You cannot use the Steam version. You need a "Pirate Rust Client." Popular releases come from:
However, for the solo player looking for "Rust for free"... rust cracked server list
Some "Top Rust Server" listing sites have a specific category for "Cracked/No-Steam." Be careful: many are dead links designed to steal clicks. You cannot use the Steam version
Discord is the primary source. Search for "Rust Cracked Community" or "Rust Pirate Servers." These servers feature a #server-list channel where admins post their IPs every few hours. Discord is the primary source
: Since there is no official authentication, other players can easily impersonate you or admins by simply using your username.
: These environments often lack the robust Anti-Cheat (EAC) protection found on official servers, leading to a higher frequency of hackers.
If you just want the survival building experience without paying, go ahead. But if you want the true Rust heartbreak—losing a box of AKs to a 12-year-old with a DB—save up the $20. The official experience is worth the price.