Hi. I’ve upgraded my linode from wheezy to jessie. Then reboot and then I see that my site is down. I tried to ping my instance, 100.0% packet was lost.

Then I connected to instance by LISH and saw ifconfig: https://www.evernote.com/l/AF5dHgLEOWNL ... EBZmw7Aa98. You see I have docker and vpn. I disable it for cleaner result, so cleaner ifconfig is:

lo        Link encap:Local Loopback
          inet addr:127.0.0.1  Mask:255.0.0.0
          inet6 addr: ::1/128 Scope:Host
          UP LOOPBACK RUNNING  MTU:65536  Metric:1
          RX packets:10 errors:0 dropped:0 overruns:0 frame:0
          TX packets:10 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0
          RX bytes:948 (948.0 B)  TX bytes:948 (948.0 B)

So I tried to configure eth0 manually:

sudo ifconfig eth0 inet 178.79.X.X
8021q: adding VLAN 0 to HW filter on device eth0

sudo ifconfig eth0 up

sudo ifconfig eth0
eth0      Link encap:Ethernet  HWaddr f2:3c:91:69:e5:58
          inet addr:178.79.X.X  Bcast:178.79.255.255  Mask:255.255.0.0
          inet6 addr: fe80::f03c:91ff:XXXX:XXXX/64 Scope:Link
          inet6 addr: 2a01:7e00::f03c:91ff:XXXX:XXXX/64 Scope:Global
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:344 errors:0 dropped:0 overruns:0 frame:0
          TX packets:6 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:26430 (25.8 KiB)  TX bytes:508 (508.0 B)

sudo route
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
10.15.0.0       *               255.255.255.0   U     0      0        0 tun0
172.17.0.0      *               255.255.0.0     U     0      0        0 docker0
178.79.0.0      *               255.255.0.0     U     0      0        0 eth0

sudo route add default gateway 178.79.157.1

And then my server became pingable from outside. There is my /etc/network/interfaces:

# This file describes the network interfaces available on your system
# and how to activate them. For more information, see interfaces(5).

# The loopback network interface
auto lo
iface lo inet loopback

# The primary network interface
allow-hotplug eth0
iface eth0 inet dhcp

It’s the same than before update. Why my server stopped getting network settings automatically on startup after upgrade?

I think you may be missing the:
auto eth0

above
iface eth0 inet dhcp