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Hosts (also called farmers) hold data on the Sia network. They get paid to perform this important task, and ultimately form the backbone of the data network called Sia.
Hosts on the Sia network are burdened with more responsibility than hosts on other networks. Sia aims to be highly reliable in an environment where people are actively attacking the network. This means that hosts need to be reliable, and there also needs to be a way to be confident that attackers pretending to be hosts are not mistaken for actual hosts.
Hosts are expected to achieve 95-98% uptime and reliability. This is actually not very much compared to traditional hosting solutions. Hosts are allowed to have hours of downtime every week, but in general must be running all the time.
Hosts are expected to put up collateral on data that they store. Typical peer to peer hosting networks struggle with churn, frequently because hosts get bored and leave. Sia handles this in two ways. First, hosts are paid for not leaving. But second, hosts promise not to leave by putting up some of their own money as collateral which is forfeit if the host leaves before the contract is completed.
Hosts are expected to burn coins to prove legitimacy. This is required to solve the Sybil Attack. Hosts ultimately are expected to burn between 0.25% and 2.5% of their total revenue to demonstrate legitimacy. What this ultimately means is that an attacker attempting to pretend to be 10 million legitimate hosts has to burn revenue equivalent to 25,000-250,000 legitimate hosts - it's extremely expensive to pretend to be a host when the proof of burn mechanic is being used. Renters will only feel comfortable using hosts that have used proof of burn to demonstrate legitimacy.
Hosts have the freedom to set their own prices. Renters then compare these prices against the features offered by the hosts and decide which hosts are most beneficial to them.
The host can set prices over 6 resources:
Renter measure hosts across a wide number of metrics. Renters that do a lot of downloading will prefer low download prices. Renters looking for infrequent-access storage will prefer low storage prices. Renters can also discriminate on other features:
Every renter forms contracts with about 50 hosts.
For a complete and update reference guide, check Using the UI for Hosting
If you're a Windows user you will have to run Sia-UI as administrator, or setting up storage won't work.
Official (but out of date) guide for setting up hosting through the terminal: How to run a host on Sia.