in ec2-plugin/src/main/java/hudson/plugins/ec2/ssh/EC2UnixLauncher.java

on line 372 in function `getEC2HostAddress()`

there is a line:

```
String host = inst.getPublicDnsName();
```

This assumes that DNS Hostnames are enabled for the VPC which the slave instance is in, however, by default DNS is not enabled. This causes an error when attempting to relaunch slave instances in VPC, but *NOT* when launching new instances.

The best fix would be to remove ALL usage of Public DNS and instead only use public IP or private DNS, both which are guaranteed to exist (I think).