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Can a server's eth & wifi share the same IP address?
(lemmy.world)
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this post was submitted on 04 Jun 2026
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It's perfectly possible on Linux to have several network adapters with the same IP address, or several default routes.
Most server applications will listen to 0.0.0.0 address, which means all network interfaces. Any incoming TCP connection will remember it's network interface, and the server will send responses to the same interface.
This will not work for UDP connections, and for outgoing TCP connections - they will always choose the network interface with the lowest metric, which you can print with
ip rcommand.This does not include advanced techniques like bridge or bonding or iptables routing - you need to run special commands in the terminal, which you cannot do just by clicking your mouse in system settings app.
Is it possible to configure interfaces this way? Yes.
Will it work? No, not without bonding, and not with WiFi as one of the interfaces.
What won't work, because at the moment everything seems to be working fine and I'm pleasantly surprised.
Oh, you are failing one over if the other fails? That's not the same thing as configuring two interfaces with the same IP, gateway, at the same time, which is what I thought you were trying to do.
I've done this for years with no failover. Linux doesn't care, zero issues.
That's because only one interface is really being used. A TCP session will reset if the hop count or metric changes all the time, the SYN/ACK wouldn't work.
Thanks for sharing your experience... so it really does work! I mean it certainly seems to be working fine and I keep thinking I'm overlooking something. I feel like I'm in a love triangle with 2 girls fully aware of the other and they're fine with it... this feels like a disaster but everyone seems to be happy!
Yeah - I mean it's technically "not ideal" but I simply don't have any issues. I did have a windows computer that once complained about there being two devices on the network with the same IP but it didn't stop it from working with it. I think that was some security software installed on that system though. This is the "less than ideal" part - it will look a bit suspicious if you have any security software that scans network traffic because "arp poisoning" is a common attack (basically stealing an IP address).
I simply wanted a fail-over but in the process got 2 interfaces with the same IP and I can't find a problem with it 😄
Will UDP packets produce an error when sent or will they simply be received twice, potentially causing errors/issues?
Nope. The server receives UDP packet from WiFi and sends reply over Ethernet, which simply gets lost in your router. From the Linux side there's no error, it sent the packet somewhere, and what happens next is your router's problem.
That's the thing about UDP packets... receipt isn't acknowledged as part of the protocol. It's send and forget. I'm not sure a UDP packet sent to my server would get lost but I'm not 100% certain there couldn't be situations where the packet is received by the server on both interfaces, essentially duplicated. It's been almost 30 years since I wrote programs that utilized UDP for communicating. I'm definitely a little rusty. 😄
Unless its rudp.