Proposal: New PROTOCOLINFO command for controllers
arma at mit.edu
Tue Aug 14 17:30:44 UTC 2007
Title: New PROTOCOLINFO command for controllers
Author: Roger Dingledine
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
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 it using?
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.
C: PROTOCOLINFO CRLF
S: "250+PROTOCOLINFO" PIVERSION CRLF
S: "250-AUTH" SP "METHODS=" AuthMethod *("," AuthMethod)
[SP "COOKIEFILE=" AuthCookieFile] CRLF
S: "250-VERSION" SP "Tor=" Version [...]
S: "250 OK"
PIVERSION is there in case we drastically change the syntax one day. For
now it should always be "1".
[XXX Is there a better way to do PIVERSION? The above way seems bad,
since what do controllers do if they hear a 2 but don't know what to
do with it? -RD]
Right now only two "topics" (AUTH and VERSION) are included, but more
may be included in the future. Controllers must accept lines with
"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
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 only if
the METHODS field contains the method "COOKIE". This field MUST be
enclosed in DQUOTEs, since the absolute path to the cookie file may
contain spaces on some platforms.
The VERSION line contains the Tor version, in DQUOTES. In the future
it might also contain Link versions, Circuit versions, or others as
described in proposal 105.
[What else might we want to include that could be useful? -RD]
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
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.
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