[VLAN] Using DHCPc on a VLAN interface

[Sylvain Beucler]

Hi,

On Thu, Sep 14, 2006 at 05:17:28PM +0200, Peter Stuge wrote:
> On Thu, Sep 14, 2006 at 12:25:43PM +0200, Sylvain Beucler wrote:
> > Shouldn't it work?
> 
> I think it should.
> 
> I would check what is actually on the wire, to confirm that nothing
> is really leaving the box. (Use a hub.)
> 
> I would also investigate what dhclient actually does by running it
> under strace.
> 
> I would also try other DHCP clients. Personally I like dhcpcd but
> there's also pump and udhcpc.

Thanks for the advice.


It turns out that something goes out of the wire, the DHCP server
machine receives it, yet does not reply (I doubt the DHCP server
daemon itself sees it). Or, the reply does not show on tcpdump.


This is strange really: tcpdump -vv shows that the initial broadcast
paquet is nearly identical for eth0 and eth0.5. Only a 'xid' field
differs, which is one-char shorter on the vlan:
* pump -i eth0.5
22:14:24.039510 IP (tos 0x0, ttl  64, id 0, offset 0, flags [none], length: 576) 0.0.0.0.bootpc > 255.255.255.255.bootps: BOOTP/DHCP, Request from 00:40:ca:d5:1c:57, length: 548, xid:0xb4c6a13, flags: [none] (0x0000)
	  Client Ethernet Address: 00:40:ca:d5:1c:57 [|bootp]
[nothing]

* pump -i eth0
22:14:26.495647 IP (tos 0x0, ttl  64, id 0, offset 0, flags [none], length: 576) 0.0.0.0.bootpc > 255.255.255.255.bootps: BOOTP/DHCP, Request from 00:40:ca:d5:1c:57, length: 548, xid:0xd54c6b50, flags: [none] (0x0000)
	  Client Ethernet Address: 00:40:ca:d5:1c:57 [|bootp]
[DHCP offer/request follows]


The behavior is the same with all DHCP clients I tries (dhclient,
dhclient3, pump, dhcpcd). I also tried dhcp3-server and udhcpd (and my
ISP's box's) :/


As for the strace, dhclient3 times out on a select:

time([1158267744])                      = 1158267744
time([1158267744])                      = 1158267744
rt_sigaction(SIGPIPE, {0xa7ea57e0, [], 0}, {SIG_DFL}, 8) = 0
send(3, "<30>Sep 14 23:02:24 dhclient: DH"..., 91, 0) = 91
rt_sigaction(SIGPIPE, {SIG_DFL}, NULL, 8) = 0
write(2, "DHCPDISCOVER on eth0.5 to 255.25"..., 61DHCPDISCOVER on eth0.5 to 255.255.255.255 port 67 interval 10) = 61
write(2, "\n", 1
)                       = 1
sendto(8, "\377\377\377\377\377\377\0@\312\325\34W\10\0E\20\1H\0\0"..., 342, 0, {sa_family=AF_PACKET, proto=0x6574, if892219496, pkttype=PACKET_HOST, addr(0)={0, }, 16) = 342
select(9, [7 8], [], [], {10, 0})       = 0 (Timeout)

time([1158267754])                      = 1158267754
time([1158267754])                      = 1158267754
[...]


A (newbie) question though: theoretically how could the eth0.5 grab a
DHCP offer?  Doesn't 'eth0' steal it first?

Thanks,

-- 
Sylvain