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I have a machine running CentOS 5.8. IPv6 was disabled by creating a file /etc/modprobe.d/disable-ipv6 with the following contents:

options ipv6 disable=1

Removing the file and rebooting the machine should re-enable IPv6. Is there a way to enable IPv6 without having to reboot?

Additional info:

I found something that said to echo 0 > /proc/sys/net/ipv6/conf/interface/disable_ipv6, but /proc/sys/net/ipv6 does not exist on this machine.

sysctl shows nothing about IPv6:

# sysctl -a | grep -i ipv6 | wc -l

The ipv6 kernel module is loaded:

#lsmod | grep ipv6
ipv6                  437857  1 cnic

Attempting to remove it throws an error:

# modprobe -vnr ipv6
FATAL: Module ipv6 is in use.

# modprobe --show-depends ipv6
insmod /lib/modules/2.6.18-308.13.1.el5/kernel/crypto/crypto_api.ko 
insmod /lib/modules/2.6.18-308.13.1.el5/kernel/net/xfrm/xfrm_nalgo.ko 
insmod /lib/modules/2.6.18-308.13.1.el5/kernel/net/ipv6/ipv6.ko

Modprobe says crypto_api depends on the ipv6 module. Unloading and loading that on a running webserver does not sound like best idea.

I tried rmmod as suggested by @Zoredache:

# rmmod -v ipv6
ERROR: Module ipv6 is in use by cnic
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Try rmmod ipv6, and then try re-inserting it? – Zoredache Apr 14 '14 at 16:19
@Zoredache Tried it, same results as modprobe -r (module in use). I could probably force it with -f, but at that point I'd rather just go the "clean" way and reboot the machine. – hcsteve Apr 14 '14 at 17:08
Looks like you're going to have to reboot. – Michael Hampton Apr 14 '14 at 18:04

It is possible to reload the ipv6 driver without a reboot

$ /sbin/lsmod |grep ipv
ipv6                  438625  3 rdma_cm,ib_addr,cnic

# loop through all the dependent modules, and unload them 
$ rmmod cnic ...
$ rmmod rdma_cm

$ modprobe ipv6 disable=0

You may need to turn off the n/w, but this should avoid the reboot.

share|improve this answer

Looks like it's not possible. I ended up rebooting the machine to enable IPv6. I'd love for someone to prove me wrong since I have a slew of other machines configured the same way.

share|improve this answer
For future readers: This is one of many reasons that we advise not disabling IPv6 even if you aren't yet deploying it. – Michael Hampton Apr 15 '14 at 21:47

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