Re: [Dnsmasq-discuss] DHCP DISCOVER and DHCPACK on different subnets getting lost

[Albert ARIBAUD]

Hi Grant,

Le Thu, 16 Mar 2017 20:36:53 +1100
Grant Traynor  a écrit:

Only on this point:

> There is no DHCPNACK offered by dnsmasq?
> 
> It almost seems as though it's ignoring the subnet when it offers the
> IP address?

Dnsmasq will send DHCPNAKs only if it is configured as authoritative
(that is, it can assume that no other DNS server can answer). How is
yours configured?

Amicalement,
-- 
Albert.

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[Dnsmasq-discuss] DHCP DISCOVER and DHCPACK on different subnets getting lost

[Grant Traynor]

Hi,

I have a few USB ethernet interfaces that can be hot plugged by users.
These are managed by NetworkManager (shared), which automatically allocates
an IP address to the interface, and starts up dnsmasq to serve any DHCP
requests on that interface.

The problem occurs with some devices which have already been assigned an IP
address come back to dnsmasq for an address. When the interface comes back
up, the device does a DHCPDISCOVER on the old subnet, but the OFFER goes
back on the new subnet. (maybe?)

06:22:50.881436 1a:2b:3c:4d:16:f3 (oui Unknown) > Broadcast, ethertype IPv4
(0x0800), length 390: 0.0.0.0.bootpc > 255.255.255.255.bootps: BOOTP/DHCP,
Request from 1a:2b:3c:4d:16:f3 (oui Unknown), length 348
06:23:08.606107 30:37:16:55:14:51 (oui Unknown) > Broadcast, ethertype IPv4
(0x0800), length 291: 10.42.2.251.bootpc > 255.255.255.255.bootps:
BOOTP/DHCP, Request from 30:37:16:55:14:51 (oui Unknown), length 249
06:23:08.606874 1a:2b:3c:4d:16:f3 (oui Unknown) > Broadcast, ethertype ARP
(0x0806), length 42: Request who-has 10.42.0.251 tell 10.42.0.1, length 28
06:23:09.600302 1a:2b:3c:4d:16:f3 (oui Unknown) > Broadcast, ethertype ARP
(0x0806), length 42: Request who-has 10.42.0.251 tell 10.42.0.1, length 28
06:23:10.600308 1a:2b:3c:4d:16:f3 (oui Unknown) > Broadcast, ethertype ARP
(0x0806), length 42: Request who-has 10.42.0.251 tell 10.42.0.1, length 28
06:23:11.110214 1a:2b:3c:4d:16:f3 (oui Unknown) > Broadcast, ethertype IPv4
(0x0800), length 342: 10.42.0.1.bootps > 255.255.255.255.bootpc:
BOOTP/DHCP, Reply, length 300

The device seems to be looking for the old DHCP server:

06:23:34.768933 30:37:16:55:14:51 (oui Unknown) > Broadcast, ethertype ARP
(0x0806), length 60: Request who-has 10.42.2.1 tell 10.42.2.251, length 46

My routing table is:
default via 192.168.1.1 dev eth0  metric 202
10.42.0.0/24 dev eth1  proto kernel  scope link  src 10.42.0.1
169.254.0.0/16 dev wlan0  proto kernel  scope link  src 169.254.214.238
 metric 303
192.168.1.0/24 dev eth0  proto kernel  scope link  src 192.168.1.20  metric
202
192.168.44.0/24 dev wlan0  proto kernel  scope link  src 192.168.44.1


There is no DHCPNACK offered by dnsmasq?

It almost seems as though it's ignoring the subnet when it offers the IP
address?

Has anyone else seen this behavior or can anyone shed some light?

Kind Regards,
Grant.

-- 
Grant Traynor
Chief Technology Officer
SwitchDin Pty Ltd
Mob +61.428408558 <+61%20428%20408%20558>
grant.tray...@switchdin.com
http://www.switchdin.com/ 
linkedin.com/in/grant-traynor-636b095


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