Directories in /proc

/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.

Device Directories

Some of the most useful data can be found in the device directories within the channel directory. Each device, such as a hard drive or CD-ROM, on that channel will have its own directory containing its own 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 different devices include:

• 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/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. Simply echo a 1 to the correct file to turn on the feature or a 0 to disable a feature.

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 come into play in order to throttle or accelerate the speed the RAID device is utilized 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 filesystem, 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, in that order.

• overflowgid and overflowuid — Defines the fixed group ID and user ID, respectively, for use with filesystems that only support 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 include:

• acct — Controls the suspension of process accounting based on the percentage of free space available on the filesystem 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 filesystem to see if logging should be suspended or resumed.

• cap-bound — Controls the capability bounding settings, which provide a list of capabilities that any process on the system can do. If a capability is not listed here, then no process, no matter how privileged, can do it. The primary 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 manual, so consult the kernel documentation for more information.

• ctrl-alt-del — Controls whether [Ctrl]-[Alt]-[Delete] 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 domain.com.

• hostname — Allows you to configure the system's host name, such as host.domain.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, which is set to 8192 bytes by default. You should be careful about raising this value, as queued messages between processes are stored in non-swappable kernel memory, and any increase in msgmax would increase RAM requirements for the system.

• msgmnb — Sets the maximum number of bytes in a single message queue. By default, 16384.

• msgmni — Sets the maximum number of message queue identifiers. By default, 16.

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

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

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

• printk — This file control 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 1024.

• 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 2097152.

• shmmax — Sets the largest shared memory segment size allowed by the kernel, in bytes. By default, this value is 33554432. The kernel supports much larger values than this, however.

• shmmni — Sets the maximum number of shared memory segments for the whole system. By default, this value is 4096

• 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 4095.

• 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 of its own concerning various networking topics, including assorted protocols and centers of emphasis. 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.

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 — Tenths of seconds required to write a new warning message. This is used to prevent Denial of Service (DoS) attacks, and 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 is that an attacker could bombard your system with requests that generate errors and fill up your logs or require all of your system's resources to handle 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.

Given the widespread use of IP networks with Linux, a look at the most important files in the ipv4 reveals additional, powerful, networking settings. Many of these settings, used in the proper conjunction with one another, are very useful in preventing attacks on your system.

	Caution
	However, be sure you know what you are doing, as well as what you expect to happen, before changing any of these settings. In addition, you should make any changes locally, as an erroneous change may affect your remote connectivity to the system.

Here are some of the most important files in the ipv4 directory:

• icmp_destunreach_rate, icmp_echoreply_rate, icmp_paramprob_rate and icmp_timeexeed_rate — Sets the maximum ICMP send packet rate, in hundredths of a second on Intel systems, to hosts under different 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 0 allows the kernel to respond, while a 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 to disable forwarding, but setting this file to 1 will enable 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 1024 to 4999 should use the 32768 to 61000 range in this file.

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

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

• tcp_retries2 — Sets the number of permitted re-transmissions of TCP packets. Default of 15.

For a complete list of files and options available, see /usr/src/linux-2.4/Documentation/networking/ip-sysctl.txt.

A number of other directories exist within the /proc/sys/net/ipv4 directory cover specific topics. The conf directory allows each of the systems interfaces to be configured in different ways, including the use of a 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 would allow you to treat systems differently that you might trust more due to their relatively proximity or relation 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.

• freepages — Displays various values related to free pages of system memory. This file looks similar to this:

  223 446 669

  The first value shows the minimum number of free pages permitted before the kernel takes over control of allocating additional memory. The second value gives the number of free pages before the kernel begins swapping aggressively to preserve performance. The third value is the number of free pages that the system attempts to keep available at all times.

• 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 65536 is appropriate.

• overcommit_memory — Contains a value that, when set to something other than the default of 0, allows the kernel to skip a standard check to see if there is enough memory before allocating it.

• 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.

Additional information on these various files can be found in /usr/src/linux-2.4/Documentation/sysctl/vm.txt.