How to block bots in Nginx by stopping Invalid Host Headers - nginx

Question

I have tried a ton of different things to stop bots from hitting my backend but cannot seem to block invalid host headers without blocking all traffic. My current configuration looks as such:
# trying to stop invalid host headers which doesn't work
server {
listen 80 default_server;
return 444;
}
upstream backend_server {
server backend:8000;
}
server {
listen 80;
location /.well-known/acme-challenge/ {
root /var/www/certbot;
}
location ~* ^/(api|admin|static|v2) {
return 301 https://$host$request_uri;
}
location / {
return 301 https://$host$request_uri;
}
}
server {
listen 443 ssl;
listen [::]:443 ssl;
server_name example.com *.example.com;
deny 143.198.76.27; # trying to stop certain IPs here which doesn't work
ssl_certificate /etc/letsencrypt/live/example.com/fullchain.pem;
ssl_certificate_key /etc/letsencrypt/live/example.com/privkey.pem;
include /etc/letsencrypt/options-ssl-nginx.conf;
ssl_dhparam /etc/letsencrypt/ssl-dhparams.pem;
location ~ ^/v2(?:/(.*))?$ {
root /usr/share/nginx/html;
index index.html;
try_files $uri $uri/ /v2/index.html =404;
}
location /backend_static/ {
alias /backend/assets/;
}
location /media/ {
alias /backend/media/;
}
location ~* ^/(api|admin) {
proxy_pass http://backend_server$request_uri;
proxy_set_header Host $host;
proxy_set_header X-Real-IP $remote_addr;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
proxy_set_header X-Forwarded-Proto $https;
proxy_connect_timeout 360s;
proxy_read_timeout 360s;
}
location ~* ^/(videos|notes|memos|images|policies|documents|files|uploads|static) {
proxy_pass http://backend_server$request_uri;
proxy_set_header Host $host;
proxy_set_header X-Real-IP $remote_addr;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
proxy_set_header X-Forwarded-Proto $https;
proxy_connect_timeout 360s;
proxy_read_timeout 360s;
# Set upload size for videos to be 500MB
client_max_body_size 500M;
}
location / {
proxy_pass http://backend_server$request_uri;
proxy_set_header Host $host;
proxy_set_header X-Real-IP $remote_addr;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
proxy_set_header X-Forwarded-Proto $https;
proxy_connect_timeout 360s;
proxy_read_timeout 360s;
}
}

Answer 1

Basically to block bots who sends invalid Host header you need something like this:
server {
listen 80 default_server;
server_name mydomain.com;
if ( $host !~* ^(mydomain.com|www.mydomain.com)$ ) {
return 444;
}
if ( $http_host !~* ^(mydomain.com|www.mydomain.com)$ ) {
return 444;
}
}
But there are plenty of other options (because almost all bots have tendency to adapt), like cookie tests, javascript or even CAPTCHa.