Linux kernel documentation for /sys/class/net

Recently I am working on a project to handle network interface failure which involves detecting the connected state of a network cable.


After searching around the Internet I found the post from Stack Overflow mentioned a good way to solve the problem. As Kent Fredric said,we can use /sys/class/net/<iface>/carrier and /sys/class/net/<iface>/operstate to identify the connected state of a network cable.


What does /sys/class/net/<iface>/carrier and /sys/class/net/<iface>/operstate means?
From the kernel documentation I found the description for them:


What:  /sys/class/net/<iface>/operstate
Date:  March 2006
KernelVersion: 2.6.17
Contact: netdev@vger.kernel.org
Description:
  Indicates the interface RFC2863 operational state as a string.
  Possible values are:
  "unknown", "notpresent", "down", "lowerlayerdown", "testing",
  "dormant", "up".


What:  /sys/class/net/<iface>/carrier
Date:  April 2005
KernelVersion: 2.6.12
Contact: netdev@vger.kernel.org
Description:
  Indicates the current physical link state of the interface.
  Posssible values are:
  0: physical link is down
  1: physical link is up

  Note: some special devices, e.g: bonding and team drivers will
  allow this attribute to be written to force a link state for
  operating correctly and designating another fallback interface.

The whole kernel documentation for /sys/class/net is attached below for reference. 


What:  /sys/class/net/<iface>/name_assign_type
Date:  July 2014
KernelVersion: 3.17
Contact: netdev@vger.kernel.org
Description:
  Indicates the name assignment type. Possible values are:
  1: enumerated by the kernel, possibly in an unpredictable way
  2: predictably named by the kernel
  3: named by userspace
  4: renamed

What:  /sys/class/net/<iface>/addr_assign_type
Date:  July 2010
KernelVersion: 3.2
Contact: netdev@vger.kernel.org
Description:
  Indicates the address assignment type. Possible values are:
  0: permanent address
  1: randomly generated
  2: stolen from another device
  3: set using dev_set_mac_address

What:  /sys/class/net/<iface>/addr_len
Date:  April 2005
KernelVersion: 2.6.12
Contact: netdev@vger.kernel.org
Description:
  Indicates the hardware address size in bytes.
  Values vary based on the lower-level protocol used by the
  interface (Ethernet, FDDI, ATM, IEEE 802.15.4...). See
  include/uapi/linux/if_*.h for actual values.

What:  /sys/class/net/<iface>/address
Date:  April 2005
KernelVersion: 2.6.12
Contact: netdev@vger.kernel.org
Description:
  Hardware address currently assigned to this interface.
  Format is a string, e.g: 00:11:22:33:44:55 for an Ethernet MAC
  address.

What:  /sys/class/net/<bridge iface>/bridge/group_fwd_mask
Date:  January 2012
KernelVersion: 3.2
Contact: netdev@vger.kernel.org
Description:
  Bitmask to allow forwarding of link local frames with address
  01-80-C2-00-00-0X on a bridge device. Only values that set bits
  not matching BR_GROUPFWD_RESTRICTED in net/bridge/br_private.h
  allowed.
  Default value 0 does not forward any link local frames.

  Restricted bits:
  0: 01-80-C2-00-00-00 Bridge Group Address used for STP
  1: 01-80-C2-00-00-01 (MAC Control) 802.3 used for MAC PAUSE
  2: 01-80-C2-00-00-02 (Link Aggregation) 802.3ad

  Any values not setting these bits can be used. Take special
  care when forwarding control frames e.g. 802.1X-PAE or LLDP.

What:  /sys/class/net/<iface>/broadcast
Date:  April 2005
KernelVersion: 2.6.12
Contact: netdev@vger.kernel.org
Description:
  Hardware broadcast address for this interface. Format is a
  string, e.g: ff:ff:ff:ff:ff:ff for an Ethernet broadcast MAC
  address.

What:  /sys/class/net/<iface>/carrier
Date:  April 2005
KernelVersion: 2.6.12
Contact: netdev@vger.kernel.org
Description:
  Indicates the current physical link state of the interface.
  Posssible values are:
  0: physical link is down
  1: physical link is up

  Note: some special devices, e.g: bonding and team drivers will
  allow this attribute to be written to force a link state for
  operating correctly and designating another fallback interface.

What:  /sys/class/net/<iface>/dev_id
Date:  April 2008
KernelVersion: 2.6.26
Contact: netdev@vger.kernel.org
Description:
  Indicates the device unique identifier. Format is an hexadecimal
  value. This is used to disambiguate interfaces which might be
  stacked (e.g: VLAN interfaces) but still have the same MAC
  address as their parent device.

What:  /sys/class/net/<iface>/dormant
Date:  March 2006
KernelVersion: 2.6.17
Contact: netdev@vger.kernel.org
Description:
  Indicates whether the interface is in dormant state. Possible
  values are:
  0: interface is not dormant
  1: interface is dormant

  This attribute can be used by supplicant software to signal that
  the device is not usable unless some supplicant-based
  authentication is performed (e.g: 802.1x). 'link_mode' attribute
  will also reflect the dormant state.

What:  /sys/clas/net/<iface>/duplex
Date:  October 2009
KernelVersion: 2.6.33
Contact: netdev@vger.kernel.org
Description:
  Indicates the interface latest or current duplex value. Possible
  values are:
  half: half duplex
  full: full duplex

  Note: This attribute is only valid for interfaces that implement
  the ethtool get_settings method (mostly Ethernet).

What:  /sys/class/net/<iface>/flags
Date:  April 2005
KernelVersion: 2.6.12
Contact: netdev@vger.kernel.org
Description:
  Indicates the interface flags as a bitmask in hexadecimal. See
  include/uapi/linux/if.h for a list of all possible values and
  the flags semantics.

What:  /sys/class/net/<iface>/ifalias
Date:  September 2008
KernelVersion: 2.6.28
Contact: netdev@vger.kernel.org
Description:
  Indicates/stores an interface alias name as a string. This can
  be used for system management purposes.

What:  /sys/class/net/<iface>/ifindex
Date:  April 2005
KernelVersion: 2.6.12
Contact: netdev@vger.kernel.org
Description:
  Indicates the system-wide interface unique index identifier as a
  decimal number. This attribute is used for mapping an interface
  identifier to an interface name. It is used throughout the
  networking stack for specifying the interface specific
  requests/events.

What:  /sys/class/net/<iface>/iflink
Date:  April 2005
KernelVersion: 2.6.12
Contact: netdev@vger.kernel.org
Description:
  Indicates the system-wide interface unique index identifier a
  the interface is linked to. Format is decimal. This attribute is
  used to resolve interfaces chaining, linking and stacking.
  Physical interfaces have the same 'ifindex' and 'iflink' values.

What:  /sys/class/net/<iface>/link_mode
Date:  March 2006
KernelVersion: 2.6.17
Contact: netdev@vger.kernel.org
Description:
  Indicates the interface link mode, as a decimal number. This
  attribute should be used in conjunction with 'dormant' attribute
  to determine the interface usability. Possible values:
  0: default link mode
  1: dormant link mode

What:  /sys/class/net/<iface>/mtu
Date:  April 2005
KernelVersion: 2.6.12
Contact: netdev@vger.kernel.org
Description:
  Indicates the interface currently configured MTU value, in
  bytes, and in decimal format. Specific values depends on the
  lower-level interface protocol used. Ethernet devices will show
  a 'mtu' attribute value of 1500 unless changed.

What:  /sys/class/net/<iface>/netdev_group
Date:  January 2011
KernelVersion: 2.6.39
Contact: netdev@vger.kernel.org
Description:
  Indicates the interface network device group, as a decimal
  integer. Default value is 0 which corresponds to the initial
  network devices group. The group can be changed to affect
  routing decisions (see: net/ipv4/fib_rules and
  net/ipv6/fib6_rules.c).

What:  /sys/class/net/<iface>/operstate
Date:  March 2006
KernelVersion: 2.6.17
Contact: netdev@vger.kernel.org
Description:
  Indicates the interface RFC2863 operational state as a string.
  Possible values are:
  "unknown", "notpresent", "down", "lowerlayerdown", "testing",
  "dormant", "up".

What:  /sys/class/net/<iface>/phys_port_id
Date:  July 2013
KernelVersion: 3.12
Contact: netdev@vger.kernel.org
Description:
  Indicates the interface unique physical port identifier within
  the NIC, as a string.

What:  /sys/class/net/<iface>/phys_port_name
Date:  March 2015
KernelVersion: 4.0
Contact: netdev@vger.kernel.org
Description:
  Indicates the interface physical port name within the NIC,
  as a string.

What:  /sys/class/net/<iface>/speed
Date:  October 2009
KernelVersion: 2.6.33
Contact: netdev@vger.kernel.org
Description:
  Indicates the interface latest or current speed value. Value is
  an integer representing the link speed in Mbits/sec.

  Note: this attribute is only valid for interfaces that implement
  the ethtool get_settings method (mostly Ethernet ).

What:  /sys/class/net/<iface>/tx_queue_len
Date:  April 2005
KernelVersion: 2.6.12
Contact: netdev@vger.kernel.org
Description:
  Indicates the interface transmit queue len in number of packets,
  as an integer value. Value depend on the type of interface,
  Ethernet network adapters have a default value of 1000 unless
  configured otherwise

What:  /sys/class/net/<iface>/type
Date:  April 2005
KernelVersion: 2.6.12
Contact: netdev@vger.kernel.org
Description:
  Indicates the interface protocol type as a decimal value. See
  include/uapi/linux/if_arp.h for all possible values.

What:  /sys/class/net/<iface>/phys_switch_id
Date:  November 2014
KernelVersion: 3.19
Contact: netdev@vger.kernel.org
Description:
  Indicates the unique physical switch identifier of a switch this
  port belongs to, as a string.
Location: San Jose, CA, USA

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