair" rate with respect to pagecache and
1157 swapcache reclaim. Decreasing vfs_cache_pressure causes the kernel to prefer
1158 to retain dentry and inode caches. Increasing vfs_cache_pressure beyond 100
1159 causes the kernel to prefer to reclaim dentries and inodes.
1161 dirty_background_ratio
1162 ----------------------
1164 Contains, as a percentage of total system memory, the number of pages at which
1165 the pdflush background writeback daemon will start writing out dirty data.
1170 Contains, as a percentage of total system memory, the number of pages at which
1171 a process which is generating disk writes will itself start writing out dirty
1174 dirty_writeback_centisecs
1175 -------------------------
1177 The pdflush writeback daemons will periodically wake up and write `old' data
1178 out to disk. This tunable expresses the interval between those wakeups, in
1179 100'ths of a second.
1181 Setting this to zero disables periodic writeback altogether.
1183 dirty_expire_centisecs
1184 ----------------------
1186 This tunable is used to define when dirty data is old enough to be eligible
1187 for writeout by the pdflush daemons. It is expressed in 100'ths of a second.
1188 Data which has been dirty in-memory for longer than this interval will be
1189 written out next time a pdflush daemon wakes up.
1194 If non-zero, this sysctl disables the new 32-bit mmap mmap layout - the kernel
1195 will use the legacy (2.4) layout for all processes.
1197 lower_zone_protection
1198 ---------------------
1200 For some specialised workloads on highmem machines it is dangerous for
1201 the kernel to allow process memory to be allocated from the "lowmem"
1202 zone. This is because that memory could then be pinned via the mlock()
1203 system call, or by unavailability of swapspace.
1205 And on large highmem machines this lack of reclaimable lowmem memory
1208 So the Linux page allocator has a mechanism which prevents allocations
1209 which _could_ use highmem from using too much lowmem. This means that
1210 a certain amount of lowmem is defended from the possibility of being
1211 captured into pinned user memory.
1213 (The same argument applies to the old 16 megabyte ISA DMA region. This
1214 mechanism will also defend that region from allocations which could use
1217 The `lower_zone_protection' tunable determines how aggressive the kernel is
1218 in defending these lower zones. The default value is zero - no
1221 If you have a machine which uses highmem or ISA DMA and your
1222 applications are using mlock(), or if you are running with no swap then
1223 you probably should increase the lower_zone_protection setting.
1225 The units of this tunable are fairly vague. It is approximately equal
1226 to "megabytes". So setting lower_zone_protection=100 will protect around 100
1227 megabytes of the lowmem zone from user allocations. It will also make
1228 those 100 megabytes unavaliable for use by applications and by
1229 pagecache, so there is a cost.
1231 The effects of this tunable may be observed by monitoring
1232 /proc/meminfo:LowFree. Write a single huge file and observe the point
1233 at which LowFree ceases to fall.
1235 A reasonable value for lower_zone_protection is 100.
1240 page-cluster controls the number of pages which are written to swap in
1241 a single attempt. The swap I/O size.
1243 It is a logarithmic value - setting it to zero means "1 page", setting
1244 it to 1 means "2 pages", setting it to 2 means "4 pages", etc.
1246 The default value is three (eight pages at a time). There may be some
1247 small benefits in tuning this to a different value if your workload is
1253 This file contains one value. The following algorithm is used to decide if
1254 there's enough memory: if the value of overcommit_memory is positive, then
1255 there's always enough memory. This is a useful feature, since programs often
1256 malloc() huge amounts of memory 'just in case', while they only use a small
1257 part of it. Leaving this value at 0 will lead to the failure of such a huge
1258 malloc(), when in fact the system has enough memory for the program to run.
1260 On the other hand, enabling this feature can cause you to run out of memory
1261 and thrash the system to death, so large and/or important servers will want to
1262 set this value to 0.
1264 nr_hugepages and hugetlb_shm_group
1265 ----------------------------------
1267 nr_hugepages configures number of hugetlb page reserved for the system.
1269 hugetlb_shm_group contains group id that is allowed to create SysV shared
1270 memory segment using hugetlb page.
1275 laptop_mode is a knob that controls "laptop mode". All the things that are
1276 controlled by this knob are discussed in Documentation/laptop-mode.txt.
1281 block_dump enables block I/O debugging when set to a nonzero value. More
1282 information on block I/O debugging is in Documentation/laptop-mode.txt.
1287 This file contains valid hold time of swap out protection token. The Linux
1288 VM has token based thrashing control mechanism and uses the token to prevent
1289 unnecessary page faults in thrashing situation. The unit of the value is
1290 second. The value would be useful to tune thrashing behavior.
1292 2.5 /proc/sys/dev - Device specific parameters
1293 ----------------------------------------------
1295 Currently there is only support for CDROM drives, and for those, there is only
1296 one read-only file containing information about the CD-ROM drives attached to
1299 >cat /proc/sys/dev/cdrom/info
1300 CD-ROM information, Id: cdrom.c 2.55 1999/04/25
1304 drive # of slots: 1 0
1308 Can change speed: 1 1
1309 Can select disk: 0 1
1310 Can read multisession: 1 1
1312 Reports media changed: 1 1
1316 You see two drives, sr0 and hdb, along with a list of their features.
1318 2.6 /proc/sys/sunrpc - Remote procedure calls
1319 ---------------------------------------------
1321 This directory contains four files, which enable or disable debugging for the
1322 RPC functions NFS, NFS-daemon, RPC and NLM. The default values are 0. They can
1323 be set to one to turn debugging on. (The default value is 0 for each)
1325 2.7 /proc/sys/net - Networking stuff
1326 ------------------------------------
1328 The interface to the networking parts of the kernel is located in
1329 /proc/sys/net. Table 2-3 shows all possible subdirectories. You may see only
1330 some of them, depending on your kernel's configuration.
1333 Table 2-3: Subdirectories in /proc/sys/net
1334 ..............................................................................
1335 Directory Content Directory Content
1336 core General parameter appletalk Appletalk protocol
1337 unix Unix domain sockets netrom NET/ROM
1338 802 E802 protocol ax25 AX25
1339 ethernet Ethernet protocol rose X.25 PLP layer
1340 ipv4 IP version 4 x25 X.25 protocol
1341 ipx IPX token-ring IBM token ring
1342 bridge Bridging decnet DEC net
1344 ..............................................................................
1346 We will concentrate on IP networking here. Since AX15, X.25, and DEC Net are
1347 only minor players in the Linux world, we'll skip them in this chapter. You'll
1348 find some short info on Appletalk and IPX further on in this chapter. Review
1349 the online documentation and the kernel source to get a detailed view of the
1350 parameters for those protocols. In this section we'll discuss the
1351 subdirectories printed in bold letters in the table above. As default values
1352 are suitable for most needs, there is no need to change these values.
1354 /proc/sys/net/core - Network core options
1355 -----------------------------------------
1360 The default setting of the socket receive buffer in bytes.
1365 The maximum receive socket buffer size in bytes.
1370 The default setting (in bytes) of the socket send buffer.
1375 The maximum send socket buffer size in bytes.
1377 message_burst and message_cost
1378 ------------------------------
1380 These parameters are used to limit the warning messages written to the kernel
1381 log from the networking code. They enforce a rate limit to make a
1382 denial-of-service attack impossible. A higher message_cost factor, results in
1383 fewer messages that will be written. Message_burst controls when messages will
1384 be dropped. The default settings limit warning messages to one every five
1390 Maximum number of packets, queued on the INPUT side, when the interface
1391 receives packets faster than kernel can process them.
1396 Maximum ancillary buffer size allowed per socket. Ancillary data is a sequence
1397 of struct cmsghdr structures with appended data.
1399 /proc/sys/net/unix - Parameters for Unix domain sockets
1400 -------------------------------------------------------
1402 There are only two files in this subdirectory. They control the delays for
1403 deleting and destroying socket descriptors.
1405 2.8 /proc/sys/net/ipv4 - IPV4 settings
1406 --------------------------------------
1408 IP version 4 is still the most used protocol in Unix networking. It will be
1409 replaced by IP version 6 in the next couple of years, but for the moment it's
1410 the de facto standard for the internet and is used in most networking
1411 environments around the world. Because of the importance of this protocol,
1412 we'll have a deeper look into the subtree controlling the behavior of the IPv4
1413 subsystem of the Linux kernel.
1415 Let's start with the entries in /proc/sys/net/ipv4.
1420 icmp_echo_ignore_all and icmp_echo_ignore_broadcasts
1421 ----------------------------------------------------
1423 Turn on (1) or off (0), if the kernel should ignore all ICMP ECHO requests, or
1424 just those to broadcast and multicast addresses.
1426 Please note that if you accept ICMP echo requests with a broadcast/multi\-cast
1427 destination address your network may be used as an exploder for denial of
1428 service packet flooding attacks to other hosts.
1430 icmp_destunreach_rate, icmp_echoreply_rate, icmp_paramprob_rate and icmp_timeexeed_rate
1431 ---------------------------------------------------------------------------------------
1433 Sets limits for sending ICMP packets to specific targets. A value of zero
1434 disables all limiting. Any positive value sets the maximum package rate in
1435 hundredth of a second (on Intel systems).
1443 This file contains the number one if the host received its IP configuration by
1444 RARP, BOOTP, DHCP or a similar mechanism. Otherwise it is zero.
1449 TTL (Time To Live) for IPv4 interfaces. This is simply the maximum number of
1450 hops a packet may travel.
1455 Enable dynamic socket address rewriting on interface address change. This is
1456 useful for dialup interface with changing IP addresses.
1461 Enable or disable forwarding of IP packages between interfaces. Changing this
1462 value resets all other parameters to their default values. They differ if the
1463 kernel is configured as host or router.
1468 Range of ports used by TCP and UDP to choose the local port. Contains two
1469 numbers, the first number is the lowest port, the second number the highest
1470 local port. Default is 1024-4999. Should be changed to 32768-61000 for
1476 Global switch to turn path MTU discovery off. It can also be set on a per
1477 socket basis by the applications or on a per route basis.
1482 Enable/disable debugging of IP masquerading.
1484 IP fragmentation settings
1485 -------------------------
1487 ipfrag_high_trash and ipfrag_low_trash
1488 --------------------------------------
1490 Maximum memory used to reassemble IP fragments. When ipfrag_high_thresh bytes
1491 of memory is allocated for this purpose, the fragment handler will toss
1492 packets until ipfrag_low_thresh is reached.
1497 Time in seconds to keep an IP fragment in memory.
1505 This file controls the use of the ECN bit in the IPv4 headers, this is a new
1506 feature about Explicit Congestion Notification, but some routers and firewalls
1507 block trafic that has this bit set, so it could be necessary to echo 0 to
1508 /proc/sys/net/ipv4/tcp_ecn, if you want to talk to this sites. For more info
1509 you could read RFC2481.
1511 tcp_retrans_collapse
1512 --------------------
1514 Bug-to-bug compatibility with some broken printers. On retransmit, try to send
1515 larger packets to work around bugs in certain TCP stacks. Can be turned off by
1518 tcp_keepalive_probes
1519 --------------------
1521 Number of keep alive probes TCP sends out, until it decides that the
1522 connection is broken.
1527 How often TCP sends out keep alive messages, when keep alive is enabled. The
1533 Number of times initial SYNs for a TCP connection attempt will be
1534 retransmitted. Should not be higher than 255. This is only the timeout for
1535 outgoing connections, for incoming connections the number of retransmits is
1536 defined by tcp_retries1.
1541 Enable select acknowledgments after RFC2018.
1546 Enable timestamps as defined in RFC1323.
1551 Enable the strict RFC793 interpretation of the TCP urgent pointer field. The
1552 default is to use the BSD compatible interpretation of the urgent pointer
1553 pointing to the first byte after the urgent data. The RFC793 interpretation is
1554 to have it point to the last byte of urgent data. Enabling this option may
1555 lead to interoperatibility problems. Disabled by default.
1560 Only valid when the kernel was compiled with CONFIG_SYNCOOKIES. Send out
1561 syncookies when the syn backlog queue of a socket overflows. This is to ward
1562 off the common 'syn flood attack'. Disabled by default.
1564 Note that the concept of a socket backlog is abandoned. This means the peer
1565 may not receive reliable error messages from an over loaded server with
1571 Enable window scaling as defined in RFC1323.
1576 The length of time in seconds it takes to receive a final FIN before the
1577 socket is always closed. This is strictly a violation of the TCP
1578 specification, but required to prevent denial-of-service attacks.
1583 Indicates how many keep alive probes are sent per slow timer run. Should not
1584 be set too high to prevent bursts.
1589 Length of the per socket backlog queue. Since Linux 2.2 the backlog specified
1590 in listen(2) only specifies the length of the backlog queue of already
1591 established sockets. When more connection requests arrive Linux starts to drop
1592 packets. When syncookies are enabled the packets are still answered and the
1593 maximum queue is effectively ignored.
1598 Defines how often an answer to a TCP connection request is retransmitted
1604 Defines how often a TCP packet is retransmitted before giving up.
1606 Interface specific settings
1607 ---------------------------
1609 In the directory /proc/sys/net/ipv4/conf you'll find one subdirectory for each
1610 interface the system knows about and one directory calls all. Changes in the
1611 all subdirectory affect all interfaces, whereas changes in the other
1612 subdirectories affect only one interface. All directories have the same
1618 This switch decides if the kernel accepts ICMP redirect messages or not. The
1619 default is 'yes' if the kernel is configured for a regular host and 'no' for a
1620 router configuration.
1625 Should source routed packages be accepted or declined. The default is
1626 dependent on the kernel configuration. It's 'yes' for routers and 'no' for
1632 Accept packets with source address 0.b.c.d with destinations not to this host
1633 as local ones. It is supposed that a BOOTP relay daemon will catch and forward
1636 The default is 0, since this feature is not implemented yet (kernel version
1642 Enable or disable IP forwarding on this interface.
1647 Log packets with source addresses with no known route to kernel log.
1652 Do multicast routing. The kernel needs to be compiled with CONFIG_MROUTE and a
1653 multicast routing daemon is required.
1658 Does (1) or does not (0) perform proxy ARP.
1663 Integer value determines if a source validation should be made. 1 means yes, 0
1664 means no. Disabled by default, but local/broadcast address spoofing is always
1667 If you set this to 1 on a router that is the only connection for a network to
1668 the net, it will prevent spoofing attacks against your internal networks
1669 (external addresses can still be spoofed), without the need for additional
1675 Accept ICMP redirect messages only for gateways, listed in default gateway
1676 list. Enabled by default.
1681 If it is not set the kernel does not assume that different subnets on this
1682 device can communicate directly. Default setting is 'yes'.
1687 Determines whether to send ICMP redirects to other hosts.
1692 The directory /proc/sys/net/ipv4/route contains several file to control
1695 error_burst and error_cost
1696 --------------------------
1698 These parameters are used to limit how many ICMP destination unreachable to
1699 send from the host in question. ICMP destination unreachable messages are
1700 sent when we can not reach the next hop, while trying to transmit a packet.
1701 It will also print some error messages to kernel logs if someone is ignoring
1702 our ICMP redirects. The higher the error_cost factor is, the fewer
1703 destination unreachable and error messages will be let through. Error_burst
1704 controls when destination unreachable messages and error messages will be
1705 dropped. The default settings limit warning messages to five every second.
1710 Writing to this file results in a flush of the routing cache.
1712 gc_elasticity, gc_interval, gc_min_interval_ms, gc_timeout, gc_thresh
1713 ---------------------------------------------------------------------
1715 Values to control the frequency and behavior of the garbage collection
1716 algorithm for the routing cache. gc_min_interval is deprecated and replaced
1717 by gc_min_interval_ms.
1723 Maximum size of the routing cache. Old entries will be purged once the cache
1724 reached has this size.
1726 max_delay, min_delay
1727 --------------------
1729 Delays for flushing the routing cache.
1731 redirect_load, redirect_number
1732 ------------------------------
1734 Factors which determine if more ICPM redirects should be sent to a specific
1735 host. No redirects will be sent once the load limit or the maximum number of
1736 redirects has been reached.
1741 Timeout for redirects. After this period redirects will be sent again, even if
1742 this has been stopped, because the load or number limit has been reached.
1744 Network Neighbor handling
1745 -------------------------
1747 Settings about how to handle connections with direct neighbors (nodes attached
1748 to the same link) can be found in the directory /proc/sys/net/ipv4/neigh.
1750 As we saw it in the conf directory, there is a default subdirectory which
1751 holds the default values, and one directory for each interface. The contents
1752 of the directories are identical, with the single exception that the default
1753 settings contain additional options to set garbage collection parameters.
1755 In the interface directories you'll find the following entries:
1757 base_reachable_time, base_reachable_time_ms
1758 -------------------------------------------
1760 A base value used for computing the random reachable time value as specified
1763 Expression of base_reachable_time, which is deprecated, is in seconds.
1764 Expression of base_reachable_time_ms is in milliseconds.
1766 retrans_time, retrans_time_ms
1767 -----------------------------
1769 The time between retransmitted Neighbor Solicitation messages.
1770 Used for address resolution and to determine if a neighbor is
1773 Expression of retrans_time, which is deprecated, is in 1/100 seconds (for
1774 IPv4) or in jiffies (for IPv6).
1775 Expression of retrans_time_ms is in milliseconds.
1780 Maximum queue length for a pending arp request - the number of packets which
1781 are accepted from other layers while the ARP address is still resolved.
1786 Maximum for random delay of answers to neighbor solicitation messages in
1787 jiffies (1/100 sec). Not yet implemented (Linux does not have anycast support
1793 Maximum number of retries for unicast solicitation.
1798 Maximum number of retries for multicast solicitation.
1800 delay_first_probe_time
1801 ----------------------
1803 Delay for the first time probe if the neighbor is reachable. (see
1809 An ARP/neighbor entry is only replaced with a new one if the old is at least
1810 locktime old. This prevents ARP cache thrashing.
1815 Maximum time (real time is random [0..proxytime]) before answering to an ARP
1816 request for which we have an proxy ARP entry. In some cases, this is used to
1817 prevent network flooding.
1822 Maximum queue length of the delayed proxy arp timer. (see proxy_delay).
1827 Determines the number of requests to send to the user level ARP daemon. Use 0
1833 Determines how often to check for stale ARP entries. After an ARP entry is
1834 stale it will be resolved again (which is useful when an IP address migrates
1835 to another machine). When ucast_solicit is greater than 0 it first tries to
1836 send an ARP packet directly to the known host When that fails and
1837 mcast_solicit is greater than 0, an ARP request is broadcasted.
1842 The /proc/sys/net/appletalk directory holds the Appletalk configuration data
1843 when Appletalk is loaded. The configurable parameters are:
1848 The amount of time we keep an ARP entry before expiring it. Used to age out
1854 The amount of time we will spend trying to resolve an Appletalk address.
1856 aarp-retransmit-limit
1857 ---------------------
1859 The number of times we will retransmit a query before giving up.
1864 Controls the rate at which expires are checked.
1866 The directory /proc/net/appletalk holds the list of active Appletalk sockets
1869 The fields indicate the DDP type, the local address (in network:node format)
1870 the remote address, the size of the transmit pending queue, the size of the
1871 received queue (bytes waiting for applications to read) the state and the uid
1874 /proc/net/atalk_iface lists all the interfaces configured for appletalk.It
1875 shows the name of the interface, its Appletalk address, the network range on
1876 that address (or network number for phase 1 networks), and the status of the
1879 /proc/net/atalk_route lists each known network route. It lists the target
1880 (network) that the route leads to, the router (may be directly connected), the
1881 route flags, and the device the route is using.
1886 The IPX protocol has no tunable values in proc/sys/net.
1888 The IPX protocol does, however, provide proc/net/ipx. This lists each IPX
1889 socket giving the local and remote addresses in Novell format (that is
1890 network:node:port). In accordance with the strange Novell tradition,
1891 everything but the port is in hex. Not_Connected is displayed for sockets that
1892 are not tied to a specific remote address. The Tx and Rx queue sizes indicate
1893 the number of bytes pending for transmission and reception. The state
1894 indicates the state the socket is in and the uid is the owning uid of the
1897 The /proc/net/ipx_interface file lists all IPX interfaces. For each interface
1898 it gives the network number, the node number, and indicates if the network is
1899 the primary network. It also indicates which device it is bound to (or
1900 Internal for internal networks) and the Frame Type if appropriate. Linux
1901 supports 802.3, 802.2, 802.2 SNAP and DIX (Blue Book) ethernet framing for
1904 The /proc/net/ipx_route table holds a list of IPX routes. For each route it
1905 gives the destination network, the router node (or Directly) and the network
1906 address of the router (or Connected) for internal networks.
1908 2.11 /proc/sys/fs/mqueue - POSIX message queues filesystem
1909 ----------------------------------------------------------
1911 The "mqueue" filesystem provides the necessary kernel features to enable the
1912 creation of a user space library that implements the POSIX message queues
1913 API (as noted by the MSG tag in the POSIX 1003.1-2001 version of the System
1914 Interfaces specification.)
1916 The "mqueue" filesystem contains values for determining/setting the amount of
1917 resources used by the file system.
1919 /proc/sys/fs/mqueue/queues_max is a read/write file for setting/getting the
1920 maximum number of message queues allowed on the system.
1922 /proc/sys/fs/mqueue/msg_max is a read/write file for setting/getting the
1923 maximum number of messages in a queue value. In fact it is the limiting value
1924 for another (user) limit which is set in mq_open invocation. This attribute of
1925 a queue must be less or equal then msg_max.
1927 /proc/sys/fs/mqueue/msgsize_max is a read/write file for setting/getting the
1928 maximum message size value (it is every message queue's attribute set during
1932 ------------------------------------------------------------------------------
1934 ------------------------------------------------------------------------------
1935 Certain aspects of kernel behavior can be modified at runtime, without the
1936 need to recompile the kernel, or even to reboot the system. The files in the
1937 /proc/sys tree can not only be read, but also modified. You can use the echo
1938 command to write value into these files, thereby changing the default settings
1940 ------------------------------------------------------------------------------
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