Ubuntu has a newer Internet protocol called IPv6 turned on by default. However, some hardware — such as NICs and modems — shows broken behavior when exposed to IPv6 related DNS requests. This leaves you wondering why DNS resolution seems slower or doesn't work at all. This guide shows how to disable this new protocol.

Wrong and/or Citation needed.

Blackhole IPv6 route

Slow responses are often due to the presence of a default route for IPv6 but which is not connected to the Internet. Routers and or other machines on the network may advertise themselves as IPv6 routers, and your own host's Linux kernel can add these as a default IPv6 gateway automatically (probably only when there is no default route yet). To counter this issue, you can block all received Router-Advertisement (RA) packets either by setting a sysctl flag or using ip6tables (tutorials for that elsewhere).

In /etc/sysctl.conf one would add:

net.ipv6.conf.all.accept_ra = 0

or with ip6tables (further actions may be needed to make this permanent across reboots),

# ip6tables -I INPUT -p ipv6-icmp --icmpv6-type router-advertisement -j DROP

Hosts having working IPv6 networking do have a default route (as shown below); conversely, if you do not have IPv6 internet, no "default.." line should be present to avoid delays. If RA is blocked from the start, no such route should spring into existence either, thereby solving the problem.

# ip -6 r
2a01:198:200:f::/64 via :: dev sit1 proto kernel metric 256 mtu 1392 advmss 1332 hoplimit 4294967295
fe80::/64 dev rtl0 proto kernel metric 256 mtu 1500 advmss 1440 hoplimit 4294967295
fe80::/64 dev sis0 proto kernel metric 256 mtu 1500 advmss 1440 hoplimit 4294967295
fe80::/64 via :: dev sit1 proto kernel metric 256 mtu 1392 advmss 1332 hoplimit 4294967295
default via 2a01:198:200:f::1 dev sit1 metric 1024 mtu 1392 advmss 1332 hoplimit 4294967295

The rationale behind this is: If there is no default IPv6 route, attempts to connect with outside IPv6 hosts immediately fails because there is no route to them, and programs can continue to try with IPv4 without delays.

Hence, completely disabling IPv6 by removing addresses on interfaces or unloading the module is often just a bad workaround. Below's claims w.r.t. DNS are therefore to be taken with lots of salt.

Open up a terminal and type:

lsmod | grep inet6

If it there is any output, which shows something like:

ipv6 123456 22 ip6_tables

then IPv6 is enabled. If there's no output, then IPv6 is disabled.


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Взято отсюда: https://help.ubuntu.com/community/WebBrowsingSlowIPv6IPv4
Судя по поведению ФФ, сработало. Сделал в Xubuntu 9.04