How to remove only IPv6 addresses with iproute2?

Question

I'm trying to write an application that can change the IP address of the local machine. When doing some experimentation, I've noticed that if I freshly connect to a wireless network, I am assigned an IPv6 address that is a function of the MAC address. Consider the following sequence of commands:

<disconnect wireless and reconnect>
$ ifconfig 
eth0      Link encap:Ethernet  HWaddr 00:0c:29:7d:7c:42  
          inet addr:192.168.2.98  Bcast:192.168.2.255  Mask:255.255.255.0
          inet6 addr: fe80::20c:29ff:fe7d:7c42/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST...
$ sudo ip addr flush dev eth0
$ ifconfig 
eth0      Link encap:Ethernet  HWaddr 00:0c:29:7d:7c:42  
          UP BROADCAST RUNNING MULTICAST...
$ sudo dhclient eth0
$ ifconfig 
eth0      Link encap:Ethernet  HWaddr 00:0c:29:7d:7c:42  
          inet addr:192.168.2.98  Bcast:192.168.2.255  Mask:255.255.255.0
          UP BROADCAST RUNNING MULTICAST

Note that after flushing, I lose both the IPv4 and IPv6 addresses, but after renewing with dhclient, I only reinitialize my IPv4 address. Why is this? What can I do to be reassigned my IPv6 address automatically?

What should I use instead if I am configuring a static ip and then want to wipe out my configurations and then return to dhcp? — jonderry, Jun 09 '14 at 22:13
Just delete the address you configured, then. Leave everything else alone. — Michael Hampton, Jun 09 '14 at 22:14
And even if that works, I'm still curious about the questions in the OP. Where did the ipv6 address come from initially, and why wasn't it re-added when I re-ran dhclient? — jonderry, Jun 09 '14 at 22:21
The initial ipv6 address.. that's the [link local](http://en.wikipedia.org/wiki/Link-local_address) address. This is set up during the network initialization (read the scripts). Assuming it's CentOS/RHEL a simple `service network restart` would bring it back. That IPv6 address has nothing to do with dhcp, that's why it wasn't re-added. There's a whole lot more to setting up the network than just running dhclient. — , Jun 10 '14 at 01:08
OK, that makes sense, but I get `stop: Job failed while stopping` and `start: Job is already running: networking`, and this fails to reset the ipv6 address. — jonderry, Jun 10 '14 at 01:13

little-dude · Answer 1 · 2017-01-30T21:36:06.830

Instead of flushing all the ip addresses from an interface, you can flush only IPv4 addresses. Here is an example where I have an interface with both an IPv4 and an IPv6 address:

# ip address show qemu-br2
5: qemu-br2: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UNKNOWN group default qlen 1000
link/ether ca:b8:97:12:0a:30 brd ff:ff:ff:ff:ff:ff
inet 192.168.20.1/24 scope global qemu-br2
   valid_lft forever preferred_lft forever
inet6 fe80::c8b8:97ff:fe12:a30/64 scope link
   valid_lft forever preferred_lft forever

To flush only the IPv4:

# ip -4 address flush qemu-br2

See that the IPv6 is still there:

# ip address show qemu-br2
5: qemu-br2: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UNKNOWN group default qlen 1000
link/ether ca:b8:97:12:0a:30 brd ff:ff:ff:ff:ff:ff
inet6 fe80::c8b8:97ff:fe12:a30/64 scope link
   valid_lft forever preferred_lft forever

Generally speaking, you can add -4 or -6 to make an iproute2 command act only on IPv4 or IPv6 addresses, respectively.

How to remove only IPv6 addresses with iproute2?

1 Answers1