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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?
248
Verify that your instance is ready Check your instance to make sure it is running and has passed its status checks. Open the Amazon EC2 console at https://console.aws.amazon.com/ec2/ . In the navigation pane, choose Instances, and then select your instance.
The following are common reasons why EC2 Instance Connect might not work as expected: EC2 Instance Connect doesn't support the OS distribution. The EC2 Instance Connect package isn't installed on the instance. There are missing or incorrect AWS Identity and Access Management (IAM) policies or permissions.
This error occurs if you created a password for your key file, but haven't manually entered the password. To resolve this error, enter the password or use ssh-agent to load the key automatically. There are a number of reasons why you might get an SSH error, like Resource temporarily unavailable.
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
I was able to fix it after enabling that service:
sudo systemctl enable --now systemd-resolved.service
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