How to point a subdomain to another "folder" on same server? - nginx

Question

So I have set up two domains with each of there repositories on one server with forge. It looks like this:
DomainA.com (179.x.x.x)
DomainB.com (179.x.x.x)
In my DNS I have pointed them both to the same ip, and forge has handled everything else and it works great.
On the server there's two folders.
DomainA.com
DomainB.com
So now I want to create a subdomain on DomainB which "loads" the code of DomainA, example: code.domainb.com will load show the contents which in on domaina.com.
I'm not sure how I can do that? So far I understand that I need to change something in the nginx config, hopefully someone can give me some pointers :)
Nginx config for domainb:
# FORGE CONFIG (DO NOT REMOVE!)
include forge-conf/domainb.com/before/*;
server {
listen 443 ssl http2;
listen [::]:443 ssl http2;
server_name .domainb.com;
root /home/forge/domainb.com/public;
# FORGE SSL (DO NOT REMOVE!)
ssl_certificate /etc/nginx/ssl/domainb.com/471043/server.crt;
ssl_certificate_key /etc/nginx/ssl/domainb.com/471043/server.key;
ssl_protocols TLSv1.2;
ssl_ciphers XXX;
ssl_prefer_server_ciphers on;
ssl_dhparam /etc/nginx/dhparams.pem;
add_header X-Frame-Options "SAMEORIGIN";
add_header X-XSS-Protection "1; mode=block";
add_header X-Content-Type-Options "nosniff";
index index.html index.htm index.php;
charset utf-8;
# FORGE CONFIG (DO NOT REMOVE!)
include forge-conf/domainb.com/server/*;
location / {
try_files $uri $uri/ /index.php?$query_string;
}
location = /favicon.ico { access_log off; log_not_found off; }
location = /robots.txt { access_log off; log_not_found off; }
access_log off;
error_log /var/log/nginx/domainb.com-error.log error;
error_page 404 /index.php;
location ~ \.php$ {
fastcgi_split_path_info ^(.+\.php)(/.+)$;
fastcgi_pass unix:/var/run/php/php7.3-fpm.sock;
fastcgi_index index.php;
include fastcgi_params;
}
location ~ /\.(?!well-known).* {
deny all;
}
}
# FORGE CONFIG (DO NOT REMOVE!)
include forge-conf/domainb.com/after/*;
Nginx conf for domain A:
# FORGE CONFIG (DO NOT REMOVE!)
include forge-conf/domaina.com/before/*;
server {
listen 443 ssl http2;
listen [::]:443 ssl http2;
server_name .domaina.com;
root /home/forge/domaina.com/public;
# FORGE SSL (DO NOT REMOVE!)
ssl_certificate /etc/nginx/ssl/domaina.com/470443/server.crt;
ssl_certificate_key /etc/nginx/ssl/domaina.com/470443/server.key;
ssl_protocols TLSv1.2;
ssl_ciphers XXX;
ssl_prefer_server_ciphers on;
ssl_dhparam /etc/nginx/dhparams.pem;
add_header X-Frame-Options "SAMEORIGIN";
add_header X-XSS-Protection "1; mode=block";
add_header X-Content-Type-Options "nosniff";
index index.html index.htm index.php;
charset utf-8;
# FORGE CONFIG (DO NOT REMOVE!)
include forge-conf/domaina.com/server/*;
location / {
try_files $uri $uri/ /index.php?$query_string;
}
location = /favicon.ico { access_log off; log_not_found off; }
location = /robots.txt { access_log off; log_not_found off; }
access_log off;
error_log /var/log/nginx/domaina.com-error.log error;
error_page 404 /index.php;
location ~ \.php$ {
fastcgi_split_path_info ^(.+\.php)(/.+)$;
fastcgi_pass unix:/var/run/php/php7.3-fpm.sock;
fastcgi_index index.php;
include fastcgi_params;
}
location ~ /\.(?!well-known).* {
deny all;
}
}
# FORGE CONFIG (DO NOT REMOVE!)
include forge-conf/domaina.com/after/*;

Answer 1

The server_name directive can have more than one value. The exact match HOST name takes precedence. See this document for details.
If code.domainb.com uses the same document root as the server block for .domaina.com, simply add its name to the server_name directive.
For example:
server {
...
server_name .domainb.com;
root /home/forge/domainb.com/public;
...
}
server {
...
server_name .domaina.com code.domainb.com;
root /home/forge/domaina.com/public;
...
}
See this document for more.