TomatoUSB

Use Buffalo WHR-G54S & Asus WL-500gP with Tomato Firmware v1.27.1797 ok
but after upgraded with tomato-NDUSB-1.27.8746-Std.trx, both model WLAN port cannot obtain IP from DHCP (DT-R460, ISP modem)
Please kindly help! Thanks!

You probably meant WAN - not WLAN…
Did you reset the router to default settings, and manually reconfigured after flashing this firmware?
Did you try to power-cycle the router?
Did you try to power-cycle your cable or DSL modem?

Yes, the WAN port.

Everythings that I have already tried but it still cannot work with ISP modem to obtain a real IP (but it works with another router LAN port to have virtual IP).

BUT I have tried to use older version of TomatoUSB, luckily, it works! Both 2.4 version 8745 & 2.6 version 9045 are ok, all version after "45" do not work. Please help to check and fix it, I really want to have update version! Thank you!

The Busybox was updated to the 1.16.x version between builds 45 and 46 - that's the only significant change which could affect DHCP. However I didn't get many DHCP-related complains after this update - yours is probably the only one (besides the other post here - but that one seems to be about a different issue).

Besides what you've already tried, please check a few more tips from this post…

Hi Teddy,

Thanks for your information. I have found out the reason of DHCP problem from WAN port:-

After Build 4.6:-
"Removed "Reduce packet size" option from the GUI DHCP settings - this is always enabled now."

Is it possible to disable the "Reduce packet size"?

Are you sure that this is a reason, and if so - why do you believe so?

The reason I removed the option is because this change (reduced packet size) finally made it's way up into the upstream Busybox, and in the next Busybox version it will be permanently enabled with no option to turn it off. OpenWRT includes the same patch for a while as well - and it also can't be disabled there.

Yes I am sure!

Because I can't never use the firmware after build 45.

I have tested with this option that when I check the box "Reduce packet size" with firmware build 45, the WAN port cannot obtain IP. But when I uncheck the box, it works again!

I have searched the internet and found somebody that also has similar problem and want to disable it.

Interesting… Who is your ISP?
Please register an account here so we can exchange PMs - I'd like to investigate this further and give you a couple of test builds to try.

Already registered. My ISP is Hutchison Global Communication.
Glad to have the test builds.

I can confirm this problem on build later than 45 (beta 11).

With the "Reduce DHCP packet size" box unchecked I have no problem receiving an IP from my modem but if it's checked I nearly always have to power cycle my modem.

Out of curiosity what would be the command be to turn off "Reduce DHCP packet size" in later builds?

Edit: Just to answer myself. The command in builds later that 45 where the "Reduce DHCP packet size" check box has been removed is:

nvram set dhcp_minpkt=0
nvram commit
reboot

I'll update when I've had time to test.