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

If you’re not using the default VPC, or a VPC created by the VPC wizard, you’ll need to make sure that the DNS hostname and DNS resolution options are enabled.

Answer 2

this answer will probably solve your issue, as it did mine. you must fix your hosts file to

127.0.0.1    localhost.localdomain localhost
127.0.1.1    my-machine

or

127.0.0.1    localhost
127.0.1.1    my-machine

where my-machine is the host name that appears at the end of the error message.
(e.g. mine was domX-A2-33-D9-CB-F4-A8)

don't forget to reboot your instance from your management console after you have changed the hosts file. ;)

Answer 3

I have encountered the same issue. This might be the reason of the issue .Disk space full so /etc/resolve.conf file gets cleared then Its unable to resolved the host

This solution works for me.

Clean the fulled spaces

Update the resolve.conf from the working ec2 instance of same vpc

Answer 4

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

Answer 5

Here is my checklist of things to verify. Are you 100% sure nobody / nothing changed the following:

• DNS Settings on the instance? (check with dig to see if name resolution is OK)
• NACL rules on the subnet where your instance is running?
• Security Group rules of the SG attached to your instances?

Seb