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James Carr

Replacing Text in Nginx with sub_filter

Sometimes you find yourself in a weird predicament. A third party application that you’ve slapped nginx in front of insists on using internal IP addresses or ports despite your reverse proxy passing all the correct headers and other pieces required. Or maybe you’ve found yourself in the situation I found myself in last week where you have a third party internal application wanting to reference a file on a CDN.

As luck would have it, that CDN has been deprecated and changing it requires rebuilding a jar where the script path is hardcoded and then repackaging the internal application that uses it. Long story short, we’ll soon be doing some yak shaving that could possibly take all day so how about we just use sub_filter instead and take a nap?

Use It

Thankfully most stock builds of nginx include ngx_http_sub_module by default. Using it is actually simple enough, just slap the following directive under your location or server directive in nginx.conf.

sub_filter 'Hello' 'Goodbye';
sub_filter_once on;

And that’s it! You can also use regular expression here as well. sub_filter_once indicates whether nginx should apply the replacement once or repeatedly. One gotcha you might have is that by default this only works with mime types of text/html. If you want to modify the mime-types that subfilter executes on set `sub_filter_types` to the desired type.

Give It a Try

I’ve put together a very simple demonstration of using subfilter for text replacement on Github.

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