!GetAtt InternalLB.DNSName - bash

Question

I am trying to fetch the dns of internal classic load balancer and pass it in the launch configuration of ec2 instance as a user data. Creating a db in ec2 instance and in the config file trying to get the dns of the internal classic load balancer but I am unable to get it. !GetAtt InternalLB.DNSName I have used this in the user data but the db is not connected but when I manually pass the dns it works.I need to fetch the dns on its on using this userdata script.
ec2instance:
Type: AWS::AutoScaling::LaunchConfiguration
Properties:
# AvailabilityZone: us-east-2a
UserData:
Fn::Base64: !Sub |
#!/bin/bash
cd /var/www
mkdir inc
cd inc
sudo echo "<?php
define('DB_SERVER', '!GetAtt InternalLB.DNSName');
define('DB_USERNAME', 'db');
define('DB_PASSWORD', 'db');
define('DB_DATABASE', 'db');
?>" > dbinfo.inc
BlockDeviceMappings:
- DeviceName: /dev/xvda
Ebs:
DeleteOnTermination: "true"
VolumeSize: "8"
VolumeType: gp2
ImageId: ami-0bdcc6c05dec346bf
InstanceType: t2.micro
KeyName: wahaj(webserver)
SecurityGroups:
- Ref: wahajwebserver
myASG:
Type: AWS::AutoScaling::AutoScalingGroup
Properties:
AvailabilityZones:
- "us-east-2a"
- "us-east-2b"
AutoScalingGroupName: myASG
LoadBalancerNames:
- Fn::ImportValue: !Sub "${elb}-MyLoadBalancer"
MinSize: "2"
MaxSize: "2"
DesiredCapacity: "2"
HealthCheckGracePeriod: 300
LaunchConfigurationName:
Ref: ec2instance
VPCZoneIdentifier:
- Fn::ImportValue: !Sub "${SourceStackName}-SubnetC"
- Fn::ImportValue: !Sub "${SourceStackName}-SubnetD"
internalelbsg:
Type: AWS::EC2::SecurityGroup
Properties:
GroupName: internal-elb
VpcId:
Fn::ImportValue:
Fn::Sub: "${SourceStackName}-VpcID"
SecurityGroupIngress:
- IpProtocol: tcp
FromPort: 3306
ToPort: 3306
SourceSecurityGroupId: !GetAtt wahajwebserver.GroupId
Description: For traffic from Internet
GroupDescription: Security Group for demo server
InternalLB:
Type: AWS::ElasticLoadBalancing::LoadBalancer
Properties:
Scheme: internal
Listeners:
- LoadBalancerPort: "80"
InstancePort: "80"
Protocol: HTTP
- LoadBalancerPort: "3306"
InstancePort: "3306"
Protocol: TCP
SecurityGroups:
- !Ref internalelbsg
LoadBalancerName: internalelbsg
Subnets:
- Fn::ImportValue: !Sub "${SourceStackName}-SubnetC"
- Fn::ImportValue: !Sub "${SourceStackName}-SubnetD"
HealthCheck:
Target: HTTP:80/index.html
HealthyThreshold: "3"
UnhealthyThreshold: "5"
Interval: "30"
Timeout: "5"
The main issue is in define('DB_SERVER', '!GetAtt InternalLB.DNSName'); this line I am not fetching the dns in the right way. Please help

Answer 1

To use GetAtt inside of Sub, you have to use the VarName: VarValue syntax as described in the Fn::Sub documentation page. Additionally, you have to wrap VarName with ${}, i.e. ${VarName}.
I believe this syntax should work:
UserData:
Fn::Base64: !Sub
- |
#!/bin/bash
cd /var/www
mkdir inc
cd inc
sudo echo "<?php
define('DB_SERVER', '${InternalLBDNSName}');
define('DB_USERNAME', 'db');
define('DB_PASSWORD', 'db');
define('DB_DATABASE', 'db');
?>" > dbinfo.inc
- InternalLBDNSName: !GetAtt InternalLB.DNSName