networkingaddressing

Loopback

loopbacknetworkingaddressing
Plain English

Loopback is a computer talking to itself. Traffic sent to 127.0.0.1 never leaves the machine, so a service listening only there cannot be reached from the network.

Technical Definition

The lo interface holds 127.0.0.0/8 and ::1. Name localhost resolves to it via /etc/hosts.

  • Binding a service to 127.0.0.1 makes it local-only, a common hardening step (for example Docker -p 127.0.0.1:8088:80 behind a reverse proxy).
  • 0.0.0.0 means all interfaces, the opposite.
  • Ubuntu’s systemd-resolved stub listens on 127.0.0.53.

Source: RFC 6890

Loopback in practice

$ ss -tln
LISTEN 0 4096 127.0.0.53%lo:53   0.0.0.0:*
LISTEN 0 511       0.0.0.0:8080 0.0.0.0:*
In the Wild

Reading ss -tln and checking for 127.0.0.1 vs 0.0.0.0 is the fastest way to know what is exposed. BNLC Phases 5 and 8 use it.

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