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A host announcement relay daemon for Sia
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shard is a stripped-down Sia node that records host announcements and basically nothing else. When run as a public service, it provides these announcements to clients, sparing them the trouble of running a node themselves.

Host announcements are the mechanism by which Sia hosts associate their public key with their current IP address or domain name. Subsequent announcements for the same public key replace earlier ones. A shard server is thus analogous to a DNS resolver: it resolves public keys to network addresses.

While this is convenient, it also introduces a degree of trust. The amount of trust is (nearly) minimized, but still present. The host announcements reported by shard are signed by their public key, so shard cannot forge the announcements themselves. However, it can report outdated information. For example, a host might switch from address A to B, and make a new announcement declaring so; but a malicious shard server can continue providing the old announcement. shard also provides the current blockheight, and it can lie about that too. Fortunately, the consequences of these lies are minor, and the lies can be easily detected by comparing the responses of two or more servers.

The API for a shard server is as follows:

Endpoint Type Description
/synced JSON bool If false, the server's responses may be outdated.
/height JSON number The current height of the Sia blockchain.
/host/:pubkey Binary The most recent announcement of the host with the request public key.

Announcements are encoded in binary using the Sia encoding format. An informal description of the announcement structure is as follows:

Field Size (bytes) Description
Magic 16 The string "HostAnnouncement".
Address Len 8 The length of the subsequent address.
Address (variable) The host's network address, as a string.
Pubkey Type 16 The string "ed25519\0\0\0\0\0\0\0\0\0".
Pubkey Len 8 The length of the subsequent key; always 32.
Pubkey 32 The raw bytes of the ed25519 public key.
Signature 64 The ed25519 signature of the BLAKE-2B hash of the preceding bytes.
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