[Dnsmasq-discuss] Incorrect broadcast address given

OB Lutz ob.lutz at gmail.com
Wed Oct 8 15:51:54 BST 2008

# ip addr show
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 16436 qdisc noqueue state UNKNOWN
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet brd scope host lo
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast
state UP qlen 100
    link/ether 00:d0:12:09:91:52 brd ff:ff:ff:ff:ff:ff
3: eth1: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc pfifo_fast
state DOWN qlen 100
    link/ether 00:d0:12:19:91:52 brd ff:ff:ff:ff:ff:ff
4: br-lan: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue
    link/ether 00:d0:12:09:91:52 brd ff:ff:ff:ff:ff:ff
    inet brd scope global br-lan
5: wifi0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast
state UNKNOWN qlen 1000
    link/ieee802.11 00:15:6d:63:d4:31 brd ff:ff:ff:ff:ff:ff
6: ath0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast
state UP qlen 1000
    link/ether 00:15:6d:63:d4:31 brd ff:ff:ff:ff:ff:ff
    inet brd scope global ath0
pfifo_fast state UNKNOWN qlen 500
    inet brd scope global bmf0

On Wed, Oct 8, 2008 at 9:58 AM, Simon Kelley <simon at thekelleys.org.uk> wrote:
> OB Lutz wrote:
>> The routing is fine. All the eth*s are bridged to br-lan, which is
>> given ( ath0 is,
>> ath1 iptables rules take care of forwarding
>> appropriate traffic around. Routing is not an issue.
>> Shoving various log messages into dnsmasq, i could see that when going
>> through the block of code at dhcp.c: complete_context,line 404-405
>> if (is_same_net(broadcast, context->start, context->netmask))
>>                    context->broadcast = broadcast;
>> Prior to sending the response back, dnsmasq would pass through here
>> about 4 times. Most passes the correct broadcast address
>> would be put in context->broadcast, but the final pass always put in
>> the incorrect I assume the only pass with relevance to
>> the broadcast address offered in  dnsmasq's response was the last one.
> That code gets called for each address associated with each interface. Maybe
> you have more than one address set up on the interface, with a different
> broadcast address for some. What does
> ip addr show
> return?
>> Now, I'm no expert in the ways of dnsmasq so maybe I'm just not seeing
>> something thats blatantly obvious to all of you. But seeing as how
>> dhcp is dhcp and ip routing is ip routing, why would my network setup
>> have any influence on the address dnsmasq gives for broadcast? If i
>> tell it to give addresses in the range of through
>> with a mask of, why should it do any
>> different? It happily respects my configuration when I force the
>> broadcast address, why should it ignore my other configs?
> If you don't explicitly configure it, it tries to use the interface
> configuration. To understand why it's doing what it is, we need to know what
> that is.
> Cheers,
> Simon.
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