通过AWS负载均衡器连接Proxy(EC2)时主机名丢失的技术求助

Got it, let's work through this hostname missing issue you're seeing when traffic routes through your Load Balancer to the EC2 proxy. I've dealt with similar proxy/LB setups before, so here's what to check and fix:

First, let's confirm the core problem: when you hit the proxy directly, your logs show the full target hostname (abehiroshi.la.coocan.jp) in the GET request line. But when going through the load balancer, that hostname vanishes. This almost always ties back to how the LB forwards requests and whether it preserves critical headers or URL details.

Here are the key steps to resolve this:

• Make sure your Load Balancer preserves the original Host header
  Most cloud LBs (like AWS ALB/NLB, GCP LB) have a setting to pass through the client's original Host header instead of replacing it with the EC2 instance's internal hostname or the LB's own name. For example, on AWS ALB, check your listener rules and ensure the "Forward Host Header" option is set to Original (not Instance). If this is off, your proxy will only see the LB's internal hostname, not the one the client requested.

• Verify your proxy is configured to respect the Host header
  If your proxy (Nginx, Apache, Squid, etc.) relies on the Host header to route requests, double-check it's not overwriting or ignoring the header sent by the LB. For Nginx, if you're using proxy_pass, make sure you include proxy_set_header Host $host; (or $http_host) in your config to retain the original Host value. If your proxy was only working directly because it parsed the full URL in the GET line, you might need to adjust it to prioritize the Host header instead.

• Check if the LB is rewriting the request URL
  Some LBs are set up to strip the domain from the request when forwarding to the backend. Instead of sending GET http://abehiroshi.la.coocan.jp/ HTTP/1.1, it might send just GET / HTTP/1.1. In this case, your proxy needs the Host header to know where to route the request. If the LB is doing this, either disable the URL rewriting on the LB, or reconfigure your proxy to use the Host header for routing instead of relying on the full URL in the request line.

• Capture the raw request on the EC2 proxy to diagnose exactly what's missing
  To get a clear picture, use tools like tcpdump or enable debug logging on your proxy to see the full request it receives from the LB. For example, run tcpdump -i any port 80 -A on the EC2 instance to capture the raw HTTP traffic. Compare this to your direct request log—you'll likely see either the Host header is missing/replaced, or the request line only has the path without the full URL.

• Use X-Forwarded-Host as a fallback if needed
  If your LB supports it, enable sending the X-Forwarded-Host header (which contains the original client's requested hostname). You can then configure your proxy to use this header as a fallback if the Host header isn't present. For Apache, you might use UseCanonicalName Off and set RequestHeader set X-Forwarded-Host "%{HTTP_HOST}s" if the LB isn't sending it automatically.

If you can share more details about your specific LB type (AWS, GCP, self-hosted?) and proxy software, I can give even more targeted advice. Hope this helps!

内容的提问来源于stack exchange，提问作者Kota Ando