How to fix Nginx "could not build server_names_hash"

nginx: [emerg] could not build server_names_hash, you should increase server_names_hash_bucket_size: 64

Common causes

• A long domain name, such as a long subdomain or cloud hostname
• Many server names across all server blocks
• server_names_hash_bucket_size left at its default, which depends on CPU cache line size (32 or 64)
• The directive was added inside a server block instead of the http block
• Duplicate or wildcard-heavy server_name lists

How to fix it

1. Raise the bucket size. In /etc/nginx/nginx.conf, inside the http { } block, add server_names_hash_bucket_size 128;. On Ubuntu there is often a commented line you can uncomment and edit.
2. Raise max size if mentioned. If the error also names server_names_hash_max_size, add server_names_hash_max_size 1024; (or 2048 for very many names) in the http block.
3. Place it in the http context. These directives are only valid in http { }. Putting them in a server block causes a different error.
4. Test the config. Run sudo nginx -t. If it still fails, double the value again (bucket size should stay a power of two, like 128 or 256).
5. Reload Nginx. Run sudo systemctl reload nginx. Check that all sites respond.

Increase hash sizes in /etc/nginx/nginx.conf

http {
    server_names_hash_bucket_size 128;
    server_names_hash_max_size 1024;
    # ...
    include /etc/nginx/conf.d/*.conf;
    include /etc/nginx/sites-enabled/*;
}

How to stop it happening again

• Set the hash sizes in your base Nginx template
• Run nginx -t before reloading on every domain addition
• Remove old domains you no longer serve
• Avoid duplicate names across server blocks