/[smeserver]/rpms/stunnel-tls/sme8/pop3-tls.README
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Contents of /rpms/stunnel-tls/sme8/pop3-tls.README

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Revision 1.1 - (show annotations) (download)
Tue Jun 12 21:18:31 2007 UTC (17 years, 4 months ago) by slords
Branch: MAIN
CVS Tags: stunnel-tls-3_22-4_el5_sme, HEAD
Import on branch sme8 of package stunnel-tls-3.22-4.el5.sme.src.rpm

1 ********************
2 *** INTRODUCTION ***
3 ********************
4
5 STLS is the standard (RFC 2595) way of doing POP3 encrypted with
6 SSL/TLS. Although it does not provide end-to-end encryption of email
7 messages, it can be useful to protect POP passwords, and to protect POP messages across the "last mile" of mail delivery.
8
9 qmail-pop3d doesn't have native support for STLS. Many people run
10 qmail for it's extremely good security record, and are reluctant to
11 have the security of their mail system depend on the many thousands of
12 lines of code in openssl. One way to avoid this is to run a proxy
13 which can handle the encryption and the STLS command itself, and
14 then have it hand off to the standard qmail-pop3d. Even better is if
15 the proxy can run in an environment secured by chroot(), setuid(), and
16 setgid().
17
18 That's the approach that this document describes, with stunnel acting
19 as the proxy. Basic POP3/STLS proxy support is already included
20 in stunnel, and that support has been extended to do a plaintext proxy
21 of the POP3 session if STLS isn't used. stunnel runs chrooted in
22 its own directory, as a special user and group. This means that even
23 a grievous security error in stunnel or openssl wouldn't allow
24 significant access to your system, or even allow interfering with
25 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 pop3-tls
53
54 4. Set up a log directory for pop3-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 "pop3-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" pop3-" tells it to act as a POP3
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 POP3 server. As in a normal qmail POP3
144 setup, qmail-popup runs checkpasswords runs qmail-pop3d.
145
146 *************
147 *** NOTES ***
148 *************
149
150 It is also possible to run this proxy as a simple TCP proxy, as long
151 as you don't care about what IP addresses your POP3 users are really
152 coming from.
153
154
155 ************
156 *** BUGS ***
157 ************
158
159 * STLS is only supported if no other commands besides CAPA are sent
160 before it.
161
162 * makesock.c is a single-purpose ugly hack. It should take more
163 command-line options, to make it a flexible tool.
164

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