Convince apt-get *not* to use IPv6 method

Question

The ISP I work at is setting up an internal IPv6 network in preparation for eventually connecting to the IPv6 internet. As a result, several of the servers in this network now try to connect to security.debian.org via its IPv6 address by default when running apt-get update, and that results in having to wait for a lengthy timeout whenever I'm downloading updates of any sort.

Is there a way to tell apt to either prefer IPv4 or ignore IPv6 altogether?

Answer 1

As Gilles says, use gai.conf

Say we have two hosts www.he.net and www.ripe.net

$ host www.he.net
www.he.net is an alias for he.net.
he.net has address 216.218.186.2
he.net has IPv6 address 2001:470:0:76::2


$ host www.ripe.net
www.ripe.net has address 193.0.6.139
www.ripe.net has IPv6 address 2001:67c:2e8:22::c100:68b

Case 1: Prefer IPV4

Append the following to /etc/gai.conf

precedence ::ffff:0:0/96  100

then we have:

$ telnet www.ripe.net 81
Trying 193.0.6.139...
^C
$ telnet www.he.net 81
Trying 216.218.186.2...

Case 2: Prefer IPV6 for specific hosts

If we append

precedence 2001:470::/32 100

then we have

$ telnet www.ripe.net 81
Trying 193.0.6.139...
^C
$ telnet www.he.net 81
Trying 2001:470:0:76::2...
^C

So we seem to prefer that network for ipv6 and ipv4 everywhere else.

Case 3: Prefer ipv4 for specific hosts

Wondering if we invert the mask the reverse will be true.

---

See also:

• http://docs.funtoo.org/wiki/IPv6_Networking#Prefer_IPv4_over_IPv6
• http://www.getipv6.info/index.php/Customer_problems_that_could_occur#GNU.2FLinux

Answer 2

You could work around this by setting up a DNS proxy server that dropped ip6 responses.

Answer 3

How about adding a line in /etc/hosts overriding the relevant addresses? e.g.,

130.89.149.226  ftp.debian.org      
195.20.242.89   security.debian.org 

Answer 4

You could setup apt-cacher-ng on a spare machine to act as a proxy/cache for all of your hosts. You can force the configuration to only use specific hosts or use the /etc/hosts trick suggested by @badp on that one machine.

apt-get install apt-cacher-ng

Once you have apt-cache-ng setup you just need to drop the following line (with IP address/hostname altered to point at your cacher machine) in /etc/apt/apt.conf.d/90httpproxy

Acquire::http { Proxy "http://[192.168.1.254]:3142"; };

I use that setup to reduce bandwidth usage but it should workaround your problem. Unfortunately I'm not aware of a way to directly disable ipv6 lookups for apt-get itself.

Answer 5

I have found a much better way to do this. Open up your sources.list file and note down the hostnames of the repos. Get their IPv4 addresses, then edit sources.list with the IPv4 addresses rather than the hostnames. Apt-get should now contact the repositories over the IPv4 addresses you specified, bypassing IPv6.

There is the disadvantage that repos usually have some sort of load balancing and/or IP geolocation set up, which this method of course bypasses. However, it shouldn't matter if only a few people are doing it. If you do find one mirror is slow, try getting another repo IP address (for instance, by using an online ping service) and use that.