Testing the effect of interface manipulation on the values of IFF_UP and IFF_RUNNING.

Links:

• operstates.txt

From include/uapi/linux/if.h:

enum net_device_flags {
/* for compatibility with glibc net/if.h */
#if __UAPI_DEF_IF_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 */
#endif /* __UAPI_DEF_IF_NET_DEVICE_FLAGS */
#if __UAPI_DEF_IF_NET_DEVICE_FLAGS_LOWER_UP_DORMANT_ECHO
        IFF_LOWER_UP                    = 1<<16, /* volatile */
        IFF_DORMANT                     = 1<<17, /* volatile */
        IFF_ECHO                        = 1<<18, /* volatile */
#endif /* __UAPI_DEF_IF_NET_DEVICE_FLAGS_LOWER_UP_DORMANT_ECHO */
};

Administrative versus operational state:

Linux distinguishes between administrative and operational state of an
interface. Administrative state is the result of "ip link set dev
<dev> up or down" and reflects whether the administrator wants to use
the device for traffic.

However, an interface is not usable just because the admin enabled it
- ethernet requires to be plugged into the switch and, depending on
a site's networking policy and configuration, an 802.1X authentication
to be performed before user data can be transferred. Operational state
shows the ability of an interface to transmit this user data.

Thanks to 802.1X, userspace must be granted the possibility to
influence operational state. To accommodate this, operational state is
split into two parts: Two flags that can be set by the driver only, and
a RFC2863 compatible state that is derived from these flags, a policy,
and changeable from userspace under certain rules.

ifconfig is getting hex value from SIOCGIFFLAGS which does dev_get_flags(dev).

4163:

enp3s0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        inet 192.168.0.15  netmask 255.255.255.0  broadcast 192.168.0.255
        inet6 fd00:f81d:f81:4062:2e56:dcff:fe25:ad47  prefixlen 64  scopeid 0x0<global>
        inet6 fe80::2e56:dcff:fe25:ad47  prefixlen 64  scopeid 0x20<link>
        ether 2c:56:dc:25:ad:47  txqueuelen 1000  (Ethernet)
        RX packets 81232  bytes 92092778 (87.8 MiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 36768  bytes 9024553 (8.6 MiB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

4098:

enp3s0: flags=4098<BROADCAST,MULTICAST>  mtu 1500
        inet 192.168.0.15  netmask 255.255.255.0  broadcast 192.168.0.255
        ether 2c:56:dc:25:ad:47  txqueuelen 1000  (Ethernet)
        RX packets 81290  bytes 92110209 (87.8 MiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 36836  bytes 9042100 (8.6 MiB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

4099:

enp3s0: flags=4099<UP,BROADCAST,MULTICAST>  mtu 1500
        inet 192.168.0.15  netmask 255.255.255.0  broadcast 192.168.0.255
        inet6 fd00:f81d:f81:4062:2e56:dcff:fe25:ad47  prefixlen 64  scopeid 0x0<global>
        inet6 fe80::2e56:dcff:fe25:ad47  prefixlen 64  scopeid 0x20<link>
        ether 2c:56:dc:25:ad:47  txqueuelen 1000  (Ethernet)
        RX packets 81318  bytes 92115343 (87.8 MiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 36917  bytes 9052750 (8.6 MiB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

/*
 *      Perform the SIOCxIFxxx calls, inside rcu_read_lock()
 */
static int dev_ifsioc_locked(struct net *net, struct ifreq *ifr, unsigned int cmd)
{
        int err;
        struct net_device *dev = dev_get_by_name_rcu(net, ifr->ifr_name);

        if (!dev)
                return -ENODEV;

        switch (cmd) {
        case SIOCGIFFLAGS:      /* Get interface flags */
                ifr->ifr_flags = (short) dev_get_flags(dev);
                return 0;