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body { font-family: Segoe UI; font-size: 12pt; }</style></head><body class="plain"><div><div>Hi Simon,</div></div><div><br /></div>
<div>Am 04.04.2019 22:10:19, "Simon Kelley" <simon@thekelleys.org.uk> schrieb:</div><div><br /></div>
<div id="xd1baa5a3ee24498"><blockquote type="cite" class="cite2">
<div class="plain_line">Sorry, I wasn't clear.</div>
<div class="plain_line"> </div>
<div class="plain_line">A DHCP client can set a bit in the DHCP DISCOVER message that asks the</div>
<div class="plain_line">DHCP server to broadcast the reply to it. The packet captures you posted</div>
<div class="plain_line">showed exactly that. It's quite possible that the ThinkPad X260,</div>
<div class="plain_line">_doesn't_ do this, so the reply is not broadcast.</div></blockquote><div id="xd3ee025ce45e42c1a285e21cbe945629"><div class="plain"><div class="plain_line"></div>
<div class="plain_line">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.</div><div class="plain_line"><br /></div></div></div><blockquote type="cite" class="cite2"><div class="plain_line">A source of problems in the past has been firewall (iptables) rules that</div>
<div class="plain_line">block packets sent to the 255.255.255.255 broadcast address. Such a rule</div>
<div class="plain_line">on the machine running dnsmasq would break DHCP but only for clients</div>
<div class="plain_line">which set the broadcast bit in the DHCPDISCOVER. I don't know if the</div>
<div class="plain_line">packet capture happens before or after iptables, for the packet to be</div>
<div class="plain_line">blocked, but still appear (as it did) in the packet capture, it would</div>
<div class="plain_line">have to be before iptables.</div>
<div class="plain_line"><br /></div></blockquote><div id="xd1baa5a3ee24498"><div id="xd27dd0e37ef14d549fc423b9e69662dc"><div class="plain"><div class="plain_line">I've run now several tests and found the cause. First, it's not iptables, they allow here broadcast, as it should be.</div><div class="plain_line">It's my switch, who is not fowarding those broadcast pakets. It's an Netgear GS116Ev2.</div><div class="plain_line">I am speechless, why this switch doesn't forward them. According to the WebUI, Broadcast Forwarding is enabled and not filtered.</div><div class="plain_line"><br /></div><div class="plain_line">Conrad</div></div></div></div><blockquote type="cite" class="cite2">
</blockquote></div>
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