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Put server_names_hash_bucket_size in NGINX’s top-level http block, raise it to the next power of two named by the error, then run nginx -t and reload NGINX. If the message instead says it cannot build an optimal hash and offers server_names_hash_max_size, increase that maximum first when many names are configured.
Identify which hash problem you have
NGINX builds server-name hash tables when it starts and whenever it processes a configuration reload. The tables let it select the virtual server for an incoming Host name without scanning every server block. A configuration can therefore be syntactically valid yet fail at startup or reload while NGINX is constructing the hash.
Read the complete error before changing anything. The wording points to two different causes:
| Error pattern | Likely cause | First change to make |
|---|---|---|
could not build the server_names_hash followed by increase server_names_hash_bucket_size: 32 (or another value) |
At least one configured name is too long for the current bucket capacity. | Set server_names_hash_bucket_size to the next power of two above the reported value. |
could not build optimal server_names_hash followed by increase either server_names_hash_max_size: 512 or server_names_hash_bucket_size: 32 |
There are many names, so the selected table size is not efficient, or a long name is also present. | Raise server_names_hash_max_size first, to a value near the number of configured names. |
A hard “could not build” error can prevent NGINX from starting. An “optimal” warning may still allow it to run, but it signals a hash layout that NGINX considers inefficient. Treat both as configuration problems and test the full configuration before reloading.
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Where the directives belong
Both directives are valid only in the top-level http context. Put them inside the http { ... } block in the main configuration, not inside an individual server or location block.
http {
server_names_hash_bucket_size 64;
server_names_hash_max_size 1024;
include mime.types;
include /etc/nginx/conf.d/*.conf;
# server { ... } blocks and other http settings
}The main file is commonly named nginx.conf, but package layouts differ. Included files are part of the same configuration tree. A directive placed in a file that is not included, or placed under the wrong context, has no effect or causes an “directive is not allowed here” error.
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Ask the installed binary which prefix and configuration it uses:
nginx -V 2>&1Inspect the output for the --conf-path option. Then inspect that file and every include it references. On a running system, the effective configuration can also be printed with:
nginx -TThe -T output includes the main file and expanded includes. Use it to verify that your hash directives appear inside the intended http block and to locate every server_name.
Fix a single long server name
NGINX’s documented example uses the name too.long.server.name.example.org. With a bucket size of 32, NGINX reports:
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could not build the server_names_hash,
you should increase server_names_hash_bucket_size: 32Change the value to the next power of two:
http {
server_names_hash_bucket_size 64;
}Use the value shown in your own error, not the example’s number. The usual progression is 32, 64, 128, 256 and so on. If the message names 64, use 128; if it names 128, use 256. NGINX’s documentation specifies increasing the bucket size to the next power of two.
Why a long name causes a hard failure
Exact names and wildcard names are stored in hash tables. A bucket must have enough capacity for the longest entry, including the internal bookkeeping NGINX uses. When one name cannot fit, NGINX cannot construct the table and aborts startup or the reconfiguration attempt.
Maxim Dounin, an NGINX maintainer, describes the limit directly: the value of server_names_hash_bucket_size limits the maximum length of a server_name used. He recommends sizing it for the maximum expected name plus additional space for two pointers. In practice, the error’s requested value and the next power-of-two rule provide the safe operational setting.
Check whether shortening is possible
Before increasing a global setting, inspect the names that caused it. Remove obsolete aliases, duplicate names and temporary migration hostnames when they are no longer needed. A shorter canonical hostname or a reduced alias list can solve the capacity problem without changing hash memory characteristics. Do not remove a name that clients, certificates or DNS records still require.
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When many names are configured, NGINX may report:
could not build optimal server_names_hash,
you should increase either server_names_hash_max_size: 512
or server_names_hash_bucket_size: 32For this form, increase server_names_hash_max_size first. The directive controls the maximum number of buckets NGINX may use while building the name table; its documented default is 512. Choose a value close to the number of configured names, then leave room for aliases added by the same deployment.
http {
server_names_hash_max_size 1024;
}There is no universal replacement number. Count the exact and wildcard names in the expanded configuration, select a practical upper bound, and retest. If NGINX still requests a larger bucket size, apply the bucket-size rule as a second change. Raising both directives immediately can hide which constraint was responsible and can allocate more hash-table space than necessary.
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Count names in all included files
Do not count only the file you last edited. Names can be declared in virtual-host files, generated snippets and distribution-specific include directories. Use nginx -T, search its output for server_name, and account for every token after the directive. A single server block can contain several aliases, and each contributes to the hash.
Understand the two directives
| Directive | Context | Documented default | What it controls | When to change it |
|---|---|---|---|---|
server_names_hash_bucket_size size; |
http |
32, 64 or 128, depending on processor cache-line size | The capacity of each hash bucket and therefore the maximum server-name length that can fit. | A long name triggers a message naming this directive, or NGINX still cannot build the table after the maximum size is increased. |
server_names_hash_max_size size; |
http |
512 | The maximum number of buckets NGINX may allocate for the server-name hash. | Many names produce an “optimal hash” message that offers this directive. |
Bucket size is aligned with the processor’s cache-line size to reduce memory accesses during lookup. Consequently, an unnecessarily large bucket is not automatically an improvement. Keep it at the smallest power of two that accommodates the longest required name, and prefer increasing the maximum table size first when the error indicates a crowding problem.
Apply the change safely
- Capture the exact error. Record whether it names
server_names_hash_bucket_size,server_names_hash_max_size, or both, and note the numeric values. - Inspect the complete configuration. Run
nginx -Tor inspect the file reported bynginx -V. Find the longest name and estimate the total count, including aliases and included files. - Choose one targeted edit. For one long name, add the bucket directive in
httpand move to the next power of two. For many names, raiseserver_names_hash_max_sizefirst. - Test before touching the running service. Run:
nginx -tA successful test reports that the syntax is okay and the test is successful. If your installation uses a nondefault configuration path, pass it explicitly, for example nginx -t -c /path/to/nginx.conf, using the same path your service manager starts.
- Reload through the installed service manager. Use the mechanism that owns your NGINX process, such as
systemctl reload nginx, the platform’sservice nginx reloadcommand, ornginx -s reload. A reload rebuilds the hash while applying the new configuration; a full stop and start is normally unnecessary after a successful test. - Check the result. Review the NGINX error log and service status. Confirm that the reload completed and that the hash message did not recur.
If the test fails, do not reload. Correct the reported syntax, context or include-path problem, then run nginx -t again. If a reload fails after a successful test because of a service-manager or permission issue, the previous worker configuration generally remains active; inspect service logs before attempting a restart.
Common failure modes and recovery
The directive was added inside a server block
NGINX will reject a bucket or maximum-size directive outside http. Move it to the enclosing top-level block in the main configuration, or to an included file that is itself included from that block. Re-run nginx -t to verify the context.
The edited file is not included
Some installations load conf.d, sites-enabled or a generated configuration directory; others use a custom path. If the value does not appear in nginx -T, NGINX is not reading the file you changed. Edit the active file or add the correct include, then test again.
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The value was increased but the same long-name error remains
Check that the new value is a power of two greater than the number in the message and that no other included file overrides it later in the http block. Look for an even longer alias than the one you first noticed. If the error names a larger requested value, follow that request rather than repeatedly changing an unrelated setting.
The “optimal hash” warning persists
Count all names from the expanded configuration and increase server_names_hash_max_size toward that count. If the warning changes to a bucket-size request, then raise the bucket size to the next power of two. Remove stale aliases and redundant patterns where possible so future deployments do not require continually larger tables.
A reload appears successful but traffic is unchanged
Check the service manager’s status and NGINX’s error log, then verify that the process used the configuration you tested. A test against one file followed by a reload of a different service instance can produce this symptom. Compare the running process arguments with the nginx -V configuration path and use the matching reload command.
Server-name patterns and lookup cost
NGINX searches exact names before wildcard names. Exact names are therefore the best choice for frequently requested hosts when you know the complete hostname. Wildcard lookups are slower because NGINX searches by domain parts. Regular-expression names are tested sequentially and scale least well as the list grows.
- Exact: use a complete hostname when the set of hosts is known and stable.
- Wildcard: use only where a pattern genuinely represents many hosts; keep the pattern set small and intentional.
- Regular expression: reserve for cases that cannot be expressed with exact or wildcard names, because sequential testing becomes more expensive as patterns accumulate.
These lookup characteristics do not replace the hash-size fix, but they explain why simply increasing values forever is poor configuration design. Consolidating unnecessary aliases and replacing broad patterns with explicit names can reduce both table pressure and lookup work.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Choosing values without over-sizing
Use the error and the configuration, not a copied number from another server. Defaults vary: the documented bucket default is 32, 64 or 128 according to processor cache-line size, while the documented maximum table size default is 512. A setting that is appropriate on one host may be excessive or insufficient on another.
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- For a long name, start with the requested bucket value and select the next power of two.
- For many names, start with a maximum size near the actual name count, then retest.
- Keep the bucket as small as the longest required name permits because cache-aware sizing is part of NGINX’s design.
- Change one directive at a time when the message offers alternatives, so the resulting behavior identifies the real constraint.
- Document the reason for an unusually high value beside the directive, especially when names are generated automatically.
The hash is rebuilt at every startup and reconfiguration. A value that works today must still accommodate names added by the next deployment, so include expected aliases in your sizing decision without allocating an unnecessarily large bucket for hypothetical names.
A repeatable diagnostic checklist
- Have you copied the complete error, including the requested number?
- Is the directive inside the top-level
httpblock? - Does
nginx -Tshow the directive and every includedserver_name? - Is the problem one unusually long name or a large number of names?
- For a long name, did you choose the next power of two?
- For many names, did you try
server_names_hash_max_sizefirst? - Did
nginx -tpass using the same configuration path as the service? - Did you reload with the service manager that owns the running process?
- After reloading, are the service status and error log clear?
Worked examples
Example: one long hostname
An error asks for server_names_hash_bucket_size: 32. The configuration contains a long fully qualified hostname and only a few aliases. Add this to http:
server_names_hash_bucket_size 64;Run nginx -t. If it succeeds, reload NGINX. If the next startup message requests 128, use 128; do not keep 64 merely because it worked in a previous configuration.
Example: thousands of aliases
The message offers server_names_hash_max_size: 512 or server_names_hash_bucket_size: 32, and the names are ordinary in length but numerous. Count the names shown by nginx -T, remove retired aliases, then set a maximum near that count, such as:
server_names_hash_max_size 2048;Test and reload. Only if NGINX then reports that a bucket cannot hold a name should you increase server_names_hash_bucket_size.
What not to do
- Do not put the directive in a
locationblock or inside an individual virtual host. - Do not reload after editing without first running
nginx -t. - Do not assume a warning about an optimal hash requires a huge bucket; increase the maximum table size first when the configuration has many names.
- Do not copy a value from a different machine without considering its longest name, name count and processor-dependent default.
- Do not delete aliases solely to silence the error if those aliases are still published in DNS, certificates or client configuration.
Final decision rule
Use server_names_hash_bucket_size for capacity problems caused by a long individual name, and choose the next power of two above the value NGINX reports. Use server_names_hash_max_size first when the issue is a crowded table containing many names. In either case, make the change in the http block, verify the complete expanded configuration with nginx -t, and reload through the correct service manager so NGINX rebuilds its hashes.
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