[Dnsmasq-discuss] dhcp doesn't work with dnsmasq in multi ip environment

Philipp Kolmann kolmann at zid.tuwien.ac.at
Thu Aug 4 08:45:48 BST 2016


Hi,

I have a special setup with two IP Subnets on one Interface:

3: eth1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast 
state UP group default qlen 1000
     inet 172.18.92.60/24 brd 172.18.92.255 scope global eth1
        valid_lft forever preferred_lft forever
     inet 172.17.32.1/24 brd 172.17.32.255 scope global eth1:0
        valid_lft forever preferred_lft forever

primary IP is 172.18.92.60, secondary is 172.17.32.1

I now want to use dnsmasq to supply clients identified by MAC Address an 
IP address in the secondary range:

interface=eth1
dhcp-range = set:power, 172.17.32.0, static, 255.255.255.0
dhcp-host = set:power, 00:19:32:00:8f:90, fh-tuck-1p, 172.17.32.8,12h
dhcp-option = tag:power, option:ntp-server, 172.17.32.1
dhcp-option = tag:power, option:dns-server, 172.17.32.1
dhcp-option = tag:power, option:router,  172.17.32.1


My issue is now, that the Network-enabled power distribution unit (GUDE 
EXPERT POWER CONTROL NET D4X 1200) seems to have an issue if dnsmasq is 
providing an IP address from the secondary range. If I turn primary and 
secondary IP, it works.

I have now also tested with ISC DHCP server and there the PDU accepts 
the IP Address. The only difference that i found is the following:

The Source IP of the packets is the primary of the interface: 172.18.92.60

in ISC DHCP the option 54 IP (DHCP Server Identifier) is the primary, 
172.18.92.60

in dnsmasq the option 54 IP is the secondary: 172.17.32.1

It seems that this is a discrepancy and therefore the PDU doesn't accept 
the IP address, since the IP source and the DHCP Server Identifier do 
not match.

Find attached the wireshark logs.

I tried to set

dhcp-option = tag:power, 54,  172.18.92.60

but this option is not used for the packet.

Any help is appriciated.
Thanks
Philipp

-- 
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DI Mag. Philipp Kolmann                  mail:kolmann at zid.tuwien.ac.at
Technische Universitaet Wien                  web:www.zid.tuwien.ac.at
Zentraler Informatikdienst (ZID)                 tel: +43(1)58801-42011
Wiedner Hauptstr. 8-10, A-1040 Wien                        DVR: 0005886
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