Client can send `DHCPDISCOVER` but cannot receive an offer

Installing and Using OpenWrt
1

Hello everyone, I'm trying to understand why a client connected to a switch can broadcast DHCPDISCOVER messages but cannot receive the DHCPOFFER from the router.
To explain my network I prepared a diagram:

                                        x86 OpenWRT "SWITCH"                                             
                                +---------------------------------+                             
                                |                      +----------|                             
                                |                      | untagged |                             
                                |               +------+    to    |                             
                                |--------+      |      |  vid 17  |                             
        ROUTER                  | trunk  |      |      +----------|             CLIENT 1        
+---------------------+         | vid 11 +---BRIDGE               |       +--------------------+
|            +--------|   +-----+ vid 17 |      |      +----------|       |                    |
|            | trunk  |   |     |--------+      |      | untagged |       |----------+         |
|            | vid 11 +---+     |               +------+    to    +-------+   DHCP   |         |
|-------+    | vid 17 |         |                      |  vid 17  |       | untagged |         |
|  wan  |    +--------|         |                      +----------|       |----------+         |
| pppoe |    +--------|         +---------------------------------+       |                    |
|-------+    | trunk  |                                                   +--------------------+
|            | vid 11 +-----------------------------------+                                     
|            | vid 17 |                                   |                                     
|            +--------|                                   |                      CLIENT 2       
+---------------------+                                   |               +--------------------+
                                                          |               |                    |
                                                          |               |--------+           |
                                                          |               |  DHCP  |           |
                                                          +---------------+ vid 17 |           |
                                                                          |--------+           |
                                                                          |                    |
                                                                          +--------------------+

You can see that my router has two LAN ports that I made into trunks with only tagged packages using VLANs 11 and 17. To one port I connected a client that I configured to use VLAN 17 and it can get an IP using DHCP. To the other port I connect a x86 OpenWRT that I want to use as a managed switch; it can get an IP using DHCP through VLAN 17, and bridges a trunk port with two ports to receive untagged packages and tag them using VLAN 17.

My problem is, when I connect a client to one of the "untagged" ports of the "switch" I can see logs on the router of that client macaddr broadcasting DHCPDISCOVER and the router offering a DHCPOFFER, but these packages never arrive back to the client. If I try running a tcpdump on the client I can only see packages going out but nothing coming in. It seems that the bridge on the "switch" is not working....

On the OpenWRT x86 "switch", I have disabled/stopped the firewall, dnsmasq and odhcpcd.

Here is the /etc/config/network:

config interface loopback
  option device lo
  option proto static
  option ipaddr 127.0.0.1
  option netmask 255.0.0.0

config device
	option name switch
	option type bridge
	option ipv6 0
	list ports eth0
	list ports eth1
	list ports eth2

config bridge-vlan
	option device switch
	option vlan 11
	list ports 'eth0:t'
	
config bridge-vlan
	option device switch
	option vlan 17
	list ports 'eth0:t'
	list ports 'eth1:u*'
	list ports 'eth2:u*'

config device
  option name main
  option type 8021q
  option ifname switch
  option vid 17
  option ipv6 0

config interface wan
  option device main
  option proto dhcp

Am I missing anything on this bridge config?

2

The logs I see on the router are:

$ logread -f
Fri Jan 24 18:53:18 2025 daemon.info dnsmasq-dhcp[1]: DHCPDISCOVER(vlan-main) CLIENT1MACADDR
Fri Jan 24 18:53:18 2025 daemon.info dnsmasq-dhcp[1]: DHCPOFFER(vlan-main) 10.0.17.198 CLIENT1MACADDR
Fri Jan 24 18:53:21 2025 daemon.info dnsmasq-dhcp[1]: DHCPDISCOVER(vlan-main) CLIENT1MACADDR
Fri Jan 24 18:53:21 2025 daemon.info dnsmasq-dhcp[1]: DHCPOFFER(vlan-main) 10.0.17.198 CLIENT1MACADDR

and they keep going forever...

3

Is this an OpenWrt based router?

If so, let's see the configs of that device to make sure we don't see any problems:

Please connect to your OpenWrt device using ssh and copy the output of the following commands and post it here using the "Preformatted text </> " button:
grafik
Remember to redact passwords, MAC addresses and any public IP addresses you may have:

ubus call system board
cat /etc/config/network
cat /etc/config/wireless
cat /etc/config/dhcp
cat /etc/config/firewall

This is a very unusual choice for a switch. Most x86 systems will lack a hardware switch. This means that you've got multiple individually routed ports that you can obviously bridge together, but it is not a proper switch. Aside from that, most x86 systems will have at least an order of magnitude more power consumption than a simple managed switch (purpose built).

Now, on to the configuration of the OpenWrt device...

If you're using it as a switch, why do you have this:

Do you want the switch to use VLAN 17 for its address? If so, do this instead (delete the above and add the next part below):

config interface 'lan'
  option device 'switch.17'
  option proto 'dhcp'

Another thing I'm noticing, although maybe it is an issue with the copy/paste of the config, is that the values are not contained in single quotes (option something 'value'). That is a bit strange and could be related to the issues.

Anyway, after changing deleting the wan and 802.1q stanzas, then adding the lan as I recommended, reboot and test again.

4

Hey, first of all thank you for looking into my problem.

I figured what was the problem and it was not related to OpenWRT, it was a driver issue.

I was trying to mimic my real network in a virtual network using VirtualBox, and it seems that the network adapters that vbox emulates have some trouble with bridges and tagged network packages. I ran the same configuration using QEMU and everything worked as expected.

For people googling this in the future: stay away from VirtualBox network adapter emulation: QEMU works as expected and allows your guest-to-guest connections to be done unix sockets.

5

Regarding your comments,

Do you want the switch to use VLAN 17 for its address? If so, do this instead (delete the above and add the next part below):

I don't think it makes a difference. Does it?

Another thing I'm noticing, although maybe it is an issue with the copy/paste of the config, is that the values are not contained in single quotes (option something 'value'). That is a bit strange and could be related to the issues.

I have been generating config files using some automation and I missed this quoting detail. So far things have worked... but I think I'm going to change it to use the standard quotes.

6

I believe that VBox works with VLANs, although it's been a while since I tried it.

However, that said, virtualizing OpenWrt in general is a bit tricky because so much is dependent on the correct host config (I.e. the Virtualizer/hypervisor software configuration, the host OS, etc.).

7

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