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Manual Reference Pages  -  PXYTEST (1)

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pxytest - test proxy server for unsecured mail relay



pxytest [ -a ] [ -h ] [ -M mail_server ] [ -m mail_addr ] [ -S smtp_banner ] [ -T mail_tag ] [ -t num_threads ] [ -v verbosity ] target_host [ port_spec ... ]


The pxytest utility performs a test on target_host (given as a host name or address) to locate an unsecured proxy that allows allow connections to a mail server. Spammers use such hosts to distribute vast amounts of junk email.

Normally, pxytest will not actually attempt to relay mail through the proxy, only verify that an open proxy exists and can connect to a mail server. If the test runs to completion without encountering an unsecured proxy, the program terminates with a message:

        Test complete - no proxies found

Normally, as soon as the program encounters an open proxy, it terminates with a message:

        Test complete - identified open proxy addr:port/protocol

The following options are available.
-a Find all open proxies. Instead of terminating as soon as an open proxy is detected, pxytest will continue on to perform the full set of tests. At completion, it will indicate the number of open proxies detected.
-h Display a help message and then exit. The help message provides information on defaults and definitions that may have been modified by your local administrator.
-M mail_server Specifies a target mail_server, given as a name or number. pxytest will attempt to connect to this server through the proxy. See Mail Server Selection for more information.
-m mail_addr A probe email message is transmitted to mail_addr. Normally, pxytest stops as soon as it verifies connection to the SMTP server. When this option is given it continues on to send an email to the indicated recipient.
-S smtp_banner Specifies string that identifies the SMTP banner from the mail server. See the Mail Server Selection section for more information.
-T mail_tag An arbitrary mail_tag is added to the probe email headers. This tag may be used, for example, to serialize the email so it may be correlated with a particular incident. This option has no effect unless -m was specified.
-t num_threads This option is experimental. The test is accelerated by running up to num_threads probes in parallel. Under best-to-normal case conditions, this will actually slow down the test, taking it longer to complete. In the worst case situation, however, where certain tests are pausing for long times waiting for server responses, this can greatly reduce the total test time.
-v verbosity Controls the amount of output messages produced. The verbosity levels are:

    0 - Display nothing but program errors.
    1 - Display final test result.
    2 - Display individual test results. 
    3 - Display details of individual tests.
    4 - Display thread management information.

The default verbosity level is 3.

The port_spec Arguments

Exhaustive testing for open proxies is impractical. Proxies may appear on any of 65,536 TCP ports. Also, there are a number of different forms of proxies, each requiring its own test. At 50msec/test, it could take over 6 hours to test a single host.

The user must direct the pxytest test sequence. This is done with port_spec arguments. These may be simply a tag name (discussed shortly) or a specification in the form:


where min is the starting port number of the scan, max is the ending port number of the scan, and proto is the proxy mechanism to test. If max is not specified (it usually isn’t), then a single-port scan is done. The possible proto values are: http-connect, http-post, http, socks4, socks5, telnet, cisco, wingate, and all. If proto is not specified then it defaults to http-connect. (The next section describes what these proxy mechanisms mean.)

The port_spec may also be a mnemonic tags. As distributed, there are three tags defined:
basic A basic set of tests that covers most common cases. If no port_spec argument is given on the command line, the default is to do a basic scan.
full All of the basic tests plus several more that have been reported in less common instances.
socks A shortcut for: 1080/socks4 1080/socks5
Your local administrator may have modified this script to change the definition of these tags or added additional tags. Run pxytest with the -h option to get a list of all the tags and their exact definitions.

    Proxy Mechanisms

There are a number of different proxy mechanisms that can be abused for mail relay. The mechanisms supported by this utility include:
http-connect A web proxy or cache that supports the HTTP CONNECT mechanism. See CERT Vulnerability Note VU#150227 ( for further information.

This is the most common type of unsecured proxy. It may appear on any TCP port. Some of the common locations are port 3128 (the well known port for squid), port 8080 (the well known port for webcache), and port 8081 (the well known port for tproxy). Unsecured or misconfigured web servers can often act as proxies, so these are often found on port 80 (the well known port for http). The AnalogX Proxy uses port 6588.

If no proto is specified in a port_spec, it defaults to http-connect.

http An alias for http-connect.
http-post A web proxy or cache that supports access to a URL via the HTTP POST mechanism. This vulnerability is not well documented, but according to the OPM stats it’s the second most prevalent type.
socks4 SOCKS version 4 proxy. See the SOCKS Version 4 Overview <> for further information on this service. TCP port 1080 is the well known port allocated to socks.
socks5 SOCKS version 5 proxy. See the SOCKS Version 5 Overview <> for further information on this service. TCP port 1080 is the well known port allocated to socks.
telnet A proxy that accepts a command in the form:

        telnet dstaddr dstport

and establishes a connection to the indicated destination.

cisco An unsecured Cisco router that allows login with the factory default values. Once a user is logged into the router, they can use it as a telnet proxy.
wingate The WinGate Internet Sharing/Proxy Server by See their corporate web site <> for further information on this product. Such a proxy accepts a specification in the form:


and establishes a connection to the indicated destination. This proxy typically appears on TCP port 23, which, confusingly enough, is the well known port reserved for the telnet service.

all This value is expanded out to all the available test mechanisms.

    Mail Server Selection

The pxytest utility attempts connection to a target mail server, and declares a proxy as open if it succeeds. The target mail server is selected by the following process:
o If the -M command line option is given, the mail_server value it specifies (host name or address) is used.
o Otherwise, if the $DEFAULT_MAIL_SERVER parameter is defined in the script, that is selected. Typically that parameter is left undefined, although the local administrator may choose to modify the script to set a value.
o Otherwise, if the perl Net::DNS module is installed, the utility will attempt to determine the mail server (MX) for the local host and use that.
If none of these methods may be used, the utility terminates with an error.

The utility will attempt to recognize the mail server by its SMTP welcome banner, which typically looks something like:

    220 ESMTP Postfix [NO UCE C=US L=TX]

By default, it declares success when it sees a line beginning with 220 (two-two-oh-space). In certain conditions, this may be a problem.

Some rare mail servers do not use the 220 code. If, for example, the mail server does not want to accept incoming mail, it may use some other code. Such a server can be used by pxytest, although the -m option won’t work.

Some proxies are actually honeypots that are used to trap spammers and crackers. These honeypots may redirect SMTP connections. So pxytest will declare success when it sees the SMTP welcome banner generated by the honeypot.

In these cases, the -S option may be used to specify a more specific match for the SMTP banner. The smtp_banner argument will specify a fixed string that appears at the start of the banner. For example,

    -S "220"

might be a good way to ensure pxytest has connected back to the server that gives the SMTP banner shown above.

    Probe Email

When the -m option is specified, the utility attempts to send a probe email message through the target mail server. Here is the header from a sample probe message:

        Subject: open proxy test
        X-Mailer: pxytest v1.17
        X-Proxy-Spec: ID-000034

The To and From headers were specified with the -m option. The X-Mailer header may be used to simplify recognition and sorting of incoming test probes. The X-Proxy-Spec header identifies the proxy, plus any tag that may have been given with the -T option.


An exit status of 0 means the test ran to completion without finding any open proxies. An exit status of 2 means that an open proxy was detected. Any other non-zero exit status indicates some sort of error.


This section provides additional explanation for selected error messages:
unknown host target_host
unknown proxy type proto
bad port specification num These all indicate a problem with the port_spec given on the command line.
error setting SIGALRM handler This utility uses the POSIX interface to set timeout alarms. This error likely indicates you are running on a non-POSIX system. If you run into this, please contact the author.
cannot locate mailserver for ‘‘hostname’’ Was unable to locate a mail exchanger (MX) for your host or your domain. This would happen if there is no MX for your host or your domain. It also could happen if there are DNS problems. This can be worked around by either using the -M option or modifying the script to define a $DEFAULT_MAIL_SERVER value.
you must define a mail server (Net::DNS unavailable) The automatic mail server lookup cannot run, because your system does not have the perl Net::DNS module installed. If you do not want to install this module, then you will need to specify the target mail server. Either use the -M option or modify the script to define define a $DEFAULT_MAIL_SERVER value.
host lookup for hostname failed The indicated host was identified as the target mail server to use, but pxytest was unable to determine the IP address of that host. This typically results from DNS problems. Either resolve the DNS problems, or specify the target mail host as an address rather than a name.
Cannot get host name of local machine This diagnostic is produced by the perl Sys::Hostname module. See the documentation on that module for information.
cannot determine your username A number of methods were attempted to determine your username, none of which worked. Please contact the author if you get this message.


Proxies may appear on any TCP port. A complete test would require an exhaustive scan of all available ports, which is infeasible. Instead, the basic and full scans cover ports that (based on past observation) are mostly likely to be bound to a proxy service. The author welcomes feedback on the ports definitions for the basic and full scans. The author also welcomes information on additional proxy mechanisms that may be used for email abuse (spam).

Ideally, the -S option should not be required. We ought be able to probe the target mail server to get the SMTP banner. We don’t do this automatically, because in some cases (e.g. running the test from a host on a network that blocks outbound port 25) it won’t work.

The threading is an ugly hack to address the inordinately long test times against a proxy that is not responding. Hell, it isn’t even real threading. It’s a lame facsimile implemented with fork().

The port 23 tests can be troublesome. If there is something listening at that port, these tests frequently will hang until timeout occurs. I ought to investigate whether there is some way they all can be combined into some smarter, optimized test.

Severely overloaded proxies are prone to false negatives. That is, pxytest might fail to connect because the proxy is throttled or dropping connections or otherwise busy puking its guts out. So it will declare this proxy as closed, even though a repeated attempt might prove otherwise.


services(5), httpd(8), sockd(8)


I found the following programs helpful in developing this utility.
Blitzed Open Proxy Monitor <>
Proxy Stress Tester <>


 Chip Rosenthal
 Unicom Systems Development

 $Id: pxytest,v 1.36 2002/12/28 20:56:55 chip Exp $
 See <> for latest version.

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