Nginx: location 404 with try_files under a different root directory

Andru asked:

I’m in the process of moving a wiki I administer (practicalplants.org) to a new server running Nginx. I’m accustomed to Apache; this is my first time working with Nginx and I’m having trouble adapting the rewrites and aliases I had set up in Apache. The directory layout is as follows

/html               (server root)
    /html/w         (mediawiki)
    /html/community (vanilla)
    /html/blog      (wordpress)
/sso                (Zend Framework SSO solution)
    /sso/public

As you can see, the sso solution has it’s own publicly accessible application root directory, which I’m attempting to set as the root for the location directive, but it seems like the following try_files still attempts to fall back to the server root.

Here’s my config:

server {
  server_name practicalplants.org;

  root /var/www/v-practicalplants/html;
  access_log /var/log/nginx/practicalplants.org-access.log;
  error_log /var/log/nginx/practicalplants.org-error.log;

  include /etc/nginx/biowikifarm_listen80_443.conf;
  include /etc/nginx/biowikifarm_generics.conf;
  include /etc/nginx/biowikifarm_generic_php.conf;

  disable_symlinks off;

  location = / {
    # Redirect domain-only access to wiki:
    rewrite "^[/]?$" /wiki/ permanent;
  }

  location /wiki {
    #rewrite "^/wiki[/]?([^?]*)(?:\?(.*))?" /w/index.php?title=$1&$args last;
    try_files $uri $uri/ /w/index.php?title=$uri&$args;
  }
  location /w/media {
    location ~ ^/w/media/thumb/(archive/)?[0-9a-f]/[0-9a-f][0-9a-f]/([^/]+)/([0-9]+)px-.*$ {
      try_files $uri @thumb_w;
    }
  }
  location @thumb_w {
    rewrite ^/w/media/thumb/[0-9a-f]/[0-9a-f][0-9a-f]/([^/]+)/([0-9]+)px-.*$ /w/thumb.php?f=$1&width=$2;
    rewrite ^/w/media/thumb/archive/[0-9a-f]/[0-9a-f][0-9a-f]/([^/]+)/([0-9]+)px-.*$ /w/thumb.php?f=$1&width=$2&archived=1;
    include /etc/nginx/fastcgi_params; fastcgi_param SCRIPT_FILENAME  $document_root/w/thumb.php; fastcgi_pass php5-fpm-sock;
  }

  location /sso {
    root /var/www/v-practicalplants/sso/public;
    try_files $uri $uri/ /index.php
  }
}

This config gives me a 404 if I navigate to /sso

Is there something obviously wrong here?