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The BOLT #9 specification shows a 52-bit field that represents various feature flags. That should require no more than 7 bytes, or less.

When I run the listnodes command, some of the nodes include features fields that are in the range of expected lengths, such as 808252a1 which is less than 7 bytes, but others are unexpectedly long, always beginning with the byte 80 followed by a long sequence of 00 bytes and then what appear to be the actual feature flags.

One of them looks like this: 800000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000802000888252a1

Is this a bug of some sort, or is there some other information to be interpreted from these long sequences of feature flags?

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The BOLT#9 specification lists all standardized or soon-to-be-standardized features, but developers are free to pick a higher bit to represent their custom feature. Oftentimes the spec engineers also use a high bit while developing a feature to avoid clashes and later drop it to a lower one for the standardization (this saves bytes as you noticed).

The above feature bitset decodes into:

['option_data_loss_protect',
 'option_upfront_shutdown_script(opt)',
 'gossip_queries(opt)',
 'var_onion_optin(opt)',
 'option_static_remotekey',
 'payment_secret',
 'basic_mpp(opt)',
 'option_anchors_zero_fee_htlc_tx(opt)',
 'option_channel_type',
 'AMP(LND)(opt)',
 'ExplicitChannelType(LND)(opt)',
 'unknown[55]',
 'ScriptEnforcedLease(LND)(opt)']

(maintaining the order the bits are set in the bitset)

The large number of 0 is due to bit 2023 being set: script_enforced_lease, an experimental feature by lnd, which iirc is used by their proprietary Pool implementation.

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