Hackery

Anycast DNS

Fri 01 Jan 2010

(Okay, brand new year - must be time to get back on the blogging wagon ...)

Linux Journal recently had a really good article by Philip Martin on Anycast DNS. It's well worth a read - I just want to point it out and record a cutdown version of how I've been setting it up recently.

As the super-quick intro, anycast is the idea of providing a network service at multiple points in a network, and then routing requests to the 'nearest' service provider for any particular client. There's a one-to-many relationship between an ip address and the hosts that are providing services on that address.

In the LJ article above, this means you provide a service on a /32 host address, and then use a(n) (interior) dynamic routing protocol to advertise that address to your internal routers. If you're a non-cisco linux shop, that means using quagga/ospf.

The classic anycast service is dns, since it's stateless and benefits from the high availability and low latency benefits of a distributed anycast service.

So here's my quick-and-dirty notes on setting up an anycast dns server on CentOS/RHEL using dnsmasq for dns, and quagga zebra/ospfd for the routing.

First, setup your anycast ip address (e.g. 192.168.255.1/32) on a random virtual loopback interface e.g. lo:0. On CentOS/RHEL, this means you want to setup a /etc/sysconfig/network-scripts/ifcfg-lo:0 file containing:
```
DEVICE=lo:0
IPADDR=192.168.255.1
NETMASK=255.255.255.255
ONBOOT=yes
```

Setup your dns server to listen to (at least) your anycast dns interface. With dnsmasq, I use an /etc/dnsmasq.conf config like:

interface=lo:0
domain=example.com
local=/example.com/
resolv.conf=/etc/resolv.conf.upstream
expand-hosts
domain-needed
bogus-priv

Use quagga's zebra/ospfd to advertise this host address to your internal routers. I use a completely vanilla zebra.conf, and an /etc/quagga/ospfd.conf config like:

hostname myhost
password mypassword
log syslog
!
router ospf
  ! Local segments (adjust for your network config and ospf areas)
  network 192.168.1.0/24 area 0
  ! Anycast address redistribution
  redistribute connected metric-type 1
  distribute-list ANYCAST out connected
!
access-list ANYCAST permit 192.168.255.1/32

That's it. Now (as root) start everything up:

ifup lo:0
for s in dnsmasq zebra ospfd; do
  service $s start
  chkconfig $s on
done
tail -50f /var/log/messages

And then check on your router that the anycast dns address is getting advertised and picked up by your router. If you're using cisco, you're probably know how to do that; if you're using linux and quagga, the useful vtysh commands are:

show ip ospf interface <interface>
show ip ospf neighbor
show ip ospf database
show ip ospf route
show ip route