`pub` message

pub messages are used for pub announcements. These are issued by pub servers so that clients discover them.

What they look like

{
    "author": "@E5lOaTD+74yeVZhyGnPW9wSykpUGj6h8OjgVIoD4QdI=.ed25519",
    "content": {
        "address": {
            "host": "188.166.252.233",
            "key": "@uRECWB4KIeKoNMis2UYWyB2aQPvWmS3OePQvBj2zClg=.ed25519",
            "port": 8008
        },
        "type": "pub"
    },
    "hash": "sha256",
    "previous": "%c6bhBJfl1zWABSmV0sVWlTkklYTLxGHfSxt2LwA1ndM=.sha256",
    "sequence": 6,
    "signature": "4zTxKyyu8Gt24AGwTUrJO8FQ0Xit5OhgHTX+u6eB4FgNTK1ugKnEy7x91657+dQrCwjLHrYuE156/dy9cTpvCA==.sig.ed25519",
    "timestamp": 1459981972234
}

They match a pub key with an accessible host. You can learn more about pub messages online.