Hi Simon,

Am 04.04.2019 22:10:19, "Simon Kelley" <si...@thekelleys.org.uk> schrieb:

Sorry, I wasn't clear.

A DHCP client can set a bit in the DHCP DISCOVER message that asks the
DHCP server to broadcast the reply to it. The packet captures you posted
showed exactly that. It's quite possible that the ThinkPad X260,
_doesn't_ do this, so the reply is not broadcast.
thank you! Now I see, what you mean. And you are absolutley right. My X260 
haven't set the broadcast flag, when looking into that packets.

A source of problems in the past has been firewall (iptables) rules that
block packets sent to the broadcast address. Such a rule
on the machine running dnsmasq would  break DHCP but only for clients
which set the broadcast bit in the DHCPDISCOVER. I don't know if the
packet capture happens before or after iptables, for the packet to be
blocked, but still appear (as it did) in the packet capture, it would
have to be before iptables.

I've run now several tests and found the cause. First, it's not iptables, they 
allow here broadcast, as it should be.
It's my switch, who is not fowarding those broadcast pakets. It's an Netgear 
I am speechless, why this switch doesn't forward them. According to the WebUI, 
Broadcast Forwarding is enabled and not filtered.

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