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Discussion about how net cable connect and disconnect are reflected in /sys.

-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

/**
 * 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)