stunnel-tls/sme8/smtp-tls.README

********************
*** INTRODUCTION ***
********************

STARTTLS is the standard (RFC 2487) way of doing SMTP encrypted with
SSL/TLS.  Although it does not provide end-to-end encryption of email
messages, it can be useful to protect the passwords of an AUTH PLAIN
login, and possibly to protect mail which will travel a very specific
route known in advance.

qmail-smtpd doesn't have native support for STARTTLS.  Many people run
qmail for it's extremely good security record, and are reluctant to
have the security of their mail system depend on the many thousands of
lines of code in openssl.  One way to avoid this is to run a proxy
which can handle the encryption and the STARTTLS command itself, and
then have it hand off to the standard qmail-smtpd.  Even better is if
the proxy can run in an environment secured by chroot(), setuid(), and
setgid().

That's the approach that this document describes, with stunnel acting
as the proxy.  Basic SMTP/STARTTLS proxy support is already included
in stunnel, and that support has been extended to do a plaintext proxy
of the SMTP session if STARTTLS isn't used.  stunnel runs chrooted in
its own directory, as a special user and group.  This means that even
a grievous security error in stunnel or openssl wouldn't allow
significant access to your system, or even allow interfering with
mail.


********************
*** INSTRUCTIONS ***
********************

WARNING: These are not for the faint-hearted.  They are confusing and
may not work for you.  This is still experimental; if you get stuck,
email me at <sgifford@suspectclass.com>.

1. Download stunnel-3.22.  Apply the patch "stunnel3.22-sg2.patch".
   Compile and install it somewhere.  This patch improves the SMTP
   proxy support, adds options to tell it to communicate via an
   already opened file descriptor, adds chroot() support, and improves
   setuid/setgid support; see:

       http://www.suspectclass.com/~sgifford/qmail-smtp-tls-proxy/stunnel3.22-sg2.README

   for a full description of the patch.

2. Compile and install "makesock.c".  

3. Create your service directory for smtp-tls

4. Set up a log directory for smtp-tls.

5. Create a user called "stunnel" with a primary group of "stunnel".

6. Create a directory in your service directory called "ssl".

   6a. Copy in your certificate as "stunnel.pem"

   6b. Copy in your SSL configuration as openssl.cnf

   6c. Create a seed file with "dd if=/dev/random of=seed count=10k"
       or something.

   6d/1. Some copies of OpenSSL will require you to create a fake
         'usr/share/ssl' directory, to placate openssl in chroot.
         Something like:

             mkdir -p usr/share/ssl

         .  If your ssl expects to find its configuration elsewhere,
         make that directory instead.

   6d/2. If your copy of OpenSSL requires it, make a symlink to
         openssl.cnf from the fake config dir.  Something like:
 
             ln -s ../../../openssl.cnf usr/share/ssl/

         should do the trick, if your openssl expects its config file in
         /usr/share/ssl normally.

   6e. Set group-ownership of the ssl directory to "stunnel" (leaving
       user-ownership at "root") and permissions to "owner read-write,
       group read, other none" on everything in the ssl directory:

           chgrp -R stunnel ssl
           chmod -R u=rwX,g=rX,o= ssl

7. Install the run file "smtp-tls-run" as "run" in your service
   directory.  Make sure it's executable.  If you've installed the
   modified stunnel somewhere other than /usr/local/bin, add that to
   the PATH near the top.

8. Run the "run" file in the service directory, and find and fix any
   errors.

9. Active the service, perhaps by symlinking it into /service.


*******************
*** EXPLANATION ***
*******************

Here's what the run script does.  It expects everything it runs to be
in your PATH.

First, it gathers up some information from control files and from the
system user and group database, and gets some hardcoded configuration
information.

softlimit limits the memory usage for each process to 5 MB.

tcpserver listens on the SMTP port.  We continue running as root from
here (so we can do chroot() and set[ug]id() later), and when we get a
connection we run...

...makesock.  This is a small C program that creates a socket with
socketpair(), and provides one end of that socket on file descriptor 3
to the first program it's asked to run, and the other end on standard
input and output to the second program it's asked to run.  The first
and second programs are separated by the command line option
"-makesock_connect_to".

The first program, the STARTTLS proxy, is stunnel.  Debugging is
turned on, since this is still experimental.  "-/ ssl" (an option
added by my patch) asks it to chroot to the "ssl" directory.  "-s
$SSLUID" asks it to change to the stunnel user.  "-g $SSLGID" asks it
to change to the stunnel group.  "-i" (an option added by my patch)
asks it to switch users immediately, instead of after binding to the
local port for listening (which we don't ask stunnel to do, since
tcpserver has done it for us).  "-R seed" tells it to get the seed for
the random number generator from the file "seed".  "-p stunnel.pem"
tells it to use the certificate in "stunnel.pem".  "-n" smtp-" tells
it to act as an SMTP proxy, and to act as a plaintext proxy if TLS
isn't negotiated.  "-f" asks it to stay in the foreground and write
its errors to stderr, perfect for running under supervise!  "-F 3" (an
option added by my patch) asks it to connect to file descriptor 3 (set
up by makesock) as the plaintext end of the proxy.

The second program is the SMTP server.  We first have to change to a
safer UID than root; in most installations, tcpserver would to this
for us, but we couldn't do that here (because we needed the ability to
chroot() and setuid()/setgid() to a different user/group in stunnel),
so "setuidgid" changes to the "qmaild" user and their primary group.
stunnel doesn't seem to be very good about passing along CR/LF's
properly, and rather than fixing that right now, I just threw in
"fixcrio" to take care of it.  Finally, the real qmail-smtpd is run.

*************
*** NOTES ***
*************

It is tempting to run this proxy as a simple TCP proxy, but that makes
authorizing relays by IP address hard.


************
*** BUGS ***
************

  * RFC 2487 requires that we restart the SMTP session completely after
    STARTTLS, in particular throwing away the EHLO information we have.
    We don't do that, because: this architecture makes it hard, qmail
    doesn't mind if it gets multiple ehlo commands, and qmail doesn't
    really do much with the ehlo argument anyways.

  * STARTTLS is only supported if the first command we get is an EHLO,
    and the second is STARTTLS.

  * makesock.c is a single-purpose ugly hack.  It should take more
    command-line options, to make it a flexible tool.

  * There's no way for the SMTP server to know whether TLS was
    negotiated.  This could be useful to, for example, only offer AUTH
    PLAIN authentication after STARTTLS.
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.