[or-cvs] some scribblings on exit policies

Roger Dingledine arma at seul.org
Fri Oct 24 11:21:22 UTC 2003

Update of /home/or/cvsroot/doc
In directory moria.mit.edu:/home2/arma/work/onion/cvs/doc

Modified Files:
	tor-design.bib tor-design.tex 
Log Message:
some scribblings on exit policies
somebody please go turn this into a section

Index: tor-design.bib
RCS file: /home/or/cvsroot/doc/tor-design.bib,v
retrieving revision 1.8
retrieving revision 1.9
diff -u -d -r1.8 -r1.9
--- tor-design.bib	23 Oct 2003 11:45:50 -0000	1.8
+++ tor-design.bib	24 Oct 2003 11:21:19 -0000	1.9
@@ -1,3 +1,9 @@
+ at misc{darkside,
+  title = {{The Dark Side of the Web: An Open Proxy's View}},
+  author = {Vivek S. Pai and Limin Wang and KyoungSoo Park and Ruoming Pang and Larry Peterson},
+  note = {Submitted to HotNets-II. \url{http://codeen.cs.princeton.edu/}},
   key =		 {anonymizer},
   title =	 {The {Anonymizer}},

Index: tor-design.tex
RCS file: /home/or/cvsroot/doc/tor-design.tex,v
retrieving revision 1.20
retrieving revision 1.21
diff -u -d -r1.20 -r1.21
--- tor-design.tex	24 Oct 2003 04:09:10 -0000	1.20
+++ tor-design.tex	24 Oct 2003 11:21:19 -0000	1.21
@@ -605,6 +605,82 @@
 \SubSection{Exit policies and abuse}
+Exit abuse is a serious barrier to wide-scale Tor deployment --- we
+must block or limit attacks and other abuse that users can do through
+the Tor network.
+Each onion router's \emph{exit policy} describes to which external
+addresses and ports the router will permit stream connections. On one end
+of the spectrum are \emph{open exit} nodes that will connect anywhere;
+on the other end are \emph{middleman} nodes that only relay traffic to
+other Tor nodes, and \emph{private exit} nodes that only connect locally
+or to addresses internal to that node's organization. This private exit
+node configuration is more secure for clients --- the adversary cannot
+see plaintext traffic leaving the network (e.g. to a webserver), so he
+is less sure of Alice's destination. More generally, nodes can require
+a variety of forms of traffic authentication \cite{onion-discex00}.
+Tor offers more reliability than the high-latency fire-and-forget
+anonymous email networks, because the sender opens a TCP stream
+with the remote mail server and receives an explicit confirmation of
+acceptance. But ironically, the private exit node model works poorly for
+email, when Tor nodes are run on volunteer machines that also do other
+things, because it's quite hard to configure mail transport agents so
+normal users can send mail normally, but the Tor process can only deliver
+mail locally. Further, most organizations have specific hosts that will
+deliver mail on behalf of certain IP ranges; Tor operators must be aware
+of these hosts and consider putting them in the Tor exit policy.
+The abuse issues on closed (e.g. military) networks are very different
+from the abuse on open networks like the Internet. While these IP-based
+access controls are still commonplace on the Internet, on closed networks,
+nearly all participants will be honest, and end-to-end authentication
+can be assumed for anything important.
+Tor is harder than minion because tcp doesn't include an abuse
+address. you could reach inside the http stream and change the agent
+or something, but that's a very specific case and probably won't help
+much anyway.
+And volunteer nodes don't resolve to anonymizer.mit.edu so it never
+even occurs to people that it wasn't you.
+Preventing abuse of open exit nodes is an unsolved problem. Princeton's
+CoDeeN project \cite{darkside} gives us a glimpse of what we're in for.
+but their solutions, which mainly involve rate limiting and blacklisting
+nodes which do bad things, don't translate directly to Tor. Rate limiting
+still works great, but Tor intentionally separates sender from recipient,
+so it's hard to know which sender was the one who did the bad thing,
+without just making the whole network wide open.
+even limiting most nodes to allow http, ssh, and aim to exit and reject
+all other stuff is sketchy, because plenty of abuse can happen over
+port 80. but it's a very good start, because it blocks most things,
+and because people are more used to the concept of port 80 abuse not
+coming from the machine's owner.
+we could also run intrusion detection system (IDS) modules at each tor
+node, to dynamically monitor traffic streams for attack signatures. it
+can even react when it sees a signature by closing the stream. but IDS's
+don't actually work most of the time, and besides, how do you write a
+signature for "is sending a mean mail"?
+we should run a squid at each exit node, to provide comparable anonymity
+to private exit nodes for cache hits, to speed everything up, and to
+have a buffer for funny stuff coming out of port 80. we could similarly
+have other exit proxies for other protocols, like mail, to check
+delivered mail for being spam.
+A mixture of open and restricted exit nodes will allow the most
+flexibility for volunteers running servers. But while a large number
+of middleman nodes is useful to provide a large and robust network,
+a small number of exit nodes still simplifies traffic analysis because
+there are fewer nodes the adversary needs to monitor, and also puts a
+greater burden on the exit nodes.
+The JAP cascade model is really nice because they only need one node to
+take the heat per cascade. On the other hand, a hydra scheme could work
+better (it's still hard to watch all the clients).
 \SubSection{Directory Servers}
@@ -747,7 +823,7 @@
 token (the introduction point can do prescreening, eg to block replays),
 and (encrypted to Bob's public key) the location of the meeting point,
 a meeting cookie Bob should tell the meeting point so he gets connected to
-Alice, an optional authentication token so Bob choose whether to respond,
+Alice, an optional authentication token so Bob can choose whether to respond,
 and the first half of a DH key exchange. When Bob connects to the meeting
 place and gets connected to Alice's pipe, his first cell contains the
 other half of the DH key exchange.

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