/[smeserver]/rpms/stunnel-tls/sme7/imap-tls.README
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Contents of /rpms/stunnel-tls/sme7/imap-tls.README

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Revision 1.1 - (show annotations) (download)
Tue Jun 12 17:44:40 2007 UTC (17 years, 4 months ago) by slords
Branch: MAIN
CVS Tags: stunnel-tls-3_22-4_el4_sme, HEAD
Import on branch sme7 of package stunnel-tls-3.22-4.el4.sme.src.rpm

1 ********************
2 *** INTRODUCTION ***
3 ********************
4
5 STARTLS is the standard (RFC 2595) way of doing IMAP encrypted with
6 SSL/TLS. Although it does not provide end-to-end encryption of email
7 messages, it can be useful to protect IMAP passwords, and to protect
8 email messages across the "last mile" of mail delivery.
9
10 While courier IMAP has native support for STARTTLS, TLS negotiations
11 are done before authentication, and are therefore done as root. That
12 means that a small bug such as a buffer overflow in the OpenSSL
13 library becomes a root exploit---yikes!
14
15 This approach uses a proxy which can handle the encryption and the
16 STARTTLS command itself, and then hands the already-encrypted
17 connection off to courier IMAP. The proxy runs in an environment
18 secured by chroot(), setuid(), and setgid().
19
20 IMAP proxy support has been added to stunnel, and also support for
21 doing a plaintext proxy of the IMAP session if STARTTLS isn't used.
22 stunnel runs chrooted in its own directory, as a special user and
23 group. This means that even a grievous security error in stunnel or
24 openssl wouldn't allow significant access to your system, or even
25 allow interfering with mail.
26
27
28 ********************
29 *** INSTRUCTIONS ***
30 ********************
31
32 WARNING: These are not for the faint-hearted. They are confusing and
33 may not work for you. This is still experimental; if you get stuck,
34 email me at <sgifford@suspectclass.com>.
35
36 1. Download stunnel-3.22. Apply the patch "stunnel3.22-sg2.patch",
37 available from:
38
39 http://www.suspectclass.com/~sgifford/stunnel-tlsproxy/stunnel3.22-sg2.patch
40
41 Compile and install it somewhere. This patch improves the proxy
42 support, adds options to tell stunnel to communicate via an already
43 opened file descriptor, adds chroot() support, and improves
44 setuid/setgid support; see:
45
46 http://www.suspectclass.com/~sgifford/qmail-smtp-tls-proxy/stunnel3.22-sg2.README
47
48 for a full description of the patch.
49
50 2. Compile and install "makesock.c".
51
52 3. Create your service directory for imap-tls
53
54 4. Set up a log directory for imap-tls.
55
56 5. Create a user called "stunnel" with a primary group of "stunnel".
57
58 6. Create a directory in your service directory called "ssl".
59
60 6a. Copy in your certificate as "stunnel.pem"
61
62 6b. Copy in your SSL configuration as openssl.cnf
63
64 6c. Create a seed file with "dd if=/dev/random of=seed count=10k"
65 or something.
66
67 6d/1. Some copies of OpenSSL will require you to create a fake
68 'usr/share/ssl' directory, to placate openssl in chroot.
69 Something like:
70
71 mkdir -p usr/share/ssl
72
73 . If your ssl expects to find its configuration elsewhere,
74 make that directory instead.
75
76 6d/2. If your copy of OpenSSL requires it, make a symlink to
77 openssl.cnf from the fake config dir. Something like:
78
79 ln -s ../../../openssl.cnf usr/share/ssl/
80
81 should do the trick, if your openssl expects its config file in
82 /usr/share/ssl normally.
83
84 6e. Set group-ownership of the ssl directory to "stunnel" (leaving
85 user-ownership at "root") and permissions to "owner read-write,
86 group read, other none" on everything in the ssl directory:
87
88 chgrp -R stunnel ssl
89 chmod -R u=rwX,g=rX,o= ssl
90
91 7. Install the run file "imap-tls-run" as "run" in your service
92 directory. Make sure it's executable. If you've installed the
93 modified stunnel somewhere other than /usr/local/sbin, add that to
94 the PATH near the top.
95
96 8. Run the "run" file in the service directory, and find and fix any
97 errors.
98
99 9. Active the service, perhaps by symlinking it into /service.
100
101
102
103 *******************
104 *** EXPLANATION ***
105 *******************
106
107 Here's what the run script does. It expects everything it runs to be
108 in your PATH.
109
110 First, it gathers up some information from control files and from the
111 system user and group database, and gets some hardcoded configuration
112 information.
113
114 softlimit limits the memory usage for each process to 5 MB.
115
116 tcpserver listens on the POP3 port. We continue running as root from
117 here (so we can do chroot() and set[ug]id() later, and also run
118 checkpassword), and when we get a connection we run...
119
120 ...makesock. This is a small C program that creates a socket with
121 socketpair(), and provides one end of that socket on file descriptor 3
122 to the first program it's asked to run, and the other end on standard
123 input and output to the second program it's asked to run. The first
124 and second programs are separated by the command line option
125 "-makesock_connect_to".
126
127 The first program, the STLS proxy, is stunnel. Debugging is turned
128 on, since this is still experimental. "-/ ssl" (an option added by my
129 patch) asks it to chroot to the "ssl" directory. "-s $SSLUID" asks it
130 to change to the stunnel user. "-g $SSLGID" asks it to change to the
131 stunnel group. "-i" (an option added by my patch) asks it to switch
132 users immediately, instead of after binding to the local port for
133 listening (which we don't ask stunnel to do, since tcpserver has done
134 it for us). "-R seed" tells it to get the seed for the random number
135 generator from the file "seed". "-p stunnel.pem" tells it to use the
136 certificate in "stunnel.pem". "-n imap-" tells it to act as an IMAP
137 proxy, and to act as a plaintext proxy if TLS isn't negotiated. "-f"
138 asks it to stay in the foreground and write its errors to stderr,
139 perfect for running under supervise! "-F 3" (an option added by my
140 patch) asks it to connect to file descriptor 3 (set up by makesock) as
141 the plaintext end of the proxy.
142
143 The second program is the IMAP server. I run my IMAP server with an
144 unusual run script which uses checkpassword for authentication (see:
145
146 http://www.suspectclass.com/~sgifford/qmail/courier-imap-checkpassword.txt
147
148 ), and I run it under tcpserver; you will probably want to modify this
149 to use a more typical courier IMAP startup, but since I don't have any
150 servers to test that on, I'm just giving you what I've got. :-) Other
151 than that, this is a pretty normal configuration.
152
153 *************
154 *** NOTES ***
155 *************
156
157 It is also possible to run this proxy as a simple TCP proxy, as long
158 as you don't care about what IP addresses your IMAP users are really
159 coming from.
160
161
162 ************
163 *** BUGS ***
164 ************
165
166 * STARTTLS is only supported if no other commands besides CAPABILITY
167 are sent before it.
168
169 * makesock.c is a single-purpose ugly hack. It should take more
170 command-line options, to make it a flexible tool.
171

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