---
title: "Traefik InFlightReq Documentation"
description: "Traefik Proxy's HTTP middleware lets you limit the number of simultaneous in-flight requests. Read the technical documentation."
---

# InFlightReq

Limiting the Number of Simultaneous In-Flight Requests
{: .subtitle }

![InFlightReq](../../assets/img/middleware/inflightreq.png)

To proactively prevent services from being overwhelmed with high load, the number of allowed simultaneous in-flight requests can be limited.

## Configuration Examples

```yaml tab="Docker & Swarm"
labels:
  - "traefik.http.middlewares.test-inflightreq.inflightreq.amount=10"
```

```yaml tab="Kubernetes"
apiVersion: traefik.io/v1alpha1
kind: Middleware
metadata:
  name: test-inflightreq
spec:
  inFlightReq:
    amount: 10
```

```yaml tab="Consul Catalog"
# Limiting to 10 simultaneous connections
- "traefik.http.middlewares.test-inflightreq.inflightreq.amount=10"
```

```yaml tab="File (YAML)"
# Limiting to 10 simultaneous connections
http:
  middlewares:
    test-inflightreq:
      inFlightReq:
        amount: 10
```

```toml tab="File (TOML)"
# Limiting to 10 simultaneous connections
[http.middlewares]
  [http.middlewares.test-inflightreq.inFlightReq]
    amount = 10
```

## Configuration Options

### `amount`

The `amount` option defines the maximum amount of allowed simultaneous in-flight request.
The middleware responds with `HTTP 429 Too Many Requests` if there are already `amount` requests in progress (based on the same `sourceCriterion` strategy).

```yaml tab="Docker & Swarm"
labels:
  - "traefik.http.middlewares.test-inflightreq.inflightreq.amount=10"
```

```yaml tab="Kubernetes"
apiVersion: traefik.io/v1alpha1
kind: Middleware
metadata:
  name: test-inflightreq
spec:
  inFlightReq:
    amount: 10
```

```yaml tab="Consul Catalog"
# Limiting to 10 simultaneous connections
- "traefik.http.middlewares.test-inflightreq.inflightreq.amount=10"
```

```yaml tab="File (YAML)"
# Limiting to 10 simultaneous connections
http:
  middlewares:
    test-inflightreq:
      inFlightReq:
        amount: 10
```

```toml tab="File (TOML)"
# Limiting to 10 simultaneous connections
[http.middlewares]
  [http.middlewares.test-inflightreq.inFlightReq]
    amount = 10
```

### `sourceCriterion`

The `sourceCriterion` option defines what criterion is used to group requests as originating from a common source.
If several strategies are defined at the same time, an error will be raised.
If none are set, the default is to use the `requestHost`.

#### `sourceCriterion.ipStrategy`

The `ipStrategy` option defines three parameters that configures how Traefik determines the client IP: `depth`, `excludedIPs` and `ipv6Subnet`.

!!! important "As a middleware, InFlightReq happens before the actual proxying to the backend takes place. In addition, the previous network hop only gets appended to `X-Forwarded-For` during the last stages of proxying, i.e. after it has already passed through the middleware. Therefore, during InFlightReq, as the previous network hop is not yet present in `X-Forwarded-For`, it cannot be used and/or relied upon."

##### `ipStrategy.depth`

The `depth` option tells Traefik to use the `X-Forwarded-For` header and select the IP located at the `depth` position (starting from the right).

- If `depth` is greater than the total number of IPs in `X-Forwarded-For`, then the client IP is empty.
- `depth` is ignored if its value is less than or equal to 0.

If `ipStrategy.ipv6Subnet` is provided and the selected IP is IPv6, the IP is transformed into the first IP of the subnet it belongs to.  
See [ipStrategy.ipv6Subnet](#ipstrategyipv6subnet) for more details.

!!! example "Example of Depth & X-Forwarded-For"

    If `depth` is set to 2, and the request `X-Forwarded-For` header is `"10.0.0.1,11.0.0.1,12.0.0.1,13.0.0.1"` then the "real" client IP is `"10.0.0.1"` (at depth 4) but the IP used as the criterion is `"12.0.0.1"` (`depth=2`).

    | `X-Forwarded-For`                       | `depth` | clientIP     |
    |-----------------------------------------|---------|--------------|
    | `"10.0.0.1,11.0.0.1,12.0.0.1,13.0.0.1"` | `1`     | `"13.0.0.1"` |
    | `"10.0.0.1,11.0.0.1,12.0.0.1,13.0.0.1"` | `3`     | `"11.0.0.1"` |
    | `"10.0.0.1,11.0.0.1,12.0.0.1,13.0.0.1"` | `5`     | `""`         |

```yaml tab="Docker & Swarm"
labels:
  - "traefik.http.middlewares.test-inflightreq.inflightreq.sourcecriterion.ipstrategy.depth=2"
```

```yaml tab="Kubernetes"
apiVersion: traefik.io/v1alpha1
kind: Middleware
metadata:
  name: test-inflightreq
spec:
  inFlightReq:
    sourceCriterion:
      ipStrategy:
        depth: 2
```

```yaml tab="Consul Catalog"
- "traefik.http.middlewares.test-inflightreq.inflightreq.sourcecriterion.ipstrategy.depth=2"
```

```yaml tab="File (YAML)"
http:
  middlewares:
    test-inflightreq:
      inFlightReq:
        sourceCriterion:
          ipStrategy:
            depth: 2
```

```toml tab="File (TOML)"
[http.middlewares]
  [http.middlewares.test-inflightreq.inflightreq]
    [http.middlewares.test-inflightreq.inFlightReq.sourceCriterion.ipStrategy]
      depth = 2
```

##### `ipStrategy.excludedIPs`

`excludedIPs` configures Traefik to scan the `X-Forwarded-For` header and select the first IP not in the list.

!!! important "If `depth` is specified, `excludedIPs` is ignored."

!!! example "Example of ExcludedIPs & X-Forwarded-For"

    | `X-Forwarded-For`                       | `excludedIPs`         | clientIP     |
    |-----------------------------------------|-----------------------|--------------|
    | `"10.0.0.1,11.0.0.1,12.0.0.1,13.0.0.1"` | `"12.0.0.1,13.0.0.1"` | `"11.0.0.1"` |
    | `"10.0.0.1,11.0.0.1,12.0.0.1,13.0.0.1"` | `"15.0.0.1,13.0.0.1"` | `"12.0.0.1"` |
    | `"10.0.0.1,11.0.0.1,12.0.0.1,13.0.0.1"` | `"10.0.0.1,13.0.0.1"` | `"12.0.0.1"` |
    | `"10.0.0.1,11.0.0.1,12.0.0.1,13.0.0.1"` | `"15.0.0.1,16.0.0.1"` | `"13.0.0.1"` |
    | `"10.0.0.1,11.0.0.1"`                   | `"10.0.0.1,11.0.0.1"` | `""`         |

```yaml tab="Docker & Swarm"
labels:
  - "traefik.http.middlewares.test-inflightreq.inflightreq.sourcecriterion.ipstrategy.excludedips=127.0.0.1/32, 192.168.1.7"
```

```yaml tab="Kubernetes"
apiVersion: traefik.io/v1alpha1
kind: Middleware
metadata:
  name: test-inflightreq
spec:
  inFlightReq:
    sourceCriterion:
      ipStrategy:
        excludedIPs:
        - 127.0.0.1/32
        - 192.168.1.7
```

```yaml tab="Consul Catalog"
- "traefik.http.middlewares.test-inflightreq.inflightreq.sourcecriterion.ipstrategy.excludedips=127.0.0.1/32, 192.168.1.7"
```

```yaml tab="File (YAML)"
http:
  middlewares:
    test-inflightreq:
      inFlightReq:
        sourceCriterion:
          ipStrategy:
            excludedIPs:
              - "127.0.0.1/32"
              - "192.168.1.7"
```

```toml tab="File (TOML)"
[http.middlewares]
  [http.middlewares.test-inflightreq.inflightreq]
    [http.middlewares.test-inflightreq.inFlightReq.sourceCriterion.ipStrategy]
      excludedIPs = ["127.0.0.1/32", "192.168.1.7"]
```

##### `ipStrategy.ipv6Subnet`

This strategy applies to `Depth` and `RemoteAddr` strategy only.
If `ipv6Subnet` is provided and the selected IP is IPv6, the IP is transformed into the first IP of the subnet it belongs to.

This is useful for grouping IPv6 addresses into subnets to prevent bypassing this middleware by obtaining a new IPv6.

- `ipv6Subnet` is ignored if its value is outside of 0-128 interval

!!! example "Example of ipv6Subnet"

    If `ipv6Subnet` is provided, the IP is transformed in the following way.

    | `IP`                      | `ipv6Subnet` | clientIP              |
    |---------------------------|--------------|-----------------------|
    | `"::abcd:1111:2222:3333"` | `64`         | `"::0:0:0:0"`         |
    | `"::abcd:1111:2222:3333"` | `80`         | `"::abcd:0:0:0:0"`    |
    | `"::abcd:1111:2222:3333"` | `96`         | `"::abcd:1111:0:0:0"` |

```yaml tab="Docker & Swarm"
labels:
  - "traefik.http.middlewares.test-inflightreq.inflightreq.sourcecriterion.ipstrategy.ipv6Subnet=64"
```

```yaml tab="Kubernetes"
apiVersion: traefik.io/v1alpha1
kind: Middleware
metadata:
  name: test-inflightreq
spec:
  inFlightReq:
    sourceCriterion:
      ipStrategy:
        ipv6Subnet: 64
```

```yaml tab="Consul Catalog"
- "traefik.http.middlewares.test-inflightreq.inflightreq.sourcecriterion.ipstrategy.ipv6Subnet=64"
```

```yaml tab="File (YAML)"
http:
  middlewares:
    test-inflightreq:
      inFlightReq:
        sourceCriterion:
          ipStrategy:
            ipv6Subnet: 64
```

```toml tab="File (TOML)"
[http.middlewares]
  [http.middlewares.test-inflightreq.inflightreq]
    [http.middlewares.test-inflightreq.inFlightReq.sourceCriterion.ipStrategy]
      ipv6Subnet = 64
```

#### `sourceCriterion.requestHeaderName`

Name of the header used to group incoming requests.

```yaml tab="Docker & Swarm"
labels:
  - "traefik.http.middlewares.test-inflightreq.inflightreq.sourcecriterion.requestheadername=username"
```

```yaml tab="Kubernetes"
apiVersion: traefik.io/v1alpha1
kind: Middleware
metadata:
  name: test-inflightreq
spec:
  inFlightReq:
	sourceCriterion:
      requestHeaderName: username
```

```yaml tab="Consul Catalog"
- "traefik.http.middlewares.test-inflightreq.inflightreq.sourcecriterion.requestheadername=username"
```

```yaml tab="File (YAML)"
http:
  middlewares:
    test-inflightreq:
      inFlightReq:
        sourceCriterion:
          requestHeaderName: username
```

```toml tab="File (TOML)"
[http.middlewares]
  [http.middlewares.test-inflightreq.inflightreq]
    [http.middlewares.test-inflightreq.inFlightReq.sourceCriterion]
      requestHeaderName = "username"
```

#### `sourceCriterion.requestHost`

Whether to consider the request host as the source.

```yaml tab="Docker & Swarm"
labels:
  - "traefik.http.middlewares.test-inflightreq.inflightreq.sourcecriterion.requesthost=true"
```

```yaml tab="Kubernetes"
apiVersion: traefik.io/v1alpha1
kind: Middleware
metadata:
  name: test-inflightreq
spec:
  inFlightReq:
    sourceCriterion:
      requestHost: true
```

```yaml tab="Consul Catalog"
- "traefik.http.middlewares.test-inflightreq.inflightreq.sourcecriterion.requesthost=true"
```

```yaml tab="File (YAML)"
http:
  middlewares:
    test-inflightreq:
      inFlightReq:
        sourceCriterion:
          requestHost: true
```

```toml tab="File (TOML)"
[http.middlewares]
  [http.middlewares.test-inflightreq.inflightreq]
    [http.middlewares.test-inflightreq.inFlightReq.sourceCriterion]
      requestHost = true
```