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My industrial server computer have two Ethernets (eth0, eth1) plus one Wi-Fi. We have decided to assign eth1 to be a configurable IP which will not collide with eth0. Eth0 will be 169.254.13.14 which is fixed so an engineer computer can easily connect to it and the PC will have fixed IP 169.254.13.13. The eth0 and wlan0 are in local area network (eth1 can be reached remote).

Question about Wi-Fi

Wi-Fi uses DHCP, as an access point. So it will assign wireless client IP. We plan to use 169.254.13.15 as a fixed IP for the server wlan0. So eth0 and wlan0 are on same block, same subnet. Now, will Linux/DHCP server be able to know not to assign the two addresses mentioned above to clients?

I may have multiple wireless client (PC, or tables), but only one PC will connect to the eth0. There is no other participants on eth0. Will my plan work?

I don't necessary need a routing between the eth0 and wlan0, and my software will not actually send packets across: which means, the PC on eth0 will not try to send anything to the wlan0 IP. Client on Wi-Fi will not try to send packets to the eth0, or the PC on eth0. The communication is point to point over the TCP/IP.

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  • In general, it's bad practice to have multiple interfaces in the same LAN. It leads to unpredictable behavior. (not counting, bonding or bridging.)
    – Ricky
    Jan 8 '16 at 20:41
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You will be misusing the link-local addressing (169.254.0.0/16). Link-local addresses are not allowed in DNS, and any traffic with a link-local source address is not allowed to be routed. Also, link-local addresses must be assigned randomly (you are not allowed to assign a specific one to your wlan0 interface, and DHCP doesn't do random assignments and allow for conflicts), and special precautions must be taken since address conflicts will happen with random address assignment, and a host must determine if there is a conflict and correct it.

See RFC 3927, Dynamic Configuration of IPv4 Link-Local Addresses.

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  • I am using eth0 and wlan0 in local area network, so no DNS problems, and no routing as I mentioned is needed. Also, my wlan0 is an access point, so it has a fixed IP itself and it assign IP to clients.
    – Splash
    Jan 8 '16 at 16:49
  • You are not allowed to assign fixed addresses in the 169.254.0.0/16 range. You should read the RFC.
    – Ron Maupin
    Jan 8 '16 at 16:54
  • How about the answer to my other question? networkengineering.stackexchange.com/questions/23697/…
    – Splash
    Jan 8 '16 at 19:30
  • I didn't advocate setting specific addresses in that answer. The RFC is pretty clear that each interface needs to randomly set its address and check for conflicts. This leaves out DHCP. I see nothing wrong with using the TEST-NET addresses even if you don't want to route beyond the LAN since those addresses may be used as you see fit (statically or DHCP assigned), and they will never be allowed on the Internet, and, if you change your mind later, you can actually route them internally. If you don't need DHCP, DNS, or fixed addressing, the link-local addresses will work just fine.
    – Ron Maupin
    Jan 8 '16 at 19:38
  • The main question is whether wlan0 can be in same block as eth0. Since the real ip address can be Test-NET, I can easily change both but still, whether wlan0 and eth0 can be in same block and one is static, and one has DHCP server.
    – Splash
    Jan 8 '16 at 19:59

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