Directories in proc


Directories in /proc/

Common groups of information concerning the kernel are grouped into directories and subdirectories within the /proc/ directory.

Process Directories

Every /proc/ directory contains a number of directories and their numerical names. A listing of them may start off like this:

dr-xr-xr-x    3 root        root         0 Feb 13 01:28 1 dr-xr-xr-x    3 root        root         0 Feb 13 01:28 1010 dr-xr-xr-x    3 xfs         xfs          0 Feb 13 01:28 1087 dr-xr-xr-x    3 daemon      daemon       0 Feb 13 01:28 1123 dr-xr-xr-x    3 root        root         0 Feb 13 01:28 11307 dr-xr-xr-x    3 apache      apache       0 Feb 13 01:28 13660 dr-xr-xr-x    3 rpc         rpc          0 Feb 13 01:28 637 dr-xr-xr-x    3 rpcuser     rpcuser      0 Feb 13 01:28 666

These directories are called process directories, as they are named after a program’s process ID and contain information specific to that process. The owner and group of each process directory is set to the user running the process. When the process is terminated, its /proc/ process directory vanishes. Each process directory contains the following files.

This file contains the command issued when starting the process.

Provides specific information about the utilization of each of the system’s CPUs. A process running on a dual-CPU system produces output similar to this:

cpu    11 3 cpu0    0 0 cpu1   11 3

A symlink to the current working directory for the process.

Gives a list of the environment variables for the process. The environment variable is given in all uppercase characters, and the value is in lowercase characters.

A symlink to the executable of this process.

A directory containing all of the file descriptors for a particular process. These are given in numbered links:

total 0 lrwx------   1 root  root  64 May   8 11:31 0 -> /dev/null lrwx------   1 root  root  64 May   8 11:31 1 -> /dev/null lrwx------   1 root  root  64 May   8 11:31 2 -> /dev/null lrwx------   1 root  root  64 May   8 11:31 3 -> /dev/ptmx lrwx------   1 root  root  64 May   8 11:31  4 -> socket:[7774817] lrwx------   1 root  root  64 May   8 11:31 5 -> /dev/ptmx lrwx------   1 root  root  64 May   8 11:31 6 -> socket:[7774829] lrwx------   1 root  root  64 May   8 11:31 7 -> /dev/ptmx

Contains memory maps to the various executables and library files associated with this process. This file can be rather long, depending upon the complexity of the process. Sample output from the sshd process begins like this:

08048000-08086000 r-xp 00000000 03:03 391479          /usr/sbin/sshd 08086000-08088000 rw-p 0003e000 03:03 391479          /usr/sbin/sshd 08088000-08095000 rwxp 00000000 00:00 0 40000000-40013000 r-xp 00000000 03:03 293205         lib/ld-2.2.5.so 40013000-40014000 rw-p 00013000 03:03 293205         /lib/ld-2.2.5.so 40031000-40038000 r-xp 00000000 03:03 293282         /lib/libpam.so.0.75 40038000-40039000 rw-p 00006000 03:03 293282         /lib/libpam.so.0.75 40039000-4003a000 rw-p 00000000 00:00 0 4003a000-4003c000 r-xp 00000000 03:03 293218         /lib/libdl-2.2.5.so 4003c000-4003d000 rw-p 00001000 03:03 293218         /lib/libdl-2.2.5.so

The memory held by the process. This file cannot be read by the user.

A link to the root directory of the process.

The status of the process.

The status of the memory in use by the process. Following is a sample /proc/statm file:

      263 210 210 5 0 205 0

The seven columns relate to different memory statistics for the process. From left to right, they report the following aspects of the memory used:

  1. Total program size, in kilobytes

  2. Size of memory portions, in kilobytes

  3. Number of pages that are shared

  4. Number of pages that are code

  5. Number of pages of data/stack

  6. Number of pages that are libraries

  7. Number of dirty pages

The status of the process in a more readable form than stat or statm. Sample output for sshd looks similar to this:

Name: sshd State: S (sleeping) Tgid: 797 Pid: 797 PPid: 1 TracerPid: 0 Uid: 0 0 0 0 Gid: 0 0 0 0 FDSize: 32 Groups: VmSize:         3072 kB VmLck:             0 kB VmRSS:          840 kB VmData:           104 kB VmStk:            12 kB VmExe:          300 kB VmLib:         2528 kB SigPnd: 0000000000000000 SigBlk: 0000000000000000 SigIgn: 8000000000001000 SigCgt: 0000000000014005 CapInh: 0000000000000000 CapPrm: 00000000fffffeff CapEff: 00000000fffffeff

Other than the process’s name and ID, the state (such as S (sleeping) or R (running) and user/group ID running the process are available, as well as much more detailed data regarding memory usage.

/proc/self/

The /proc/self/ directory is a link to the currently running process. This allows a process to look at itself without having to know its process ID. Within a shell environment, a listing of the /proc/self/ directory produces the same contents as listing the process directory for that process.

/proc/bus/

This directory contains information specific to the various buses available on the system. So, for example, on a standard system containing ISA, PCI, and USB buses, current data on each of these buses is available in its directory under /proc/bus/. The contents of the subdirectories and files available vary greatly on the precise configuration of your system. However, each of the directories for each of the bus types has at least one directory for each bus of that type. These individual bus directories, usually signified with numbers, such as 00, contain binary files that refer to the various devices available on that bus.

So, for example, a system with a USB bus but no USB devices connected to it has a /proc/bus/usb/ directory containing several files:

total 0 dr-xr-xr-x    1 root     root       0 May     3 16:25 001 -r--r--r--    1 root     root       0 May     3 16:25 devices -r--r--r--    1 root     root       0 May     3 16:25 drivers

The /proc/bus/usb/ directory contains files that track the various devices on any USB buses, as well as the drivers required to use them. The /proc/bus/usb/001/ directory contains all devices on the first USB bus. By looking at the contents of the devices file, you can identify the USB root hub on the motherboard:

T:     Bus=01 Lev=00 Prnt=00 Port=00 Cnt=00 Dev#=       1 Spd=12    MxCh= 2 B:     Alloc=     0/900 us > 0%), #Int=       0, #Iso=     0 D:     Ver= 1.00 Cls=09(hub    ) Sub=00 Prot=00 MxPS= 8 #Cfgs=            1 P:     Vendor=0000 ProdID=0000 Rev= 0.00 S:     Product=USB UHCI Root Hub S:     SerialNumber=d400 C:* #Ifs= 1 Cfg#= 1 Atr=40 MxPwr=             0mA I:     If#= 0 Alt= 0 #EPs= 1 Cls=09(hub         ) Sub=00 Prot=00 Driver=hub E:     Ad=81(I) Atr=03(Int.) MxPS=           8 Ivl=255ms

/proc/driver/

This directory contains information for specific drivers in use by the kernel. A common file found here is rtc, which provides output from the driver for the system’s Real Time Clock (RTC), the device that keeps the time while the system is switched off. Sample output from /proc/driver/rtc looks like this:

rtc_time : 01:38:43 rtc_date : 1998-02-13 rtc_epoch : 1900 alarm    : 00:00:00 DST_enable : no BCD     : yes 24hr     : yes square_wave : no alarm_IRQ : no update_IRQ : no periodic_IRQ : no periodic_freq : 1024 batt_status : okay

For more information about the RTC, review /usr/src/linux-2.4/Documentation/rtc.txt.

/proc/fs

This directory shows which file systems are exported. If you are running an NFS server, you can type cat /proc/fs/nfs/exports to view the file systems being shared and the permissions granted for the those file systems. For more on sharing file system with NFS, see Chapter 20.

/proc/ide/

This directory holds information about IDE devices on the system. Each IDE channel is represented as a separate directory, such as /proc/ide/ide0 and /proc/ide/ide1. In addition, a drivers file is also available, providing the version number of the various drivers used on the IDE channels:

ide-cdrom version 4.59 ide-floppy version 0.97 ide-disk version 1.10

Many chipsets also provide an informational file in this directory that gives additional data concerning the drives connected through the the channels. For example, a generic Intel PIIX4 Ultra 33 chipset produces a /proc/ide/piix file that will tell you whether DMA or UDMA is enabled for the devices on the IDE channels:

Intel PIIX4 Ultra 33 Chipset. --------- Primary Channel ------- Secondary Channel --------               enabled                enabled ------------- drive0 ---- drive1 ---- drive0 ---- drive1 --- DMA enabled:    yes         no          yes         no UDMA enabled:   yes         no           no         no UDMA enabled:    2           X            X          X UDMA DMA PIO

Navigating into the directory for an IDE channel, such as ide0, provides additional information. The channel file provides the channel number, while the model tells you the bus type for the channel (such as pci).

The Device Directory

Within each IDE channel directory is a device directory. The name of the device directory corresponds to the drive letter in the /dev/ directory. For instance, the first IDE drive on ide0 would be hda.

Note

There is a symlink to each of these device directories in the /proc/ide/ directory.

Each device directory contains a collection of information and statistics. The contents of these directories vary according to the type of device connected. Some of the more useful files common to many devices are:

  • cache — The device’s cache.

  • capacity — The capacity of the device, in 512-byte blocks.

  • driver — The driver and version used to control the device.

  • geometry — The physical and logical geometry of the device.

  • media — The type of device, such as a disk.

  • model — The model name or number of the device.

  • settings — A collection of current parameters of the device. This file usually contains quite a bit of useful technical information. A sample settings file for a standard IDE hard disk looks similar to this:

    name               value    min     max        mode ----               -----    ---     ---        ---- bios_cyl            784      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 current_speed        66      0        69        rw file_readahead        0      0   2097151        rw ide_scsi              0      0         1        rw init_speed           66      0        69        rw io_32bit              0      0         3        rw keepsettings          0      0         1        rw lun                   0      0         7        rw max_kb_per_request   64      1       127        rw multcount             8      0         8        rw nice1                 1      0         1        rw nowerr                0      0         1        rw number                0      0         3        rw pio_mode         write-only  0       255         w slow                  0      0         1        rw unmaskirq             0      0         1        rw using_dma             1      0         1        rw

/proc/irq/

This directory is used to set IRQ to CPU affinity, which allows you to connect a particular IRQ to only one CPU. Alternatively, you can exclude a CPU from handling any IRQs. Each IRQ has its own directory, allowing for individual configuration of each IRQ. The /proc/irq/prof_cpu_mask file is a bitmask that contains the default values for the smp_affinity file in the IRQ directory. The values in smp_affinity specify which CPUs handle that particular IRQ. /usr/src/linux-2.4/Documentation/filesystems/proc.txt contains more information.

/proc/net/

This directory provides a comprehensive look at various networking parameters and statistics. Each of the files covers a specific range of information related to networking on the system. Following is a partial listing of these virtual files:

  • arp — Contains the kernel’s ARP table. This file is particularly useful for connecting a hardware address to an IP address on a system.

  • atm — A directory containing files with various Asynchronous Transfer Mode (ATM) settings and statistics. This directory is primarily used with ATM networking and ADSL cards.

  • dev — Lists the various network devices configured on the system, complete with transmit and receive statistics. This file will quickly tell you the number of bytes each interface has sent and received, the number of packets inbound and outbound, the number of errors seen, the number of packets dropped, and more.

  • dev_mcast — Displays the various Layer2 multicast groups each device is listening to.

  • igmp — Lists the IP multicast addresses that this system joined.

  • ip_fwchains — If ipchains are in use, this virtual file reveals any current rule.

  • ip_fwnames — If ipchains are in use, this virtual file lists all firewall chain names.

  • ip_masquerade — Provides a table of masquerading information under ipchains.

  • ip_mr_cache — List of the multicast routing cache.

  • ip_mr_vif — List of multicast virtual interfaces.

  • netstat — Contains a broad yet detailed collection of networking statistics, including TCP time-outs, SYN cookies sent and received, and much more.

  • psched — List of global packet scheduler parameters.

  • raw — List of raw device statistics.

  • route — Displays the kernel’s routing table.

  • rt_cache — Contains the current routing cache.

  • snmp — List of Simple Network Management Protocol (SNMP) data for various networking protocols in use.

  • sockstat — Provides socket statistics.

  • tcp — Contains detailed TCP socket information.

  • tr_rif — The token ring RIF routing table.

  • udp — Contains detailed UDP socket information.

  • unix — Lists UNIX domain sockets currently in use.

  • wireless — Lists wireless interface data.

/proc/scsi/

This directory is analogous to the /proc/ide/ directory, only it is for connected SCSI devices. The primary file in this directory is /proc/scsi/scsi, which contains a list of every recognized SCSI device. From this listing, the type of devices, as well as the model name, vendor, SCSI channel and ID data, is available. For example, if a system contains a SCSI CD-ROM, tape drive, hard drives, and RAID controller, this file will look similar to this:

Attached devices: Host: scsi1 Channel: 00 Id: 05 Lun: 00   Vendor: NEC           Model: CD-ROM DRIVE:466 Rev: 1.06   Type:      CD-ROM                ANSI SCSI revision: 02 Host: scsi1 Channel: 00 Id: 06 Lun: 00   Vendor: ARCHIVE       Model: Python 04106-XXX Rev: 7350   Type:      Sequential-Access     ANSI SCSI revision: 02 Host: scsi2 Channel: 00 Id: 06 Lun: 00   Vendor: DELL          Model: 1x6 U2W SCSI BP    Rev: 5.35   Type:      Processor             ANSI SCSI revision: 02 Host: scsi2 Channel: 02 Id: 00 Lun: 00   Vendor: MegaRAID      Model: LD0 RAID5 34556R Rev: 1.01   Type:      Direct-Access         ANSI SCSI revision: 02

Each SCSI driver used by the system has its own directory in /proc/scsi/, which contains files specific to each SCSI controller using that driver. So, for the example system above, aic7xxx and megaraid directories are present, as those two drivers are being utilized. The files in each of the directories typically contain IO address range, IRQ, and statistics for the particular SCSI controller using that driver. Each controller can report a different type and amount of information. The Adaptec AIC-7880 Ultra SCSI host adapter’s file in this example system produces the following output:

Adaptec AIC7xxx driver version: 5.1.20/3.2.4 Compile Options:       TCQ Enabled By Default : Disabled       AIC7XXX_PROC_STATS     : Enabled       AIC7XXX_RESET_DELAY    : 5 Adapter Configuration:      SCSI Adapter: Adaptec AIC-7880 Ultra SCSI host adapter      Ultra Narrow Controller  PCI MMAPed I/O Base: 0xfcffe000  Adapter SEEPROM Config: SEEPROM found and used.           Adaptec SCSI BIOS: Enabled                             IRQ: 30            SCBs: Active 0, Max Active 1, Allocated 15,                    HW 16, Page 255                   Interrupts: 33726           BIOS Control Word: 0x18a6       Adapter Control Word: 0x1c5f       Extended Translation: Enabled Disconnect Enable Flags: 0x00ff         Ultra Enable Flags: 0x0020  Tag Queue Enable Flags: 0x0000 Ordered Queue Tag Flags: 0x0000 Default Tag Queue Depth: 8     Tagged Queue By Device array for aic7xxx host instance 1:           {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 1:           {1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1} Statistics: (scsi1:0:5:0)    Device using Narrow/Sync transfers at 20.0 MByte/sec, offset 15       Transinfo settings: current(12/15/0/0), goal(12/15/0/0), user(12/15/0/0)       Total transfers 0 (0 reads and 0 writes)       < 2K     2K+    4K+   8K+   16K+    32K+   64K+   128K+ Reads:   0      0      0     0     0       0      0       0 Writes:  0      0      0     0     0       0      0       0 (scsi1:0:6:0)       Device using Narrow/Sync transfers at 10.0 MByte/sec, offset 15       Transinfo settings: current(25/15/0/0), goal(12/15/0/0), user(12/15/0/0)       Total transfers 132 (0 reads and 132 writes)        < 2K    2K+    4K+    8K+   16K+   32K+   64K+   128K+ Reads:   0      0      0      0     0      0      0       0 Writes:  0      0      0      1    131     0      0       0

From this screen, you can see the transfer speed to the various SCSI devices connected to the controller based on channel ID, as well as detailed statistics concerning the amount and sizes of files read or written by that device. For instance, from the output above you can see that this controller is communicating with the CD-ROM at 20 megabytes per second, while the tape drive is connected at only 10 megabytes per second.

/proc/sys/

The /proc/sys/ directory is different from others in /proc/ because it not only provides information about the system but also allows you to make configuration changes to the kernel. This allows the administrator of the machine to immediately enable and disable kernel features.

Warning

Use caution when changing settings on a production system using the various files in the /proc/sys/ directory. Changing the wrong setting may render the kernel unstable, requiring a reboot of the system. For this reason, be sure you know the valid options for that file and the expected outcome before attempting to change a value in /proc/sys/.

A good way to determine if a particular file can be configured or is designed only to provide information is to list it with the -l flag in the terminal. If the file is writable, you may use it to configure the kernel. For example, a partial listing of /proc/sys/fs looks like this:

-r--r--r--   1 root   root   0 May 10 16:14 dentry-state -rw-r--r--   1 root   root   0 May 10 16:14 dir-notify-enable -r--r--r--   1 root   root   0 May 10 16:14 dquot-nr -rw-r--r--   1 root   root   0 May 10 16:14 file-max -r--r--r--   1 root   root   0 May 10 16:14 file-nr

In this listing, the files dir-notify-enable and file-max can be written to and, therefore, can be used to configure the kernel. The other files only provide feedback on current settings. Changing a value within a /proc/sys/ file is done by echoing the new value into the file. For example, to enable the System Request Key on a running kernel, type the command

echo 1 > /proc/sys/kernel/sysrq

This will change the sysrq file’s value from 0 (off) to 1 (on). The purpose of the System Request Key is to allow you to immediately instruct the kernel to do a number of important activities by using a simple key combination, such as immediately shutting down or restarting a system, syncing all mounted file systems, or dumping important information to your console. This feature is most useful when using a development kernel or if you are experiencing system freezes. However, it is considered a security risk for an unattended console and is therefore turned off by default under Red Hat Linux. Refer to /usr/src/linux-2.4/Documentation/sysrq.txt for more information on the System Request Key.

A few /proc/sys/ configuration files contain more than one value. In order to correctly send new values to them, place a space character between each value passed with the echo command, such as is done in this example:

echo 4 2 45 > /proc/sys/kernel/acct
Note

Any configuration changes you make using the echo command will disappear when the system is restarted. To make your configuration changes take effect at the time the system is booted, see Chapter 2.

The /proc/sys/ directory contains several subdirectories controlling different aspects of a running kernel.

/proc/sys/dev/

This directory provides parameters for particular devices on the system. Most systems have at least two directories, cdrom and raid, but customized kernels can have others, such as parport, which provides the ability to share one parallel port between multiple device drivers. The cdrom directory contains a file called info, which reveals a number of important CD-ROM parameters:

CD-ROM information, Id: cdrom.c 3.12 2000/10/18 drive name:             hdc drive speed:            32 drive # of slots:       1 Can close tray:         1 Can open tray:          1 Can lock tray:          1 Can change speed:       1 Can select disk:        0 Can read multisession:  1 Can read MCN:           1 Reports media changed:  1 Can play audio:         1 Can write CD-R:         0 Can write CD-RW:        0 Can read DVD:           0 Can write DVD-R:        0 Can write DVD-RAM:      0

This file can be quickly scanned to discover the qualities of an unknown CD-ROM, at least in the eyes of the kernel. If multiple CD-ROMs are available on a system, each device is given its own column of information.

Various files in /proc/sys/dev/cdrom, such as autoclose and checkmedia, can be used to control the system’s CD-ROM. Use the echo command to enable or disable these features. If RAID support is compiled into the kernel, a /proc/sys/dev/raid/ directory will be available with at least two files in it: speed_limit_min and speed_limit_max. These settings determine how much to accelerate the RAID device for particularly I/O-intensive tasks, such as resyncing the disks.

/proc/sys/fs/

This directory contains an array of options and information concerning various aspects of the file system, including quota, file handle, inode, and dentry information. The binfmt_misc directory is used to provide kernel support for miscellaneous binary formats. The important files in /proc/sys/fs include:

  • dentry-state — Provides the status of the directory cache. The file looks similar to this:

    57411 52939 45 0 0 0
  • The first number reveals the total number of directory cache entries, while the second number displays the number of unused entries. The third number tells the number of seconds between when a directory has been freed and when it can be reclaimed, and the fourth measures the pages currently requested by the system. The last two numbers are not used and currently display only zeros.

  • dquot-nr — Shows the maximum number of cached disk quota entries.

  • file-max — Allows you to change the maximum number of file handles that the kernel will allocate. Raising the value in this file can resolve errors caused by a lack of available file handles.

  • file-nr — Displays the number of allocated file handles, used file handles, and the maximum number of file handles.

  • overflowgid and overflowuid — Define the fixed group ID and user ID, respectively, for use with file systems that support only 16-bit group and user IDs.

  • super-max — Controls the maximum number of superblocks available.

  • super-nr — Displays the current number of superblocks in use.

/proc/sys/kernel/

This directory contains a variety of different configuration files that directly affect the operation of the kernel. Some of the most important files are:

  • acct — Controls the suspension of process accounting based on the percentage of free space available on the file system containing the log. By default, the file looks like this:

    4 2 30
  • The second value sets the threshold percentage of free space when logging will be suspended, while the first value dictates the percentage of free space required for logging to resume. The third value sets the interval in seconds that the kernel polls the file system to see if logging should be suspended or resumed.

  • cap-bound — Controls the capability bounding settings, which provide a list of capabilities for any process on the system. If a capability is not listed here, then no process, no matter how privileged, can use that capability. The idea is to make the system more secure by ensuring that certain things cannot happen, at least beyond a certain point in the boot process. The various values that are possible here are beyond the scope of this book, so consult the kernel documentation for more information.

  • ctrl-alt-del — Controls whether Ctrl-Alt-Del will gracefully restart the computer using init (value 0) or force an immediate reboot without syncing the dirty buffers to disk (value 1).

  • domainname — Allows you to configure the system’s domain name, such as subgenius.com.

  • hostname — Allows you to configure the system’s hostname, such as bob.subgenius.com.

  • hotplug — Configures the utility to be used when a configuration change is detected by the system. This is primarily used with USB and Cardbus PCI. The default value of /sbin/hotplug should not be changed unless you are testing a new program to fulfill this role.

  • modprobe — Sets the location of the program to be used to load kernel modules when necessary. The default value of /sbin/modprobe signifies that kmod will call it to actually load the module when a kernel thread calls kmod.

  • msgmax — Sets the maximum size of any message sent from one process to another. This value is set to 8,192 bytes by default. Be careful about raising this value, as queued messages between processes are stored in nonswappable kernel memory. Any increase in msgmax would increase RAM requirements for the system.

  • msgmnb — Sets the maximum number of bytes in a single message queue. The default is 16,384.

  • msgmni — Sets the maximum number of message queue identifiers. The default is 16.

  • osrelease — Lists the Linux kernel release number. This file can be altered only by changing the kernel source and recompiling.

  • ostype — Displays the type of operating system. By default, this file is set to Linux, and this value can be changed only by changing the kernel source and recompiling.

  • overflowgid and overflowuid — Define the fixed group ID and user ID, respectively, for use with system calls on architectures that support only 16-bit group and user IDs.

  • panic — Defines the number of seconds the kernel will postpone rebooting when the system experiences a kernel panic. By default, the value is set to 0, which disables automatic rebooting after a panic.

  • printk — This file controls a variety of settings related to printing or logging error messages. Each error message reported by the kernel has a loglevel associated with it that defines the importance of the message. The loglevel values break down in this order:

    • 0 — Kernel emergency. The system is unusable.

    • 1 — Kernel alert. Action must be taken immediately.

    • 2 — Condition of the kernel is considered critical.

    • 3 — General kernel error condition.

    • 4 — General kernel warning condition.

    • 5 — Kernel notice of a normal but significant condition.

    • 6 — Kernel informational message.

    • 7 — Kernel debug-level messages.

  • Four values are found in the printk file:

    6 4 1 7
  • Each of these values defines a different rule for dealing with error messages. The first value, called the console loglevel, defines the lowest priority of messages that will be printed to the console. (Note that the lower the priority, the higher the loglevel number.) The second value sets the default loglevel for messages without an explicit loglevel attached to them. The third value sets the lowest possible loglevel configuration for the console loglevel. The last value sets the default value for the console loglevel.

  • rtsig-max — Configures the maximum number of POSIX realtime signals that the system may have queued at any one time. The default value is 1,024.

  • rtsig-nr — The current number of POSIX realtime signals queued by the kernel.

  • sem — This file configures semaphore settings within the kernel. A semaphore is a System V IPC object that is used to control utilization of a particular process.

  • shmall — Sets the total amount of shared memory that can be used at one time on the system, in bytes. By default, this value is 2,097,152.

  • shmmax — Sets the largest shared memory segment size allowed by the kernel, in bytes. By default, this value is 33,554,432. However, the kernel supports much larger values than this.

  • shmmni — Sets the maximum number of shared memory segments for the whole system. By default, this value is 4,096.

  • sysrq — Activates the System Request Key, if this value is set to anything other than the default of 0.

  • threads-max — Sets the maximum number of threads to be used by the kernel, with a default value of 2,048.

  • version — Displays the date and time the kernel was last compiled. The first field in this file, such as #3, relates to the number of times a kernel was built from the source base.

The random directory stores a number of values related to generating random numbers for the kernel.

/proc/sys/net/

This directory contains assorted directories concerning various networking topics. Various configurations at the time of kernel compilation make available different directories here, such as appletalk, ethernet, ipv4, ipx, and ipv6. Within these directories, you can adjust the assorted networking values for that configuration on a running system.

Note

Given the wide variety of possible networking options available with Linux and the great amount of space required to discuss them, only the most common /proc/sys/net/ directories will be discussed.

The core directory contains a variety of settings that control the interaction between the kernel and networking layers. The most important files there are:

  • message_burst — The amount of time in tenths of a second required to write a new warning message. This is used to prevent Denial of Service (DoS) attacks. The default setting is 50.

  • message_cost — Also used to prevent DoS attacks by placing a cost on every warning message. The higher the value of this file (default of 5), the more likely the warning message will be ignored. The idea of a DoS attack is to bombard your system with requests that generate errors and fill up disk partitions with log files or require all of your system’s resources to handle the error logging. The settings in message_burst and message_cost are designed to be modified based on your system’s acceptable risk versus the need for comprehensive logging.

  • netdev_max_backlog — Sets the maximum number of packets allowed to queue when a particular interface receives packets faster than the kernel can process them. The default value for this file is 300.

  • optmem_max — Configures the maximum ancillary buffer size allowed per socket.

  • rmem_default — Sets the receive socket buffer’s default size in bytes.

  • rmem_max — Sets the receive socket buffer’s maximum size in bytes.

  • wmem_default — Sets the send socket buffer’s default size in bytes.

  • wmem_max — Sets the send socket buffer’s maximum size in bytes.

The /ipv4 directory contains additional networking settings. Many of these settings, used in conjunction with one another, are very useful in preventing attacks on the system or using the system to act as a router.

Warning

An erroneous change to these files may affect your remote connectivity to the system.

Here are some of the most important files in the /proc/sys/net/ipv4/ directory:

  • icmp_destunreach_rate, icmp_echoreply_rate, icmp_paramprob_rate, and icmp_timeexeed_rate — Set the maximum ICMP send packet rate, in hundredths of a second, to hosts under certain conditions. A setting of 0 removes any delay and is not a good idea.

  • icmp_echo_ignore_all and icmp_echo_ignore_broadcasts — Allows the kernel to ignore ICMP ECHO packets from every host or only those originating from broadcast and multicast addresses, respectively. A value of 0 allows the kernel to respond, while a value of 1 ignores the packets.

  • ip_default_ttl — Sets the default Time To Live (TTL), which limits the number of hops a packet may make before reaching its destination. Increasing this value can diminish system performance.

  • ip_forward — Permits interfaces on the system to forward packets to one other. By default, this file is set to 0. Setting this file to 1 will enable network packet forwarding.

  • ip_local_port_range — Specifies the range of ports to be used by TCP or UDP when a local port is needed. The first number is the lowest port to be used, and the second number specifies the highest port. Any systems that expect to require more ports than the default 1,024 to 4,999 should use the 32,768 to 61,000 range in this file.

  • tcp_syn_retries — Provides a limit on the number of times your system will retransmit a SYN packet when attempting to make a connection.

  • tcp_retries1 — Sets the number of permitted retransmissions attempting to answer an incoming connection. Default of 3.

  • tcp_retries2 — Sets the number of permitted retransmissions of TCP packets. Default of 15.

The /usr/src/linux-2.4/Documentation/networking/ip-sysctl.txt file contains a complete list of files and options available in the /proc/sys/net/ipv4/ directory. A number of other directories that exist within the /proc/sys/net/ipv4/ directory cover specific topics.

The conf directory allows each of the system interfaces to be configured in different ways, including the use of default settings for unconfigured devices (in the default subdirectory) and settings that override all special configurations (in the all subdirectory). In order to control connections between direct neighbors, meaning any other system directly connected to your system, the neigh directory allows special configurations for each interface. This will allow you to treat differently systems that you trust more because of their relative proximity to your system. At the same time, it also makes it easy to put strict rules in place for systems several hops away.

Routing over IPV4 also has its own directory, route. Unlike conf and neigh, the route directory contains specifications that apply to routing with any interfaces on the system. Many of these settings, such as max_size, max_delay, and min_delay, relate to controlling the size of the routing cache. To clear the routing cache, simply write any value to the flush file. Additional information about these directories and the possible values for their configuration files can be found in /usr/src/linux-2.4/Documentation/filesystems/proc.txt.

/proc/sys/vm/

This directory facilitates the configuration of the Linux kernel’s virtual memory (VM) subsystem. The kernel makes extensive and intelligent use of virtual memory, which is commonly called swap space. The following files are commonly found in the /proc/sys/vm/ directory:

  • bdflush — Sets various values related to the bdflush kernel daemon.

  • buffermem — Allows you to control the percentage amount of total system memory to be used for buffer memory. Typical output for this file looks like this:

    2        10        60

The first and last values set the minimum and maximum percentage of memory to be used as buffer memory, respectively. The middle value sets the percentage of system memory dedicated to buffer memory where the memory management subsystem will begin to clear buffer cache more than other kinds of memory to compensate for a general lack of free memory.

  • kswapd — Sets various values concerned with the kernel swap-out daemon, kswapd. This file has three values:

    512 32 8
  • The first value sets the maximum number of pages that kswapd will attempt to free in a single attempt. The larger this number, the more aggressively the kernel can move to free pages. The second value sets the minimum number of times that kswapd attempts to free a page. The third value sets the number of pages kswapd attempts to write in a single attempt. Proper tuning of this final value can improve performance on a system using a lot of swap space by telling the kernel to write pages in large chunks, minimizing the number of disk seeks.

  • max_map_count — Configures the maximum number of memory map areas a process may have. In most cases, the default value of 65,536 is appropriate.

  • overcommit_memory — When this is set to the default value of 0, the kernel estimates the amount of memory available and fails requests that are blatantly invalid. Unfortunately, since memory is allocated using a heuristic rather than a precise algorithm, it can sometimes overload the system. If overcommit_memory is set to 1, then the potential for system overload is increased, but so is the performance for memory-intensive tasks, such as those used by some scientific software. For customers who need more control over the risk of overcommitted memory, the following two options have been added. Setting overcommit_memory to 2 fails if a memory request adds up to more than half of the physical RAM, plus swap. Setting it to 3 fails if a memory request adds up to more than swap alone can hold.

  • pagecache — Controls the amount of memory used by the page cache. The values in pagecache are percentages, and they work in a similar way as buffermem to enforce minimums and maximums of available page cache memory.

  • page-cluster — Sets the number of pages read in a single attempt. The default value of 4, which actually relates to 16 pages, is appropriate for most systems.

  • pagetable_cache — Controls the number of page tables that are cached on a per-processor basis. The first and second values relate to the minimum and maximum number of page tables to set aside, respectively.

The /usr/src/linux-2.4/Documentation/sysctl/vm.txt file contains additional information on these various files.

/proc/sysvipc/

This directory contains information about System V IPC resources. The files in this directory relate to System V IPC calls for messages (msg), semaphores (sem), and shared memory (shm).

/proc/tty/

This directory contains information about the available and currently used tty devices on the system. Originally called a teletype device, character-based data terminals are called tty devices. In Linux, there are three different kinds of tty devices. Serial devices are used with serial connections, such as over a modem or using a serial cable. Virtual terminals create the common console connection, such as the virtual consoles available when pressing Alt- F at the system console. Pseudo terminals create a two-way communication that is used by some higher level applications, such as X11. The drivers file is a list of the current tty devices in use:

serial        /dev/cua       5    64-127   serial:callout serial        /dev/ttyS      4    64-127   serial 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 /dev/vc/0     /dev/vc/0      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/vc/%d     4      1-63   console

The /proc/tty/driver/serial file lists the usage statistics and status of each of the serial tty lines. In order for tty devices to be used in a similar way as network devices, the Linux kernel enforces line discipline on the device. This allows the driver to place a specific type of header with every block of data transmitted over the device, making it possible for the remote end of the connection to be able to see that block of data as just one in a stream of data blocks. SLIP and PPP are common line disciplines, and each is commonly used to connect systems to one other over a serial link. Registered line disciplines are stored in the ldiscs file, with detailed information available in the ldisc directory.




Official Red Hat Linux Administrator's Guide
Official Red Hat Linux Administrators Guide
ISBN: 0764516957
EAN: 2147483647
Year: 2002
Pages: 278
Authors: Red Hat Inc

flylib.com © 2008-2017.
If you may any questions please contact us: flylib@qtcs.net