Networking Question

[Matt Graham]

On 2019-12-09 13:48, kitepilot at kitepilot.com wrote:
> Matt Graham writes:
>> I think I turned on both wired and wireless networking
>> on my laptop at some point, and it didn't break
>> everything. [...] This is *not* recommended, but it
>> should not be the horrible failure you got in the
>> 2000s if you had 2 wired Ethernet devices on the same
>> machine in the same subnet.
> Finally, this explanation is watered down to dilution because I don't
> have a lot of time right now.

TL;DR: AFAICT, what you said would happen either does not happen or is 
not nearly as problematic as you said it was.

> There is no guarantee (that I know of) that the kernel will search
> the network interfaces in any particular order for a matching subnet,
> and the search will stop as the first one is found.
> 
> That creates a race condition with the ARP table where a packed may
> be sent while the ARP table gets refreshed and moved to the other
> interface, and those packets will die a slow death.  That will create
> random connection drops and transmission slowdowns.

I took my laptop and did "dhcpcd -d wlp3s0" on it since it was already 
plugged in to a wired network.  This resulted in its enp1s0 having 
192.168.2.26/24 and its wlp3s0 having 192.168.2.23/24 .  (The IPv6 addrs 
were different, but that's probably not a factor.)  Subsequent going to 
web pages, pinging things, scp'ing large files, and other network 
operations on the laptop showed zero performance degradation.  Or maybe 
I just got lucky?  It's difficult to tell with a very limited time test, 
but usually networking trouble gives you stalls and failures instead of 
fast scp'ing and web pages showing up quickly.

Please note that doing this with 2 wired interfaces in the 2000s caused 
all network operations to barf ingloriously until the subnets got fixed. 
This laptop and desktop are running pretty recent kernels though.  The 
kernel and/or networking people may have put some code in somewhere that 
says "Have the users done something idiotic like have 2 NICs in the same 
subnet?  Try not to barf," because people can be pretty dumb.

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