[Dnsmasq-discuss] DHCP lease-time weirdness

Johan Kruger johan at thekrugers.com
Thu Nov 28 19:49:43 GMT 2019


Hi,



I have a fairly simple DHCP setup on a Raspbian box, using dnsmasq 2.80. The relavant options in /etc/dnsmasq.conf are (yes, my home LAN is 10.168.8.0/24):



dhcp-range=10.168.8.10,10.168.8.200 # Also tried with 12h on the end, no difference

dhcp-authoritative

log-dhcp



I'm testing by running "dnsmasq -d" in a shell, so I can see what's going on.



When dnsmasq gets a DHCP request (I'm using "nmap --script broadcast-dhcp-discover" on another box to test), I see the following:



dnsmasq-dhcp: 811185697 broadcast response

dnsmasq-dhcp: 811185697 sent size:  1 option: 53 message-type  2

dnsmasq-dhcp: 811185697 sent size:  4 option: 54 server-identifier  10.168.8.254

dnsmasq-dhcp: 811185697 sent size:  4 option: 51 lease-time  2m

dnsmasq-dhcp: 811185697 sent size:  4 option: 58 T1  1m

dnsmasq-dhcp: 811185697 sent size:  4 option: 59 T2  1m45s

dnsmasq-dhcp: 811185697 sent size:  4 option:  1 netmask  255.255.255.0

dnsmasq-dhcp: 811185697 sent size:  4 option: 28 broadcast  10.168.8.255

dnsmasq-dhcp: 811185697 sent size:  4 option:  3 router  10.168.8.254

dnsmasq-dhcp: 811185697 sent size:  4 option:  6 dns-server  10.168.8.254

dnsmasq-dhcp: 811185697 sent size: 16 option: 15 domain-name  myhome



Notice the lease-time, T1 and T2 responses... 2 minutes??? Default should be 1 hour, and I get the same 2m if I specify 12h in the dhcp-range line.



I then added the following to the conf file:



dhcp-option=51,12h

dhcp-option=58,12h # DHCP Renewal (T1) Time

dhcp-option=59,12h # DHCP Rebinding (T2) Time



Then the response looked like this:



dnsmasq-dhcp: 1994188113 broadcast response

dnsmasq-dhcp: 1994188113 sent size:  1 option: 53 message-type  2

dnsmasq-dhcp: 1994188113 sent size:  4 option: 54 server-identifier  10.168.8.254

dnsmasq-dhcp: 1994188113 sent size:  4 option: 51 lease-time  2m

dnsmasq-dhcp: 1994188113 sent size:  4 option: 58 T1  1m

dnsmasq-dhcp: 1994188113 sent size:  4 option: 59 T2  1m45s

dnsmasq-dhcp: 1994188113 sent size:  4 option:  1 netmask  255.255.255.0

dnsmasq-dhcp: 1994188113 sent size:  4 option: 28 broadcast  10.168.8.255

dnsmasq-dhcp: 1994188113 sent size:  4 option:  3 router  10.168.8.254

dnsmasq-dhcp: 1994188113 sent size:  4 option:  6 dns-server  10.168.8.254

dnsmasq-dhcp: 1994188113 sent size: 16 option: 15 domain-name  myhome

dnsmasq-dhcp: 1994188113 sent size:  4 option: 51 lease-time  12h



Two lease-time responses, at least the correct 12h one is second so the client uses that, but notice that T1 and T2 are still wrong.



The upshot is that most DHCP clients (Android and Windows anyway) just reject the response.



For now, I guess I'll have to install isc-dhcp-server, since 2.80 is the only version of dnsmasq available for Raspdian 10 (buster). Bit of overkill for a little home LAN only running a couple of devices, but there we have it.
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