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I have a Linux machine with one Ethernet interface eth0 which already has one primary static address and one secondary static address (eth0:1) assigned. I would like to use DHCP to dynamically assign a third address to eth0:2. This is an old question and the canonical answer is, no, you can't do that; dhclient and dhcpcd don't work on alias interfaces. See this related question "How to request dhcp (using dhclient) on a virtual interface (i.e. eth0:1)" for which kce's answer gives a good explanation of the fundamental impossibility. (There's also mention there of a potential solution involving a VLAN, but this isn't an option for me.)

Now, AFAIK, the DHCP protocol doesn't care about interfaces (alias or otherwise); its job is simply to lease IP addresses to clients. So I'm thinking of trying to rig up a configuration involving dhclient or dhcpcd where they use eth0 for the packet-level communications with the local DHCP server, but then, once an address is obtained, use that address to configure eth0:2. Has anyone ever done this? Should it be possible? Are there pitfalls or other impossibilities I'm overlooking? (I'm prepared to modify the dhclient or dhcpcd source if I need to, if there aren't fundamental impossibilities. I'm also prepared to use a custom dhclient-script to set up eth0:2 and its routing correctly, once I have an address.)

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  • What on earth is going on with your network, then? – Michael Hampton Oct 26 '15 at 15:01
  • The machine is normally on a teeny-tiny isolated network with a handful of other machines and with nothing but hand-assigned 10.* and 192.168.* addresses. But occasionally it would be useful to connect it to a larger network, and only then might DHCP come into play. – Steve Summit Oct 26 '15 at 15:12
  • Beginning to answer my own question: It looks like lightly modifying dbclient-script to configure a specific interface -- not necessarily the primary one that dhclient used for lease negotiation -- will suffice to solve my problem. (Now I just need to, like, actually test it.) – Steve Summit Oct 26 '15 at 16:21

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