How to see the resolved proxy_pass URL in Nginx? - nginx

Question

I have a setup like this for reflecting requests:
location ~ ^/prefix/string/(.*)$
{
proxy_pass https://abc.example.com/$1;
access_log /var/log/nginx/unique.log logformat
}
Visit my site at /prefix/string/more_stuff
Logs show 404 with $upstream_addr only containing the IP of abc.example.com,
not the full URL
Log lines show up in unique.log, so I know I'm hitting this location block.
I want to eliminate the possibility that the URL construction with $1 is wrong.
I would like to be able to log the resolved complete proxy_pass URL so I can
fix it or move on to other types of debugging.
Note: I realize that I can read docs to figure out nginx's behavior in this case. But just logging the resolved URL would be way faster if possible.
Question: How to log the resolved URL that NGINX actually visits for a proxy_pass like this?

Answer 1

You can use named capture group to capture your URI suffix as some unique variable instead of $1:
location ~ ^/prefix/string/(?<suffix>.*)$ {
access_log /var/log/nginx/unique.log unique;
proxy_pass https://abc.example.com/$suffix;
}
Define a new unique log format at the http context level (for example, adding this $suffix value to the default combined log format):
# should be defined outside the server block!
log_format unique '$remote_addr - $remote_user [$time_local] '
'"$request" $status $body_bytes_sent '
'"$http_referer" "$http_user_agent"'
' Proxy path: "$suffix"';
server {
...
}