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I have connected a USB network interface to my RPi 4 running Raspbian 10 Lite (headless) and I am trying to assign a fixed IP address to it. The onboard eth0 interface should continue to use DHCP as per default.

I tried the following, all of which failed:

  1. Add these lines to /etc/dhcpcd.conf + reboot:
interface eth1
static ip_address=192.168.60.10/24
  1. Add /etc/network/interfaces.d with the following content + reboot:
interface eth1
static ip_address=192.168.60.1/24
  1. Add the above lines directly to /etc/network/interfaces and reboot.

In all cases, eth1 remains unconfigured (no IP address), as witnessed by the output of ifconfig eth1:

eth1: flags=4099<UP,BROADCAST,MULTICAST>  mtu 1500
        ether 00:e0:4c:39:bf:d8  txqueuelen 1000  (Ethernet)
        RX packets 0  bytes 0 (0.0 B)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 0  bytes 0 (0.0 B)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

The only thing that seems to work is sudo ifconfig eth1 192.168.60.10/24, but then the result is not persistent, and adding this command to /etc/rc.local feels like a hack.

What is the correct way to do this?

eth1 should automatically get this fixed IP address when the USB network interface is plugged in.

EDIT: After some experiments triggered by suggestions from @seamus, I realized that dhcpcd does assign the requested address to eth1, but only when a carrier is detected, i.e. when eth1 is physically connected to another device using an ethernet cable. This answers my original question.

There is an alternative way to do this, also suggested by @seamus: instead of static, use inform in dhcpcd.conf. This has the added advantage that a dhcp server running on the 192.168.60.0/24 subnet is informed of the fixed IP, avoiding potential conflicts. In my case, there is no dhcp server, so this is irrelevant.

There is one remaining issue: connections to other hosts on the 192.168.60.0/24 subnet are not routed to eth1. Since this is a different question, not about assigning a fixed IP, I posted a separate question: How to setup point-to-point ethernet connection to PLC in addition to standard network

ygramoel
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2 Answers2

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Please refer to the system manual for dhcpcd - man dhcpcd.conf says:

"For IPv4, you should use the inform ipaddress option instead of setting a static address."

Also see these other Q&A: 1, 2, 3

Seamus
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The correct answer, if you don't want an interface to have a gateway is to tell dhcpcd not to assign one to an interface.

If you want an interface to not install any default routes (often used in conjunction with a static IP address) specify

nogateway

See Prevent dhcpcd from configuring a gateway on an interface in How to set up networking/WiFi.

Incidentally unless you use Predictable Network Interface Names results will be unreliable as there is a race condition in interface enumeration.

Milliways
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