Posts Tagged ‘dovecot’

Conversion to Dual Stack IPv6

Monday, April 25th, 2011

Over the weekend we took a large step and converted 25% of our network over to DualStack IPv4/IPv6. We aren’t just saying that we’re ready for IPv6, we are actually using IPv6 internally and starting to move some public facing sites so that they can serve both IPv4 and IPv6 enabled web surfers. We primarily run Debian, but, have a few Windows, FreeBSD and other OSs. Our current efforts are switching our Debian machines over since that comprises 95% of our network. We run 2.6.38 with a few config changes rather than the default Debian kernel, but, much of the testing was done with a stock Debian kernel.

We ran into a few minor issues, but, currently backups for the first group of machines that we converted are running over IPv6. Additionally, email that is handled from our MX servers is handed off to the machines over IPv6. Currently, only the actual machines have IPv6 addresses so, we don’t have many public facing sites running, but, of the few that are announcing both IPv4 and IPv6, they amount to almost 4% of our traffic. Clients that access the machines directly for SSH, FTP, POP3, IMAP, SMTP will use IPv6 if they are able. Most clients don’t use the actual devicename for FTP/POP3/IMAP/SMTP, so, most won’t use IPv6 until their public facing site is IPv6 enabled.

Our network is relatively flat which makes our deployment a little easier, but, the basic structure is:

edge router -> chassis switch
                        chassis switch
                        chassis switch
edge router -> chassis switch

We use VRRP to announce a x:x::1/64, each machine gets a /128 from that /64, then, using static routes, we route a /64 to each machine. Due to an issue with OSPF3 on our current network, we had to fall back to static routes. Each machine is allocated a /128 from our main network, and a /64 to the client. Virtual webhost machines, we might allocate /80s to each virtual client out of the /64, but we haven’t made a firm decision on that. We’ve actually cheated and run IPv6 on their own connections to the chassis switch to make traffic and flow monitoring a little easier.

Our basic network now consists of every machine in a single /64 which cuts down on arp requests and VLAN issues, but, requires a slightly different configuration than our existing IPv4 network which used VLANs.

When we configure a machine, we need to add the admin IP to it and push the config changes using our management software. We’ve not automated putting the initial IP address on each machine as it requires route entries into our edge routers. Once OSPF3 is fixed later this week, I expect the process to be more automated.

The first step is to take a /128 out of the /64 for the ‘device’ network and assign it to the machine:

ifconfig eth0 add xxxx:xxxx::c:1/64
route --inet6 add default gateway xxxx:xxxx::1

We opted to use ::ad:xxxx for admin machines, ::c:xxxx for client servers. Since you can use hexadecimal, you could actually assign cabinet numbers, or switch numbers to make identifying the machine location a little quicker. Perhaps some identifier for the building, cabinet/rack, switch it is connected to, etc. could be used. For now, we’re using :c: and :ad: to signify client and admin. Our primary storage server is :ad:bac1, our development machine is :ad:de, etc. Our admin network is unlikely to exceed 65536 machines, but, there is a lot of flexibility if you want to get creative.

Once we’ve added the initial IP, our management software inserts the following into /etc/network/interfaces:

iface eth0 inet6 static
        address xxxx:xxxx::c:1
        netmask 64
        endpoint any
        gateway xxxx:xxxx::1

At this point, the AAAA record for the device is published, and we can access the machine over ssh using IPv6.

For Postfix, we needed to add the following to /etc/postfix/

inet_protocols = ipv4, ipv6

Additionally, we needed to modify /etc/postfix/mynetworks.txt to add:


which allows machines on our local network to communicate with the server and ‘relay’. It is possible that the line to be modified might not refer to a config file and is specified in /etc/postfix/

mynetworks =

Dovecot required changes to /etc/dovecot/dovecot.conf:

listen=[::], *

Pure-FTPD had problems with IPv6 reverse lookups:

echo yes > /etc/pure-ftpd/conf/DontResolve;/etc/init.d/pure-ftpd restart

And of course, /etc/resolv.conf:

nameserver xx.xx.xx.xx
nameserver xx.xx.xx.xx
nameserver xxxx:xxxx::ad:1
nameserver xxxx:xxxx::ad:2

We’ve had minor customer impact and lost one email during our conversions due to missing the mynetworks parameter in postfix and bouncing one message. Debian’s version of Dovecot doesn’t listen to both interfaces with listen=[::] as one might imagine by reading the documentation, but, that was tested on a test machine and didn’t affect any clients.

Many of the config files require [] around the IPv6 addresses such as Apache and Varnish. When you need to specify Listen ports on those machines since they have multiple services listening on port 80 on the same machine on separate IPs, it is something to remember.

Most of the server software we’ve run across hasn’t had any issues. However, client software that uses char(15) to store IP addresses probably needs to be fixed.

So far, I think we’ll be ready for World IPv6 Day with over 98% of our machines running DualStack and we’re shooting for 20% of our client’s public facing sites to have IPv6 support by June 8, 2011.

We have two machines running Tux that are stuck on 2.4.37 and regrettably, Tux appears to segfault when it receives IPv6 traffic. It is a shame since Varnish and Nginx are still outclassed by a webserver written twelve years ago.

So far, the conversion process has been quite straightforward with the minor issues you would expect when introducing IPv6 to applications and server stacks that weren’t written to handle it. I suspect we’ll have 98% of our machines dualstack by May 7 which will give us a month to get 20% of our client base educated and convinced to turn on IPv6.

Entries (RSS) and Comments (RSS).
Cluster host: li