Bon finalement, après moultes essais, c'était bien un problème d'options DHCP, j'ai réduit le dhclient.conf et c'est passé:
# /etc/dhcp3/dhclient6_eth5_832.conf
option dhcp6.auth code 11 = string;
option dhcp6.vendorclass code 16 = string;
option dhcp6.userclass code 15 = string;
option dhcp6.vendor-specific-info code 17 = string;
#External interface (VLAN must be 832 for Orange)
interface "eth5.832" {
send dhcp6.vendorclass 00:00:04:0e:00:05:73:61:67:65:6d;
send dhcp6.userclass 00:2b:46:53:56:44:53:4c:5f:6c:69:76:65:62:6f:78:2e:49:6e:74:65:72:6e:65:74:2e:73:6f:66:74:61:74:68:6f:6d:65:2e:6c:69:76:65:62:6f:78:34:0a;
send dhcp6.auth <chaine_auth_longue_idem_ipv4;
request dhcp6.name-servers, dhcp6.vendorclass, dhcp6.userclass, dhcp6.auth;
}