[Dnsmasq-discuss] DHCP server not assigning IP to RTMU86

Fri May 20 16:43:43 UTC 2022

I have a theory:

In the logs, it takes between 7 and 9 seconds after carrier detect on 
eth0 (I guess that's when you plug the RTMU86 in?) before eth0 is 
allocated IP address 192.168.6.1. Until the address is allocated, 
dnsmasq will ignore DHCP requests and log

"DHCP packet received on eth0 which has no address"

If the RTMU86 send a DHCP discover faster than 7 seconds and doesn't 
retry in a reasonable time, that could explain the problem.

ISC dhcpd may not need the local interface to be configured with an IP 
address, which would explain the different behaviour.

The packet capture below shows just one DHCPDISCOVER being sent from
  0.0.0.0.bootpc > 255.255.255.255.bootps

and it's before the  IP 192.168.6.1 > igmp.mcast.net which markswhen 
eth0 gets an IP address.

If you repeat the same test and leave the capture running for a few 
minutes, do you see any more  0.0.0.0.bootpc > 255.255.255.255.bootps 
packets, and if you do, do they get a reply?

If I'm right, the solution may be to configure eth0 with a statis IP 
address, so it doesn't have to wait for dhcpcd to go through a lengthy 
process to give it an IP address every time the cable gets plugged in.

Simon.

On 15/05/2022 15:40, Kamil via Dnsmasq-discuss wrote:
> 
>     On Sun, May 15, 2022 at 01:10:40AM +0200, Kamil via Dnsmasq-discuss
>     wrote:
>      > On Sat, May 14, 2022 at 11:27:34AM +0200, Geert Stappers via
>     Dnsmasq-discuss wrote:
>      > > On Sat, May 14, 2022 at 02:53:51AM +0200, Kamil via
>     Dnsmasq-discuss wrote:
>        ...
>      > > > 1. I plug one device - it works.
>      > > > 2. I unplug it and plug the RTMU86 - doesn't work.
>      > > > 3. I Unplug RTMU86 and plug any other device - it works.
>      > > >
>        ...
>      > > Be yourself, accept yourself. Full focus on the challenge that
>     you are
>      > > facing.
>      >
>      > Sure
>      >
> 
>     What proof is available of
>     the RTMU86 sending DHCP packages
>     when pluging the ethercable back in?
> 
> 
> RTMU86 has a led indicator that is lid when the address has been 
> assigned to it.
> 1. If I plug RTMU86 to the home router - the led is on, I can find its 
> IP in the router's lease list and I can ping this IP
> 2. If I plug RTMU86 to the RPi with isc-dhcp-server configured - the led 
> is on, I can find its IP in the isc-dhcp-server's lease list and I can 
> ping this IP
> 3. If I plug RTMU86 to the RPi with Dnsmasq configured - I can use 
> tcpdump which gives me this output (I changed MAC addresses):
>  > 16:17:31.125779 IP 0.0.0.0.bootpc > 255.255.255.255.bootps: 
> BOOTP/DHCP, Request from fa:XX:XX:XX:XX:1c (oui Unknown), length 300
> So I presume that RTMU86 indeed sends DHCP packages, but the led is off 
> and I can not find it in the Dnsmasq lease list.
> 
> Full tcpdump log (with changed MAC addresses) below:
>  > 16:17:28.175124 IP6 :: > ff02::16: HBH ICMP6, multicast listener 
> report v2, 2 group record(s), length 48
>  > 16:17:28.187120 IP6 :: > ff02::16: HBH ICMP6, multicast listener 
> report v2, 4 group record(s), length 88
>  > 16:17:28.199412 ARP, Request who-has 192.168.6.1 tell 0.0.0.0, length 28
>  > 16:17:28.235076 IP6 :: > ff02::16: HBH ICMP6, multicast listener 
> report v2, 4 group record(s), length 88
>  > 16:17:28.483118 IP6 :: > ff02::1:fffb:fe9: ICMP6, neighbor 
> solicitation, who has fe80::XX:XX:XX:fe9, length 32
>  > 16:17:29.507285 IP6 fe80::XX:XX:XX:fe9 > ff02::16: HBH ICMP6, 
> multicast listener report v2, 5 group record(s), length 108
>  > 16:17:29.507966 IP6 fe80::XX:XX:XX:fe9 > ip6-allrouters: ICMP6, 
> router solicitation, length 16
>  > 16:17:29.519049 IP6 fe80::XX:XX:XX:fe9 > ff02::16: HBH ICMP6, 
> multicast listener report v2, 1 group record(s), length 28
>  > 16:17:29.891065 IP6 fe80::XX:XX:XX:fe9 > ff02::16: HBH ICMP6, 
> multicast listener report v2, 5 group record(s), length 108
>  > 16:17:30.019064 IP6 fe80::XX:XX:XX:fe9 > ff02::16: HBH ICMP6, 
> multicast listener report v2, 1 group record(s), length 28
>  > 16:17:30.077265 IP6 fe80::XX:XX:XX:fe9.mdns > ff02::fb.mdns: 0*- [0q] 
> 2/0/0 (Cache flush) PTR myhost.local., (Cache flush) AAAA 
> fe80::XX:XX:XX:fe9 (139)
>  > 16:17:30.151935 ARP, Request who-has 192.168.6.1 tell 0.0.0.0, length 28
>  > 16:17:31.125779 IP 0.0.0.0.bootpc > 255.255.255.255.bootps: 
> BOOTP/DHCP, Request from fa:XX:XX:XX:XX:1c (oui Unknown), length 300
>  > 16:17:31.728625 ARP, Request who-has 192.168.6.1 tell 0.0.0.0, length 28
>  > 16:17:32.152301 IP6 fe80::XX:XX:XX:fe9.mdns > ff02::fb.mdns: 0*- [0q] 
> 2/0/0 (Cache flush) PTR myhost.local., (Cache flush) AAAA 
> fe80::XX:XX:XX:fe9 (139)
>  > 16:17:33.510623 IP6 fe80::XX:XX:XX:fe9 > ip6-allrouters: ICMP6, 
> router solicitation, length 16
>  > 16:17:33.730051 ARP, Request who-has 192.168.6.1 tell 192.168.6.1, 
> length 28
>  > 16:17:33.739141 IP 192.168.6.1 > igmp.mcast.net 
> <http://igmp.mcast.net>: igmp v3 report, 1 group record(s)
> 
> PS. I've read in FAQ about 255.255.255.255 broadcast and invalid 
> firewall rules, but I don't have any firewall rules and I'm not changing 
> network config files when switching to isc-dhcp-server (which I use as a 
> benchmark)
> 
> Kind regards,
> Kamil
> 
> 
> 
>     Groeten
>     Geert Stappers
>     -- 
>     Silence is hard to parse
> 
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