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user5265160
Asked: 2019-11-07 01:55:20 +0800 CST

How to configure a fallback static IP address when there is no DHCP server present

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I am working with an embedded linux device, which we typically connect to by static IP address during development. However, on-site we have a requirement to connect using DHCP and so, I would like to setup a primary DHCP connection, with a fallback static IP address on a different network. Is this possible?

I can use nmcli to configure a DHCP connection (IPV4.method=manual) and can successfully add an additional static IP address. Both will exist when simultaneously when the DHCP server is present, however when there is no DHCP server, the connection is disabled including the static IP address.

I have also tried setting up multiple connections, assigned to eth0, i.e. one static and one DHCP, and can manually enable them using 

nmcli con up ConnectionName

but this does not meet the requirement because of the need to manually enable them in the event of the other failing. I can, of course run a script to check the connection status and enable the other if required but thought this would be a realistic expectation of a network manager.

Is there a way to tell network manager to attempt one connection and if this fails try another?

I am using Ubuntu 18.04.

Thanks.

network-manager dhcp static-ip

3 Answers

  1. Use nmcli command line to create connection profile and linked to the interface.

    nmcli connection add con-name staticx-03 connection.autoconnect-priority -1 connection.autoconnect-retries -1 ifname enp0s25 type ethernet ip4 192.168.62.74/22 gw4 192.168.60.1 ipv4.dns 208.67.222.222

    create another connection profile and linked to the same interface but with higher priority.

    nmcli connection add con-name dhcpx-02 connection.autoconnect-priority 1 connection.autoconnect-retries 2 ifname enp0s25 type ethernet

    Then check the connection created.

    nmcli con show

    Use other commands such as below to troubleshoot, up/down connection, remove connection.

    nmcli con reload
nmcli con del profilename
nmcli con up profilename 
nmcli con down profilename

    Always remember to remove the default connection, for my case is

    'Wired connection 1'

    or in the /etc/NetworkManager/system-connections it will be

    'Wired connection 1.nmconnection'
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  2. Best Answer

    You can solve this by creating two profiles (aka. connection).

    1. A DHCP config with high priority that tries for some time
    2. A static config with lower priority that connects if DHCP fails

    In config files (e.g. /etc/NetworkManager/system-connections/*.nmconnection) :

    The DHCP profile

    [connection]
id=dhcp
uuid=<an UUID>
type=ethernet
interface-name=eth0
autoconnect-priority=1
autoconnect-retries=2

[ipv4]
dns-search=
method=auto
dhcp-timeout=10
may-fail=false

    The static profile

    [connection]
id=static
uuid=<an UUID>
type=ethernet
interface-name=eth0
autoconnect-priority=0
autoconnect-retries=-1

[ipv4]
address1=192.168.1.10/24,192.168.1.1
dns=192.168.1.1
dns-search=
method=manual

    Note : by default, autoconnect=true when not specified, otherwise it wouldn't work.

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  3. I've also tried the same configuration on Ubuntu 18.04 - it just doesn't work (played with ipv4.addresses, connection.autoconnect, connection.autoconnect-retries, ipv4.dhcp-timeout, ipv4.method, ipv4.may-fail, etc.)

    The only workable option was:

    1) disable NetworkManager

    2) /etc/network/interfaces.d/some_name

        auto lo eth0
    iface lo inet loopback
    iface eth0 inet dhcp

    3) /etc/dhcp/dhclient.conf

        timeout 10;
    alias {
      interface "eth0";
      fixed-address 192.168.1.1;
      option subnet-mask 255.255.255.0;
    }
    • 1