How to check if IPv6 is enabled on a Debian Linux?

To check if IPv6 is disabled through sysctl :

sysctl -a | grep disable_ipv6

If the output of above command shows configuration lines with ...disable_ipv6 = 1 then IPv6 is disabled.

To enable IPv6, edit the /etc/sysctl.conf file:

nano /etc/sysctl.conf

• and then edit+change the 1 into 0 for below system configurations, and add below lines if does not-exist in the /etc/sysctl.conf file:

  net.ipv6.conf.all.disable_ipv6 = 0  
  net.ipv6.conf.default.disable_ipv6 = 0  
  net.ipv6.conf.lo.disable_ipv6 = 0  
  net.ipv6.conf.eth0.disable_ipv6 = 0  
  

• additionally also add these configuration lines (if does not pre-exist), to enable manual configurations of IPv6:

  net.ipv6.conf.all.autoconf = 0  
  net.ipv6.conf.all.accept_ra = 0  
  net.ipv6.conf.eth0.autoconf = 0  
  net.ipv6.conf.eth0.accept_ra = 0
  

Yes, you can:

echo net.ipv6.conf.all.disable_ipv6=1  > /etc/sysctl.d/disableipv6.conf 
sysctl --system

But you might need to do several other things, as described in the related article in the Debian wiki. The description for squeeze should still be valid for Wheezy.