@VirtuBox Je viens de créer un nouveau conteneur LXC (sans privilège) .
J'ai créé dans Proxmox 4.4, les 8 interfaces :
auto lo
iface lo inet loopback
iface lo inet6 loopback
# interfaces(5) file used by ifup(8) and ifdown(8)
# Include files from /etc/network/interfaces.d:
source-directory /etc/network/interfaces.d
auto eth0
iface eth0 inet static
address 123.123.5.144
netmask 255.255.255.255
# — BEGIN PVE —
post-up ip route add 123.123.5.254 dev eth0
post-up ip route add default via 123.123.5.254 dev eth0
pre-down ip route del default via 123.123.5.254 dev eth0
pre-down ip route del 123.123.5.254 dev eth0
# — END PVE —
auto eth1
iface eth1 inet static
address 123.123.5.145
netmask 255.255.255.255
# — BEGIN PVE —
post-up ip route add 123.123.5.254 dev eth1
post-up ip route add default via 123.123.5.254 dev eth1
pre-down ip route del default via 123.123.5.254 dev eth1
pre-down ip route del 123.123.5.254 dev eth1
# — END PVE —
auto eth2
iface eth2 inet static
address 123.123.5.146
netmask 255.255.255.255
# — BEGIN PVE —
post-up ip route add 123.123.5.254 dev eth2
post-up ip route add default via 123.123.5.254 dev eth2
pre-down ip route del default via 123.123.5.254 dev eth2
pre-down ip route del 123.123.5.254 dev eth2
# — END PVE —
auto eth3
iface eth3 inet static
address 123.123.5.147
netmask 255.255.255.255
# — BEGIN PVE —
post-up ip route add 123.123.5.254 dev eth3
post-up ip route add default via 123.123.5.254 dev eth3
pre-down ip route del default via 123.123.5.254 dev eth3
pre-down ip route del 123.123.5.254 dev eth3
# — END PVE —
auto eth4
iface eth4 inet static
address 123.123.5.148
netmask 255.255.255.255
# — BEGIN PVE —
post-up ip route add 123.123.5.254 dev eth4
post-up ip route add default via 123.123.5.254 dev eth4
pre-down ip route del default via 123.123.5.254 dev eth4
pre-down ip route del 123.123.5.254 dev eth4
# — END PVE —
auto eth5
iface eth5 inet static
address 123.123.5.149
netmask 255.255.255.255
# — BEGIN PVE —
post-up ip route add 123.123.5.254 dev eth5
post-up ip route add default via 123.123.5.254 dev eth5
pre-down ip route del default via 123.123.5.254 dev eth5
pre-down ip route del 123.123.5.254 dev eth5
# — END PVE —
auto eth6
iface eth6 inet static
address 123.123.5.150
netmask 255.255.255.255
# — BEGIN PVE —
post-up ip route add 123.123.5.254 dev eth6
post-up ip route add default via 123.123.5.254 dev eth6
pre-down ip route del default via 123.123.5.254 dev eth6
pre-down ip route del 123.123.5.254 dev eth6
# — END PVE —
auto eth7
iface eth7 inet static
address 123.123.5.151
netmask 255.255.255.255
# — BEGIN PVE —
post-up ip route add 123.123.5.254 dev eth7
post-up ip route add default via 123.123.5.254 dev eth7
pre-down ip route del default via 123.123.5.254 dev eth7
pre-down ip route del 123.123.5.254 dev eth7
# — END PVE —
Je n'ai pas modifié manuellement, juste ajouté les interfaces avec leurs MAC Adresses virtuelles OVH depuis Proxmox 4.4.
… et ça fonctionne !
Tous les pings répondent 
Mais … pourtant j'avais fait un test avec un conteneur avec 3 IPv4 et le ping était aléatoire d'un reboot à l'autre…
Je vais comparer les fichiers /etc/network/interfaces …