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Pipelining

HTTP Pipelining support has been added which allows multiple HTTP requests to be sent on the same connection inspired from http-desync-attacks-request-smuggling-reborn. Before running HTTP pipelining based templates, make sure the running target supports HTTP Pipeline connection, otherwise nuclei engine fallbacks to standard HTTP request engine. If you want to confirm the given domain or list of subdomains supports HTTP Pipelining, httpx has a flag -pipeline to do so. An example configuring showing pipelining attributes of nuclei. 
    unsafe: true
    pipeline: true
    pipeline-concurrent-connections: 40
    pipeline-requests-per-connection: 25000
An example template demonstrating pipelining capabilities of nuclei has been provided below- 
id: pipeline-testing
info:
  name: pipeline testing
  author: pdteam
  severity: info

http:
  - raw:
      - |+
        GET /{{path}} HTTP/1.1
        Host: {{Hostname}}
        Referer: {{BaseURL}}

    attack: batteringram
    payloads:
      path: path_wordlist.txt

    unsafe: true
    pipeline: true
    pipeline-concurrent-connections: 40
    pipeline-requests-per-connection: 25000

    matchers:
      - type: status
        part: header
        status:
          - 200

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Connection pooling

While the earlier versions of nuclei did not do connection pooling, users can now configure templates to either use HTTP connection pooling or not. This allows for faster scanning based on requirement. To enable connection pooling in the template, threads attribute can be defined with respective number of threads you wanted to use in the payloads sections. Connection: Close header can not be used in HTTP connection pooling template, otherwise engine will fail and fallback to standard HTTP requests with pooling. An example template using HTTP connection pooling- 
id: fuzzing-example
info:
  name: Connection pooling example
  author: pdteam
  severity: info

http:

  - raw:
      - |
        GET /protected HTTP/1.1
        Host: {{Hostname}}
        Authorization: Basic {{base64('admin:§password§')}}

    attack: batteringram
    payloads:
      password: password.txt
    threads: 40

    matchers-condition: and
    matchers:
      - type: status
        status:
          - 200

      - type: word
        words:
          - "Unique string"
        part: body