My Broadcast [The UNIX rwall problem]

rwall.md

Originally from: http://catless.ncl.ac.uk/Risks/4.73.html#subj10.1

Jordan K. Hubbard jkh@violet.Berkeley.EDU
Thu, 2 Apr 87 10:45:46 PST

 [The following message was submitted to RISKS by 6 different people.
 I initially thought it might already have been widely circulated, but 
 its repeated receipt has led me to include it here anyway.  PGN]

By now, many of you have heard of (or seen) the broadcast message I sent to the net two days ago. I have since received 743 messages and have replied to every one (either with a form letter, or more personally when questions were asked). The intention behind this effort was to show that I wasn't interested in doing what I did maliciously or in hiding out afterwards and avoiding the repercussions. One of the people who received my message was Dennis Perry, the Inspector General of the ARPAnet (in the Pentagon), and he wasn't exactly pleased. (I hear his Interleaf windows got scribbled on)

So now everyone is asking: "Who is this Jordan Hubbard, and why is he on my screen??"

I will attempt to explain.

I head a small group here at Berkeley called the "Distributed Unix Group". What that essentially means is that I come up with Unix distribution software for workstations on campus. Part of this job entails seeing where some of the novice administrators we're creating will hang themselves, and hopefully prevent them from doing so. Yesterday, I finally got around to looking at the "broadcast" group in /etc/netgroup which was set to "(,,)". It was obvious that this was set up for rwall to use, so I read the documentation on "netgroup" and "rwall". A section of the netgroup man[ual] page said:

...

 Any of three fields can be empty, in which case it signifies
 a wild card.  Thus

            universal (,,)

 defines a group to which everyone belongs.  Field names that ...

...

Now "everyone" here is pretty ambiguous. Reading a bit further down, one sees discussion on yellow-pages domains and might be led to believe that "everyone" was everyone in your domain. I know that rwall uses point-to-point RPC connections, so I didn't feel that this was what they meant, just that it seemed to be the implication.

Reading the rwall man page turned up nothing about "broadcasts". It doesn't even specify the communications method used. One might infer that rwall did indeed use actual broadcast packets.

Failing to find anything that might suggest that rwall would do anything nasty beyond the bounds of the current domain (or at least up to the IMP), I tried it. I knew that rwall takes awhile to do its stuff, so I left it running and went back to my office. I assumed that anyone who got my message would let me know.. Boy, was I right about that! After the first few mail messages arrived from Purdue and Utexas, I begin to understand what was really going on and killed the rwall. I mean, how often do you expect to run something on your machine and have people from Wisconsin start getting the results of it on their screens?

All of this has raised some interesting points and problems.

1. Rwall will walk through your entire hosts file and blare at anyone and everyone if you use the (,,) wildcard group. Whether this is a bug or a feature, I don't know.

2. Since rwall is an RPC service, and RPC doesn't seem to give a damn who you are as long as you're root (which is trivial to be, on a work- station), I have to wonder what other RPC services are open holes. We've managed to do some interesting, unauthorized, things with the YP service here at Berkeley, I wonder what the implications of this are.

3. Having a group called "broadcast" in your netgroup file (which is how it comes from sun) is just begging for some novice admin (or operator with root) to use it in the mistaken belief that he/she is getting to all the users. I am really surprised (as are many others) that this has taken this long to happen.

4. Killing rwall is not going to solve the problem. Any fool can write rwall, and just about any fool can get root priviledge on a Sun workstation. It seems that the place to fix the problem is on the receiving ends. The only other alternative would be to tighten up all the IMP gateways to forward packets only from "trusted" hosts. I don't like that at all, from a standpoint of reduced convenience and productivity. Also, since many places are adding hosts at a phenominal rate (ourselves especially), it would be hard to keep such a database up to date. Many perfectly well- behaved people would suffer for the potential sins of a few.

I certainly don't intend to do this again, but I'm very curious as to what will happen as a result. A lot of people got wall'd, and I would think that they would be annoyed that their machine would let someone from the opposite side of the continent do such a thing!

Jordan Hubbard, jkh@violet.berkeley.edu, (ucbvax!jkh) Computer Facilities & Communications, U.C. Berkeley