+++
date = 2022-12-07
title = "Redirect Nginx Subdomains and Trailing Content with Regex"
description = ""
draft = false
+++

# 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:

``` txt
# 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.

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

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

``` conf
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.

``` conf
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`).

``` conf
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.

```sh
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.