Azure应用网关：主机名相关配置疑问咨询

Let's break down your questions one by one based on Azure's backend pool behavior and Linux networking basics:

1. What value should I configure for the Target field in BackendPool? #

For Azure Load Balancer's Backend Pool, the target depends on your scenario, but here's the most common and recommended approach for your VM:

• If you're using an internal Load Balancer (traffic stays within your Azure VNet)：Use either your VM's private IP (YYY.YYY.YYY.YYY) or its internal fully qualified domain name (FQDN) returned by hostname -f (trialinstance.somerandomguid.dx.internal.cloudapp.net). The internal FQDN is preferred if your VM has a dynamic private IP, as Azure's internal DNS will automatically resolve it to the correct IP even if it changes.
• If you're using a public Load Balancer：You still want to use the private IP or internal FQDN as the target. The public IP (XXX.XXX.XXX.XXX) is for external traffic to reach your VM, but the Load Balancer communicates directly with your VM over the private network, so using the private address is more efficient and avoids unnecessary public network hops.

Avoid using the public FQDN (trialinstance.westus.cloudapp.azure.com) as the backend target—it will force traffic through the public internet, which adds latency and potential security risks.

2. What exactly does "hostname" refer to? #

The term "hostname" can have two related but distinct meanings, depending on context:

• Operating system hostname: This is the short name assigned to your VM's OS (you can check it with just hostname, which would return trialinstance in your case). It's used internally by the OS to identify itself.
• Fully Qualified Domain Name (FQDN): This is the complete hostname including the domain suffix. Your VM has two FQDNs:
  • The internal FQDN (trialinstance.somerandomguid.dx.internal.cloudapp.net): Managed by Azure's internal DNS, used for resolving VNet-internal traffic between resources.
  • The public FQDN (trialinstance.westus.cloudapp.azure.com): Used for public internet access to your VM, mapped to its public IP.

In most Azure networking configurations (like backend pools), when they refer to a "hostname", they're typically asking for the FQDN—either internal or public, depending on the use case.

3. What's the purpose of setting 127.0.0.1 as a hostname? #

Mapping 127.0.0.1 (the loopback address) to a hostname (usually done in the /etc/hosts file) tells your local OS to resolve that hostname to the local machine itself, instead of querying a DNS server. Common use cases include:

• Testing local services: If you're running a service bound to the loopback address (e.g., a development server), using the hostname to access it lets you verify the service works as expected without exposing it to the network.
• Avoiding DNS issues: If your applications rely on accessing the local machine via hostname, setting this mapping ensures the resolution works even if DNS is down or misconfigured.
• Security: If a service is only intended to be accessed locally, binding it to 127.0.0.1 and mapping the hostname to that address prevents external devices from reaching the service.
• Configuration compatibility: Some applications are hardcoded to use the VM's hostname to access local resources—this mapping ensures those applications connect to the correct local instance.

内容的提问来源于stack exchange，提问作者BlindSniper