If due to a misconfiguration a v4tunnel interface you have brought up with ifup has failed, you need to manually delete it before trying again or this annoying error will happen:


# ifup somev6tunnel
ioctl: No buffer space available
# ip tunnel del somev6tunnel
# ifup somev6tunnel
#


I was trying to configure a 6to4 tunnel but without specifying a local interface address by using local any endpoint any, but that gives an unhelpful and yet increasingly familiar error message.


# ifup 6to4
ioctl: No buffer space available


Linux doesn’t like both local and remote values unset, so I thought, aha I’ll just use local any endpoint 192.88.99.1, it appeared to work. I have since realised that it only works for talking to non 6to4 hosts, if I tried to talk to another 6to4 host, it routed the packets through the gateway instead of directly and the return packets were also lost. So, I just specified the local address and it works.

My working 6to4 debian /etc/network/interfaces


auto 6to4
iface 6to4 inet6 v4tunnel
address 2002:561e:XXXX::1 # ipv6calc -I ipv4addr -O ipv6addr -A conv6to4 86.30.XX.XX
netmask 16
local 192.168.1.2 # address assigned by wifi router
endpoint any
gateway ::192.88.99.1 # 6to4 anycast address


Though its best to use a managed tunnel, like Hurricane Electrics tunnelbroker.net

The is the Debian network interfaces config i used to connect to my tunnelbroker.net ipv6 tunnel, reconfigure the tunnel endpoint dynamically, and also add one of my routed /48 subnets to the interface (so i can use pretty reverse dns addresses from my host).


auto he-ipv6
iface he-ipv6 inet6 v4tunnel
address 2001:470:1f08:xxxx::2
netmask 64
endpoint 216.66.80.26
gateway ::216.66.80.26
# Docs to generate pass etc from http://ipv4.tunnelbroker.net/ipv4_end.php
up wget --no-check-certificate -O - 'https://ipv4.tunnelbroker.net/ipv4_end.php?ipv4b=AUTO&pass=9c4db7a186c8xxxxxxxxxxxxxx&user_id=ef2ffab0c775dxxxxxx&tunnel_id=19xxx' 2>/dev/null
up ip addr add 2001:470:XXXX:1::1/64 dev $IFACE


Highly recommend working your way through their course if this interests you.