THREDDS and Apache ProxyPass

I have not seen this come up in the THREDDS discussion or documentation, and I 
was scratching my head for a while today to get it to work right, so here's a 
tech note for consideration in the THREDDS documentation:

THREDDS and Apache ProxyPass

If you are using Tomcat on port 8080 (for example) and Apache on the default 
port 80, you can use the Apache proxy module to hide the Tomcat port in the 
URLs used to connect to THREDDS.  This may be critical if your server is behind 
a firewall that only allows port 80 access.  It is also important if THREDDS is 
running on a machine that is completely behind a firewall, but you have a 
server running Apache that can talk to the machine.

You will need to edit the Apache httpd.conf file (on Linux, it is stored in 

Find the lines that start with "<IfModule mod_proxy.c>" and end with 
"</IfModule>".  These lines may need to be uncommented.  Between these lines, 
make sure the following line exists and is uncommented:

    ProxyRequests On

Then add the following lines:

    ProxyPreserveHost On
    <Location /thredds>
        ProxyPass http://myhost.mydomain:8080/thredds
        ProxyPassReverse http://myhost.mydomain:8080/thredds
    <Proxy http://myhost.mydomain:8080/thredds>
        AllowOverride None
        Order allow,deny
        Allow from All

You of course need to replace "http://myhost.mydomain:8080/thredds"; with the 
full URL of the THREDDS web application.

The section between <Proxy... And </Proxy> can usually be omitted, but in some 
cases a server is sufficiently locked down that you need to explicitly allow 
access to resources.  It can also be edited to restrict access to registered 
users via Apache authentication mechanisms.

Finally, on the server command line (as a super/admin user), execute:

apachectl graceful

In my configuration, I initially failed to add the line "ProxyPreserveHost On", 
which caused me many headaches as links generated by THREDDS kept directing the 
client back to port 8080 (which was blocked by a firewall). 

Best of luck, 
Michael A. Godin
Software Engineer
Monterey Bay Aquarium Research Institute

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