diff options
| author | Marcus Brinkmann <marcus@gnu.org> | 2002-10-03 13:29:32 +0000 |
|---|---|---|
| committer | Marcus Brinkmann <marcus@gnu.org> | 2002-10-03 13:29:32 +0000 |
| commit | 1420c8de10bf98c35d753e86da81d40dccee6ffa (patch) | |
| tree | 4bfd95f626c60d9664a59e78c40164fb15204d50 | |
| parent | 63cf0655c1617c826f5dcf93ac76ffd3dc08aa1f (diff) | |
Add Wolfgang's talk about the auth server.
| -rw-r--r-- | auth.html | 248 |
1 files changed, 248 insertions, 0 deletions
diff --git a/auth.html b/auth.html new file mode 100644 index 00000000..aa4e6b67 --- /dev/null +++ b/auth.html @@ -0,0 +1,248 @@ +<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0//EN" + "http://www.w3.org/TR/REC-html40/strict.dtd"> +<HTML> +<HEAD> +<TITLE>The GNU Hurd - GNU Project - Free Software Foundation (FSF)</TITLE> +<LINK REV="made" HREF="mailto:web-hurd@gnu.org"> +<META NAME="keywords" CONTENT="hurd"> +</HEAD> +<BODY BGCOLOR="#FFFFFF" TEXT="#000000" LINK="#1F00FF" ALINK="#FF0000" VLINK="#9900DD"> +<TABLE width="100%" border="0" cellspacing="5" cellpadding="15"> +<TR> +<TD COLSPAN="2"> +<IMG SRC="/graphics/hurd_sm_mf.jpg" ALT=" [image of the Hurd logo] "> +[ + <A HREF="/software/hurd/auth.html">English</A> +] +</TD> +</TR> +<TR> +<TD ALIGN="LEFT" VALIGN="TOP" BGCOLOR="#eeeeee"> +<A HREF="/software/hurd/whatsnew.html"><STRONG>What's New</STRONG></A><BR> + <BR> +<A HREF="/software/hurd/changelogs.html">ChangeLogs</A><BR> +<P> +<A HREF="/software/hurd/hurd.html"><STRONG>The GNU Hurd</STRONG></A><BR> + <BR> +<A HREF="/software/hurd/docs.html">Documentation</A><BR> +<A HREF="/software/hurd/install.html">Installation</A><BR> +<A HREF="/software/hurd/help.html">Getting Help</A><BR> +<A HREF="/software/hurd/download.html">Source Code</A><BR> +<A HREF="/software/hurd/devel.html">Development</A><BR> +<A HREF="/software/hurd/history.html">History</A><BR> + <BR> +<A HREF="/software/hurd/gnumach.html"><STRONG>GNU Mach</STRONG></A><BR> + <BR> +<A HREF="/software/hurd/gnumach-docs.html">Documentation</A><BR> +<A HREF="/software/hurd/gnumach-install.html">Installation</A><BR> +<A HREF="/software/hurd/gnumach-download.html">Source Code</A><BR> + <BR> +<A HREF="/software/hurd/mig.html"><STRONG>MiG</STRONG></A><BR> + <BR> +<A HREF="/software/hurd/mig-download.html">Source Code</A><BR> +</TD> +<TD ALIGN="LEFT" VALIGN="TOP"> +<HR> +<H3><A NAME="contents">Table of Contents</A></H3> +<UL> + <LI><A HREF="#intro" NAME="TOCintro">Introduction</A> + <LI><A HREF="#ids" NAME="TOCids">How IDs are represented and used</A> + <LI><A HREF="#posix" NAME="TOCposix">POSIX and beyond</A> + <LI><A HREF="#servers" NAME="TOCservers">Related servers</A> +</UL> +<HR> + +<H3><A HREF="#TOCintro" NAME="intro">Introduction</A></H3> +<P> +In this text, which mostly resembles the talk I gave at Libre Software +Meeting 2002 in Bordeaux, I will describe what the auth server does, +why it is so important and which cool things you can do with it, both +on the programming and the user side. I will also describe related +programs like the password and fakeauth servers. Note that this text +is targeted at programmers who want to understand the auth mechanism +in detail and are already familiar with concepts like Remote Procedure +Calls (RPCs) as well as the way User- and Group-IDs are used in the +POSIX world. + +<P> +The auth server is a very small server, therefore it gives a useful +example when you want to learn how a server typically looks like. One +reason why it is so small is that the auth interface, which it +implements, consists of only four RPCs. You can find the interface in +hurd/hurd/auth.defs and the server itself in hurd/auth/. + +<H3><A HREF="#TOCids" NAME="ids">How IDs are represented and used</A></H3> +<P> +Each process holds (usually) one port to auth (an auth_t in C source, +which actually is a mach_port_t, of course). The purpose of auth is +to manage User-IDs and Group-IDs, which is the reason why users often +will have no choice but to make use of the systems main auth server, +which does not listen on /servers/auth; instead you inherit a port to +auth from your parent process. Each such port is (internally in the +auth server) associated with a set of effective User- and Group-IDs as +well as a set of available User- and Group-IDs. So we have four sets +of IDs in total. The available IDs can be turned into corresponding +effective IDs at any time. + +<P> +When you send an auth_getids RPC on the port you hold, you will get +information about which IDs are associated with it, so you can figure +out which permissions you have. But how will a server know that you +have these permissions and therefore know which actions (e.g. writing +into file "foo") it is supposed to do on your behalf and which not? +The establishing of a trusted connection to a server works as follows: + +<P><OL> +<LI>A user wants a server to know its IDs</LI> +<LI>The user requests a reauthentication from the server</LI> +<LI>In this request the user will include a port</LI> +<LI>Both will hand this port to auth</LI> +<LI>The user uses auth_user_authenticate</LI> +<LI>The server uses auth_server_authenticate</LI> +<LI>The server also passes a new port to auth</LI> +<LI>auth matches these two requests</LI> +<LI>The user gets the new port from auth</LI> +<LI>The server learns about the IDs of the user</LI> +<LI>The user uses the new port for further communication</LI> +</OL> + +<P> +We have different RPCs for users and servers because what we pass and +what we get back differs for them: Users get a port, and servers get +the sets of IDs, and have to specify the port which the user will get. + +<P> +It is interesting to note that auth can match the requests by +comparing two integers, because when you get the same port from two +people, you will have the same mach_port_t (which is nothing but an +integer). + +<P> +All of this of course only works if they use the same auth server, +which is why I said often you have no choice other than to use the +one main auth server. But this is no serious restriction, as the auth server has +almost no functionality one might want to replace. In fact, there is +one replacement for the default auth implementation, but more on that +later. + +<H3><A HREF="#TOCposix" NAME="posix">POSIX and beyond</A></H3> +<P> +Before we examine what is possible with this design, let us take a +short look at how the POSIX semantics are implemented on top of this +design. When a program that comes out of POSIX-land asks for its own +effective User- or Group-ID, we will tell it about the first of the +effective IDs. In the same sense, the POSIX real User- or Group-ID is +the first available ID and the POSIX saved User- or Group-ID is the +second available ID, which is why you have the same ID two times in +the available IDs when you log into your GNU/Hurd machine (you can +figure out which IDs you have with the program "ids", that basically +just does an auth_getauth RPC). When you lack one of those IDs (for +example when you have no effective Group-ID), a POSIX program asking +for this particular information will get "-1" as the ID. + +<P> +But as you can imagine, we can do more than what POSIX specifies. Fox +example, we can modify our permissions. This is always done with the +auth_makeauth RPC. In this RPC, you specify the IDs that should be +associated with the new port. All of these IDs must be associated +with either the port where the RPC is sent to or one of the additional +ports you can specify; an exception is the superuser root, which is +allowed to creat ports that are associated with arbitrary IDs. +Hereby you can convert available into effective IDs. + +<P> +This opens the door to a bunch of nice features. For example, we have +the addauth program in the Hurd, which makes it possible to add an ID +to either a single process or a group of processes if you hold the ID or know the +appropriate password, and there is a corresponding rmauth program that +removes an ID. So when you are working on your computer with GNU +Emacs and want to edit a system configuration file, you switch to +Emacs' shell-mode, do an "addauth root", enter the password, edit the +file, and when you are done switch back to shell-mode and do "rmauth +root". These programs have some interesting options, and there are +various other programs, for setting the complete list of IDs (setauth) +and so on. + +<H3><A HREF="#TOCservers" NAME="servers">Related servers</A></H3> +<P> +Finally, I want to explain two servers which are related to auth. The +first is the password server, which listens on /servers/password. If +you pass to it a User- or Group-ID and the correct password for it, it +will return a port to auth to you which is associated with the ID you +passed to it. It can create such a port because it is running as +root. So let us assume you are an FTP server process. You will start +as root, because you want to use port 21 (in this case, "port" does +not refer to a mach_port_t, of course). But then, you can drop all +your permissions so that you run without any ID. This makes it far +less dangerous to communicate with yet unknown users over the +network. But when someone now hands a username and password to you, +you can ask the password server for a new auth port. The password +server will check the data you pass to it, for example by looking into +/etc/shadow, and if it is valid, it will ask the auth server for a new +port. It receives this port from auth and then passes it on to you. +So you have raised your permissions. (And for the very curious: Yes, +we are well aware of the differences between this concept and +capabilities; and we also do have some kinds of capabilities in +various parts of the Hurd.) + +<P> +My second example is the fakeauth server. It also implements the auth +protocol. It is the part of the fakeroot implementation that gives a +process the impression that it runs as root, even if it doesn't. So +when the process asks fakeauth about its own IDs, fakeauth will tell +the process that it runs as root. But when the process wants to make +use of the authentication protocol described earlier in this text, +fakeauth will forward the request to its own auth server, which will +usually be the systems main auth server, which will then be able to +match the auth_*_authenticate requests. So what fakeauth does is +acting as a proxy auth server that gives someone the impression to run +as root, while not modifying what that one is allowed to do. + +<P> +At this point, I have said at least most of what can be said about the +auth server and the protocol it implements, so I will finish by saying +that it might be an interesting task (for you) to modify some existing +software to take advantage of the features I described here. + +</TD> +</TR> +</TABLE> + +<HR> + +[ + <A HREF="/software/hurd/auth.html">English</A> +] + +<HR> + +<P> +Return to <A HREF="/home.html">GNU's home page</A>. +<P> + +Please send FSF & GNU inquiries & questions to + +<A HREF="mailto:gnu@gnu.org"><EM>gnu@gnu.org</EM></A>. +There are also <A HREF="/home.html#ContactInfo">other ways to +contact</A> the FSF. +<P> + +Please send comments on these web pages to + +<A HREF="mailto:web-hurd@gnu.org"><EM>web-hurd@gnu.org</EM></A>, +send other questions to +<A HREF="mailto:gnu@gnu.org"><EM>gnu@gnu.org</EM></A>. +<P> +Copyright (C) 2002 Free Software Foundation, Inc., +59 Temple Place - Suite 330, Boston, MA 02111, USA +<P> +Verbatim copying and distribution of this entire article is +permitted in any medium, provided this notice is preserved. +<P> +Updated: +<!-- timestamp start --> +$Date$ $Author$ +<!-- timestamp end --> +<HR> +</BODY> +</HTML> |