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UDP 631

ProtocolUDP
Port631
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Synopsis

  • UDP port 631 is used by the legacy CUPS Browsing protocol in the CUPS (Common UNIX Printing System) stack to advertise and discover shared printers on local networks.
  • Real examples include the cupsd daemon in older CUPS releases (e.g., CUPS ≤1.5/1.6 era) and the cups-browsed daemon from OpenPrinting (packaged on Linux distros like Ubuntu and Debian), which can listen and send announcements on UDP 631.
  • Many UNIX/Linux systems and older macOS installations running CUPS historically used this UDP 631 traffic for printer discovery before migrating to DNS-SD/mDNS.
  • Modern CUPS typically prefers DNS-SD/mDNS (UDP 5353), but cups-browsed still supports UDP 631 for backward compatibility.

Observed activity

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