Filename: 119-protocolinfo-on-controlport.txt Title: New PROTOCOLINFO command for controllers Version: $Revision$ Last-Modified: $Date$ Author: Roger Dingledine Created: 14-Aug-2007 Status: Closd Overview: Here we describe how to help controllers locate the cookie authentication file when authenticating to Tor, so we can a) require authentication by default for Tor controllers and b) still keep things usable. Also, we propose an extensible, general-purpose mechanism for controllers to learn about a Tor instance's protocol and authentication requirements before authenticating. The Problem: When we first added the controller protocol, we wanted to make it easy for people to play with it, so by default we didn't require any authentication from controller programs. We allowed requests only from localhost as a stopgap measure for security. Due to an increasing number of vulnerabilities based on this approach, it's time to add authentication in default configurations. We have a number of goals: - We want the default Vidalia bundles to transparently work. That means we don't want the users to have to type in or know a password. - We want to allow multiple controller applications to connect to the control port. So if Vidalia is launching Tor, it can't just keep the secrets to itself. Right now there are three authentication approaches supported by the control protocol: NULL, CookieAuthentication, and HashedControlPassword. See Sec 5.1 in control-spec.txt for details. There are a couple of challenges here. The first is: if the controller launches Tor, how should we teach Tor what authentication approach it should require, and the secret that goes along with it? Next is: how should this work when the controller attaches to an existing Tor, rather than launching Tor itself? Cookie authentication seems most amenable to letting multiple controller applications interact with Tor. But that brings in yet another question: how does the controller guess where to look for the cookie file, without first knowing what DataDirectory Tor is using? Design: We should add a new controller command PROTOCOLINFO that can be sent as a valid first command (the others being AUTHENTICATE and QUIT). If PROTOCOLINFO is sent as the first command, the second command must be either a successful AUTHENTICATE or a QUIT. If the initial command sequence is not valid, Tor closes the connection. Spec: C: "PROTOCOLINFO" *(SP PIVERSION) CRLF S: "250+PROTOCOLINFO" SP PIVERSION CRLF *InfoLine "250 OK" CRLF InfoLine = AuthLine / VersionLine / OtherLine AuthLine = "250-AUTH" SP "METHODS=" AuthMethod *(",")AuthMethod *(SP "COOKIEFILE=" AuthCookieFile) CRLF VersionLine = "250-VERSION" SP "Tor=" TorVersion [SP Arguments] CRLF AuthMethod = "NULL" / ; No authentication is required "HASHEDPASSWORD" / ; A controller must supply the original password "COOKIE" / ; A controller must supply the contents of a cookie AuthCookieFile = QuotedString TorVersion = QuotedString OtherLine = "250-" Keyword [SP Arguments] CRLF For example: C: PROTOCOLINFO CRLF S: "250+PROTOCOLINFO 1" CRLF S: "250-AUTH Methods=HASHEDPASSWORD,COOKIE COOKIEFILE="/tor/cookie"" CRLF S: "250-VERSION Tor=0.2.0.5-alpha" CRLF S: "250 OK" CRLF Tor MAY give its InfoLines in any order; controllers MUST ignore InfoLines with keywords it does not recognize. Controllers MUST ignore extraneous data on any InfoLine. PIVERSION is there in case we drastically change the syntax one day. For now it should always be "1", for the controller protocol. Controllers MAY provide a list of the protocol versions they support; Tor MAY select a version that the controller does not support. Right now only two "topics" (AUTH and VERSION) are included, but more may be included in the future. Controllers must accept lines with unexpected topics. AuthCookieFile = QuotedString AuthMethod is used to specify one or more control authentication methods that Tor currently accepts. AuthCookieFile specifies the absolute path and filename of the authentication cookie that Tor is expecting and is provided iff the METHODS field contains the method "COOKIE". Controllers MUST handle escape sequences inside this string. The VERSION line contains the Tor version. [What else might we want to include that could be useful? -RD] Compatibility: Tor 0.1.2.16 and 0.2.0.4-alpha hang up after the first failed command. Earlier Tors don't know about this command but don't hang up. That means controllers will need a mechanism for distinguishing whether they're talking to a Tor that speaks PROTOCOLINFO or not. I suggest that the controllers attempt a PROTOCOLINFO. Then: - If it works, great. Authenticate as required. - If they get hung up on, reconnect and do a NULL AUTHENTICATE. - If it's unrecognized but they're not hung up on, do a NULL AUTHENTICATE. Unsolved problems: If Torbutton wants to be a Tor controller one day... talking TCP is bad enough, but reading from the filesystem is even harder. Is there a way to let simple programs work with the controller port without needing all the auth infrastructure? Once we put this approach in place, the next vulnerability we see will involve an attacker somehow getting read access to the victim's files --- and then we're back where we started. This means we still need to think about how to demand password-based authentication without bothering the user about it.