X3000 WAN failover with 5G SA IPv6 only

Technical Support for Routers
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My Ethernet1 WAN is IPv4 only, T-Mobile US SA mode is IPv6 only

T-Mobile 5G SA gets 192.0.0.2, no IPv4 connectivity. But IPv6 pass-through mode works and routers gets a /64 on interface and working v6 connectivity. How can I use this cellular connection as backup for v4 only Ethernet?

  • do I need to set up 464xlat to get failover to Cellular working? dns64 to 192.0.0.30 works. is something like 464xlatcfg tmo_sa rmnet_mhi0 2607:7700:0:4:0:2::/96 enough? Is the GL.iNet GL-MT300N-V2 the glue between T-Mobil Home internet and multiplayer Xbox Series X? - #5 by lincoln
  • what if Cellular switches between T-Mobile SA and NSA?
  • in v6 passthrough mode, clients seem to get v6 prefix via RA (sharing the same /64) and IPv6 connectivity works on clients. However this is not the intended behavior, as my primary Eth1 is v4 only, I don't want clients to use v6 over Cellular when Eth1 is up.
  • ip6tables has no effect in IPv6 passthrough mode? how can I modify TTL going out cellular interface?

So how can I set up cellular as backup only in the case of v4 only on Eth1 and v6 only on Cellular?

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I figured out how to get ip6tables with in IPv6 pass-through mode

I need to override with /etc/sysctl.d/11-br-netfilter.conf
sysctl net.bridge.bridge-nf-call-ip6tables=1

related discussion [OpenWrt-Devel] [patch] disable bridge firewalling in sysctl.conf by default

what is the correct way to persistently set / override sysctl? putting it in /etc/sysctl.conf ?

is there a more elegant way? I guess I could add a rule to accept any v6 traffic where input and output interface both br-lan

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