OpenWRT WAP seemingly not forwarding DHCPOFFER

Installing and Using OpenWrt
1

I am running OpenWRT as a “dumb” wifi access point / “bridged” AP, configured according to https://openwrt.org/docs/guide-user/network/wifi/wifiextenders/bridgedap#configuration_via_luci

I am having trouble connecting clients to it, and I’ve narrowed it down the the DHCPOFFER getting lost. I have no clue why this should happen, and this is where I’d like to ask for help.

My home network is not particularly special:

[internet] ←DSL→ [FritzBox /w stock firmware] ←LAN→ [FritzBox /w OpenWRT]

For my analysis, I ran tcpdump on my laptop’s wifi (upper half of the screenshot) and also on the OpenWRT’s br-lan interface (lower half of the screenshot).

2025-12-23_09-54 dhcp
2025-12-23_09-54 dhcp1338×668 133 KB

The marked packet (black line) is the same packet, it is also the time reference for all other packets. This is ~30 seconds of my laptop trying to get a IPv4 via DHCP. In the upper half, you can see my laptop shout out into the world for an address. Seemingly no reply is offered.

In the lower screenshot (as seen from the br-lan interface), we see that besides the DHCP Request there are also packets for Discover and most importantly: Offer! So my stock-firmware FritzBox actually offers an address, but it never gets forwarded to my laptop, it is somehow lost in the OpenWRT FritzBox.

There are a lot of devices that report that they don’t get an IP address, where I don’t have the means to capture on-device, but the pattern is effectively the same. On this AP I can’t get DHCP addresses.

Here is further, hopefully relevant, information:

ubus call system board 
{
	"kernel": "6.6.119",
	"hostname": "wifi-ap-2",
	"system": "ARMv7 Processor rev 5 (v7l)",
	"model": "AVM FRITZ!Box 7530",
	"board_name": "avm,fritzbox-7530",
	"rootfs_type": "squashfs",
	"release": {
		"distribution": "OpenWrt",
		"version": "24.10.5",
		"revision": "r29087-d9c5716d1d",
		"target": "ipq40xx/generic",
		"description": "OpenWrt 24.10.5 r29087-d9c5716d1d",
		"builddate": "1766005702"
	}
}
cat /etc/config/wireless 
config wifi-device 'radio0'
	option type 'mac80211'
	option path 'platform/soc/a000000.wifi'
	option band '2g'
	option channel 'auto'
	option htmode 'HT20'
	option country 'DE'
	option cell_density '0'

config wifi-iface 'default_radio0'
	option device 'radio0'
	option network 'lan'
	option mode 'ap'
	option ssid '---redacted---'
	option encryption 'sae-mixed'
	option key '---redacted---'
	option ieee80211r '1'
	option mobility_domain '4757'
	option ft_over_ds '0'
	option ocv '0'

config wifi-device 'radio1'
	option type 'mac80211'
	option path 'platform/soc/a800000.wifi'
	option band '5g'
	option channel '36'
	option htmode 'VHT80'
	option country 'DE'
	option cell_density '0'

config wifi-iface 'default_radio1'
	option device 'radio1'
	option network 'lan'
	option mode 'ap'
	option ssid '---redacted---'
	option encryption 'sae-mixed'
	option key '---redacted---'
	option ieee80211r '1'
	option mobility_domain '4757'
	option ft_over_ds '0'
	option ocv '0'
cat /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 globals 'globals'
	option ula_prefix 'something::/48'
	option packet_steering '1'

config atm-bridge 'atm'
	option vpi '1'
	option vci '32'
	option encaps 'llc'
	option payload 'bridged'
	option nameprefix 'dsl'

config dsl 'dsl'
	option annex 'j'
	option tone 'b'
	option ds_snr_offset '0'

config device
	option name 'br-lan'
	option type 'bridge'
	list ports 'lan1'
	list ports 'lan2'
	list ports 'lan3'
	list ports 'lan4'
	option mtu '9000'
	option mtu6 '9000'

config interface 'lan'
	option device 'br-lan'
	option proto 'static'
	option ipaddr '192.168.42.12'
	option netmask '255.255.255.0'
	option ip6assign '60'
	option gateway '192.168.42.1'
	list dns '192.168.42.1'

config device
	option name 'dsl0'
	option macaddr '---redacted---'

config interface 'wan'
	option device 'dsl0'
	option proto 'pppoe'
	option username 'username'
	option password 'password'
	option ipv6 '1'

config interface 'wan6'
	option device '@wan'
	option proto 'dhcpv6'

config device
	option name 'eth0'
	option mtu '9000'
	option mtu6 '9000'

config device
	option name 'lan1'
	option mtu '9000'

config device
	option name 'lan2'
	option mtu '9000'

config device
	option name 'lan3'
	option mtu '9000'

config device
	option name 'lan4'
	option mtu '9000'
cat /etc/config/dhcp 
config dnsmasq
	option domainneeded '1'
	option boguspriv '1'
	option filterwin2k '0'
	option localise_queries '1'
	option rebind_protection '1'
	option rebind_localhost '1'
	option local '/lan/'
	option domain 'lan'
	option expandhosts '1'
	option nonegcache '0'
	option cachesize '1000'
	option authoritative '1'
	option readethers '1'
	option leasefile '/tmp/dhcp.leases'
	option resolvfile '/tmp/resolv.conf.d/resolv.conf.auto'
	option nonwildcard '1'
	option localservice '1'
	option ednspacket_max '1232'
	option filter_aaaa '0'
	option filter_a '0'

config dhcp 'lan'
	option interface 'lan'
	option start '100'
	option limit '150'
	option leasetime '12h'
	option dhcpv4 'server'
	option ignore '1'

config dhcp 'wan'
	option interface 'wan'
	option ignore '1'

config odhcpd 'odhcpd'
	option maindhcp '0'
	option leasefile '/tmp/hosts/odhcpd'
	option leasetrigger '/usr/sbin/odhcpd-update'
	option loglevel '4'
	option piofolder '/tmp/odhcpd-piofolder'
cat /etc/config/firewall 
config defaults
	option syn_flood	1
	option input		REJECT
	option output		ACCEPT
	option forward		REJECT
# Uncomment this line to disable ipv6 rules
#	option disable_ipv6	1

config zone
	option name		lan
	list   network		'lan'
	option input		ACCEPT
	option output		ACCEPT
	option forward		ACCEPT

config zone
	option name		wan
	list   network		'wan'
	list   network		'wan6'
	option input		REJECT
	option output		ACCEPT
	option forward		REJECT
	option masq		1
	option mtu_fix		1

config forwarding
	option src		lan
	option dest		wan

# We need to accept udp packets on port 68,
# see https://dev.openwrt.org/ticket/4108
config rule
	option name		Allow-DHCP-Renew
	option src		wan
	option proto		udp
	option dest_port	68
	option target		ACCEPT
	option family		ipv4

# Allow IPv4 ping
config rule
	option name		Allow-Ping
	option src		wan
	option proto		icmp
	option icmp_type	echo-request
	option family		ipv4
	option target		ACCEPT

config rule
	option name		Allow-IGMP
	option src		wan
	option proto		igmp
	option family		ipv4
	option target		ACCEPT

# Allow DHCPv6 replies
# see https://github.com/openwrt/openwrt/issues/5066
config rule
	option name		Allow-DHCPv6
	option src		wan
	option proto		udp
	option dest_port	546
	option family		ipv6
	option target		ACCEPT

config rule
	option name		Allow-MLD
	option src		wan
	option proto		icmp
	option src_ip		fe80::/10
	list icmp_type		'130/0'
	list icmp_type		'131/0'
	list icmp_type		'132/0'
	list icmp_type		'143/0'
	option family		ipv6
	option target		ACCEPT

# Allow essential incoming IPv6 ICMP traffic
config rule
	option name		Allow-ICMPv6-Input
	option src		wan
	option proto	icmp
	list icmp_type		echo-request
	list icmp_type		echo-reply
	list icmp_type		destination-unreachable
	list icmp_type		packet-too-big
	list icmp_type		time-exceeded
	list icmp_type		bad-header
	list icmp_type		unknown-header-type
	list icmp_type		router-solicitation
	list icmp_type		neighbour-solicitation
	list icmp_type		router-advertisement
	list icmp_type		neighbour-advertisement
	option limit		1000/sec
	option family		ipv6
	option target		ACCEPT

# Allow essential forwarded IPv6 ICMP traffic
config rule
	option name		Allow-ICMPv6-Forward
	option src		wan
	option dest		*
	option proto		icmp
	list icmp_type		echo-request
	list icmp_type		echo-reply
	list icmp_type		destination-unreachable
	list icmp_type		packet-too-big
	list icmp_type		time-exceeded
	list icmp_type		bad-header
	list icmp_type		unknown-header-type
	option limit		1000/sec
	option family		ipv6
	option target		ACCEPT

config rule
	option name		Allow-IPSec-ESP
	option src		wan
	option dest		lan
	option proto		esp
	option target		ACCEPT

config rule
	option name		Allow-ISAKMP
	option src		wan
	option dest		lan
	option dest_port	500
	option proto		udp
	option target		ACCEPT


### EXAMPLE CONFIG SECTIONS
# do not allow a specific ip to access wan
#config rule
#	option src		lan
#	option src_ip	192.168.45.2
#	option dest		wan
#	option proto	tcp
#	option target	REJECT

# block a specific mac on wan
#config rule
#	option dest		wan
#	option src_mac	00:11:22:33:44:66
#	option target	REJECT

# block incoming ICMP traffic on a zone
#config rule
#	option src		lan
#	option proto	ICMP
#	option target	DROP

# port redirect port coming in on wan to lan
#config redirect
#	option src			wan
#	option src_dport	80
#	option dest			lan
#	option dest_ip		192.168.16.235
#	option dest_port	80
#	option proto		tcp

# port redirect of remapped ssh port (22001) on wan
#config redirect
#	option src		wan
#	option src_dport	22001
#	option dest		lan
#	option dest_port	22
#	option proto		tcp

### FULL CONFIG SECTIONS
#config rule
#	option src		lan
#	option src_ip	192.168.45.2
#	option src_mac	00:11:22:33:44:55
#	option src_port	80
#	option dest		wan
#	option dest_ip	194.25.2.129
#	option dest_port	120
#	option proto	tcp
#	option target	REJECT

#config redirect
#	option src		lan
#	option src_ip	192.168.45.2
#	option src_mac	00:11:22:33:44:55
#	option src_port		1024
#	option src_dport	80
#	option dest_ip	194.25.2.129
#	option dest_port	120
#	option proto	tcp
2

Additional information: This might be related more generally to UDP. Connected machines that have an ip address already can communicate with servers whose IP they have, including new connections (relatively certain about that, but haven't captured it). But in those cases, DNS (boring old port 53 udp DNS) does not work, so they will not establish connections to "unknown" servers.

Still not sure what to make of this or how to debug this further.

Any help, including pointers, documentation, things to look for ... appreciated!

3

current suspect is my MTU setting on all APs. Not because of OpenWRT, but maybe because of my stock-firmware FritzBox, which, apparently, can't do MTU > 1518 bytes.

I'll investigate this further. For now, I have set all my MTUs to 1500 (default). If you don't read anything past this post, consider this at least as part of the solution. If it turns out not to help, I'll probably be back here whining about it.

Nope. MTU 1500 didn't help.

4

That DHCPOFFER doesnt look right. The dest IP should be 255.255.255.255 i.e broadcast, since the client hasnt got an IP yet

5

Interesting! I will record a couple hours of packets, looking specifically at this. That would be a problem on the stock firmware FB, then.

6

I have completeley reflashed, "factory"-reset and reconfigured one of my OpenWRT APs, and yet it still does the same weird thing. Sometimes DHCP works fine, and then it just doesn't. And devices that do not get an IP address can't get one for hours. And then it suddenly works fine again.

I am at a complete loss right now.

7

Does the device actually support mtu 9000?

8

I was still experiencing the issue even when I completely went back to 1500 on MTU.

9

I have currently disconnected two out of three APs, all of which were configured for 802.11r roaming. I have not experienced this particular problem in a while. I will keep investigating.

10

Okay it's been days where wifi with a single AP worked better than with three. My next step is to bring one other AP back into the mix and see if the problems return. After that, I'm going to try deactivating 802.11r roaming - I have read dozens of threads now, where this seems to have been (at least) part of the problem.

11

I have permanently disabled one of three OpenWRT APs, and it seems to work pretty well with two APs right now, even including the 802.11r roaming. I'm currently thinking, this might have been a hardware failure on that AP. Not sure how this could extend to other APs (the whole network did not work correctly), but it has worked pretty well for a couple days now.