curl - amazon ec2 instance unable to resolve host

Question

I have a bunch of amazon ec2 instances. Today, for no reason I can determine, three of them became unable to resolve a specific url. When I try curl <url>, those three instances say "curl: (6) Could not resolve host", while all the other instances are able to resolve the url and return the correct data. It's only that specific url, as well - all other urls work fine.

What could have caused this? I haven't changed anything on any of the instances, so it's not something I did. How do I find out why it happened?

Answer 1

Check and compare /etc/resolv.conf (for Linux instances) between working and non-working hosts. here is info from my Linux instance:

# Dynamic resolv.conf(5) file for glibc resolver(3) generated by resolvconf(8)
#     DO NOT EDIT THIS FILE BY HAND -- YOUR CHANGES WILL BE OVERWRITTEN
nameserver 10.0.0.2
search us-west-2.compute.internal

In case of Windows instances, check and compare the output of ipconfig /all command for DNS entries between working and non-working hosts. here is info from my windows instance:

Windows IP Configuration

   Host Name . . . . . . . . . . . . : WIN-************
   Primary Dns Suffix  . . . . . . . :
   Node Type . . . . . . . . . . . . : Hybrid
   IP Routing Enabled. . . . . . . . : No
   WINS Proxy Enabled. . . . . . . . : No
   DNS Suffix Search List. . . . . . : us-west-2.ec2-utilities.amazonaws.com
                                       ec2.internal
                                       us-east-1.ec2-utilities.amazonaws.com
                                       compute-1.internal
                                       us-west-2.compute.internal

Ethernet adapter Local Area Connection:

   Connection-specific DNS Suffix  . : us-west-2.compute.internal
   Description . . . . . . . . . . . : Citrix PV Ethernet Adapter #0
   Physical Address. . . . . . . . . : 02-E6-AB-83-C6-8A
   DHCP Enabled. . . . . . . . . . . : Yes
   Autoconfiguration Enabled . . . . : Yes
   IPv4 Address. . . . . . . . . . . : 10.0.0.251(Preferred)
   Subnet Mask . . . . . . . . . . . : 255.255.255.0
   Lease Obtained. . . . . . . . . . : Friday, March 21, 2014 11:36:17 AM
   Lease Expires . . . . . . . . . . : Tuesday, March 25, 2014 5:07:59 AM
   Default Gateway . . . . . . . . . : 10.0.0.1
   DHCP Server . . . . . . . . . . . : 10.0.0.1
   DNS Servers . . . . . . . . . . . : 10.0.0.2
   NetBIOS over Tcpip. . . . . . . . : Disabled