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In my setup I have 2 different interfaces, one for WiFi and another for 4G/LTE. Both are USB dongles. I'm using Ubuntu 16.04 at the moment, but that can change. In my problem I need to mantain an IP session, meaning that when I go out of a WiFi hotspot coverage area I need to automatically change to LTE (and viceversa) in a transparent and lossless way. That is supposed to be acomplished having the same IP for both interfaces, since a change in the IP would bring packet loss.

Both interfaces have different subnets and gateways. WiFi is in 10.102.81.0/24 while LTE is in 10.10.0.148/30. This is a fixed setup, since WiFi addres depends on the WiFi network, and LTE addres depend on the cellular provider.

I have tried bonding, but I've read that it is necessary to have both interfaces in the same subnet, since it is not "IP level routing". I tried using aliases or configuring two different gateways, but nothing has worked so far. I also saw something about making a bridge, but I read it wouldn't work with WiFi.

So my question is: How could I make both WiFi and LTE interfaces on different subnets work as one unique interface, or at least make them work as a fault-tolerant system? Is it possible through bonding? If not, is there another way?

Thank you very much in advance.

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There’s no chance it can work that way.

IP addresses in 10.0.0.0/8 are not routable on the Internet. This means they are not “real” IP addresses, and get rewritten by some middlebox in you ISPs’ networks to “real” IP addresses, in a process that is called NAT44 (or, more accurately, NAPT44). Hence, even if both interfaces were using the same IP address in 10.0.0.0/8, a connection could not migrate flawlessly between networks.

What you’re willing to do is called multi-homing. It could be possible if the same ISP were managing the WiFi and LTE networks, although I doubt any mainstream ISP will do it.

With distinct ISPs, it should still be possible if you could obtain a PI (provider-independent) IP address from your RIR (regional internet registry) and then convince both ISPs to allow you to use your PI address and establish a BGP session with them.

I think the most realistic option is to use a VPN so the inner address can be stable when you change your outer connection.

  • I have acces to the core LTE network, like the ePDG or the PGW, so I may be able to see those "real" IP addresses you mention. Couldn't something similar to NAT work? Like making the NAT change the IP address when going in/out the inner network. – Iván G Apr 17 '17 at 12:09
  • I don’t know much about LTE networks and have no idea what you call ePDG and PGW. – user2233709 Apr 17 '17 at 12:14
  • Those elementes are Packet Gateways that give the LTE network internet connectivity. Anyway I think it is not that relevant. I will try to investigate about PMIP, that seems to be what I'm looking for. Thanks for your response, although not solving my problem I learnt from what you told me! – Iván G Apr 17 '17 at 12:44
  • I don’t know PMIP but at first sight, I agree that it looks relevant. Seems it looks somewhat like a (non-encrypting) VPN. Anyway, I’m glad if my answer could help you. – user2233709 Apr 17 '17 at 13:03

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