Strange Issue with Linksys routers

Ben Wiechman ben.w at
Mon May 18 01:41:43 UTC 2009

We've seen similar issues in the past after lightning storms where the
routers seem to be affected by a brownout or other power issue. Typically a
reboot seems to cure those issues. In this case these routers are scattered
across several separate power companies and 50-60 miles of geography.
Rebooting the router does not clear up the issue. Upgrading the firmware
seems to be the only real solution that has a long term affect. 

Ben Wiechman
Network Administrator

> -----Original Message-----
> From: dhcp-users-bounces at [mailto:dhcp-users-
> bounces at] On Behalf Of Frank Bulk
> Sent: Sunday, May 17, 2009 8:17 PM
> To: Users of ISC DHCP
> Subject: RE: Strange Issue with Linksys routers
> Ben:
> We have hundreds of WRT54G's attached to cable modems and not noticed this
> issue.  Any chance that there was a common powering/lightning issue that
> affected these routers' power supplies?  We have seen several cases where
> Linksys router acts marginally with a bad power supply.
> Frank
> -----Original Message-----
> From: dhcp-users-bounces at
> [mailto:dhcp-users-bounces at] On Behalf Of Ben Wiechman
> Sent: Saturday, May 16, 2009 5:21 PM
> To: Users of ISC DHCP
> Subject: Strange Issue with Linksys routers
> This doesn't appear to be an issue that is specific to ISC, but since this
> is most readily visible in the dhcp logs maybe someone else here has seen
> is seeing this issue as well.
> We have a number of subscribers on our network (75-100 - possibly more)
> are running WRT54G/GS routers with older firmware (1.00.6/7, etc.) with
> ethernet controllers that appear to lock up. When the router is power
> they tend to work properly for several hours and then the ethernet
> controller appears to lock up again. This keeps repeating.
> When the condition exists any computers attached to the wired interfaces
> will eventually report limited or no connectivity. Any traffic sent to the
> router on the LAN ports receives no response. However it is possible to
> connect to the routers using the wireless interface and access the web
> management interface. With remote management enabled and pings allowed
> will not respond on the WAN interface when this condition exists. We have
> done packet captures to verify that the icmp/ip packets are being
> to the WAN interface, however the router generates no response.
> This is where it gets weird. The router will continue to send
> DHCPREQUEST/DHCPOFFER packets but does not appear to receive the response.
> This is how we initially noticed the issue. Large numbers of routers were
> hammering our dhcp server hundreds of times every hour with DHCPDISCOVER
> broadcasts.
> When the router is power cycled it will broadcast a DHCPDISCOVER packet to
> the dhcp server, receive the offer, broadcast the request and receive the
> ack. Our default lease time is 12 hours. Normally the router would send a
> unicast DHCPREQUEST to the server half way through the lease time and
> receives a unicast DHCPACK. Under normal conditions this would simply
> repeat. Here the ethernet controller appears to lock up at some point. So
> the router will send the DHCPREQUEST packet at the midway point, then with
> increasing frequency as the end of the lease period nears. Doing a packet
> capture will show the DHCPACK is received at the WAN interface of the
> router. In the last couple of minutes before the lease expires the router
> will broadcast a series of DHCPREQUEST packets and receive broadcast
> responses from the server. Once the dhcp lease expires the router will
> continue to broadcast a series of DHCPDISCOVER messages every minute or so
> and receives the DHCPOFFERs in return. This will repeat until the router
> power cycled. Even if the ethernet ports are disconnected the lost link is
> not detected.
> The dicover/offer cycle can be tripped by logging into the router via the
> wireless interface and changing the hostname. This causes the router to
> a DHCPRELEASE request, followed by a DHCPDISCOVER. It receives a
> however does not appear to process the offer and once again enters a loop
> where it continues to broadcast a series of DHCPDISCOVER packets every
> minute or so.
> We have not seen this on WRT54GL routers to this point. It appears to have
> begun at a very defined point on Monday 5/11. Is this some new exploit? We
> have yet to track down anything that appears to trigger this condition.
> Has anyone seen anything like this in the past?
> Ben Wiechman
> Network Administrator
> Wisper High Speed Internet
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