Overview
Discussion about how net cable connect and disconnect are reflected in /sys.
/sys/class/net/enp3s0/ files
-r--r--r--. 1 root root 4096 Aug 1 10:41 addr_assign_type -r--r--r--. 1 root root 4096 Aug 1 10:41 address -r--r--r--. 1 root root 4096 Aug 1 10:41 addr_len -r--r--r--. 1 root root 4096 Aug 1 10:41 broadcast -rw-r--r--. 1 root root 4096 Aug 1 10:41 carrier -r--r--r--. 1 root root 4096 Aug 1 10:41 carrier_changes -r--r--r--. 1 root root 4096 Aug 1 10:41 carrier_down_count -r--r--r--. 1 root root 4096 Aug 1 10:41 carrier_up_count lrwxrwxrwx. 1 root root 0 Aug 1 07:13 device -> ../../../0000:03:00.0 -r--r--r--. 1 root root 4096 Aug 1 10:41 dev_id -r--r--r--. 1 root root 4096 Aug 1 10:41 dev_port -r--r--r--. 1 root root 4096 Aug 1 10:41 dormant -r--r--r--. 1 root root 4096 Aug 1 10:41 duplex -rw-r--r--. 1 root root 4096 Aug 1 10:41 flags -rw-r--r--. 1 root root 4096 Aug 1 10:41 gro_flush_timeout -rw-r--r--. 1 root root 4096 Aug 1 10:41 ifalias -r--r--r--. 1 root root 4096 Aug 1 10:41 ifindex -r--r--r--. 1 root root 4096 Aug 1 10:41 iflink -r--r--r--. 1 root root 4096 Aug 1 10:41 link_mode -rw-r--r--. 1 root root 4096 Aug 1 10:41 mtu -r--r--r--. 1 root root 4096 Aug 1 10:41 name_assign_type -rw-r--r--. 1 root root 4096 Aug 1 10:41 netdev_group -r--r--r--. 1 root root 4096 Aug 1 10:41 operstate -r--r--r--. 1 root root 4096 Aug 1 10:41 phys_port_id -r--r--r--. 1 root root 4096 Aug 1 10:41 phys_port_name -r--r--r--. 1 root root 4096 Aug 1 10:41 phys_switch_id drwxr-xr-x. 2 root root 0 Aug 1 10:41 power -rw-r--r--. 1 root root 4096 Aug 1 10:41 proto_down drwxr-xr-x. 4 root root 0 Aug 1 10:41 queues -r--r--r--. 1 root root 4096 Aug 1 10:41 speed drwxr-xr-x. 2 root root 0 Aug 1 10:41 statistics lrwxrwxrwx. 1 root root 0 Aug 1 07:13 subsystem -> ../../../../../../class/net -rw-r--r--. 1 root root 4096 Aug 1 10:41 tx_queue_len -r--r--r--. 1 root root 4096 Aug 1 10:41 type -rw-r--r--. 1 root root 4096 Aug 1 10:41 uevent
In particular, we care about the attributes that reflect physical connection (note how carrier is writeable):
-rw-r--r--. 1 root root 4096 Aug 1 10:41 carrier [0 or 1] -r--r--r--. 1 root root 4096 Aug 1 10:41 operstate [up or down] -rw-r--r--. 1 root root 4096 Aug 1 10:41 flags
flags
include/uapi/linux/if.h
/**
* enum net_device_flags - &struct net_device flags
*
* These are the &struct net_device flags, they can be set by drivers, the
* kernel and some can be triggered by userspace. Userspace can query and
* set these flags using userspace utilities but there is also a sysfs
* entry available for all dev flags which can be queried and set. These flags
* are shared for all types of net_devices. The sysfs entries are available
* via /sys/class/net/<dev>/flags. Flags which can be toggled through sysfs
* are annotated below, note that only a few flags can be toggled and some
* other flags are always always preserved from the original net_device flags
* even if you try to set them via sysfs. Flags which are always preserved
* are kept under the flag grouping @IFF_VOLATILE. Flags which are volatile
* are annotated below as such.
*
* You should have a pretty good reason to be extending these flags.
*
* @IFF_UP: interface is up. Can be toggled through sysfs.
* @IFF_BROADCAST: broadcast address valid. Volatile.
* @IFF_DEBUG: turn on debugging. Can be toggled through sysfs.
* @IFF_LOOPBACK: is a loopback net. Volatile.
* @IFF_POINTOPOINT: interface is has p-p link. Volatile.
* @IFF_NOTRAILERS: avoid use of trailers. Can be toggled through sysfs.
* Volatile.
* @IFF_RUNNING: interface RFC2863 OPER_UP. Volatile.
* @IFF_NOARP: no ARP protocol. Can be toggled through sysfs. Volatile.
* @IFF_PROMISC: receive all packets. Can be toggled through sysfs.
* @IFF_ALLMULTI: receive all multicast packets. Can be toggled through
* sysfs.
* @IFF_MASTER: master of a load balancer. Volatile.
* @IFF_SLAVE: slave of a load balancer. Volatile.
* @IFF_MULTICAST: Supports multicast. Can be toggled through sysfs.
* @IFF_PORTSEL: can set media type. Can be toggled through sysfs.
* @IFF_AUTOMEDIA: auto media select active. Can be toggled through sysfs.
* @IFF_DYNAMIC: dialup device with changing addresses. Can be toggled
* through sysfs.
* @IFF_LOWER_UP: driver signals L1 up. Volatile.
* @IFF_DORMANT: driver signals dormant. Volatile.
* @IFF_ECHO: echo sent packets. Volatile.
*/
enum net_device_flags {
IFF_UP = 1<<0, /* sysfs */
IFF_BROADCAST = 1<<1, /* volatile */
IFF_DEBUG = 1<<2, /* sysfs */
IFF_LOOPBACK = 1<<3, /* volatile */
IFF_POINTOPOINT = 1<<4, /* volatile */
IFF_NOTRAILERS = 1<<5, /* sysfs */
IFF_RUNNING = 1<<6, /* volatile */
IFF_NOARP = 1<<7, /* sysfs */
IFF_PROMISC = 1<<8, /* sysfs */
IFF_ALLMULTI = 1<<9, /* sysfs */
IFF_MASTER = 1<<10, /* volatile */
IFF_SLAVE = 1<<11, /* volatile */
IFF_MULTICAST = 1<<12, /* sysfs */
IFF_PORTSEL = 1<<13, /* sysfs */
IFF_AUTOMEDIA = 1<<14, /* sysfs */
IFF_DYNAMIC = 1<<15, /* sysfs */
IFF_LOWER_UP = 1<<16, /* volatile */
IFF_DORMANT = 1<<17, /* volatile */
IFF_ECHO = 1<<18, /* volatile */
};
#define IFF_UP IFF_UP
#define IFF_BROADCAST IFF_BROADCAST
#define IFF_DEBUG IFF_DEBUG
#define IFF_LOOPBACK IFF_LOOPBACK
#define IFF_POINTOPOINT IFF_POINTOPOINT
#define IFF_NOTRAILERS IFF_NOTRAILERS
#define IFF_RUNNING IFF_RUNNING
#define IFF_NOARP IFF_NOARP
#define IFF_PROMISC IFF_PROMISC
#define IFF_ALLMULTI IFF_ALLMULTI
#define IFF_MASTER IFF_MASTER
#define IFF_SLAVE IFF_SLAVE
#define IFF_MULTICAST IFF_MULTICAST
#define IFF_PORTSEL IFF_PORTSEL
#define IFF_AUTOMEDIA IFF_AUTOMEDIA
#define IFF_DYNAMIC IFF_DYNAMIC
#define IFF_LOWER_UP IFF_LOWER_UP
#define IFF_DORMANT IFF_DORMANT
#define IFF_ECHO IFF_ECHO
#define IFF_VOLATILE (IFF_LOOPBACK|IFF_POINTOPOINT|IFF_BROADCAST|IFF_ECHO|\
IFF_MASTER|IFF_SLAVE|IFF_RUNNING|IFF_LOWER_UP|IFF_DORMANT)