Just enable IPv6 on my RT-N16 with the MTU change and testing

To Tomato Firmware users,

My setup
Asus RT-N16
Tomato Firmware v1.28.7501 MIPSR2Toastman-RT K26 USB VLAN-VPN-NOCAT

Here's my config:
Basic > IPv6
- IPv6 Service Type: DHCPv6 with Prefix Delegation
- Prefix Length: 64
- Static DNS:
|-> 2001:470:20::2
|-> 2001:4860:4860::8888
|-> 2001:4860:4860::8844
- Enable Router Advertisements: Enable
- Accept RA from: WAN

After tinkering around, I found editing "/proc/sys/net/ipv6/conf/br0/mtu" directly gives me mix result.
I solved my problem by adding "AdvLinkMTU 1480;" into "/etc/radvd.conf"
But I'm not sure about other Tomato builds but I found out that the router advertisement daemon (radvd) in my version overwrites "/etc/radvd.conf" everytime its starts up. I couldn't find the setting in the nvram dump that toggles AdvLinkMTU for radvd.conf.

So I wrote is this simple script and added it into
Administration > Scripts > WAN Up

or you can put it on a cronjob or Administration > Scheduler to constantly check on it
in which you can remove "sleep 5;"

also, I didn't change the WAN (ppp0) MTU setting. (still on default, 1492)

This post has been edited by Raki: Dec 16 2013, 10:37 AM

I'm sorry, do you mean IPv6 would only be accessible after 10-15 minutes of authenticated into screamyx ?

If so, no. I can access IPv6 sites immediately after login.
I think the problem might be becuz the OS still retains the previous IPv6 address. (becuz a different prefix is dynamically delegated each time you connect to TM). In theory, "ipconfig /release6" and "ipconfig /renew6" into command line should solve the problem. But in my case, it doesnt work so my workaround is to go to the network's Properties and uncheck"Internet Protocol Version 6 (TCP/IPv6) to disable IPv6. Than go back and reenable it .


Side note: I modified the script cuz there was a logic error

when you do, try going into CMD and check ipconfig to see whether or not you have 2 sets "IPv6 Address" and "Temporary IPv6 Address" with different /64 subnets
but after 10-15minutes (when you can access ipv6), try again and check if only 1 set of ip address are left

I modify my original post but here is again

Going thru it, I think enabling DeprecatePrefix should at least address RFC4862 Section-5.5.3 Router Advertisement Processing and should fix the 10-15min delay??? which deprecates the prefix by announcing it in the radvd shutdown




I'm not sure whether or not to enable DecrementLifetimes (default off) for RFC3633, "IPv6 Prefix Options for Dynamic Host Configuration Protocol (DHCP) version 6".

This post has been edited by Raki: Dec 16 2013, 01:32 PM

some1 replied and seems like dnsmasq 2.68 should fix the issue

On dnsmasq changelog


I don't think you should update without nvram flush unless you are certain.

I might try the latest version of toastman's build just to try out dnsmasq instead of radvd for RA

http://en.wikipedia.org/wiki/Tomato_(firmware)
Its not up to date but should at least give you an idea
Mostly should be feature differences but its open source so other authors borrow code from others

Moogle Stiltzkin Apparently, I'm now facing a similar problem that you mentioned, it takes 5-15min b4 TM decides to give a IPv6 gateway while the IPv6 prefix delegation is working as expected. I swear I didn't had this issue during the evening when I was testing.

I'm not sure what's the issue, or whether its temporary problem at TM's side as I took the time to flash my Tomato router and found that Toastman has removed radvd in his newer builds. So now, I'm relying on dnsmasq for DHCPv6. The only problem that I'm facing with using dnsmasq is that from 2.66 onwards, it based the MTU on the local interface which is "/proc/sys/net/ipv6/conf/br0/mtu".
and enabling IPv6 on LAN announcement with this in dnsmasq.conf


Side note: I might need to ask my fren to let me experiment more on his Unifi becuz I pulled this out of the modem

Interesting part is that TM sets Unifi's MTU to 1472

This post has been edited by Raki: Jan 4 2014, 12:45 AM

Not a script, just the config I used. I no longer using the previous script becuz Radvd is remove from the newer builds since by now, dnsmaqs "in theory" is more capable than radvd by the authors

if you have shell access into your router


if you can, let us know whether your router is running dnsmasq or radvd for router advertisement

This post has been edited by Raki: Feb 9 2014, 10:49 PM

I post it previously. but it seems that on unifi, TM's default setting for MTU as 1472 but I'm been running 1480 for weeks already without any issue.
Another issue(?) I'm seeing is that it takes awhile(few minutes after connected into streamyx) before wan ipv6 is available. I'm not quite sure why is that the case.

I thought using only TMNut provided modem router setup should work out of the box when IPv6 is enable

in cmd, type "ipconfig /all"
how many ipv6 address are you assigned for your network adapter?

also try "netsh interface ipv6 show subinterfaces" and see your MTU for your internet bound network interface
try ping with this "ping google.com -6 -l 1432"


This post has been edited by Raki: Mar 6 2014, 12:59 PM