The library runs a number of validation checks to look for common configuration issues. If a validation check fails, an error is logged to the console. Note that validation checks may be disabled globally by passing validate: false in the options, or disabled individually with the settings documented below. Additionally, specific checks will be disabled in some circumstances, also discussed below. Finally, all validation checks are automatically disabled after the first request. It is to be noted that if multiple ‘first’ requests come in simultaneously, the validation checks may run more than once.
The ability to disable individual checks was added in version 7.0.0.

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Errors

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ERR_ERL_UNDEFINED_IP_ADDRESS

Added in 6.8.0.
This error is logged whenever request.ip is undefined. It could indicate a misconfiguration in server settings, or that the client disconnected before the request could be processed. It could also indicate that a server other than Express is being used, which is not a supported use-case. This check will be prevented if a custom keyGenerator is supplied. Set validate: {ip: false} in the options to disable the check.

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ERR_ERL_INVALID_IP_ADDRESS

Added in 6.8.0.
This error is logged whenever the IP address is not a valid IPV4/IPV6 address, or when the IP address contains a port number. The port from which a request comes can be changed simply by opening or closing a browser, or when using an Azure proxy, thus opening up avenues for bypassing the rate limit. Consider using a custom keyGenerator function that strips out the port number (request.ip.replace(/:\d+[^:]*$/, ''), or that uses something like an API key or bearer token to count the hits against. See #234 for more information on this issue. This check will be prevented if a custom keyGenerator is supplied. Set validate: {ip: false} in the options to disable the check.

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ERR_ERL_PERMISSIVE_TRUST_PROXY

Added in 6.8.0.
This error is logged when the trust proxy setting is set to true. If this is set to true, it will cause express to return the leftmost entry in the X-Forwarded-For header as the client’s IP. This header could be set by the proxy or a malicious client, opening up avenues for bypassing the rate limiter. Refer to the troubleshooting proxy issues page for a guide to set the trust proxy value correctly. This check will be prevented if a custom keyGenerator is supplied. Set validate: {trustProxy: false} in the options to disable the check.

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ERR_ERL_UNEXPECTED_X_FORWARDED_FOR

Added in 6.8.0.
This error is logged when the X-Forwarded-For header is set (indicating use of a proxy), but the trust proxy setting is false (which is the default value). This usually indicates a configuration issue that will cause express-rate-limit to apply it’s limits global rather than on a per-user basis. Refer to the troubleshooting proxy issues page for a guide to set the trust proxy value correctly. If this error occurs only rarely, and you do not have a reverse proxy, it may indicate a malicious user probing for vulnerabilities. This check will be prevented if a custom keyGenerator is supplied. Set validate: {xForwardedForHeader: false} in the options to disable the check.

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ERR_ERL_INVALID_HITS

Added in 6.10.0.
This indicates an issue with the Store and/or it’s underlying data storage mechanism. Ensure the Store returns a positive integer for the totalHits value when increment() is called. If this is not possible, the store should throw an error. Set validate: {positiveHits: false} in the options to disable the check.

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ERR_ERL_STORE_REUSE

Added in 7.3.0.
This indicates that the single Store instance was used in more than one rate limiter. This can lead to problems such as initialization logic running multiple times, and inconsistent reset times. Instead, create a new store instance for each rate limiter. (Generally with a unique prefix value for each one.) Set validate: {unsharedStore: false} in the options to disable the check.

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ERR_ERL_DOUBLE_COUNT

Added in 6.9.0.
This indicates that the hit count for a given IP or key was incremented more than once for a single request. It could happen if the same instance of express-rate-limit is called more than once, or if multiple instances are called that use the same Store.
  • If only a single rate limit is desired, find and remove the extra rateLimit call(s).
  • If multiple rate limits are desired, consider one of the following options:
In rare circumstances this error can be a false positive. This would include situations where multiple rate limiters are configured to use the same key but different backed databases. Setting a unique key per rate limiter would prevent this.
Prior to version 6.11.1, the library did not take a store prefixes into account, and thus could log this incorrectly if two instances use different prefixes.
Set validate: {singleCount: false} in the options to disable the check.

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ERR_ERL_HEADERS_UNSUPPORTED_DRAFT_VERSION

Added in 7.5.0
The library only implements the sixth, seventh and eighth drafts of the IETF RateLimit headers specification. If draft version other than draft-6, draft-7, or draft-8 is passed to the standardHeaders option, this error is raised.

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ERR_ERL_HEADERS_NO_RESET

Added in 7.0.0.
The seventh draft of the IETF RateLimit headers specification makes reset a required field. However, the legacy Store API did not have a way for stores to provide this information. The library attempts to account for this by using the windowMs time, which has a default value of 60 seconds if not explicitly set. To resolve this, either change to a different headers version, such as draft-6, or a different store. Set validate: {headersResetTime: false} in the options to disable the check.

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ERR_ERL_UNKNOWN_VALIDATION

Added in 7.0.0.
The library allows for specific validation checks to be enabled or disabled, but only ones that it knows about. If an unknown validation check is referenced in the validate configuration object, this error will be logged. This could be due to a typo in the validate configuration, or the configuration could be intended for a different version of the library that includes the unrecognized validation check. To resolve this, remove the specified key from your configuration. To prevent the validation check that logs this error, set the validate.validationsConfig option to false: 
const limiter = rateLimit({
	validate: {
		validationsConfig: false,
		// ...
		default: true,
	},
	// ...
})
Set validate: {validationsConfig: false} in the options to disable the check.

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ERR_ERL_CREATED_IN_REQUEST_HANDLER

Instances of express-rate-limit should be created before a request comes in, not in response to one. Incorrect example: 
import { rateLimit } from 'express-rate-limit'

app.use((req, res, next) => {
	// This won't work!
	rateLimit({
		/*...*/
	})
	next()
})
Correct example: 
import { rateLimit } from 'express-rate-limit'

app.use(rateLimit({/*...*/});
To only apply the rate limit to some requests, use the skip, skipSuccessfulRequests, or skipFailedRequests options. Example where the rate limit only applies to users who are not logged in: 
// first determine the user's logged-in status
app.use((req, res, next) => {
	if (/* user is logged in */) {
		req.isLoggedIn = true
	} else {
		req.isLoggedIn = false
	}
	next()
})

// next configure express-rate-limit to skip logged in users
app.use(rateLimit({
	skip: (req, res) => req.isLoggedIn,
	// ...
}))
If using a factory function to create express-rate-limit instances, ensure that it is called at app initialization, not in response to each request: 
  function rateLimitFactory() {
  	return rateLimit({/*...*/});
  }

- app.use(rateLimitFactory); // broken
+ app.use(rateLimitFactory()); // works
In certain advanced use cases, such as serving multiple websites from a single app, it may be necessary to dynamically create a rate limiter in response to a request. In this case the validation check may be disabled, but care should be taken to ensure everything works as expected. Consider caching the limiter after it’s first usage to avoid repeating the initialization cost and any background work the store may require. Set validate: {creationStack: false} in the options to disable the check.

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ERR_ERL_IPV6_SUBNET

Added in 8.0.0.
The ipv6Subnet option must be set to an integer in the range of 1 to 128 (inclusive), or a function that returns such an integer. Set it to false to disable the subnet masking. However this validation check is more restrictive in that it only allows for a number in the more common range of 32 to 64. Subnet values less than /32 will block a large portion of the internet, and on the other side, ISPs very rarely give out subnets larger than /64 because certain IPv6 features (SLAAC) depend on it. Disable this check if you are confident that larger or smaller subnets are correct for your use-case. Set validate: {ipv6Subnet: false} in the options to disable the check.

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ERR_ERL_IPV6SUBNET_OR_KEYGENERATOR

Added in 8.0.0.
The ipv6Subnet and keyGenerator options are mutually exclusive. A custom keyGenerator function does respect the value of the ipv6Subnet option - only the default keyGenerator makes use of it. When using a custom keyGenerator, the ipv6Subnet should instead be set or calculated inside the keyGenerator and passed as a second parameter to the ipKeyGenerator helper function like so: 
import { rateLimit, ipKeyGenerator } from 'express-rate-limit

const limiter = rateLimit({
	// ...
	keyGenerator: (req, res) => {
 		if (req.query.apiKey) return req.query.apiKey

 		const ipv6Subnet = 64 // calculate or set a fixed value here
		return ipKeyGenerator(req.ip, ipv6Subnet)
	}
})
Set validate: {ipv6SubnetOrKeyGenerator: false} in the options to disable the check.

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ERR_ERL_KEY_GEN_IPV6

Added in 8.0.0.
This error indicates that your custom keyGenerator appears to have a vulnerability where IPv6 users can easily bypass rate-limiting by rotating through their available IP addresses. (See ipv6Subnet setting for more info.) Correct this error by wrapping your use of req.ip with the ipKeyGenerator helper function like so: 
 import { rateLimit, ipKeyGenerator } from 'express-rate-limit

 const limiter = rateLimit({
 	// ...
 	keyGenerator: (req, res) => {
 		// Use API key (or some other identifier) for authenticated users
 		if (req.query.apiKey) return req.query.apiKey

 		// fallback to IP for unauthenticated users
		// return req.ip // vulnerable
		return ipKeyGenerator(req.ip) // better
	}
 })
(See the example above for a custom ipv6Subnet.) Disable this check if your keyGenerator already takes IPv6 subnets into account. Set validate: {keyGeneratorIpFallback: false} in the options to disable the check.

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Warnings

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WRN_ERL_MAX_ZERO

Added in 6.10.0.
In express-rate-limit version 6 and older, the rate limiter would be disabled when setting limit (previously max until version 7.0.0) to 0 in the options. Starting with version 7.0.0, this is no longer the case - the rate limiting will apply from the very first request instead. See #369 for more information. To recreate the original behavior of disabling the rate limiter entirely, use the skip function instead. Set validate: {limit: false} in the options to disable the check.

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WRN_ERL_DEPRECATED_ON_LIMIT_REACHED

Added in 6.10.0.
The onLimitReached configuration option was deprecated in express-rate-limit v6 and removed in version 7.0.0. To replicate it’s behavior, set a custom handler like so: 
const limiter = rateLimit({
	// ...
	handler: (request, response, next, options) => {
		if (request.rateLimit.used === request.rateLimit.limit + 1) {
			// onLimitReached code here
		}
		response.status(options.statusCode).send(options.message)
	},
})
(Alternatives for express-slow-down users) Set validate: {onLimitReached: false} in the options to disable the check.

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WRN_ERL_DEPRECATED_DRAFT_POLLI_HEADERS

Added in 6.10.0.
The draft_polli_ratelimit_headers option was deprecated in express-rate-limit v6 and removed in version 7.0.0. Please use the standardHeaders: 'draft-6' option to replicate its behaviour, or use the recommended standardHeaders: 'draft-7' option instead. Set validate: {draftPolliHeaders: false} in the options to disable the check.