/[smeserver]/rpms/stunnel-tls/sme8/smtp-tls.README
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Contents of /rpms/stunnel-tls/sme8/smtp-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 STARTTLS is the standard (RFC 2487) way of doing SMTP encrypted with
6 SSL/TLS. Although it does not provide end-to-end encryption of email
7 messages, it can be useful to protect the passwords of an AUTH PLAIN
8 login, and possibly to protect mail which will travel a very specific
9 route known in advance.
10
11 qmail-smtpd doesn't have native support for STARTTLS. Many people run
12 qmail for it's extremely good security record, and are reluctant to
13 have the security of their mail system depend on the many thousands of
14 lines of code in openssl. One way to avoid this is to run a proxy
15 which can handle the encryption and the STARTTLS command itself, and
16 then have it hand off to the standard qmail-smtpd. Even better is if
17 the proxy can run in an environment secured by chroot(), setuid(), and
18 setgid().
19
20 That's the approach that this document describes, with stunnel acting
21 as the proxy. Basic SMTP/STARTTLS proxy support is already included
22 in stunnel, and that support has been extended to do a plaintext proxy
23 of the SMTP session if STARTTLS isn't used. stunnel runs chrooted in
24 its own directory, as a special user and group. This means that even
25 a grievous security error in stunnel or openssl wouldn't allow
26 significant access to your system, or even allow interfering with
27 mail.
28
29
30 ********************
31 *** INSTRUCTIONS ***
32 ********************
33
34 WARNING: These are not for the faint-hearted. They are confusing and
35 may not work for you. This is still experimental; if you get stuck,
36 email me at <sgifford@suspectclass.com>.
37
38 1. Download stunnel-3.22. Apply the patch "stunnel3.22-sg2.patch".
39 Compile and install it somewhere. This patch improves the SMTP
40 proxy support, adds options to tell it to communicate via an
41 already opened file descriptor, adds chroot() support, and improves
42 setuid/setgid support; see:
43
44 http://www.suspectclass.com/~sgifford/qmail-smtp-tls-proxy/stunnel3.22-sg2.README
45
46 for a full description of the patch.
47
48 2. Compile and install "makesock.c".
49
50 3. Create your service directory for smtp-tls
51
52 4. Set up a log directory for smtp-tls.
53
54 5. Create a user called "stunnel" with a primary group of "stunnel".
55
56 6. Create a directory in your service directory called "ssl".
57
58 6a. Copy in your certificate as "stunnel.pem"
59
60 6b. Copy in your SSL configuration as openssl.cnf
61
62 6c. Create a seed file with "dd if=/dev/random of=seed count=10k"
63 or something.
64
65 6d/1. Some copies of OpenSSL will require you to create a fake
66 'usr/share/ssl' directory, to placate openssl in chroot.
67 Something like:
68
69 mkdir -p usr/share/ssl
70
71 . If your ssl expects to find its configuration elsewhere,
72 make that directory instead.
73
74 6d/2. If your copy of OpenSSL requires it, make a symlink to
75 openssl.cnf from the fake config dir. Something like:
76
77 ln -s ../../../openssl.cnf usr/share/ssl/
78
79 should do the trick, if your openssl expects its config file in
80 /usr/share/ssl normally.
81
82 6e. Set group-ownership of the ssl directory to "stunnel" (leaving
83 user-ownership at "root") and permissions to "owner read-write,
84 group read, other none" on everything in the ssl directory:
85
86 chgrp -R stunnel ssl
87 chmod -R u=rwX,g=rX,o= ssl
88
89 7. Install the run file "smtp-tls-run" as "run" in your service
90 directory. Make sure it's executable. If you've installed the
91 modified stunnel somewhere other than /usr/local/bin, add that to
92 the PATH near the top.
93
94 8. Run the "run" file in the service directory, and find and fix any
95 errors.
96
97 9. Active the service, perhaps by symlinking it into /service.
98
99
100
101 *******************
102 *** EXPLANATION ***
103 *******************
104
105 Here's what the run script does. It expects everything it runs to be
106 in your PATH.
107
108 First, it gathers up some information from control files and from the
109 system user and group database, and gets some hardcoded configuration
110 information.
111
112 softlimit limits the memory usage for each process to 5 MB.
113
114 tcpserver listens on the SMTP port. We continue running as root from
115 here (so we can do chroot() and set[ug]id() later), and when we get a
116 connection we run...
117
118 ...makesock. This is a small C program that creates a socket with
119 socketpair(), and provides one end of that socket on file descriptor 3
120 to the first program it's asked to run, and the other end on standard
121 input and output to the second program it's asked to run. The first
122 and second programs are separated by the command line option
123 "-makesock_connect_to".
124
125 The first program, the STARTTLS proxy, is stunnel. Debugging is
126 turned on, since this is still experimental. "-/ ssl" (an option
127 added by my patch) asks it to chroot to the "ssl" directory. "-s
128 $SSLUID" asks it to change to the stunnel user. "-g $SSLGID" asks it
129 to change to the stunnel group. "-i" (an option added by my patch)
130 asks it to switch users immediately, instead of after binding to the
131 local port for listening (which we don't ask stunnel to do, since
132 tcpserver has done it for us). "-R seed" tells it to get the seed for
133 the random number generator from the file "seed". "-p stunnel.pem"
134 tells it to use the certificate in "stunnel.pem". "-n" smtp-" tells
135 it to act as an SMTP proxy, and to act as a plaintext proxy if TLS
136 isn't negotiated. "-f" asks it to stay in the foreground and write
137 its errors to stderr, perfect for running under supervise! "-F 3" (an
138 option added by my patch) asks it to connect to file descriptor 3 (set
139 up by makesock) as the plaintext end of the proxy.
140
141 The second program is the SMTP server. We first have to change to a
142 safer UID than root; in most installations, tcpserver would to this
143 for us, but we couldn't do that here (because we needed the ability to
144 chroot() and setuid()/setgid() to a different user/group in stunnel),
145 so "setuidgid" changes to the "qmaild" user and their primary group.
146 stunnel doesn't seem to be very good about passing along CR/LF's
147 properly, and rather than fixing that right now, I just threw in
148 "fixcrio" to take care of it. Finally, the real qmail-smtpd is run.
149
150 *************
151 *** NOTES ***
152 *************
153
154 It is tempting to run this proxy as a simple TCP proxy, but that makes
155 authorizing relays by IP address hard.
156
157
158 ************
159 *** BUGS ***
160 ************
161
162 * RFC 2487 requires that we restart the SMTP session completely after
163 STARTTLS, in particular throwing away the EHLO information we have.
164 We don't do that, because: this architecture makes it hard, qmail
165 doesn't mind if it gets multiple ehlo commands, and qmail doesn't
166 really do much with the ehlo argument anyways.
167
168 * STARTTLS is only supported if the first command we get is an EHLO,
169 and the second is STARTTLS.
170
171 * makesock.c is a single-purpose ugly hack. It should take more
172 command-line options, to make it a flexible tool.
173
174 * There's no way for the SMTP server to know whether TLS was
175 negotiated. This could be useful to, for example, only offer AUTH
176 PLAIN authentication after STARTTLS.

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