LEDE IPV6 behind SKY SR102

Does anybody have any hints for using IPV6 behind a SKY SR102?

I do know this is not really the recommended configuration, but I have a LAN port on my SKY SR102 connected to the WAN port of a R7800 running hnyman's LEDE 17.01 build. Everything seems to work fine except that none of the devices on the R7800 LAN are getting a valid global IPV6 address. The R7800 WAN6 interface sporadically gets a valid IPV6 address (local and global), but this also seems a bit unreliable. Messing about with Wireshark it seems that the SR102 DHCPv6 always responds with 'No Prefix Available' and so the DHCPv6 server on the R7800 cannot allocate any to its LAN clients (if that makes sense). I've tried using odhcp6c with different prefix sizes (-P0, -P64, -P56) but none seem to work.

All I know is that on Voxel's modified stock firmware the IPv6 seemed to 'just work' with this same physical configuration (SR102 LAN to R7800 WAN), the R7800 LAN clients received an IPV6 address.

I am almost completely clueless in this area and was hoping for some hints as to what configuration might work. I've tried setting odhcp to 'relay' mode on both the LAN and WAN6 interfaces which didn't seem to work either. I want to use LEDE because the QoS seems better than the stock QoS.

DHCPv6-PD (prefix delegation) needs to be supported by your gateway (SKY SR102), so you get delegated at least a /62 to your LEDE router.

thanks. as i say it seems to 'just work' with the stock(ish) firmware, any ideas what might be different?

SSH in to the router and run the following...

cat /etc/config/network

Put wireshark on the interface the modem is connected to. After installing tcpdump-mini

opkg update
opkg install tcpdump-mini

you should be able to do something along the lines of

your.big.box $ ssh root@your.openwrt.box "tcpdump -i br-wan <some filter expression>" | wireshark -k -i -

and watch the DHCPv6 negotiation. I might have gotten the quoting off there, but that should be close.

thanks for the suggestions all, i had done tcpdump/wireshark already and watched the negotiation, that's how i worked out it was sending a 'No Prefix Available' status code. i have a feeling on the stock firmware i had to set it to passthrough mode to make it work.

good news is i've managed to make it work on LEDE by setting odhcpd to relay mode. i'd thought that's what i needed with PD not available, but couldn't get it to work. i missed the example relay config in https://openwrt.org/docs/guide-user/network/ipv6/start, i was missing the master interface designation and the relay on the wan interface.

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