From DD-WRT Wiki
Installation of DD-WRT
The Netgear WNDR3700 is an extremely powerful consumer 802.11 a/b/g/n router, sporting a lightning fast Atheros CPU, lots of RAM, lots of flash space, and capable of high transmit power that even gives 2Wire a run for the money makes this router is ready for the future, and DD-WRT makes it even better. Installation of DD-WRT is very simple and can be done in a few steps. To date there are two hardware revisions of this router, v1 and v2, each of which requires its own firmware build, whether you use the stock firmware or DD-WRT.
v1 units are not labeled at all and are just called "WNDR3700".
v2 units are labeled on the side of the box and in firmware as "WNDR3700v2".
v3 units are labeled on the side of the box and in firmware as "WNDR3700v3" (v3 is currently NOT supported & with no ETA).
1a. Download DD-WRT for the WNDR3700 v1 from here depending on the region you purchased your router from:
1b. Or if you have a WNDR3700 v2, download from here following the same flashing steps:
2. Make sure you are connected to the router with an ethernet cable. Then upgrade your firmware through the stock firmware web interface, the process will take a few minutes. Do not interrupt the upgrade process and wait at least 2 minutes after 100% is reached.
3. The wireless LEDs should now be flashing, and if your NIC card is gigabit, you should also see one orange LAN led, if you see these your flash is complete and you can now proceed to the 30/30/30 or triple 30 reset (step 4.).
4. Hold the reset button under the router for 30 seconds and do not release it, while still holding the reset button unplug the router and leave it unplugged for 30 more seconds. Still holding the reset button, plug the router back in and wait yet another 30 seconds. Unplug the router again and release the reset button, plug it back in and wait at least 2 minutes, this is critical as the router is building NVRAM settings.
5. Repeat step 4 one final time.
6. Power cycle the router: Unplug it for 30 seconds and plug it back in, or use the switch. Installation is now complete.
The webflash.bin doesn't work on some WNDR3700's and could brick the router, but most are fine. Most likely cause the router is faulty to begin with. To restore from the brick TFTP the stock firmware or a known working build of DD-WRT back from it (-factory.img file).
To upgrade to a newer build of DD-WRT, use the web interface and selecting the "wndr3700-webflash.bin" file instead. eg from:  Do not select "restore defaults" or you may have a semi bricked device. There has been many reports of "restore defaults" bricking multiple different routers yet some have no issue, so USE AT YOUR OWN RISK! You should not have to restore to defaults just from upgrading builds, to restore to defaults instead use the far superior triple 30 reset. Also, if you do partially brick your router, use this page to unbrick it.
Now that DD-WRT is fully installed and NVRAM is cleared, ensure your PC does not have any internal static IP then point your browser to 192.168.1.1, this is the DD-WRT default IP and can be changed to any IP you like. You should be at the username and password screen, set them accordingly then proceed to setting up your new DD-WRT WNDR3700.
- NOTE: Some people have run into problems following the directions above, usually due to waiting only a few seconds after sending the firmware to the unit to flash, when its not finished flashing the unit probably gets reset or loss of power. This forum thread describes an alternative method - http://www.dd-wrt.com/phpBB2/viewtopic.php?t=140667&postdays=0&postorder=asc&start=30
- TFTP & recovery directions - http://www.dd-wrt.com/phpBB2/viewtopic.php?t=79802&highlight=stock+wndr+3700
Since DD-WRT is still in beta there is some issues with the WNDR3700, having followed the proper safe procedure above, you should have minimal or no issues. If luck is not on your side then take a look at the current known most critical issues and fixes for them if there is any.
There has been some reports of very weak ath0/ath1 (both radios 2.4GHz and 5GHz) after flashing DD-WRT, but there is also reports of weak wireless with those that never flashed DD-WRT. The issue is still under investigation but it is probably faulty hardware related, one can only speculate the cause but it could be TX power related. Owners with this problem report the wireless is so weak one can't move any further away from the router than 1 meter. A chart has been made by a WNDR3700 owner that is linked at the bottom of this page, you have over 85% chance to get a good working router. See this thread on netgear forums for some low S/N information: http://forum1.netgear.com/showthread.php?t=54442
According to this thread TA numbers ending in 01R15 are likely to be bad. 01R17 are better, 07R18 is 50/50, and it appears 01R21+ are usually fine. Anything starting with 02R is from the "2nd generation" and no issues have been reported with the 2.4Ghz/5.0Ghz bands.
Fix: None short of exchanging the router for another one. Looking for the "Red Box" no longer works, both V1 Second Generation and v2's are in red boxes. Best case scenario is to find one in a "Red Box" but has 8MB of flash. This will be a V1 Second Generation.
Some report MAC addresses changing to FF:FF:FF:FF:FF:FF, this can be caused by a improper flash. Reflash following the steps above before attempting the fix. If reflashing did not fix the problem replacing caldata should, see fix link.
Fix: http://www.dd-wrt.com/phpBB2/viewtopic.php?p=468313#468313 (updated)
This no longer seems to be a problem as of Firmware: DD-WRT v24-sp2 (06/14/11) std. I used the GUI to set the USB Flash Drive mountpoint to /jffs and it is working
The newest versions of DD-WRT have an incorrect mount point for USB. The causes the USB drive to not be detected correctly. The drive is no longer found under /dev/discs/*, it is now found under /dev/sda*.
Fix: Simplest way is to have a startup script that mounts the drive for you. Add a line similar to the following: "mount /dev/sda3 /mnt"
The will mount the third partition of your disk to the /mnt directory. After this your regular samba share should work (assuming it was set to use /mnt)
As of Firmware: DD-WRT v24-sp2 (06/14/11) std this does not appear to be a problem. I have enabled USB access through the GUI and now can see /dev/sda at the mountpoint i set through the GUI -- /jffs.
USB.asb has a bug in it. You can't enable USB support from the Web GUI because of the following errors:
Message: Syntax error Line: 36 Char: 43 Code: 0 URI: http://192.168.1.1/USB.asp
Message: Object expected Line: 146 Char: 1 Code: 0 URI: http://192.168.1.1/USB.asp
CPU: AR7161 680 MHz
RAM: 64 MB
Flash: 8 MB (16 MB on v2)
Switch: Realtek RTL8366SR
Radio (2.4 GHz): AR9223
Radio (5 GHz): AR9220
Antenna Type: Internal
# Of Antennas: 8
Antenna Gain (2.4 GHz): 2.8 dBi
Antenna Gain (5 GHz): 3.9 dBi
Max TX Power (2.4 GHz): 17dBm
Max TX Power (5 GHz): 24dBm
Antenna Chains (TX/RX): 1+2/1+2
- Max TX Power is assuming 40 MHz channel width is in use & regulatory domain is Canada/USA. Under 20 MHz operation 2.4 GHz may have higher TX power due to reduced channel width.
Q: After flashing DD-WRT, the LAN LEDs are now green for 10/100M and orange 1000M?
A: This is an intentional change by the DD-WRT developers, Netgear had the colors backwards as gigabit is usually orange (check your NIC card!).
Q: I heard some WNDR3700 radios were dying shortly after flashing DD-WRT? Does the firmware damage routers?
A: No DD-WRT does not damage routers, there is also reports as said above, of WNDR3700's having weak wireless even when DD-WRT was never put on it. It is most likely a faulty hardware issue.
Q: Is there going to be a special version after DD-WRT is out of beta for this router?
A: Maybe, maybe not. Probably not because Netgear already paid for special support *for us*, this is why WNDR3700 has extra features such as RP-PPPOE server and per user/subnet/MAC QoS speed control as these are special build features.
Q: My wireless performance dropped after installing DD-WRT?
A: Refer to the Atheros wireless settings wiki page, you are probably using defaults or incorrect settings.
Q: What is the default username and password for DD-WRT?
A: Username is "root" and password is "admin".
Q: Are my wireless LEDs supposed to be flashing constantly? Stock firmware didn't do this.
A: Yes they are, that is another intentional change to DD-WRT. The slow steady flashing means the radios are on with no activity, when there is data being transferred they will rapidly flash like stock firmware.
Q: With stock firmware I was able to setup a guest AP or virtual AP, can I do this as well with DD-WRT?
A: Absolutely, and much much more!
Q: How come I cannot get Wireless-N Networks?
A: You must use WPA(2) AES when encrypting your Interface, otherwise you will only have A or BG networks. WPA(2) AES is the only approved encryption method in the Wireless-N standard.
Q: Is it possible to restore the factory or stock firmware that came with my router?
A: Yes. Refer to WNDR3700: Restore Factory Firmware in Five Easy Steps.
- Refer to this wiki article for a guide to wireless settings for Atheros.
- Refer to this thread containing various info regarding WNDR3700.
- Refer to this thread about the possibility of different revisions of the WNDR3700, weak wireless problem could be tied to revision.
- Refer to this thread for a guide to restoring the Netgear factory firmware.
- Link to WNDR3700 revision list and which ones so far had the "radio death" problem.
- Software Installation on USB enabled Atheros routers 
- Refer to this thread for a guide compiling kernel modules