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Port 9Discard Protocol

The Discard Protocol is a simple and obsolete network service that accepts connections and discards any data received without any response. Originally designed for testing, measuring, or debugging, it silently consumes incoming traffic on both TCP and UDP port 9, confirming network reachability without providing feedback. Due to its minimal function and lack of authentication, it's mostly disabled on modern systems for security reasons..

inNetwork ServicesSystem
transport
tcp · udp

2 transports registered

in transit
cleartext

payload readable on path

assignment
official

registered with iana

risk
4/10

caution

lookups
12,911

rank 294 of 993 · top 30%

Technical Details

what runs on :9

The Discard Protocol, defined in RFC 863, is a straightforward service designed primarily for diagnostics and simple testing of network communications. It listens on TCP or UDP port 9 and accepts any transmitted data, which it discards immediately without any response or data returned. This simplicity allows developers and network engineers to test reachability, data transmission, and connection establishment without concern for protocol processing overhead.

Typical operation involves a client sending arbitrary data to the Discard service, which consumes it silently regardless of size or content. The absence of any reply is by design, enabling detection of issues such as packet loss, transmission delays, or connection failures purely through the connection state and local reception. Because the protocol is stateless in its UDP form and connection-oriented in its TCP incarnation, it supports flexible testing scenarios.

Historically included on UNIX and other operating systems as part of the 'inetd' superserver, the Discard Protocol serves no application function beyond development and diagnostics. It has been deprecated in most environments due to both redundancy — better tools and commands exist — and security considerations.

Security Information

exposure of :9

risk score

4/ 10caution

worth attention. how exposed you are depends on configuration — don't leave it reachable from the internet without a reason.

network services averages 3.9 across 604 ports — this one sits 0.1 above.

in transit

cleartext

credentials and payloads are readable by anyone on path

assignment

official

registered with iana for this service — scanners fingerprint it by number

reachable over

tcp · udp

udp is connectionless — source addresses are trivially spoofed and it is a reflection favourite

security overview

Common Vulnerabilities:

  • Its simplicity can be exploited in Denial-of-Service (DoS) attacks where an attacker floods the service to consume network or system resources.
  • Can be abused as an amplification vector in UDP reflection attacks due to its stateless nature.
  • Provides no authentication, encryption, or logging, making monitoring for misuse difficult.
  • Running unnecessary network services increases the attack surface.

Common Mitigations:

  • Disable the Discard Protocol service entirely unless explicitly needed.
  • Filter port 9 traffic using firewalls or access control lists (ACLs) to block unsolicited external connections.
  • Monitor network traffic for unusual port 9 activity indicating scanning or abuse.
  • Restrict permissions on test services to internal trusted users and segments only.
  • Use more secure and appropriate network diagnostic tools (e.g., ping, traceroute, packet captures) instead.

Related Ports

all 604 in network services →

the 8 most looked-up other ports in network services — 604 ports carry that label.

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risk mix of the 8 listed

  • caution100%

0 of 8 encrypted