[Dnsmasq-discuss] DHCP renew and rebind time weirdness

Geert Stappers stappers at stappers.nl
Thu Nov 28 21:09:15 GMT 2019


On Thu, Nov 28, 2019 at 09:49:43PM +0200, Johan Kruger wrote:
> 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.

Whatever works for you and thanks for the joke





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