Redirect Nginx Subdomains & Trailing Content with Regex

Problem

I recently migrated domains and replaced the old webpage with a simple info page with instructions to users on how to edit their bookmarks and URLs to get to the page they were seeking.

This was not ideal as it left the work up to the user and may have caused friction for users who accessed my RSS feed.

Solution

Instead, I finally found a solution that allows me to redirect both subdomains AND trailing content. For example, both of these URLs now redirect properly using the logic I'll explain below:

# Example 1 - Simple base domain redirect with trailing content
https://domain1.com/blog/alpine-linux/ -> https://domain2.com/blog/alpine-linux/

# Example 2 - Complex redirect with both a subdomain and trailing content
https://libreddit.domain1.com/r/history/comments/7z8cbg/new_discovery_mode_turns_video_game_assassins/
->
https://libreddit.domain2.com/r/history/comments/7z8cbg/new_discovery_mode_turns_video_game_assassins/

Go ahead, try the URLs if you want to test them.

Nginx Config

To make this possible. I needed to configure a proper redirect scheme in my Nginx configuration.

doas nano /etc/nginx/http.d/domain1.conf

Within this file, I had one block set to redirect HTTP requests to HTTPS for the base domain and all subdomains.

server {
        listen [::]:80;
        listen 80;
        server_name domain1.com *.domain1.com;

        if ($host = domain1.com) {
                return 301 https://$host$request_uri;
        }

        if ($host = *.domain1.com) {
                return 301 https://$host$request_uri;
        }

        return 404;
}

For the base domain, I have another server block dedicated to redirecting all base domain requests. You can see that the rewrite line is instructing Nginx to gather all trailing content and append it to the new domain2.com URL.

server {
        listen [::]:443 ssl http2;
        listen 443 ssl http2;

        server_name domain1.com;

        rewrite ^/(.*)$ https://domain2.com/$1 permanent;

        ssl_certificate     /etc/letsencrypt/live/domain1.com/fullchain.pem;
        ssl_certificate_key /etc/letsencrypt/live/domain1.com/privkey.pem;
}

Finally, the tricky part is figuring out how to tell Nginx to redirect while keeping both a subdomain and trailing content intact. I found that the easiest way to do this is to give it a server block of its own.

Within this block, we need to do some regex on the server_name line before we can rewrite anything. This creates a variable called subdomain.

Once the server gets to the rewrite line, it pulls the subdomain variable from above and uses it on the new domain2.com domain before appending the trailing content ($request_uri).

server {
        listen [::]:443 ssl http2;
        listen 443 ssl http2;

        server_name ~^(?<subdomain>\w+)\.domain1\.com$;

        rewrite ^ https://$subdomain.domain2.com$request_uri permanent;

        ssl_certificate     /etc/letsencrypt/live/domain1.com/fullchain.pem;
        ssl_certificate_key /etc/letsencrypt/live/domain1.com/privkey.pem;
}

That's all there is to it. With this, I simply restarted Nginx and watched the redirections work in-action.

doas rc-service nginx restart

Looking back on it, I wish I had done this sooner. Who knows how many people went looking for my sites or bookmarks and gave up when they saw the redirect instructions page.

Oh well, it's done now. Live and learn.