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

What the message looks like

nginx: [emerg] could not build server_names_hash, you should increase server_names_hash_bucket_size: 64
nginx: configuration file /etc/nginx/nginx.conf test failed

Common causes

• One server_name is very long (for example a long subdomain), and a bucket is too small for it.
• Many server names were added across server blocks, so the table needs a larger size.
• The same name is listed in several places because a file is included twice, which inflates the table.
• A generated config (wildcard lists, many aliases per site) was added without raising the hash settings.

Check

1. Run the configuration test
   nginx -tCopy
2. Current hash settings, if any
   nginx -T 2>/dev/null | grep -nE 'server_names_hash'Copy
3. The longest configured server names
   nginx -T 2>/dev/null | grep -E '^\s*server_name' | tr -s ' ;' '\n' | awk 'length > 25' | sort -uCopy
4. Is a file included twice?
   nginx -T 2>/dev/null | grep -E '^# configuration file' | sort | uniq -dCopy

Fix

• Add to the http block in /etc/nginx/nginx.conf: server_names_hash_bucket_size 128; (a power of two; the message names the current value).
• If the error stays or you have hundreds of names, also raise server_names_hash_max_size, for example 1024.
• Remove duplicate includes so no name is defined twice.
• Test and apply: nginx -t && systemctl reload nginx.

FAQ

Which value should I use?

Double the value from the message and try again: 64 becomes 128, then 256 if needed. The bucket size must be a power of two.

Why does it appear after adding one more domain?

The table is sized when the configuration loads. One more or one longer name can be the one that no longer fits, even though the old setup worked.