I am doing some source code porting works on linux kernel recently.
After I reboot my server (ubuntu 14.04) into new kernel version of 3.19.8, it can’t be connected by using ssh but only by using serial port.
The server is using a USB network card, so firstly I suspect some kernel driver for USB NIC has missing in .config file. Therefore I boot back into the old version kernel and try to find some useful information in ‘dmesg’:

asix 3-1.4:1.0 eth5: register 'asix' at usb-0000:00:11.0-1.4, ASIX AX88772B USB 2.0 Ethernet, 00:24:9c:04:44:28
......
eth3: link up, 100Mbps, full-duplex, lpa 0x45E1

The eth3 is using MII port, so when I try to grep “link up …lpa” in 3.19.8 kernel source code, I find out it must be printed by these codes in drivers/net/mii.c:

unsigned int mii_check_media (struct mii_if_info *mii,
                  unsigned int ok_to_print,
                  unsigned int init_media)
{
......
    if (ok_to_print)
        netdev_info(mii->dev, "link up, %uMbps, %s-duplex, lpa 0x%04X\n",
                lpa2 & (LPA_1000FULL | LPA_1000HALF) ? 1000 :
                media & (ADVERTISE_100FULL | ADVERTISE_100HALF) ?
                100 : 10,
                duplex ? "full" : "half",
                lpa);
......

The only place “ASIX AX88772B” driver call mii_check_media is in drivers/net/usb/asix_devices.c:

static int ax88772_link_reset(struct usbnet *dev)
{
    u16 mode;
    struct ethtool_cmd ecmd = { .cmd = ETHTOOL_GSET };

    mii_check_media(&dev->mii, 1, 1);
    mii_ethtool_gset(&dev->mii, &ecmd);
    mode = AX88772_MEDIUM_DEFAULT;
......
static const struct driver_info ax88772b_info = {
    .description = "ASIX AX88772B USB 2.0 Ethernet",                       
    .bind = ax88772_bind,
    .unbind = ax88772_unbind,                                              
    .status = asix_status,
    .link_reset = ax88772_link_reset,                                      
    .reset = ax88772_reset,
    .flags = FLAG_ETHER | FLAG_FRAMING_AX | FLAG_LINK_INTR |               
             FLAG_MULTI_PACKET,
    .rx_fixup = asix_rx_fixup_common,                                      
    .tx_fixup = asix_tx_fixup,                                             
    .data = FLAG_EEPROM_MAC,                                               
};                                                                         

So far, the reason is that: the system does not “reset” the USB network card after booting up. But why it only forget to reset USB NIC in 3.19.8 kernel? After checking the /etc/network/interfaces:

auto eth0
iface eth0 inet dhcp
auto eth3
iface eth3 inet dhcp
auto usbnet0

the answer is: the device name of the USB NIC has been changed to “eth5” by udevd in 3.9.18 kernel (new version kernel recognise new network port so eth0/eth1/eth2/eth3/eth4 all has been occupied by system) so the network scripts can’t start it up.

Solution:
Fix the name of USB NIC by adding below content into /etc/udev/rules.d/70-persistent-net.rules:

SUBSYSTEM=="net", ACTION=="add", DRIVERS=="?*", ATTR{address}=="00:24:9c:04:44:28", ATTR{dev_id}=="0x0", ATTR{type}=="1", KERNEL=="eth*", NAME="usbnet0"

and add configurations into /etc/network/interfaces to start “usbnet0” up automatically:

auto usbnet0
iface usbnet0 inet dhcp