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authorLinus Torvalds <torvalds@ppc970.osdl.org>2005-04-16 15:20:36 -0700
committerLinus Torvalds <torvalds@ppc970.osdl.org>2005-04-16 15:20:36 -0700
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tree0bba044c4ce775e45a88a51686b5d9f90697ea9d /Documentation/filesystems/proc.txt
downloadconfigs-2.6.12-rc2.tar.gz
Linux-2.6.12-rc2v2.6.12-rc2
Initial git repository build. I'm not bothering with the full history, even though we have it. We can create a separate "historical" git archive of that later if we want to, and in the meantime it's about 3.2GB when imported into git - space that would just make the early git days unnecessarily complicated, when we don't have a lot of good infrastructure for it. Let it rip!
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+------------------------------------------------------------------------------
+ T H E /proc F I L E S Y S T E M
+------------------------------------------------------------------------------
+/proc/sys Terrehon Bowden <terrehon@pacbell.net> October 7 1999
+ Bodo Bauer <bb@ricochet.net>
+
+2.4.x update Jorge Nerin <comandante@zaralinux.com> November 14 2000
+------------------------------------------------------------------------------
+Version 1.3 Kernel version 2.2.12
+ Kernel version 2.4.0-test11-pre4
+------------------------------------------------------------------------------
+
+Table of Contents
+-----------------
+
+ 0 Preface
+ 0.1 Introduction/Credits
+ 0.2 Legal Stuff
+
+ 1 Collecting System Information
+ 1.1 Process-Specific Subdirectories
+ 1.2 Kernel data
+ 1.3 IDE devices in /proc/ide
+ 1.4 Networking info in /proc/net
+ 1.5 SCSI info
+ 1.6 Parallel port info in /proc/parport
+ 1.7 TTY info in /proc/tty
+ 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
+
+------------------------------------------------------------------------------
+Preface
+------------------------------------------------------------------------------
+
+0.1 Introduction/Credits
+------------------------
+
+This documentation is part of a soon (or so we hope) to be released book on
+the SuSE Linux distribution. As there is no complete documentation for the
+/proc file system and we've used many freely available sources to write these
+chapters, it seems only fair to give the work back to the Linux community.
+This work is based on the 2.2.* kernel version and the upcoming 2.4.*. I'm
+afraid it's still far from complete, but we hope it will be useful. As far as
+we know, it is the first 'all-in-one' document about the /proc file system. It
+is focused on the Intel x86 hardware, so if you are looking for PPC, ARM,
+SPARC, 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 bb@ricochet.net. We'll be happy to add them to this
+document.
+
+The latest version of this document is available online at
+http://skaro.nightcrawler.com/~bb/Docs/Proc as HTML version.
+
+If the above direction does not works for you, ypu could try the kernel
+mailing list at linux-kernel@vger.kernel.org and/or try to reach me at
+comandante@zaralinux.com.
+
+0.2 Legal Stuff
+---------------
+
+We don't guarantee the correctness of this document, and if you come to us
+complaining about how you screwed up your system because of incorrect
+documentation, we won't feel responsible...
+
+------------------------------------------------------------------------------
+CHAPTER 1: COLLECTING SYSTEM INFORMATION
+------------------------------------------------------------------------------
+
+------------------------------------------------------------------------------
+In This Chapter
+------------------------------------------------------------------------------
+* Investigating the properties of the pseudo file system /proc and its
+ ability to provide information on the running Linux system
+* Examining /proc's structure
+* Uncovering various information about the kernel and the processes running
+ on the system
+------------------------------------------------------------------------------
+
+
+The proc file system acts as an interface to internal data structures in the
+kernel. It can be used to obtain information about the system and to change
+certain kernel parameters at runtime (sysctl).
+
+First, we'll take a look at the read-only parts of /proc. In Chapter 2, we
+show you how you can use /proc/sys to change settings.
+
+1.1 Process-Specific Subdirectories
+-----------------------------------
+
+The directory /proc contains (among other things) one subdirectory for each
+process running on the system, which is named after the process ID (PID).
+
+The link self points to the process reading the file system. Each process
+subdirectory has the entries listed in Table 1-1.
+
+
+Table 1-1: Process specific entries in /proc
+..............................................................................
+ File Content
+ cmdline Command line arguments
+ cpu Current and last cpu in wich it was executed (2.4)(smp)
+ cwd Link to the current working directory
+ environ Values of environment variables
+ exe Link to the executable of this process
+ fd Directory, which contains all file descriptors
+ maps Memory maps to executables and library files (2.4)
+ mem Memory held by this process
+ root Link to the root directory of this process
+ stat Process status
+ statm Process memory status information
+ status Process status in human readable form
+ wchan If CONFIG_KALLSYMS is set, a pre-decoded wchan
+..............................................................................
+
+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
+ CPU0
+ 0: 8728810 XT-PIC timer
+ 1: 895 XT-PIC keyboard
+ 2: 0 XT-PIC cascade
+ 3: 531695 XT-PIC aha152x
+ 4: 2014133 XT-PIC serial
+ 5: 44401 XT-PIC pcnet_cs
+ 8: 2 XT-PIC rtc
+ 11: 8 XT-PIC i82365
+ 12: 182918 XT-PIC PS/2 Mouse
+ 13: 1 XT-PIC fpu
+ 14: 1232265 XT-PIC ide0
+ 15: 7 XT-PIC ide1
+ NMI: 0
+
+In 2.4.* a couple of lines where added to this file LOC & ERR (this time is the
+output of a SMP machine):
+
+ > cat /proc/interrupts
+
+ CPU0 CPU1
+ 0: 1243498 1214548 IO-APIC-edge timer
+ 1: 8949 8958 IO-APIC-edge keyboard
+ 2: 0 0 XT-PIC cascade
+ 5: 11286 10161 IO-APIC-edge soundblaster
+ 8: 1 0 IO-APIC-edge rtc
+ 9: 27422 27407 IO-APIC-edge 3c503
+ 12: 113645 113873 IO-APIC-edge PS/2 Mouse
+ 13: 0 0 XT-PIC fpu
+ 14: 22491 24012 IO-APIC-edge ide0
+ 15: 2183 2415 IO-APIC-edge ide1
+ 17: 30564 30414 IO-APIC-level eth0
+ 18: 177 164 IO-APIC-level bttv
+ NMI: 2457961 2457959
+ LOC: 2457882 2457881
+ ERR: 2155
+
+NMI is incremented in this case because every timer interrupt generates a NMI
+(Non Maskable Interrupt) which is used by the NMI Watchdog to detect 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/
+ smp_affinity
+
+The contents of the prof_cpu_mask file and each smp_affinity file for each IRQ
+is the same by default:
+
+ > cat /proc/irq/0/smp_affinity
+ ffffffff
+
+It's a bitmask, in wich you can specify wich CPUs can handle the IRQ, you can
+set it by doing:
+
+ > echo 1 > /proc/irq/prof_cpu_mask
+
+This means that only the first CPU will handle the IRQ, but you can also echo 5
+wich means that only the first and fourth CPU can handle the IRQ.
+
+The way IRQs are routed is handled by the IO-APIC, and it's Round Robin
+between all the CPUs which are allowed to handle it. As usual the kernel has
+more info than you and does a better job than you, so the defaults are the
+best choice for almost everyone.
+
+There are three more important subdirectories in /proc: net, scsi, and sys.
+The general rule is that the contents, or even the existence of these
+directories, depend on your kernel configuration. If SCSI is not enabled, the
+directory scsi may not exist. The same is true with the net, which is there
+only when networking support is present in the running kernel.
+
+The slabinfo file gives information about memory usage at the slab level.
+Linux uses slab pools for memory management above page level in version 2.2.
+Commonly used objects have their own slab pool (such as network buffers,
+directory cache, and so on).
+
+..............................................................................
+
+> 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...
+
+..............................................................................
+
+meminfo:
+
+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
+ HighTotal:
+ 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.
+ LowTotal:
+ 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
+ 'vm.overcommit_memory').
+ 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
+ tables.
+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
+directories.
+
+
+Table 1-5: IDE device information
+..............................................................................
+ File Content
+ cache The cache
+ capacity Capacity of the medium (in 512Byte blocks)
+ driver driver and version
+ geometry physical and logical geometry
+ identify device identify block
+ media media type
+ model device identifier
+ settings device setup
+ smart_thresholds IDE disk management thresholds
+ smart_values IDE disk management values
+..............................................................................
+
+The most interesting file is settings. This file contains a nice overview of
+the drive parameters:
+
+ # cat /proc/ide/ide0/hda/settings
+ name value min max mode
+ ---- ----- --- --- ----
+ bios_cyl 526 0 65535 rw
+ bios_head 255 0 255 rw
+ bios_sect 63 0 63 rw
+ breada_readahead 4 0 127 rw
+ bswap 0 0 1 r
+ file_readahead 72 0 2097151 rw
+ io_32bit 0 0 3 rw
+ keepsettings 0 0 1 rw
+ max_kb_per_request 122 1 127 rw
+ multcount 0 0 8 rw
+ nice1 1 0 1 rw
+ nowerr 0 0 1 rw
+ pio_mode write-only 0 255 w
+ slow 0 0 1 rw
+ unmaskirq 0 0 1 rw
+ using_dma 0 0 1 rw
+
+
+1.4 Networking info in /proc/net
+--------------------------------
+
+The subdirectory /proc/net follows the usual pattern. Table 1-6 shows the
+additional values you get for IP version 6 if you configure the kernel to
+support this. Table 1-7 lists the files and their meaning.
+
+
+Table 1-6: IPv6 info in /proc/net
+..............................................................................
+ File Content
+ udp6 UDP sockets (IPv6)
+ tcp6 TCP sockets (IPv6)
+ raw6 Raw device statistics (IPv6)
+ igmp6 IP multicast addresses, which this host joined (IPv6)
+ if_inet6 List of IPv6 interface addresses
+ ipv6_route Kernel routing table for IPv6
+ rt6_stats Global IPv6 routing tables statistics
+ sockstat6 Socket statistics (IPv6)
+ snmp6 Snmp data (IPv6)
+..............................................................................
+
+
+Table 1-7: Network info in /proc/net
+..............................................................................
+ File Content
+ arp Kernel ARP table
+ dev network devices with statistics
+ dev_mcast the Layer2 multicast groups a device is listening too
+ (interface index, label, number of references, number of bound
+ addresses).
+ dev_stat network device status
+ ip_fwchains Firewall chain linkage
+ ip_fwnames Firewall chain names
+ ip_masq Directory containing the masquerading tables
+ ip_masquerade Major masquerading table
+ netstat Network statistics
+ raw raw device statistics
+ route Kernel routing table
+ rpc Directory containing rpc info
+ rt_cache Routing cache
+ snmp SNMP data
+ sockstat Socket statistics
+ tcp TCP sockets
+ tr_rif Token ring RIF routing table
+ udp UDP sockets
+ unix UNIX domain sockets
+ wireless Wireless interface data (Wavelan etc)
+ igmp IP multicast addresses, which this host joined
+ psched Global packet scheduler parameters.
+ netlink List of PF_NETLINK sockets
+ ip_mr_vifs List of multicast virtual interfaces
+ ip_mr_cache List of multicast routing cache
+..............................................................................
+
+You can use this information to see which network devices are available in
+your system and how much traffic was routed over those devices:
+
+ > cat /proc/net/dev
+ Inter-|Receive |[...
+ face |bytes packets errs drop fifo frame compressed multicast|[...
+ lo: 908188 5596 0 0 0 0 0 0 [...
+ ppp0:15475140 20721 410 0 0 410 0 0 [...
+ eth0: 614530 7085 0 0 0 0 0 1 [...
+
+ ...] Transmit
+ ...] bytes packets errs drop fifo colls carrier compressed
+ ...] 908188 5596 0 0 0 0 0 0
+ ...] 1375103 17405 0 0 0 0 0 0
+ ...] 1703981 5535 0 0 0 3 0 0
+
+In addition, each Channel Bond interface has it's own directory. For
+example, the bond0 device will have a directory called /proc/net/bond0/.
+It will contain information that is specific to that bond, such as the
+current slaves of the bond, the link status of the slaves, and how
+many times the slaves link has failed.
+
+1.5 SCSI info
+-------------
+
+If you have a SCSI host adapter in your system, you'll find a subdirectory
+named after the driver for this adapter in /proc/scsi. You'll also see a list
+of all recognized SCSI devices in /proc/scsi:
+
+ >cat /proc/scsi/scsi
+ Attached devices:
+ Host: scsi0 Channel: 00 Id: 00 Lun: 00
+ Vendor: IBM Model: DGHS09U Rev: 03E0
+ Type: Direct-Access ANSI SCSI revision: 03
+ Host: scsi0 Channel: 00 Id: 06 Lun: 00
+ Vendor: PIONEER Model: CD-ROM DR-U06S Rev: 1.04
+ Type: CD-ROM ANSI SCSI revision: 02
+
+
+The directory named after the driver has one file for each adapter found in
+the system. These files contain information about the controller, including
+the used IRQ and the IO address range. The amount of information shown is
+dependent on the adapter you use. The example shows the output for an Adaptec
+AHA-2940 SCSI adapter:
+
+ > cat /proc/scsi/aic7xxx/0
+
+ Adaptec AIC7xxx driver version: 5.1.19/3.2.4
+ Compile Options:
+ TCQ Enabled By Default : Disabled
+ AIC7XXX_PROC_STATS : Disabled
+ AIC7XXX_RESET_DELAY : 5
+ Adapter Configuration:
+ SCSI Adapter: Adaptec AHA-294X Ultra SCSI host adapter
+ Ultra Wide Controller
+ PCI MMAPed I/O Base: 0xeb001000
+ Adapter SEEPROM Config: SEEPROM found and used.
+ Adaptec SCSI BIOS: Enabled
+ IRQ: 10
+ SCBs: Active 0, Max Active 2,
+ Allocated 15, HW 16, Page 255
+ Interrupts: 160328
+ BIOS Control Word: 0x18b6
+ Adapter Control Word: 0x005b
+ Extended Translation: Enabled
+ Disconnect Enable Flags: 0xffff
+ Ultra Enable Flags: 0x0001
+ Tag Queue Enable Flags: 0x0000
+ Ordered Queue Tag Flags: 0x0000
+ Default Tag Queue Depth: 8
+ Tagged Queue By Device array for aic7xxx host instance 0:
+ {255,255,255,255,255,255,255,255,255,255,255,255,255,255,255,255}
+ Actual queue depth per device for aic7xxx host instance 0:
+ {1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1}
+ Statistics:
+ (scsi0:0:0:0)
+ Device using Wide/Sync transfers at 40.0 MByte/sec, offset 8
+ Transinfo settings: current(12/8/1/0), goal(12/8/1/0), user(12/15/1/0)
+ Total transfers 160151 (74577 reads and 85574 writes)
+ (scsi0:0:6:0)
+ Device using Narrow/Sync transfers at 5.0 MByte/sec, offset 15
+ Transinfo settings: current(50/15/0/0), goal(50/15/0/0), user(50/15/0/0)
+ Total transfers 0 (0 reads and 0 writes)
+
+
+1.6 Parallel port info in /proc/parport
+---------------------------------------
+
+The directory /proc/parport contains information about the parallel ports of
+your system. It has one subdirectory for each port, named after the port
+number (0,1,2,...).
+
+These directories contain the four files shown in Table 1-8.
+
+
+Table 1-8: Files in /proc/parport
+..............................................................................
+ File Content
+ autoprobe Any IEEE-1284 device ID information that has been acquired.
+ devices list of the device drivers using that port. A + will appear by the
+ name of the device currently using the port (it might not appear
+ against any).
+ hardware Parallel port's base address, IRQ line and DMA channel.
+ irq IRQ that parport is using for that port. This is in a separate
+ file to allow you to alter it by writing a new value in (IRQ
+ number or none).
+..............................................................................
+
+1.7 TTY info in /proc/tty
+-------------------------
+
+Information about the available and actually used tty's can be found in the
+directory /proc/tty.You'll find entries for drivers and line disciplines in
+this directory, as shown in Table 1-9.
+
+
+Table 1-9: Files in /proc/tty
+..............................................................................
+ File Content
+ drivers list of drivers and their usage
+ ldiscs registered line disciplines
+ driver/serial usage statistic and status of single tty lines
+..............................................................................
+
+To see which tty's are currently in use, you can simply look into the file
+/proc/tty/drivers:
+
+ > cat /proc/tty/drivers
+ pty_slave /dev/pts 136 0-255 pty:slave
+ pty_master /dev/ptm 128 0-255 pty:master
+ pty_slave /dev/ttyp 3 0-255 pty:slave
+ pty_master /dev/pty 2 0-255 pty:master
+ serial /dev/cua 5 64-67 serial:callout
+ serial /dev/ttyS 4 64-67 serial
+ /dev/tty0 /dev/tty0 4 0 system:vtmaster
+ /dev/ptmx /dev/ptmx 5 2 system
+ /dev/console /dev/console 5 1 system:console
+ /dev/tty /dev/tty 5 0 system:/dev/tty
+ unknown /dev/tty 4 1-63 console
+
+
+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
+interrupt.
+
+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
+CPUs.
+
+The "procs_blocked" line gives the number of processes currently blocked,
+waiting for I/O to complete.
+
+
+------------------------------------------------------------------------------
+Summary
+------------------------------------------------------------------------------
+The /proc file system serves information about the running system. It not only
+allows access to process data but also allows you to request the kernel status
+by reading files in the hierarchy.
+
+The directory structure of /proc reflects the types of information and makes
+it easy, if not obvious, where to look for specific data.
+------------------------------------------------------------------------------
+
+------------------------------------------------------------------------------
+CHAPTER 2: MODIFYING SYSTEM PARAMETERS
+------------------------------------------------------------------------------
+
+------------------------------------------------------------------------------
+In This Chapter
+------------------------------------------------------------------------------
+* Modifying kernel parameters by writing into files found in /proc/sys
+* Exploring the files which modify certain parameters
+* Review of the /proc/sys file tree
+------------------------------------------------------------------------------
+
+
+A very interesting part of /proc is the directory /proc/sys. This is not only
+a source of information, it also allows you to change parameters within the
+kernel. Be very careful when attempting this. You can optimize your system,
+but you can also cause it to crash. Never alter kernel parameters on a
+production system. Set up a development machine and test to make sure that
+everything works the way you want it to. You may have no alternative but to
+reboot the machine once an error has been made.
+
+To change a value, simply echo the new value into the file. An example is
+given below in the section on the file system data. You need to be root to do
+this. You can create your own boot script to perform this every time your
+system boots.
+
+The files in /proc/sys can be used to fine tune and monitor miscellaneous and
+general things in the operation of the Linux kernel. Since some of the files
+can inadvertently disrupt your system, it is advisable to read both
+documentation and source before actually making adjustments. In any case, be
+very careful when writing to any of these files. The entries in /proc may
+change slightly between the 2.1.* and the 2.2 kernel, so if there is any doubt
+review the kernel documentation in the directory /usr/src/linux/Documentation.
+This chapter is heavily based on the documentation included in the pre 2.2
+kernels, and became part of it in version 2.2.1 of the Linux kernel.
+
+2.1 /proc/sys/fs - File system data
+-----------------------------------
+
+This subdirectory contains specific file system, file handle, inode, dentry
+and quota information.
+
+Currently, these files are in /proc/sys/fs:
+
+dentry-state
+------------
+
+Status of the directory cache. Since directory entries are dynamically
+allocated and deallocated, this file indicates the current status. It holds
+six values, in which the last two are not used and are always zero. The others
+are listed in table 2-1.
+
+
+Table 2-1: Status files of the directory cache
+..............................................................................
+ File Content
+ nr_dentry Almost always zero
+ nr_unused Number of unused cache entries
+ age_limit
+ in seconds after the entry may be reclaimed, when memory is short
+ want_pages internally
+..............................................................................
+
+dquot-nr and dquot-max
+----------------------
+
+The file dquot-max shows the maximum number of cached disk quota entries.
+
+The file dquot-nr shows the number of allocated disk quota entries and the
+number of free disk quota entries.
+
+If the number of available cached disk quotas is very low and you have a large
+number of simultaneous system users, you might want to raise the limit.
+
+file-nr and file-max
+--------------------
+
+The kernel allocates file handles dynamically, but doesn't free them again at
+this time.
+
+The value in file-max denotes the maximum number of file handles that the
+Linux kernel will allocate. When you get a lot of error messages about running
+out of file handles, you might want to raise this limit. The default value is
+10% of RAM in kilobytes. To change it, just write the new number into the
+file:
+
+ # cat /proc/sys/fs/file-max
+ 4096
+ # echo 8192 > /proc/sys/fs/file-max
+ # cat /proc/sys/fs/file-max
+ 8192
+
+
+This method of revision is useful for all customizable parameters of the
+kernel - simply echo the new value to the corresponding file.
+
+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).
+
+nr_inodes
+~~~~~~~~~
+
+Denotes the number of inodes the system has allocated. This number will
+grow and shrink dynamically.
+
+nr_free_inodes
+--------------
+
+Represents the number of free inodes. Ie. The number of inuse inodes is
+(nr_inodes - nr_free_inodes).
+
+super-nr and super-max
+----------------------
+
+Again, super block structures are allocated by the kernel, but not freed. The
+file super-max contains the maximum number of super block handlers, where
+super-nr shows the number of currently allocated ones.
+
+Every mounted file system needs a super block, so if you plan to mount lots of
+file systems, you may want to increase these numbers.
+
+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
+binary.
+
+It works by maintaining a linked list of structs that contain a description of
+a binary format, including a magic with size (or the filename extension),
+offset and mask, and the interpreter name. On request it invokes the given
+interpreter with the original program as argument, as binfmt_java and
+binfmt_em86 and binfmt_mz do. Since binfmt_misc does not define any default
+binary-formats, you have to register an additional binary-format.
+
+There are two general files in binfmt_misc and one file per registered format.
+The two general files are register and status.
+
+Registering a new binary format
+-------------------------------
+
+To register a new binary format you have to issue the command
+
+ echo :name:type:offset:magic:mask:interpreter: > /proc/sys/fs/binfmt_misc/register
+
+
+
+with appropriate name (the name for the /proc-dir entry), offset (defaults to
+0, if omitted), magic, mask (which can be omitted, defaults to all 0xff) and
+last but not least, the interpreter that is to be invoked (for example and
+testing /bin/echo). Type can be M for usual magic matching or E for filename
+extension matching (give extension in place of magic).
+
+Check or reset the status of the binary format handler
+------------------------------------------------------
+
+If you do a cat on the file /proc/sys/fs/binfmt_misc/status, you will get the
+current status (enabled/disabled) of binfmt_misc. Change the status by echoing
+0 (disables) or 1 (enables) or -1 (caution: this clears all previously
+registered binary formats) to status. For example echo 0 > status to disable
+binfmt_misc (temporarily).
+
+Status of a single handler
+--------------------------
+
+Each registered handler has an entry in /proc/sys/fs/binfmt_misc. These files
+perform the same function as status, but their scope is limited to the actual
+binary format. By cating this file, you also receive all related information
+about the interpreter/magic of the binfmt.
+
+Example usage of binfmt_misc (emulate binfmt_java)
+--------------------------------------------------
+
+ cd /proc/sys/fs/binfmt_misc
+ echo ':Java:M::\xca\xfe\xba\xbe::/usr/local/java/bin/javawrapper:' > register
+ echo ':HTML:E::html::/usr/local/java/bin/appletviewer:' > register
+ echo ':Applet:M::<!--applet::/usr/local/java/bin/appletviewer:' > register
+ echo ':DEXE:M::\x0eDEX::/usr/bin/dosexec:' > register
+
+
+These four lines add support for Java executables and Java applets (like
+binfmt_java, additionally recognizing the .html extension with no need to put
+<!--applet> to every applet file). You have to install the JDK and the
+shell-script /usr/local/java/bin/javawrapper too. It works around the
+brokenness of the Java filename handling. To add a Java binary, just create a
+link to the class-file somewhere in the path.
+
+2.3 /proc/sys/kernel - general kernel parameters
+------------------------------------------------
+
+This directory reflects general kernel behaviors. As I've said before, the
+contents depend on your configuration. Here you'll find the most important
+files, along with descriptions of what they mean and how to use them.
+
+acct
+----
+
+The file contains three values; highwater, lowwater, and frequency.
+
+It exists only when BSD-style process accounting is enabled. These values
+control its behavior. If the free space on the file system where the log lives
+goes below lowwater percentage, accounting suspends. If it goes above
+highwater percentage, accounting resumes. Frequency determines how often you
+check the amount of free space (value is in seconds). Default settings are: 4,
+2, and 30. That is, suspend accounting if there is less than 2 percent free;
+resume it if we have a value of 3 or more percent; consider information about
+the amount of free space valid for 30 seconds
+
+ctrl-alt-del
+------------
+
+When the value in this file is 0, ctrl-alt-del is trapped and sent to the init
+program to handle a graceful restart. However, when the value is greater that
+zero, Linux's reaction to this key combination will be an immediate reboot,
+without syncing its dirty buffers.
+
+[NOTE]
+ When a program (like dosemu) has the keyboard in raw mode, the
+ ctrl-alt-del is intercepted by the program before it ever reaches the
+ kernel tty layer, and it is up to the program to decide what to do with
+ it.
+
+domainname and hostname
+-----------------------
+
+These files can be controlled to set the NIS domainname and hostname of your
+box. For the classic darkstar.frop.org a simple:
+
+ # echo "darkstar" > /proc/sys/kernel/hostname
+ # echo "frop.org" > /proc/sys/kernel/domainname
+
+
+would suffice to set your hostname and NIS domainname.
+
+osrelease, ostype and version
+-----------------------------
+
+The names make it pretty obvious what these fields contain:
+
+ > cat /proc/sys/kernel/osrelease
+ 2.2.12
+
+ > cat /proc/sys/kernel/ostype
+ Linux
+
+ > cat /proc/sys/kernel/version
+ #4 Fri Oct 1 12:41:14 PDT 1999
+
+
+The files osrelease and ostype should be clear enough. Version needs a little
+more clarification. The #4 means that this is the 4th kernel built from this
+source base and the date after it indicates the time the kernel was built. The
+only way to tune these values is to rebuild the kernel.
+
+panic
+-----
+
+The value in this file represents the number of seconds the kernel waits
+before rebooting on a panic. When you use the software watchdog, the
+recommended setting is 60. If set to 0, the auto reboot after a kernel panic
+is disabled, which is the default setting.
+
+printk
+------
+
+The four values in printk denote
+* console_loglevel,
+* default_message_loglevel,
+* minimum_console_loglevel and
+* default_console_loglevel
+respectively.
+
+These values influence printk() behavior when printing or logging error
+messages, which come from inside the kernel. See syslog(2) for more
+information on the different log levels.
+
+console_loglevel
+----------------
+
+Messages with a higher priority than this will be printed to the console.
+
+default_message_level
+---------------------
+
+Messages without an explicit priority will be printed with this priority.
+
+minimum_console_loglevel
+------------------------
+
+Minimum (highest) value to which the console_loglevel can be set.
+
+default_console_loglevel
+------------------------
+
+Default value for console_loglevel.
+
+sg-big-buff
+-----------
+
+This file shows the size of the generic SCSI (sg) buffer. At this point, you
+can't tune it yet, but you can change it at compile time by editing
+include/scsi/sg.h and changing the value of SG_BIG_BUFF.
+
+If you use a scanner with SANE (Scanner Access Now Easy) you might want to set
+this to a higher value. Refer to the SANE documentation on this issue.
+
+modprobe
+--------
+
+The location where the modprobe binary is located. The kernel uses this
+program to load modules on demand.
+
+unknown_nmi_panic
+-----------------
+
+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.
+
+[NOTE]
+ 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
+ non-zero.
+
+
+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.
+
+vfs_cache_pressure
+------------------
+
+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.
+
+dirty_background_ratio
+----------------------
+
+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.
+
+dirty_ratio
+-----------------
+
+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
+data.
+
+dirty_writeback_centisecs
+-------------------------
+
+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.
+
+dirty_expire_centisecs
+----------------------
+
+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.
+
+legacy_va_layout
+----------------
+
+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.
+
+lower_zone_protection
+---------------------
+
+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
+------------
+
+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
+swap-intensive.
+
+overcommit_memory
+-----------------
+
+This file contains one value. The following algorithm is used to decide if
+there's enough memory: if the value of overcommit_memory is positive, then
+there's always enough memory. This is a useful feature, since programs often
+malloc() huge amounts of memory 'just in case', while they only use a small
+part of it. Leaving this value at 0 will lead to the failure of such a huge
+malloc(), when in fact the system has enough memory for the program to run.
+
+On the other hand, enabling this feature can cause you to run out of memory
+and thrash the system to death, so large and/or important servers will want to
+set this value to 0.
+
+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
+-----------
+
+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
+----------
+
+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.
+
+swap_token_timeout
+------------------
+
+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.
+
+2.5 /proc/sys/dev - Device specific parameters
+----------------------------------------------
+
+Currently there is only support for CDROM drives, and for those, there is only
+one read-only file containing information about the CD-ROM drives attached to
+the system:
+
+ >cat /proc/sys/dev/cdrom/info
+ CD-ROM information, Id: cdrom.c 2.55 1999/04/25
+
+ drive name: sr0 hdb
+ drive speed: 32 40
+ drive # of slots: 1 0
+ Can close tray: 1 1
+ Can open tray: 1 1
+ Can lock tray: 1 1
+ Can change speed: 1 1
+ Can select disk: 0 1
+ Can read multisession: 1 1
+ Can read MCN: 1 1
+ Reports media changed: 1 1
+ Can play audio: 1 1
+
+
+You see two drives, sr0 and hdb, along with a list of their features.
+
+2.6 /proc/sys/sunrpc - Remote procedure calls
+---------------------------------------------
+
+This directory contains four files, which enable or disable debugging for the
+RPC functions NFS, NFS-daemon, RPC and NLM. The default values are 0. They can
+be set to one to turn debugging on. (The default value is 0 for each)
+
+2.7 /proc/sys/net - Networking stuff
+------------------------------------
+
+The interface to the networking parts of the kernel is located in
+/proc/sys/net. Table 2-3 shows all possible subdirectories. You may see only
+some of them, depending on your kernel's configuration.
+
+
+Table 2-3: Subdirectories in /proc/sys/net
+..............................................................................
+ Directory Content Directory Content
+ core General parameter appletalk Appletalk protocol
+ unix Unix domain sockets netrom NET/ROM
+ 802 E802 protocol ax25 AX25
+ ethernet Ethernet protocol rose X.25 PLP layer
+ ipv4 IP version 4 x25 X.25 protocol
+ ipx IPX token-ring IBM token ring
+ bridge Bridging decnet DEC net
+ ipv6 IP version 6
+..............................................................................
+
+We will concentrate on IP networking here. Since AX15, X.25, and DEC Net are
+only minor players in the Linux world, we'll skip them in this chapter. You'll
+find some short info on Appletalk and IPX further on in this chapter. Review
+the online documentation and the kernel source to get a detailed view of the
+parameters for those protocols. In this section we'll discuss the
+subdirectories printed in bold letters in the table above. As default values
+are suitable for most needs, there is no need to change these values.
+
+/proc/sys/net/core - Network core options
+-----------------------------------------
+
+rmem_default
+------------
+
+The default setting of the socket receive buffer in bytes.
+
+rmem_max
+--------
+
+The maximum receive socket buffer size in bytes.
+
+wmem_default
+------------
+
+The default setting (in bytes) of the socket send buffer.
+
+wmem_max
+--------
+
+The maximum send socket buffer size in bytes.
+
+message_burst and message_cost
+------------------------------
+
+These parameters are used to limit the warning messages written to the kernel
+log from the networking code. They enforce a rate limit to make a
+denial-of-service attack impossible. A higher message_cost factor, results in
+fewer messages that will be written. Message_burst controls when messages will
+be dropped. The default settings limit warning messages to one every five
+seconds.
+
+netdev_max_backlog
+------------------
+
+Maximum number of packets, queued on the INPUT side, when the interface
+receives packets faster than kernel can process them.
+
+optmem_max
+----------
+
+Maximum ancillary buffer size allowed per socket. Ancillary data is a sequence
+of struct cmsghdr structures with appended data.
+
+/proc/sys/net/unix - Parameters for Unix domain sockets
+-------------------------------------------------------
+
+There are only two files in this subdirectory. They control the delays for
+deleting and destroying socket descriptors.
+
+2.8 /proc/sys/net/ipv4 - IPV4 settings
+--------------------------------------
+
+IP version 4 is still the most used protocol in Unix networking. It will be
+replaced by IP version 6 in the next couple of years, but for the moment it's
+the de facto standard for the internet and is used in most networking
+environments around the world. Because of the importance of this protocol,
+we'll have a deeper look into the subtree controlling the behavior of the IPv4
+subsystem of the Linux kernel.
+
+Let's start with the entries in /proc/sys/net/ipv4.
+
+ICMP settings
+-------------
+
+icmp_echo_ignore_all and icmp_echo_ignore_broadcasts
+----------------------------------------------------
+
+Turn on (1) or off (0), if the kernel should ignore all ICMP ECHO requests, or
+just those to broadcast and multicast addresses.
+
+Please note that if you accept ICMP echo requests with a broadcast/multi\-cast
+destination address your network may be used as an exploder for denial of
+service packet flooding attacks to other hosts.
+
+icmp_destunreach_rate, icmp_echoreply_rate, icmp_paramprob_rate and icmp_timeexeed_rate
+---------------------------------------------------------------------------------------
+
+Sets limits for sending ICMP packets to specific targets. A value of zero
+disables all limiting. Any positive value sets the maximum package rate in
+hundredth of a second (on Intel systems).
+
+IP settings
+-----------
+
+ip_autoconfig
+-------------
+
+This file contains the number one if the host received its IP configuration by
+RARP, BOOTP, DHCP or a similar mechanism. Otherwise it is zero.
+
+ip_default_ttl
+--------------
+
+TTL (Time To Live) for IPv4 interfaces. This is simply the maximum number of
+hops a packet may travel.
+
+ip_dynaddr
+----------
+
+Enable dynamic socket address rewriting on interface address change. This is
+useful for dialup interface with changing IP addresses.
+
+ip_forward
+----------
+
+Enable or disable forwarding of IP packages between interfaces. Changing this
+value resets all other parameters to their default values. They differ if the
+kernel is configured as host or router.
+
+ip_local_port_range
+-------------------
+
+Range of ports used by TCP and UDP to choose the local port. Contains two
+numbers, the first number is the lowest port, the second number the highest
+local port. Default is 1024-4999. Should be changed to 32768-61000 for
+high-usage systems.
+
+ip_no_pmtu_disc
+---------------
+
+Global switch to turn path MTU discovery off. It can also be set on a per
+socket basis by the applications or on a per route basis.
+
+ip_masq_debug
+-------------
+
+Enable/disable debugging of IP masquerading.
+
+IP fragmentation settings
+-------------------------
+
+ipfrag_high_trash and ipfrag_low_trash
+--------------------------------------
+
+Maximum memory used to reassemble IP fragments. When ipfrag_high_thresh bytes
+of memory is allocated for this purpose, the fragment handler will toss
+packets until ipfrag_low_thresh is reached.
+
+ipfrag_time
+-----------
+
+Time in seconds to keep an IP fragment in memory.
+
+TCP settings
+------------
+
+tcp_ecn
+-------
+
+This file controls the use of the ECN bit in the IPv4 headers, this is a new
+feature about Explicit Congestion Notification, but some routers and firewalls
+block trafic that has this bit set, so it could be necessary to echo 0 to
+/proc/sys/net/ipv4/tcp_ecn, if you want to talk to this sites. For more info
+you could read RFC2481.
+
+tcp_retrans_collapse
+--------------------
+
+Bug-to-bug compatibility with some broken printers. On retransmit, try to send
+larger packets to work around bugs in certain TCP stacks. Can be turned off by
+setting it to zero.
+
+tcp_keepalive_probes
+--------------------
+
+Number of keep alive probes TCP sends out, until it decides that the
+connection is broken.
+
+tcp_keepalive_time
+------------------
+
+How often TCP sends out keep alive messages, when keep alive is enabled. The
+default is 2 hours.
+
+tcp_syn_retries
+---------------
+
+Number of times initial SYNs for a TCP connection attempt will be
+retransmitted. Should not be higher than 255. This is only the timeout for
+outgoing connections, for incoming connections the number of retransmits is
+defined by tcp_retries1.
+
+tcp_sack
+--------
+
+Enable select acknowledgments after RFC2018.
+
+tcp_timestamps
+--------------
+
+Enable timestamps as defined in RFC1323.
+
+tcp_stdurg
+----------
+
+Enable the strict RFC793 interpretation of the TCP urgent pointer field. The
+default is to use the BSD compatible interpretation of the urgent pointer
+pointing to the first byte after the urgent data. The RFC793 interpretation is
+to have it point to the last byte of urgent data. Enabling this option may
+lead to interoperatibility problems. Disabled by default.
+
+tcp_syncookies
+--------------
+
+Only valid when the kernel was compiled with CONFIG_SYNCOOKIES. Send out
+syncookies when the syn backlog queue of a socket overflows. This is to ward
+off the common 'syn flood attack'. Disabled by default.
+
+Note that the concept of a socket backlog is abandoned. This means the peer
+may not receive reliable error messages from an over loaded server with
+syncookies enabled.
+
+tcp_window_scaling
+------------------
+
+Enable window scaling as defined in RFC1323.
+
+tcp_fin_timeout
+---------------
+
+The length of time in seconds it takes to receive a final FIN before the
+socket is always closed. This is strictly a violation of the TCP
+specification, but required to prevent denial-of-service attacks.
+
+tcp_max_ka_probes
+-----------------
+
+Indicates how many keep alive probes are sent per slow timer run. Should not
+be set too high to prevent bursts.
+
+tcp_max_syn_backlog
+-------------------
+
+Length of the per socket backlog queue. Since Linux 2.2 the backlog specified
+in listen(2) only specifies the length of the backlog queue of already
+established sockets. When more connection requests arrive Linux starts to drop
+packets. When syncookies are enabled the packets are still answered and the
+maximum queue is effectively ignored.
+
+tcp_retries1
+------------
+
+Defines how often an answer to a TCP connection request is retransmitted
+before giving up.
+
+tcp_retries2
+------------
+
+Defines how often a TCP packet is retransmitted before giving up.
+
+Interface specific settings
+---------------------------
+
+In the directory /proc/sys/net/ipv4/conf you'll find one subdirectory for each
+interface the system knows about and one directory calls all. Changes in the
+all subdirectory affect all interfaces, whereas changes in the other
+subdirectories affect only one interface. All directories have the same
+entries:
+
+accept_redirects
+----------------
+
+This switch decides if the kernel accepts ICMP redirect messages or not. The
+default is 'yes' if the kernel is configured for a regular host and 'no' for a
+router configuration.
+
+accept_source_route
+-------------------
+
+Should source routed packages be accepted or declined. The default is
+dependent on the kernel configuration. It's 'yes' for routers and 'no' for
+hosts.
+
+bootp_relay
+~~~~~~~~~~~
+
+Accept packets with source address 0.b.c.d with destinations not to this host
+as local ones. It is supposed that a BOOTP relay daemon will catch and forward
+such packets.
+
+The default is 0, since this feature is not implemented yet (kernel version
+2.2.12).
+
+forwarding
+----------
+
+Enable or disable IP forwarding on this interface.
+
+log_martians
+------------
+
+Log packets with source addresses with no known route to kernel log.
+
+mc_forwarding
+-------------
+
+Do multicast routing. The kernel needs to be compiled with CONFIG_MROUTE and a
+multicast routing daemon is required.
+
+proxy_arp
+---------
+
+Does (1) or does not (0) perform proxy ARP.
+
+rp_filter
+---------
+
+Integer value determines if a source validation should be made. 1 means yes, 0
+means no. Disabled by default, but local/broadcast address spoofing is always
+on.
+
+If you set this to 1 on a router that is the only connection for a network to
+the net, it will prevent spoofing attacks against your internal networks
+(external addresses can still be spoofed), without the need for additional
+firewall rules.
+
+secure_redirects
+----------------
+
+Accept ICMP redirect messages only for gateways, listed in default gateway
+list. Enabled by default.
+
+shared_media
+------------
+
+If it is not set the kernel does not assume that different subnets on this
+device can communicate directly. Default setting is 'yes'.
+
+send_redirects
+--------------
+
+Determines whether to send ICMP redirects to other hosts.
+
+Routing settings
+----------------
+
+The directory /proc/sys/net/ipv4/route contains several file to control
+routing issues.
+
+error_burst and error_cost
+--------------------------
+
+These parameters are used to limit how many ICMP destination unreachable to
+send from the host in question. ICMP destination unreachable messages are
+sent when we can not reach the next hop, while trying to transmit a packet.
+It will also print some error messages to kernel logs if someone is ignoring
+our ICMP redirects. The higher the error_cost factor is, the fewer
+destination unreachable and error messages will be let through. Error_burst
+controls when destination unreachable messages and error messages will be
+dropped. The default settings limit warning messages to five every second.
+
+flush
+-----
+
+Writing to this file results in a flush of the routing cache.
+
+gc_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.
+
+
+max_size
+--------
+
+Maximum size of the routing cache. Old entries will be purged once the cache
+reached has this size.
+
+max_delay, min_delay
+--------------------
+
+Delays for flushing the routing cache.
+
+redirect_load, redirect_number
+------------------------------
+
+Factors which determine if more ICPM redirects should be sent to a specific
+host. No redirects will be sent once the load limit or the maximum number of
+redirects has been reached.
+
+redirect_silence
+----------------
+
+Timeout for redirects. After this period redirects will be sent again, even if
+this has been stopped, because the load or number limit has been reached.
+
+Network Neighbor handling
+-------------------------
+
+Settings about how to handle connections with direct neighbors (nodes attached
+to the same link) can be found in the directory /proc/sys/net/ipv4/neigh.
+
+As we saw it in the conf directory, there is a default subdirectory which
+holds the default values, and one directory for each interface. The contents
+of the directories are identical, with the single exception that the default
+settings contain additional options to set garbage collection parameters.
+
+In the interface directories you'll find the following entries:
+
+base_reachable_time, 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
+unreachable.
+
+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.
+
+unres_qlen
+----------
+
+Maximum queue length for a pending arp request - the number of packets which
+are accepted from other layers while the ARP address is still resolved.
+
+anycast_delay
+-------------
+
+Maximum for random delay of answers to neighbor solicitation messages in
+jiffies (1/100 sec). Not yet implemented (Linux does not have anycast support
+yet).
+
+ucast_solicit
+-------------
+
+Maximum number of retries for unicast solicitation.
+
+mcast_solicit
+-------------
+
+Maximum number of retries for multicast solicitation.
+
+delay_first_probe_time
+----------------------
+
+Delay for the first time probe if the neighbor is reachable. (see
+gc_stale_time)
+
+locktime
+--------
+
+An ARP/neighbor entry is only replaced with a new one if the old is at least
+locktime old. This prevents ARP cache thrashing.
+
+proxy_delay
+-----------
+
+Maximum time (real time is random [0..proxytime]) before answering to an ARP
+request for which we have an proxy ARP entry. In some cases, this is used to
+prevent network flooding.
+
+proxy_qlen
+----------
+
+Maximum queue length of the delayed proxy arp timer. (see proxy_delay).
+
+app_solcit
+----------
+
+Determines the number of requests to send to the user level ARP daemon. Use 0
+to turn off.
+
+gc_stale_time
+-------------
+
+Determines how often to check for stale ARP entries. After an ARP entry is
+stale it will be resolved again (which is useful when an IP address migrates
+to another machine). When ucast_solicit is greater than 0 it first tries to
+send an ARP packet directly to the known host When that fails and
+mcast_solicit is greater than 0, an ARP request is broadcasted.
+
+2.9 Appletalk
+-------------
+
+The /proc/sys/net/appletalk directory holds the Appletalk configuration data
+when Appletalk is loaded. The configurable parameters are:
+
+aarp-expiry-time
+----------------
+
+The amount of time we keep an ARP entry before expiring it. Used to age out
+old hosts.
+
+aarp-resolve-time
+-----------------
+
+The amount of time we will spend trying to resolve an Appletalk address.
+
+aarp-retransmit-limit
+---------------------
+
+The number of times we will retransmit a query before giving up.
+
+aarp-tick-time
+--------------
+
+Controls the rate at which expires are checked.
+
+The directory /proc/net/appletalk holds the list of active Appletalk sockets
+on a machine.
+
+The fields indicate the DDP type, the local address (in network:node format)
+the remote address, the size of the transmit pending queue, the size of the
+received queue (bytes waiting for applications to read) the state and the uid
+owning the socket.
+
+/proc/net/atalk_iface lists all the interfaces configured for appletalk.It
+shows the name of the interface, its Appletalk address, the network range on
+that address (or network number for phase 1 networks), and the status of the
+interface.
+
+/proc/net/atalk_route lists each known network route. It lists the target
+(network) that the route leads to, the router (may be directly connected), the
+route flags, and the device the route is using.
+
+2.10 IPX
+--------
+
+The IPX protocol has no tunable values in proc/sys/net.
+
+The IPX protocol does, however, provide proc/net/ipx. This lists each IPX
+socket giving the local and remote addresses in Novell format (that is
+network:node:port). In accordance with the strange Novell tradition,
+everything but the port is in hex. Not_Connected is displayed for sockets that
+are not tied to a specific remote address. The Tx and Rx queue sizes indicate
+the number of bytes pending for transmission and reception. The state
+indicates the state the socket is in and the uid is the owning uid of the
+socket.
+
+The /proc/net/ipx_interface file lists all IPX interfaces. For each interface
+it gives the network number, the node number, and indicates if the network is
+the primary network. It also indicates which device it is bound to (or
+Internal for internal networks) and the Frame Type if appropriate. Linux
+supports 802.3, 802.2, 802.2 SNAP and DIX (Blue Book) ethernet framing for
+IPX.
+
+The /proc/net/ipx_route table holds a list of IPX routes. For each route it
+gives the destination network, the router node (or Directly) and the network
+address of the router (or Connected) for internal networks.
+
+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).
+
+
+------------------------------------------------------------------------------
+Summary
+------------------------------------------------------------------------------
+Certain aspects of kernel behavior can be modified at runtime, without the
+need to recompile the kernel, or even to reboot the system. The files in the
+/proc/sys tree can not only be read, but also modified. You can use the echo
+command to write value into these files, thereby changing the default settings
+of the kernel.
+------------------------------------------------------------------------------