Unique Node List (UNL) and Validators
How validators are chosen and trusted on the XRP Ledger, and what a Unique Node List is.
What a validator is
A validator is a server running the rippled software in validating mode, which participates in the consensus process by proposing and voting on transaction sets. Running a validator is permissionless — anyone can operate one — but simply running one does not automatically give it influence over the network; other participants have to choose to listen to it.
What a Unique Node List is
A Unique Node List (UNL) is the list of validators that a given server chooses to trust when determining consensus. Rather than one single, network-wide official list, each rippled server operator configures their own UNL (or, in practice, most operators adopt a default UNL (dUNL) published and cryptographically signed by a trusted publisher, most commonly Ripple, though other independent publishers exist too).
The key property that keeps the network coherent is overlap: as long as the UNLs used by different participants overlap enough, and the validators on those lists are honest, everyone's view of "what happened" converges on the same answer, even though no single list is globally mandatory.
Who runs validators
Validators are run by a mix of participants, including:
- Ripple itself
- Exchanges, wallet providers, and other XRPL-focused businesses
- Universities and research institutions
- Independent individuals and community members
A validator's importance to the network comes from how many other UNLs include it and trust it — in effect, a reputation system rather than a stake-weighted or hash-power-weighted one.
How validator trust changes over time
The composition of default UNLs is not permanently fixed. The organization publishing a dUNL can add or remove validators based on their uptime, reliability, and correctness over time, and node operators are always free to build a custom UNL of their own choosing instead of using a published default.
Why this differs from "anyone can validate" on other chains
On a proof-of-work chain, anyone with enough hashing power can eventually produce a valid block — influence is bought with computation. On many proof-of-stake chains, influence is bought with locked capital. On the XRPL, influence over consensus is earned through being trusted by other participants, which is a fundamentally different (and more social/reputational) mechanism. This is one of the most frequently debated aspects of the network's design — see The Centralization Debate for the arguments on both sides.