emailrelay/doc/reference.txt

E-MailRelay Reference
=====================

Command line usage
------------------
The "emailrelay" program supports the following command-line usage:

	emailrelay [<option> [<option> ...]]

where <option> is:
# --admin (-a)
  Enables the administration interface and specifies its listening port number.

# --admin-terminate (-Q)
  Enables the terminate command on the admin interface.

# --anonymous (-A)
  Disables the smtp vrfy command and sends less verbose smtp responses.

# --as-client (-q)
  Runs as a client, forwarding all spooled mail to <host>: equivalent to "--log --no-syslog --no-daemon --dont-serve --forward --forward-to".

# --as-proxy (-y)
  Runs as a proxy server, forwarding each mail immediately to <host>: equivalent to "--log --close-stderr --poll=0 --forward-to".

# --as-server (-d)
  Runs as a server, storing mail in the spool directory: equivalent to "--log --close-stderr".

# --client-auth (-C)
  Enables smtp authentication with the remote server, using the given secrets file.

# --client-filter (-Y)
  Specifies an external program to process messages when they are forwarded.

# --client-tls (-j)
  Enables negotiated tls/ssl for smtp client (if openssl built in).

# --client-tls-connection (-b)
  Enables smtp over tls/ssl for smtp client (if openssl built in).

# --close-stderr (-e)
  Closes the standard error stream soon after start-up.

# --connection-timeout (-U)
  Sets the timeout (in seconds) when connecting to a remote server (default is 40).

# --debug (-g)
  Generates debug-level logging if built in.

# --domain (-D)
  Sets an override for the host's fully qualified domain name.

# --dont-serve (-x)
  Disables acting as a server on any port (part of --as-client and usually used with --forward).

# --filter (-z)
  Specifies an external program to process messages as they are stored.

# --filter-timeout (-W)
  Sets the timeout (in seconds) for running the --filter processor (default is 300).

# --forward (-f)
  Forwards stored mail on startup (requires --forward-to).

# --forward-to (-o)
  Specifies the remote smtp server (required by --forward, --poll, --immediate and --admin).

# --help (-h)
  Displays help text and exits.

# --immediate (-m)
  Enables immediate forwarding of messages as soon as they are received (requires --forward-to).

# --interface (-I)
  Defines the listening interface(s) for incoming connections (comma-separated list with optional smtp=,pop=,admin= qualifiers).

# --log (-l)
  Writes log information on standard error and syslog (but see --close-stderr and --no-syslog).

# --log-time (-L)
  Adds a timestamp to the logging output.

# --no-daemon (-t)
  Does not detach from the terminal.

# --no-smtp (-X)
  Disables listening for smtp connections (usually used with --admin or --pop).

# --no-syslog (-n)
  Disables syslog output (always overridden by --syslog).

# --pid-file (-i)
  Defines a file for storing the daemon process-id.

# --poll (-O)
  Enables polling of the spool directory for messages to be forwarded with the specified period (zero means on client disconnection) (requires --forward-to).

# --pop (-B)
  Enables the pop server.

# --pop-auth (-F)
  Defines the pop server secrets file (default is "/etc/emailrelay.auth").

# --pop-by-name (-J)
  Modifies the pop spool directory according to the pop user name (requires --pop).

# --pop-no-delete (-G)
  Disables message deletion via pop (requires --pop).

# --pop-port (-E)
  Specifies the pop listening port number (default is 110) (requires --pop).

# --port (-p)
  Specifies the smtp listening port number (default is 25).

# --prompt-timeout (-w)
  Sets the timeout (in seconds) for getting an initial prompt from the server (default is 20).

# --remote-clients (-r)
  Allows remote clients to connect.

# --response-timeout (-T)
  Sets the response timeout (in seconds) when talking to a remote server (default is 1800).

# --server-auth (-S)
  Enables authentication of remote clients, using the given secrets file.

# --server-tls (-K)
  Enables negotiated tls/ssl for smtp server using the given openssl certificate file (which must be in the directory trusted by openssl).

# --size (-M)
  Limits the size of submitted messages.

# --spool-dir (-s)
  Specifies the spool directory (default is "/var/spool/emailrelay").

# --syslog (-k)
  Forces syslog output if logging is enabled (overrides --no-syslog).

# --tls-config (-0)
  Sets tls configuration flags (eg. 2 for SSLv2 support).

# --user (-u)
  Names the effective user to switch to if started as root (default is "daemon").

# --verbose (-v)
  Generates more verbose output (works with --help and --log).

# --verifier (-Z)
  Specifies an external program for address verification.

# --version (-V)
  Displays version information and exits.

Under Windows there are a few minor differences. Use "--help --verbose" to see
the complete list.

Message store
-------------
Mail messages are stored as text files in the configured spool directory. Each
message is represented as an envelope file and a content file. The envelope
file contains parameters relevant to the SMTP dialogue, and the content file
contains the RFC822 headers and body text.

The filenames used in the message store have a prefix of "emailrelay", followed
by a process-id, timestamp and sequence number, and then "envelope" or
"content". The envelope files then have an additional suffix to implement a
simple locking scheme.

The envelope file suffixes are:

* ".new" -- while the envelope is first being written
* <none> -- while the message is spooled
* ".busy" -- while the message is being forwarded
* ".bad" -- if the message cannot be forwarded
* ".local" -- for copies of the envelope file for delivery to local recipients

If a message cannot be forwarded the envelope file is given a ".bad" suffix,
and the failure reason is written into the file.

Forwarding
----------
Spooled messages can be forwarded at various times, depending on the
command-line options:

* when E-MailRelay first starts up ("--as-client" or "--forward")
* as each message is submitted, just before receipt is acknowledged ("--immediate")
* as soon as the submitting client connection disconnects ("--poll=0")
* periodically ("--poll=<seconds>")
* on demand using the administration interface's "flush" command ("--admin=<port>")

These modes of operation can be mixed, although more than one "--poll" option is
not allowed.

When using "--as-client" the spooled messages begin to be forwarded as soon as
the program starts up, and the program terminates once they have all been sent.

A pending "--poll" timer can be forced to expire immediately if a "--filter"
script exits with a special value of 103, as described below.

All receipent addresses must be accepted by the remote server when E-MailRelay
forwards a message. If any one recipient is rejected then the message will be
left in the spool directory with a ".bad" suffix on the envelope file.

Mail processing
---------------
The "--filter" command-line option allows you to specify a mail pre-processor
program which operates on mail messages as they pass through the E-MailRelay
system. The mail pre-processor program is run as soon as the mail message has
been stored in the spool directory, with the full path of the content file
added onto the end of the given command-line.

For example, the following command will start a proxy server on port 10025
which processes mail using the specified filter program, and then forwards the
mail on to the system's default MTA (on port 25):

	emailrelay --as-proxy=localhost:smtp --port=10025 --no-syslog \
	  --filter=$HOME/myfilter --spool-dir=$HOME/spool

The pre-processor program should terminate with an exit code of zero to
indicate success, or a value between 1 and 99 to indicate failure. Exit codes
between 100 and 107 are reserved for special processing: 100 is used to cancel
all further processing of the message, and 103 has the effect of immediately
expiring any "--poll" timer.

If the pre-processor program terminates with a non-zero exit code then the
first few thousand characters of the standard output stream are searched for a
line starting with "<<" or "[[" followed by ">>" or "]]" respectively. The text
in between is taken as a failure reason, and passed back to the SMTP client.

The pre-processor program can edit any part of the message's envelope file or
content file: E-MailRelay remembers nothing about the message while the
pre-processor is running except the filename. But if the message is deleted
by the pre-processor then E-MailRelay may be upset, so to avoid the error
message use an exit code of 100.

If the pre-processor program creates completely new messages in the spool
directory then they may not be processed immediately, or they may be completely
ignored. To get E-MailRelay to pick up any new messages you create in the spool
directory use the "--poll" option, or run "emailrelay --as-client" from within
the pre-processor program.

As an example of a simple pre-processor this shell script examines the sending
client's IP address and conditionally passes the message into "sendmail" (using
the sendmail command-line interface rather than SMTP):

	#!/bin/sh
	# filter.sh
	content="${1}"
	envelope="`echo \"${content}\" | sed 's/content/envelope.new/'`"
	ip="`awk '/MailRelay-Client:/ {print $2;exit}' \"${envelope}\"`"
	if test "${ip}" = "192.168.0.2"
	then
		cat "${content}" | /usr/sbin/sendmail -t
		rm -f "${envelope}" "${content}"
		exit 100 # <= cancel further processing by emailrelay
	fi
	exit 0

The first thing this script does is convert the path of the content file which
it is given, into the corresponding envelope file. It then extracts the
client's IP address out of the envelope file using "awk". If this matches the
fixed address then it pipes the message content into sendmail, deletes the
message and exits with a value of 100. The exit value of 100 tells E-MailRelay
to forget the message, and not to complain about the files disappearing.

For Windows this example can be rewritten in JavaScript:

	// filter.js
	var content = WScript.Arguments(0) ;
	var envelope = content.substr(0,content.length-7) + "envelope.new" ;
	var fs = WScript.CreateObject( "Scripting.FileSystemObject" ) ;
	var ts = fs.OpenTextFile( envelope , 1 , false ) ;
	var e = ts.ReadAll() ;
	ts.Close() ;
	var re = new RegExp( "MailRelay-Client: \(.*\)" ) ;
	var ip = e.match(re)[1] ;
	if( ip == "192.168.0.2" )
	{
		var sh = WScript.CreateObject( "Wscript.Shell" ) ;
		sh.Run( "sendmail " + content ) ; // bogus
		fs.DeleteFile( content ) ;
		fs.DeleteFile( envelope ) ;
		WScript.Quit( 100 )
	}
	WScript.Quit( 0 ) ;

Windows pre-processor programs written in JavaScript can be run using
"cscript", with an E-MailRelay "--filter" option something like this:

	--filter="c:/program\ files/emailrelay/filter.js"

Note how the space character in "program files" is escaped with a backslash.

Example "--filter" scripts are included in the distribution, including
"emailrelay-process.sh" which does rot-13 masking of messages. This script also
does some simple MIME encoding, so that the masked message appears as an
attachment within a plaintext bearer message.

E-MailRelay also has a "--client-filter" option that enables pre-processing of
messages just before they are forwarded, rather then after they are stored. The
disadvantage is that by then it is too late to notify the submitting SMTP client
of any processing failures, so in many applications using "--filter" is more
useful. The special exit code of 100 can be used to ignore the current message,
and 102 to stop looking for spooled messages after processing the current one.

Bear in mind the following points when writing "--filter" programs:

* The standard input and output are not used; the content filename is passed on the command-line.
* Programs run with a reduced set of environment variables.
* The E-MailRelay process is completely blocked while the "--filter" program runs so it should be fast.
* E-MailRelay files use CR-LF line terminators, as required by the RFCs.
* Envelope files will have a file extension of ".new" or ".busy" when the program runs.
* Windows scripts may need to be run via "cscript" or a batch file wrapper.

It is also possible to do message pre-processing in a separate process by using
"net:<address>:<port>" as the "--filter" or "--client-filter" option parameter.
E-MailRelay connects to this address and then uses a simple line-based dialog as
each e-mail message is processed where it sends the full path of the message
content file in one line and expects the remote process to respond with an "ok"
line if the message is to be accepted or an error message. One advantage of
using a network pre-processor compared to running a program is that the
E-MailRelay server is not blocked while the messages are being pre-processed.

Authentication
--------------
E-MailRelay supports the SMTP "AUTH" extension, as defined in RFC2554, on both
the server-side and client-side.

The authentication mechanisms currently supported are:

# "PLAIN"

  Passwords are stored in clear-text, sent over the network in clear-text, and
  are replayable. Defined in RFC2595.

# "LOGIN"

  Similar to "PLAIN". Officially obsolete although widely used.

# "CRAM-MD5"

  Passwords are not stored in clear-text, not sent over the network, and are
  not replayable. Defined in RFC2195.

Authentication is enabled with the "--client-auth" and "--server-auth"
command-line options and the option's parameter is the name of a secrets file,
containing usernames and passwords:

	emailrelay --as-server --server-auth=/etc/emailrelay-clients.auth
	emailrelay --as-client=myisp.net:smtp --client-auth=/etc/emailrelay-myisp.auth

The secrets file has a line-based format: blank lines are ignored and the hash
character (#) is used for comments.

Lines have four white-space delimited fields:

* "client-or-server"
* "mechanism"
* "userid"
* "secret"

The "mechanism" field must be "LOGIN", "APOP" or "CRAM-MD5" (case-insensitive);
the "client-or-server" field must be "client" or "server" (also
case-insensitive); the "userid" field is xtext-encoded user identifier; and the
"secret" field is the xtext-encoded "LOGIN" password, or the "CRAM-MD5" digest
key.

The "xtext" encoding scheme is defined properly in RFC1891, but basically it
says that non alphanumeric characters, space, "+" and "=" should be represented
in hexadecimal ascii as "+XX".

The client-side secrets file specified with "--client-auth" is used when
E-MailRelay acts as a client to talk to a remove server. The file should contain
at least one "client LOGIN" or "client CRAM-MD5" entry.

A server-side secrets file specified with "--server-auth" is used when a remote
client tries to authenticate with the E-MailRelay server. The file should
normally contain several "server LOGIN" or "server CRAM-MD5" entries, one for
each client.

The same secrets file may be specified for both "--client-auth" and
"--server-auth" options.

The "CRAM-MD5" keys can be generated using the "emailrelay-passwd" utility.

As an example, the following secrets file defines "jsmith" as the username to be
used when E-MailRelay authenticates with a remote SMTP server, and defines two
usernames ("user1" and "user2") which can be used by clients when they
authenticate with the E-MailRelay server:

	#
	# emailrelay secrets file
	#
	client LOGIN jsmith my+20password
	server LOGIN user1 secret
	server LOGIN user2 e+3Dmc2

A "CRAM-MD5" version would look like this:

	#
	# emailrelay secrets file
	#
	client CRAM-MD5 jsmith 688498119.2977922305.1278051807.3015243256.2216875978.2833592318.2902375592.3156808220
	server CRAM-MD5 user1 4059553961.2316091643.3282746241.1444639637.3735501773.3404060330.2760590371.1201092398
	server CRAM-MD5 user2 2798539199.3144534242.3784876256.2879973305.2327113479.216533878.2436460291.2361831919

When using the "LOGIN" mechanism you have to store plaintext passwords in the
secrets file, so you should make sure that the secrets file has tight
permissions and the passwords in it are not also used for anything important.

On the server side authentication is advertised by E-MailRealy in the response
to the SMTP "EHLO" command if the "--server-auth" command-line option is used.
Authentication by the client is then mandatory unless the client's IP address is
configured as a trusted address.

Trusted IP addresses are configured with lines in the secrets file having "NONE"
in the first field, "server" in the second field, a wildcarded IP address in
the third field, and an arbitrary keyword in the fourth field. The keyword field
is passed to any external address verifier program specified by the "--verifier"
command-line option; it is not used for any other purpose. Wildcarded IPv4
addresses can use a format like 192.168.0.0/24 or 192.168.0.*; wildcarded IPv6
addresses must have the full set of 32 lowercase hex digits followed by the CIDR
netmask.

For example, this secrets file allows any client connecting from the
192.168.0.0/24 domain to connect without authentication desipte the
"--server-auth" option:

	#
	# emailrelay secrets file
	#
	server NONE 192.168.0.* localdomain
	server LOGIN user1 secret
	server LOGIN user2 e+3Dmc2

On the client side authentication is performed when E-MailRelay has connected to
a server which implements the AUTH extension with one of the supported
mechanisms. If client authentication is enabled (with the "--client-auth"
option) but the remote server does not support the AUTH extension, or does not
support the LOGIN or CRAM-MD5 mechanism, then E-MailRelay will log an error
and not forward any messages.

If E-MailRelay successfully authenticates with the remote server then the
client's authentication name (if any) is passed as a parameter to the MAIL
command when the message is forwarded. Some SMTP servers are reported to reject
the message if the name on the MAIL command does not match the name used to
authenticate with them. However, the name passed as a parameter to the MAIL
command is stored in the message's envelope file, so a "--filter" script can be
used to change this to match whatever the remote server expects. An example
script is provided in the distribution.

Note that some ISPs require separate POP/IMAP authentication before SMTP access
from a particular IP address is allowed. This type of POP-before-SMTP
authentication can be done outside the E-MailRelay system by POP/IMAP utilities
such as "fetchmail".

TLS/SSL
-------
E-MailRelay can use negotiated TLS/SSL to encrypt SMTP and POP3 sessions: to
enable TLS/SSL encryption when E-MailRelay is acting as an SMTP client use the
"--client-tls" command-line option, and to enable TLS/SSL when E-MailRelay is
acting as an SMTP or POP3 server use "--server-tls". The connections start off
as unencrypted and use the STARTTLS command to negotiate TSL/SSL encryption
before any plaintext passwords are exchanged.

The "--server-tls" option requires the name of an X.509 certificate in the "PEM"
format. This file must be in a directory that the OpenSSL library considers to
be secure. On some systems this is "/etc/ssl/certs".

Certificate files can be created with the "openssl" utility using the
"req -x509" sub-command. Refer to the OpenSSL documentation for the full details
but something like this might work for testing purposes:

	$ cd /etc/ssl/certs
	$ openssl req -x509 -nodes -days 365 -subj "/O=`uname -n`/CN=$USER" -newkey rsa:1024 -keyout emailrelay.pem  -out emailrelay.pem
	$ ln -s emailrelay.pem `openssl x509 -noout -hash -in emailrelay.pem`.0

E-MailRelay can also make outgoing SMTP connections using TLS/SSL encryption
where the whole SMTP dialog is encrypted from the start, without a negotiation
step ("--client-tls-connection"). This is sometimes called SMTP-over-TLS or
secure SMTP (ssmtp) and it is normally used with port number 465.

The behaviour of the TLS/SSL layer can be tweaked by using "--tls-config" option
with a special number that is made up of the following:
* 2 - allow SSL v2 or later
* 3 - allow SSL v3 or later
* 4 - ask for peer certificates but just log them
* 8 - ask for peer certificates and verify them

PAM Authentication
------------------
E-MailRelay supports the use of PAM (Pluggable Authentication Modules) for
authentication if it has been built with the "--with-pam" configure option.

PAM authentication can be used by E-MailRelay to authenticate SMTP and POP3
sessions coming in from remote clients; it cannot be used by E-MailRelay to
supply passwords when acting as an SMTP client.

Use "--server-auth=/pam" and/or "--pop-auth=/pam" on the command-line to enable
PAM authentication at run-time. The E-MailRelay server will then advertise an
SMTP authentication mechanism of PLAIN and do the actual authentication via PAM.

The PAM system itself must be configured with a service of "emailrelay". This
normally involves creating a file "/etc/pam.d/emailrelay" containing somthing
like the following:

	auth requisite pam_unix.so nullok_secure
	session required pam_permit.so
	account required pam_permit.so
	password required pam_deny.so

With this configuration the E-MailRelay server will use normal unix system
account names and passwords to authenticate remote clients, but note that on
some systems this will require special permissioning to allow the E-MailRelay
server to read the shadow password database.

The passwords for system accounts are usually very sensitive so E-MailRelay
requires the "--server-tls" command-line option when using PAM authentication,
and clients are required to establish a TLS/SSL session before authenticating.

SOCKS
-----
E-MailRelay can use a SOCKS 4a proxy for establishing outgoing SMTP connections
if the SOCKS proxy address is appended to the SMTP server's address separated by
"@".

For example, this could be used to send e-mails via the Tor network, assuming
there is a local Tor server listening on port 9050:

	emailrelay --forward-to myisp.net:smtp@localhost:9050 ...

In this example the target SMTP server will receive a connection from the Tor
exit node rather than from E-MailRelay directly.

Pop server
----------
E-MailRelay can be used as a POP3 server so that POP clients have access to
spooled messages.

The following command-line options are used:

# --pop

  Enables the POP3 server. Negotiated TLS/SSL (using "STLS") will be enabled if
  the "--client-tls" or "--server-tls" options are also given.

# --pop-port=<port>

  Changes the POP3 listening port from its default of 110.

# --pop-auth=<path>

  Changes the authentication secrets file. The default is typically
  "/etc/emailrelay.auth".

  The format of the authentication secrets file is the same as for
  "--server-auth" and the same file can be used for both SMTP and POP3
  authentication.

  "server APOP" entries are used for "APOP" authentication, "server LOGIN"
  entries are used for "USER/PASS" authentication, and "server CRAM-MD5" entries
  are used for "AUTH" authentication.

  Note that the basic POP3 protocol defines only "APOP" and "USER/PASS"
  authentication, so some POP client programs may not be able to cope with
  CRAM-MD5 authentication using the POP3 "AUTH" extension.

  The CRAM-MD5 authentication mechanism will only be advertised to clients
  if there are "server CRAM-MD5" entries in the secrets file.

  If E-MailRelay was built with PAM support then a special value of "/pam" can
  be used to enable authentication using the PAM infrastructure rather than a
  secrets file. In this case only the PLAIN authentication mechanism will be
  advertised to the client but transport level encryption using TLS/SSL will be
  mandatory.

# --pop-by-name

  Modifies the POP spool directory according to the name used by the client
  to authenticate with the E-MailRelay server. The client name is used as a
  sub-directory off the standard spool directory. So, for example, if a client
  authenticates as "bob" then the POP3 server will serve messages from the "bob"
  sub-directory, "/var/spool/emailrelay/bob".

  If E-MailRelay finds only the envelope file in the sub-directory and not the
  content file then it will look for the content file in the main spool
  directory, eg. "/var/spool/emailrelay". This feature can be used to save
  disk space if serving the same message to multiple POP clients.

# --pop-no-delete

  Disables message deletion: the POP3 DELE command will appear to succeed, but
  no files will be deleted from the spool directory.

Address verification
--------------------
By default the E-MailRelay server will accept all recipient addresses for
incoming e-mails as valid. This default behaviour can be modified by using an
external verifier program, specified with the "--verifier" command-line option,
so that you get to choose which recipient addresses are accepted as valid and
which are rejected.

Your verifier script is passed a command-line containing: (1) the full e-mail
address as supplied by the remote client, (2) the user-name part of the address
in upper-case, (3) the host-name part in upper-case, (4) the local host's fully
qualified domain name in upper-case, (5) the "MAIL" command's "FROM:" address as
supplied by the client or the empty string in the case of the "VRFY" command,
(6) the IP address of the client connection, (7) the authentication mechanism
used by the client ("NONE" if trusted), and (8) either the authentication name
or the fourth field from authentication secrets file if a trusted IP address.

So, for example, a verifier script called "myverifier" might be run with the
following command-line:

	myverifier me@myhost.mydomain ME MYHOST.MYDOMAIN MYHOST.MYDOMAIN bob@other.net 192.168.0.1 LOGIN bob

The verifier script is expected to generate two lines of output on the standard
output stream and then terminate with a specific exit code.

For valid addresses the first line of output is ignored, the second line should
be copied from the first command-line argument, and the exit value should be
one.

	#!/bin/sh
	# address verifier -- accept all (252)
	echo ""
	echo $1
	exit 1

If the address is valid but it should be delivered to a local mailbox rather
than forwarded then the verifier script should write two lines to the standard
output -- the full name associated with the mailbox, and the canonical mailbox
name -- and then exit with a value of zero.

	#!/bin/sh
	# address verifier -- accept as local (250)
	echo Local Postmaster '<postmaster@localhost>'
	echo postmaster
	exit 0

For E-MailRelay local delivery just means that the message file in the spool
directory is given a ".local" filename suffix. This can be used to create a
separate channel for adminstrative messages such as delivery reports.

For invalid addresses the exit value should be non-zero and anything written to
the standard output is taken as the reason for the failure.

	#!/bin/sh
	# address verifier -- reject as invalid (550)
	echo invalid address: $1
	exit 2

To indicate a temporary failure this can be changed to an exit code of 3.

	#!/bin/sh
	# address verifier -- reject as temporarily invalid (450)
	echo address unavailable: $1
	exit 3

If the verifier exit code is 100 then the connection is aborted immediately,
which may be useful in limiting the impact of denial of service attacks:

	#!/bin/sh
	# address verifier -- abort
	exit 100

In this more complete example the verifier script accepts all addresses as valid
as long as they contain an "at" character:

	#!/bin/sh
	# address verifier -- accept only if containing an at sign
	address="$1"
	expr "${address}" : ".*@" > /dev/null || exit 2
	echo ""
	echo "${address}"
	exit 1 # accept

As another example, this verifier script accepts all recipient addresses by
default but rejects remote addresses if the client has bypassed authentication
by connecting on a trusted IP address:

	#!/bin/sh
	# address verifier
	address="$1"
	host="$3"
	local_domain="$4"
	auth_mechanism="$7"
	if test -z "${auth_mechanism}" ; then auth_mechanism="NONE" ; fi
	if test "${auth_mechanism}" = "NONE" -a "${host}" != "${local_domain}"
	then
		echo "cannot relay without authentication"
		exit 2 # reject the recipient address
	fi
	echo ""
	echo "${address}"
	exit 1 # accept the recipient address

or written in JavaScript for Windows:

	// verifier.js
	var address = WScript.Arguments(0) ;
	var host = WScript.Arguments(2) ;
	var local_domain = WScript.Arguments(3) ;
	var auth_mechanism = WScript.Arguments(6) ;
	if( ( auth_mechanism == "NONE" || auth_mechanism == "" ) && host != local_domain )
	{
		WScript.Stdout.WriteLine( "cannot relay without authentication" ) ;
		WScript.Quit( 2 ) ;
	}
	WScript.Stdout.WriteLine( "" ) ;
	WScript.Stdout.WriteLine( address ) ;
	WScript.Quit( 1 ) ;

If this verifier script is used with a suitable "--server-auth" file then it can
be used to prevent open relay without restricting authenticated clients.

It is also possible to verify addresses in a separate daemon process by using a
"--verifier" option of the form "net:<address>:<port>". In this case E-MailRelay
will connect to the specified verifier daemon over the network and send address
verification requests as lines with pipe-delimited fields. The expected response
is another pipe-delimited line containing the same information as returned by
verifier scripts but in reverse, such as "3|address unavailable" or
"0|postmaster|Local Postmaster <postmaster@localhost>". The "inetd" super-server
daemon would be a convenient way to connect up the pieces in this case.

Security issues
---------------
The following are some security issues that have been taken into consideration:

# Effective userid

  Suid privileges are revoked at start-up, switching the effective
  userid/groupid to be the real userid/groupid values. If started as "root"
  then the effective userid/groupid are switched at start-up to those of user
  "daemon". Special privileges are only reclaimed when needed to bind sockets
  and do file i/o. Normally this means temporarily switching the userid and
  groupid back to what they were at start-up. However, when writing spool files
  after being started as "root" only the effective userid is changed, not the
  groupid, so that new files have group ownership corresponding to the
  "daemon" user.

# Execution environment

  The external pre-processor programs are run with an almost empty set of
  environment variables ("PATH" and "IFS"), and with no open file descriptors
  other than "stdin" and "stderr" open onto "/dev/null", and "stdout" open onto a
  pipe.

# Configuration

  The mail pre-processor filenames have to be configured using a full path, so
  there is no dependence on the current working directory or the PATH
  environment variable.

# Umask

  The program runs for most of the time with a "umask" of 177, switching to 117
  when creating spool files.

# Buffer overflow

  Strings are dynamically allocated in c++, so buffer overflow/truncation issues
  are avoided.

# Remote clients

  By default connections will be rejected if they come from remote machines.

# Remote configuration

  No configuration parameters can be changed through the administrative
  interface.

# Use of exec() and system()

  No exec(), system() or popen() calls are used other than execve() to spawn the
  mail pre-processor and/or address verifier.

# File permissions

  After a normal installation the spool directory is has ownership of
  "root.daemon" with permissions of "-rwxrwxr-x" and messages files are created
  with permissions of "-rw-rw----". This allows normal users to list messages
  files but not read them.

  The "emailrelay-submit" program is given group ownership of "daemon" with its
  group set-user-id flag set. This allows it to create message files in the
  spool directory, and the files created end up owned by the submitter but with
  group ownership of "daemon".

# Logging

  Logging output is conditioned so that ANSI escape sequences cannot appear
  in the log.

  Passwords and message content are not logged (except if using the "--debug"
  option at run time with debug logging enabled at build time).

# Information leakage

  The "--anonymous" option can be used to reduce the amount of information
  released to remote clients, especially with an empty "--domain".

# Mandatory encryption

  When using Linux PAM for authentication all clients are required to use
  TLS/SSL encryption.

Security issues which relate to the SMTP protocol itself are beyond the scope of
this document, but RFC2821 makes the following observation: "SMTP mail is
inherently insecure in that it is feasible for even [..] casual users to [..]
create messages that will trick a [..] recipient into believing that they came
from somewhere else. [..] Real [..] security lies [..] in end-to-end methods
[..] such as those which use digital signatures."

The "Authentication" and "Linux PAM" sections above also relate to security.

Administration interface
------------------------
If enabled with the "--admin" command-line option, the E-MailRelay server will
provide a network interface for performing administration tasks. This is a
simple command-line interface which is compatible with "telnet":

	$ emailrelay --as-server --port=125 --forward-to=localhost:25 --admin=10026
	$ telnet localhost 10026
	E-MailRelay> help
	E-MailRelay> quit

The "flush" command is used to get the E-MailRelay server to forward spooled
mail to the next SMTP server, as an alternative to running
"emailrelay --as-client" as a separate process. In proxy mode it is a way of
getting the proxy server to scan the spool-directory for new messages.

The "list" command lists the messages in the spool directory, "info" provides
network status information and activity statistics, and "notify" enables
asynchronous event notification.

Files and directories
---------------------
Following a normal build from source, a "make install" puts files in the
following locations:
* /usr/local/etc/emailrelay.conf
* /usr/local/etc/emailrelay.conf.template
* /usr/local/etc/pam.d/emailrelay
* /usr/local/libexec/emailrelay/emailrelay-filter-copy
* /usr/local/libexec/emailrelay/emailrelay-poke
* /usr/local/libexec/emailrelay/examples/*
* /usr/local/libexec/emailrelay/init/emailrelay
* /usr/local/man/man1/emailrelay*.1.gz
* /usr/local/sbin/emailrelay
* /usr/local/sbin/emailrelay-passwd
* /usr/local/sbin/emailrelay-submit
* /usr/local/share/emailrelay/doc/*
* /usr/local/share/emailrelay/doc/index.html
* /usr/local/share/emailrelay/doc/README
* /usr/local/var/spool/emailrelay/emailrelay.*.content
* /usr/local/var/spool/emailrelay/emailrelay.*.envelope

For finer control of the directory structure the following can be specified on
the "configure" command-line:
* --mandir=<dir>
* --sbindir=<dir>
* --datadir=<dir>
* e_libexecdir=<dir>
* e_sysconfdir=<dir>
* e_docdir=<dir>
* e_examplesdir=<dir>
* e_initdir=<dir>
* e_spooldir=<dir>
* e_pamdir=<dir>
* e_icondir=<dir>

For example, running "./configure --prefix=/usr e_spooldir=/tmp/spool" will
install to a directory structure under "/usr" rather than "/usr/local",
and create the E-MailRelay spool directory as "/tmp/spool" rather than
"/usr/local/var/spool/emailrelay".

For a directory structure conforming more closely to the FHS use this configure
command:

	./configure --prefix=/usr --localstatedir=/var --libexecdir=/usr/lib --sysconfdir=/etc e_init_dir=/etc/init.d

As usual, it is possible to change the installation root directory after
building by using "make DESTDIR=<root> install" or
"DESTDIR=<root> make -e install". However, this will not affect the default
spool directory path built into the scripts and executables so the correct spool
directory will have to be specified at run-time with the "--spool-dir" option.




Copyright (C) 2001-2013 Graeme Walker <graeme_walker@users.sourceforge.net>. All rights reserved.