Fix the error in the top-level http block of NGINX’s main configuration. If the message identifies server_names_hash_bucket_size, raise it to the next power of two above the value shown—for example, change 32 to 64. If NGINX reports that it could not build an optimal hash and offers server_names_hash_max_size or server_names_hash_bucket_size, increase server_names_hash_max_size first to a value close to the number of configured server names. Run nginx -t, then reload NGINX through the service manager used by your installation.
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Identify which hash problem you have
NGINX builds server-name hash tables when it starts and whenever it processes a configuration reload. The wording of the error tells you whether the problem is one unusually long name or a collection of names that does not fit efficiently in the selected table.
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A hard bucket-size error
A typical startup failure looks like this:
could not build the server_names_hash,
you should increase server_names_hash_bucket_size: 32This usually means that at least one configured server_name is too long for the current bucket capacity. NGINX’s documented example uses too.long.server.name.example.org with a bucket size of 32 and recommends changing the setting to 64. The value must be moved to the next power of two above the value reported by the error.
An optimal-hash warning
When many names are configured, the message can instead say:
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could not build optimal server_names_hash,
you should increase either
server_names_hash_max_size: 512
or server_names_hash_bucket_size: 32This is a different tuning case. NGINX recommends trying a larger server_names_hash_max_size first, using a value near the number of configured names. Increase the bucket size only if that does not resolve the message or if building the table takes an unacceptable amount of time.
What the server-names hash does
Each virtual host declares one or more names in a server_name directive. NGINX places exact names and wildcard names into hash tables so it can select the appropriate virtual server without scanning every configuration entry for each request.
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Why one long hostname can prevent startup
A hash bucket has a finite capacity. A single long hostname, a long alias, or a name included from another virtual-host file can exceed that capacity. If NGINX cannot construct a valid table, it stops with the hard could not build the server_names_hash error rather than accepting a partially working configuration.
NGINX aligns the bucket size with the processor’s cache-line size to reduce memory accesses during lookups. The setting therefore represents more than an arbitrary character limit: it controls the size of each hash bucket, with alignment and pointer overhead affecting how much of a name can fit.
Why many names produce a different message
A large set of names can fit individually but still require a larger hash table to achieve an efficient arrangement. In that situation, NGINX may be able to start but warns that it could not build an optimal hash, or it may ask for a larger table size and bucket size. This is primarily a table-capacity problem, not evidence that one hostname is too long.
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Find the setting in the correct configuration scope
Both directives are valid only in the top-level http context. They do not belong inside a server block or a location block.
http {
server_names_hash_bucket_size 64;
server_names_hash_max_size 1024;
# include files and server blocks
}Use the main configuration file selected by your NGINX installation and inspect every file brought in with an include directive. Distribution packages commonly split virtual hosts across multiple files, so the longest name may not be in the file you first opened. A setting placed in an unrelated file, or inside a server block, will either be ignored or produce a context error.
Inventory the names before changing values
- Read all
server_namedirectives in the main file and included virtual-host files. - Look for long aliases, development names, migration domains, and wildcard entries that are no longer used.
- Count the configured names when the error mentions
server_names_hash_max_size. - Check generated configuration files as well as files maintained by hand; an automated include can reintroduce a name after you edit the apparent source file.
Removing obsolete aliases or shortening an unnecessarily elaborate name is preferable to increasing limits indefinitely. It reduces the hash’s work and prevents a temporary workaround from becoming permanent configuration debt.
Fix a single long server name
Use this path when the error explicitly asks for server_names_hash_bucket_size, or when inspection finds one hostname that is substantially longer than the rest.
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- Record the value in the error. If NGINX reports 32, that is the current bucket-size value it wants you to change.
- Choose the next power of two. For 32, use 64; for 64, use 128. Do not choose a value below the requested threshold.
- Edit the top-level
httpblock. Add or change the directive there, not in an individual virtual host. - Save the complete configuration set. Ensure that an included file is not overriding your intended arrangement or that a generated file will not replace the edit.
- Validate before reloading. Run
nginx -tusing the production configuration. - Reload NGINX. Use the service manager supplied with the installation so the running process rebuilds its hash tables.
A minimal correction is:
http {
server_names_hash_bucket_size 64;
}The number 64 is appropriate for an error that reports 32; it is not a universal recommendation. Follow the value and progression shown by your own error message.
Account for expected future names
NGINX maintainer Maxim Dounin described the bucket size as the limit on the maximum length of a server_name. He advised allowing the maximum expected name length plus additional room for two pointers. That guidance is useful when you are adding a known long hostname rather than reacting to a one-time failure. Select the smallest power-of-two value that leaves that headroom, because an unnecessarily large bucket consumes more memory and can make hash construction less efficient.
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Fix a large collection of names
Use this path when the message says could not build optimal server_names_hash and presents both directives as alternatives.
- Estimate the actual name count. Include exact names, wildcard names, and names supplied by included files. Count names, not merely virtual-host files.
- Raise
server_names_hash_max_sizefirst. Choose a value near the number of configured names rather than jumping immediately to an extreme value. - Keep the directive in the
httpblock. The default maximum table size is 512, so a configuration with substantially more names may need a higher value. - Test the full configuration with
nginx -t. A syntax check also causes NGINX to parse the included files and attempt to build the relevant hashes. - Reload and inspect the error log. If NGINX still requests a larger bucket size, apply the next-power-of-two rule to
server_names_hash_bucket_size.
For example:
http {
server_names_hash_max_size 1024;
}
This is an example of the order of operations, not a fixed value for every deployment. The appropriate setting depends on the names actually present in your configuration.
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A deployment can have both a high name count and one or more unusually long names. In that case, increase server_names_hash_max_size first when NGINX reports an optimal-hash problem, then increase the bucket size only if the subsequent test specifically requests it. Treat each message as a separate constraint instead of changing both values blindly.
Directive reference
| Directive | Valid context | Documented default | Use it when | Selection rule |
|---|---|---|---|---|
server_names_hash_bucket_size |
http |
32, 64, or 128, depending on processor cache-line size | A name exceeds bucket capacity, or NGINX continues to request a larger bucket | Use the next power of two above the value reported; allow headroom for the longest expected name |
server_names_hash_max_size |
http |
512 | Many names prevent an optimal hash | Start with a value near the number of configured names; raise bucket size only if necessary |
The bucket-size default is not always 32: NGINX selects 32, 64, or 128 according to processor cache-line characteristics. The maximum-table default is 512. These are configuration defaults, not performance guarantees or measurements of a typical deployment.
Validate safely before and after the change
Run the configuration test
Execute:
nginx -tRun the command in the same environment and against the same configuration that the production service uses. A successful result confirms that NGINX can parse the complete configuration and build the required structures. If the command still names a bucket or maximum-table value, use that new message rather than assuming the first edit was sufficient.
Reload instead of stopping the service
After nginx -t succeeds, reload NGINX through the service manager used by your installation. A reload is the appropriate operational test because hash tables are rebuilt during reconfiguration while existing workers can continue handling traffic according to the service manager’s normal reload behavior.
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The exact reload command varies by packaging and operating system. Use the service unit or management command that controls your running NGINX instance, and verify that it reports success. Do not assume that editing a file has changed the running process.
Check the result
- Confirm that the reload completed without a new hash-building error.
- Review the NGINX error log for the same message or a different included-file problem.
- Request representative hostnames and verify that each reaches the intended virtual server.
- Keep the validated configuration and the previous working version available for rollback.
Troubleshoot common failed fixes
The directive is in a server block
Move it to the enclosing top-level http block. These directives are not per-virtual-host settings, so placing them inside server { ... } does not tune the hash used by all virtual hosts.
You changed a file NGINX does not load
Follow the include directives from the active main configuration and identify the files actually parsed by the service. Package layouts often contain example files, disabled files, and generated directories beside the active configuration. Testing the wrong file can produce a misleading success.
You raised the value but the error is unchanged
Check that the edited directive is in the active http context and that a later generated configuration step is not replacing it. Then run nginx -t again and read the exact value in the new error. For a bucket error, the chosen value must be the next power of two above the reported threshold.
You increased the bucket size for a name-count problem
If the message says “could not build optimal” and offers server_names_hash_max_size, start with the maximum table size. A larger bucket is aimed at accommodating longer names; it is not the first response to a table that is crowded with many ordinary names.
The error returns after a reload
Look for configuration generation or deployment automation that restores the old value, and inspect every included file for a newly added long name. Because NGINX rebuilds hashes on each reconfiguration, a name introduced by a deployment can expose the same error even when an earlier reload succeeded.
NGINX starts, but the warning remains
Distinguish a hard startup failure from an “optimal” warning. A warning indicates that the selected arrangement is less efficient than NGINX’s preferred hash, not necessarily that requests are failing. Follow the documented order: increase server_names_hash_max_size first, then retest. Remove unnecessary names if the table has grown through stale aliases.
Improve the server-name configuration instead of only raising limits
Prefer exact names for frequent traffic
NGINX searches exact names before wildcard names. Explicit names make the intended virtual-server mapping clear and are the best fit for hosts that receive regular traffic.
Use wildcards deliberately
Wildcard lookups are slower because NGINX searches by domain parts. Keep wildcard entries when they express a genuine naming policy, but avoid adding broad patterns merely to avoid listing a small, stable set of exact names.
Limit regular expressions
Regular-expression server names are tested sequentially, making them the least scalable choice as the list grows. Use them only when the matching rule cannot be represented with exact or wildcard names, and place the configuration so that the intended match is unambiguous.
Remove stale aliases
Old campaign domains, temporary migration names, and duplicate aliases increase the table’s work and can be the source of an unexpectedly long entry. Removing names that no longer resolve is often a cleaner fix than setting a very large bucket or maximum-table value.
Worked scenarios
Scenario 1: one long hostname
The error reports server_names_hash_bucket_size: 32, and inspection finds a long fully qualified hostname in one included virtual-host file.
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- Open the main configuration and add
server_names_hash_bucket_size 64;insidehttp { ... }. - Run
nginx -t. - Reload through the installation’s service manager.
- Confirm that the error log no longer reports a server-names hash failure.
Scenario 2: hundreds of ordinary names
The message reports that NGINX could not build an optimal hash and offers server_names_hash_max_size: 512 or server_names_hash_bucket_size: 32. No individual hostname is unusually long, but many names are present across included files.
- Count the names, including aliases and wildcard entries.
- Raise
server_names_hash_max_sizein thehttpblock to a value near that count. - Run
nginx -tand reload. - If NGINX then requests a larger bucket, increase the bucket to the next power of two and test again.
Scenario 3: a long name and a crowded table
The configuration contains both a long migration hostname and a large alias inventory. Address the constraints in the order shown by NGINX: use server_names_hash_max_size for the crowded table, then apply the next-power-of-two bucket adjustment if the long name still exceeds capacity. Also remove aliases that are no longer required.
Operational checklist
- Read the complete error and identify whether it names bucket size, maximum table size, or both.
- Inspect every active configuration file and included virtual-host file.
- Find the longest
server_nameand count the configured names. - Delete obsolete aliases or simplify unnecessarily long names where possible.
- For one long name, set
server_names_hash_bucket_sizeto the next power of two above the reported value. - For many names, raise
server_names_hash_max_sizefirst to a value near the name count. - Keep both directives in the top-level
httpblock. - Run
nginx -tagainst the production configuration. - Reload with the service manager used by the installation.
- Check logs and representative hostnames after the reload.
Key distinctions to remember
server_names_hash_bucket_size addresses the maximum length that a bucket can accommodate, while server_names_hash_max_size controls the maximum size of the hash table. A long hostname commonly causes the first hard startup error. A large collection of names commonly causes the second optimization warning. Choosing the directive that matches the message avoids wasting memory and preserves cache-aware lookup behavior.
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