Je viens de vérifier, quasi sur que c'est ca.
Voila mon tcpdump du client dhcp sur le bridge et la réponse instantanée.
ubnt@ubnt:~$ sudo tcpdump -v -i br0 -s 65535
tcpdump: WARNING: br0: no IPv4 address assigned
tcpdump: listening on br0, link-type EN10MB (Ethernet), capture size 65535 bytes
22:53:45.867775 IP (tos 0x10, ttl 128, id 0, offset 0, flags [none], proto UDP (17), length 343)
0.0.0.0.bootpc > 255.255.255.255.bootps: BOOTP/DHCP, Request from dc:9f:db:28:fa:74 (oui Unknown), length 315, xid 0x14da405, Flags [none]
Client-Ethernet-Address dc:9f:db:28:fa:74 (oui Unknown)
Vendor-rfc1048 Extensions
Magic Cookie 0x63825363
DHCP-Message Option 53, length 1: Request
Requested-IP Option 50, length 4: 10.54.56.154
Parameter-Request Option 55, length 5:
Subnet-Mask, Default-Gateway, NTP, WWW
Classless-Static-Route
Vendor-Class Option 60, length 5: "sagem"
Client-ID Option 61, length 7: ether 2c:39:96:31:30:b0
CLASS Option 77, length 40: "'FSVDSL_livebox.MLTV.softathome.Livebox3"
22:53:46.040948 IP (tos 0xc0, ttl 64, id 40232, offset 0, flags [none], proto UDP (17), length 401)
10.54.56.254.bootps > 10.54.56.154.bootpc: BOOTP/DHCP, Reply, length 373, xid 0x14da405, Flags [none]
Your-IP 10.54.56.154
Server-IP 192.168.3.254
Gateway-IP 10.54.131.94
Client-Ethernet-Address dc:9f:db:28:fa:74 (oui Unknown)
Vendor-rfc1048 Extensions
Magic Cookie 0x63825363
DHCP-Message Option 53, length 1: ACK
Server-ID Option 54, length 4: 192.168.3.254
Lease-Time Option 51, length 4: 98742
Subnet-Mask Option 1, length 4: 255.255.255.128
RB Option 59, length 4: 86399
RN Option 58, length 4: 49371
Classless-Static-Route Option 121, length 85: (172.23.12.0/22:10.54.56.254),(172.20.224.167/32:10.54.56.254),(172.19.20.0/23:10.54.56.254),(193.253.67.88/29:10.54.56.254),(80.10.117.120/31:10.54.56.254),(81.253.206.0/24:10.54.56.254),(81.253.210.0/23:10.54.56.254),(193.253.153.228/32:10.54.56.254),(193.253.153.227/32:10.54.56.254),(81.253.214.0/23:10.54.56.254)
NTP Option 42, length 4: 172.20.224.167
WWW Option 72, length 4: 193.253.67.89
22:53:51.607603 ARP, Ethernet (len 6), IPv4 (len 4), Request who-has 10.54.56.254 tell 10.54.56.154, length 28
22:53:51.608743 ARP, Ethernet (len 6), IPv4 (len 4), Reply 10.54.56.254 is-at 00:03:fa:08:38:01 (oui Unknown), length 46
22:53:51.608827 IP (tos 0xc0, ttl 64, id 0, offset 0, flags [DF], proto UDP (17), length 76)
10.54.56.154.ntp > 172.20.224.167.ntp: NTPv4, length 48
Client, Leap indicator: clock unsynchronized (192), Stratum 0 (unspecified), poll 6s, precision -14
Root Delay: 0.000000, Root dispersion: 0.000000, Reference-ID: (unspec)
Reference Timestamp: 0.000000000
Originator Timestamp: 0.000000000
Receive Timestamp: 0.000000000
Transmit Timestamp: 3607541631.607360363 (2014/04/26 22:53:51)
Originator - Receive Timestamp: 0.000000000
Originator - Transmit Timestamp: 3607541631.607360363 (2014/04/26 22:53:51)
22:53:51.611473 IP (tos 0x90, ttl 57, id 0, offset 0, flags [DF], proto UDP (17), length 76)
172.20.224.167.ntp > 10.54.56.154.ntp: NTPv4, length 48
Server, Leap indicator: (0), Stratum 4 (secondary reference), poll 6s, precision -18
Root Delay: 0.018417, Root dispersion: 0.032363, Reference-ID: 10.96.61.134
Reference Timestamp: 3607541596.645148992 (2014/04/26 22:53:16)
Originator Timestamp: 3607541631.607360363 (2014/04/26 22:53:51)
Receive Timestamp: 3607541631.610662996 (2014/04/26 22:53:51)
Transmit Timestamp: 3607541631.610695004 (2014/04/26 22:53:51)
Originator - Receive Timestamp: +0.003302616
Originator - Transmit Timestamp: +0.003334616
^C
6 packets captured
6 packets received by filter
0 packets dropped by kernel