Getting 2 IP addresses on one network card, using DHCP

All we need is an easy explanation of the problem, so here it is.

Given: A computer running Linux (CentOS 5.x), one network card connected to LAN switch, a second one to cable modem via crossover cable. This computer acts as a router/firewall/traffic shaper for the local network.

Computer gets its public IP address from an ISP via DHCP. I know my ISP can supply 2 IP addresses to clients (when cable modem is connected directly to a hub, etc.). I would like to get 2 IP addresses on one computer that is connected to the ISP. It will simplify traffic segregation, NAT and shaping.

I know I can probably “fake” DHCP requests and bang together some scripts that will do this, but I wonder if somebody knows an easier and proper way.

Question: How can I make a Linux computer request two IP addresses via DHCP and assign both of them to one network card?

How to solve :

I know you bored from this bug, So we are here to help you! Take a deep breath and look at the explanation of your problem. We have many solutions to this problem, But we recommend you to use the first method because it is tested & true method that will 100% work for you.

Method 1

As a DHCP reply is based on the MAC address of the requesting interface, with a single physical interface, the answer is “you can’t”. The only way to do this would be with a script.

Perhaps something like (with a subinterface defined on the primary):

  1. Primary interface issues DHCP and gets IP address
  2. macchanger changes MAC address of interface
  3. Sub interface issues DHCP and gets IP address
  4. Revert mac address with macchanger

Kill the dhcp client, so that it doesn’t automatically run later. Work out the lease time of the IP address you are given, and schedule this script to run again before the lease expires.

Update

For this you will need iproute2 installed. The following command adds a virtual interface bound to an existing eth0 interface:

ip link add link eth0 address 00:11:22:33:44:55 virtual0 type macvlan

Replace the mac and “virtual0” name of the interface to whatever you like. Turn it on:

ip link set virtual0 up

Then configure using dhcpd or dhclient or ifconfig as needed. I have tested this on Debian squeeze – your distro may not have everything needed enabled in the kernel (macvlan particularly).

Method 2

As @JesseChisholm suggested, it is way easier to ask the right thing to the DHCP server instead of making complex network setups.

For OpenWRT I could simply run:

 # udhcpc -i eth0:1 -x 0x3d:0100BEEFC0FFEE

Which resulted in having two ip addresses from the same dhcp server.

The 0x36 is option 61 which is the client id option. After the : there is an hex byte option. This came from the help option of udhcpc:

 # udhcp --help
 BusyBox v1.22.1 (2014-10-08 16:34:50 HKT) multi-call binary.

 Usage: udhcpc [-fbqRB] [-t N] [-T SEC] [-A SEC/-n]
 [-i IFACE] [-s PROG] [-p PIDFILE]
 [-oC] [-r IP] [-V VENDOR] [-F NAME] [-x OPT:VAL]... [-O OPT]...

 ...
 -x OPT:VAL     Include option OPT in sent packets (cumulative)
                Examples of string, numeric, and hex byte opts:
                -x hostname:bbox - option 12
                -x lease:3600 - option 51 (lease time)
                -x 0x3d:0100BEEFC0FFEE - option 61 (client id)

Note: Use and implement method 1 because this method fully tested our system.
Thank you 🙂

All methods was sourced from stackoverflow.com or stackexchange.com, is licensed under cc by-sa 2.5, cc by-sa 3.0 and cc by-sa 4.0

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