doscan1

Usage

Overview

doscan is a tool to discover TCP services ony our network. It is designed for scanning a single ports on a large network. (There are better tools for scanning many ports on a small set of hosts, for example nmap(8).)

The prefix parameter instructs doscan to scan all addresses in this prefix. Prefix notation is, as usual, A.B.C.D/L, where A.B.C.D is an IP address in dotted-quad notation, and L is a prefix length from 1 to 32. If the /L part is omitted, /32 is assumed (and a single host is scanned).

doscan uses a random scatter technology to distribute the load across the network. Within a given prefix, hosts are not scanned sequentially, but in a random-looking, but reproducible order. As a result, doscan will not stress-test the network edge (just the next hop). (The prefixes themselves are scanned in order.)

Options

The --port option is mandatory, all other options are optional.

-a timeout, --add-timeout timeout

-A count, --add-burst count

These options specify the timeout (in milliseconds) before new connections are added, and the number of new connections or hosts to add in one burst. Each timeout millisecond, count new hosts are contacted. (The per-host timeout controlled by the --timeout option is independent. It specifies the timeout once the first packet has been sent.)

-b count, --banner count

doscan reads at most count bytes from the remote host. The exact effect of this option varies among protocol modules, see the PROTOCOL MODULES section for details.

-c count, --connections count

At most count connections are established in parallel. See CAVEATS below for problems resulting from system file descriptor limits, and instructions for choosing this parameter. By default, at most 50 parallel connections are established.

-E, --no-epoll

Do not use the epoll kernel interface even if it's available (useful for debugging).

-f, --file name

doscan reads prefixes from the file name, in addition to the command line. The file shall contain one prefix per line. See the DESCRIPTION section above for the prefix format. To better distribute scanning of long prefix lists, all prefixies are reorded randomly if the --file option is used.

-i, --indicator

Display a progress indicator. If doscan is invoked with this option, the number of connections which have been established so far, the total number of addresses to be scanned, the number of currently active connections, and the number of hosts for which a report entry has been generated are displayed periodically.

-n, --net-errors

Instructs doscan to report network errors even if they prevent a connection. Normally, such errors are suppressed.

-o format, --output format

This option changes the format which doscan uses to report its findings. See the OUTPUT FORMAT section below for details.

-p port, --port port

The --port option controls to which TCP port doscan connects when scanning a host.

--protocol Istring, -P Istring

Chooses the protocol module string. See the PROTOCOL MODULES section for information on available protocol modules.

--send string, -s string

--receive regexp, -r regexp

The effects of these options depend on the protocol module. See the PROTOCOL MODULES section for details.

--style style, -S style

This option controls the output style. See the OUTPUT FORMAT section for details.

-t timeout, --timeout timeout

This option sets the connect timeout to timeout milliseconds. If this time passes without a successfully established connection, doscan skips the hosts.

-v, --verbose

Turn on additional reporting to standard error.

-h, --help

Display help message and exit.

-V, --version

Output version information and exit.

Protocol modules

doscan supports several protocol modules. By default, the generic tcp module is used, but you can choose another module using the --protocol option. The effect of the --banner, --send and --receive options depends on the protocol module. Available modules include:

http

This module causes doscan to connect to HTTP servers, send a request, and collect the server identification from the response.

The --banner option specifies the maximum receive buffer size. It defaults to 4000 bytes.

The --send option specifies the request that is send to the server. The string can include C escape sequences to send control characters. By default, the request GET / HTTP/1.0\r\n\r\n (that is, GET / HTTP/1.0 followed by the four characters CR, LF, CR, LF) is sent.

The --receive option is not supported by this protocol module.

http_proxy

This protocol module probes hosts for open HTTP proxies. The --port option controls the port that is probed. The required --receive option must be an integer in the range from 1 to 65534, the number specifies the port on which doscan listens for the connections from open proxies. The required --send option specifies the HTTP request method, either "GET" or "CONNECT".

The --banner option is not supported by this protocol module.

Warning: In the worst case, the amount of file descriptors is slightly more than twice the number of parallel connections given by the --connections options. The additional file descriptors are used by doscan's HTTP server component to process the connections from open proxies.

See the EXAMPLES section for some convenient combinations of those otions.

ident_check

This protocol module reports hosts which have TCP service listening on the specified port which is not a proper IDENT/AUTH daemon. It is most useful with a --port 113 command line argument. None of the --banner, --receive and --send options are supported.

tcp

This module is intended for generic TCP service probing and fingerprinting.

The --banner option controls the maximum length of banner strings which are collected. If its argument is zero or if the option is not specified, no banner strings are collected. In this case, doscan closes connections immediately after they have been established (which results in an increased scanning rate).

After establishing a connection, doscans sends the string specified by the --send option to the remote host. The string can contain the usual C escape sequences (including \000), to send non-printable characters.

The --receive option specifies a Perl-compatible regular expression (PCRE), and doscan uses it to analyze the data returned by a remote host. The regular expression may contain at least one capturing subpattern, it is always anchored at the beginning of the received data. The character . (period) matches all characters (including newline). $ (dollar sign) matches the very end of the received data (which may, however, still be incomplete). See pcrepattern(3) for details about the syntax of Perl-compatible regular expression.

The --receive regular expression is used by doscan for several purposes. If data is received from a remote host, and if the regular expression ends with $, doscan immediately closes the connection if all the data received so far from this host matches the regular expression. (doscan assumes that the reply is complete; increased scanning speed is the result.) When a connection is terminated for any reason, doscan checks if the regular expression matches the collected data. If it doesn't, a no match error is recorded (if no other error occured). If it does, and the regular expression contains a capturing subpattern, that subpattern is recorded. Otherwise, the whole data is recorded.

In order to use the --receive option, you have to specify the --banner option as well.

udp

This module is a generic UDP scanner, as far such a thing is possible. It sends up to five UDP packets (whose payload is controlled by the mandatory --send option) to the specified port. Replies are collected. The --banner option is implicit and set to the maximum payload size. Retransmission is stopped when the first reply is received.

In verbose mode (with both --verbose and --net-errors options), a warning like "stray UDP packet from 192.0.2.4:7" is printed to standard error when an unexpected UDP packets is received. Packets to sent to network or broadcast adresses trigger such packets, and poorly implemented UDP services on multi-homed machines answer with a different source IP address.

Output format

doscan prints all gathered data about scanned prefixes to standard output, just before the program terminates. The output format can be changed with the --output option. The format argument of this option is a string which includes % substitions, similar to printf(3). The following substitions are supported

%%

A literal percent character.

%a

The address of the remote host.

%b

The banner return by the host.

%e

The error code as a string, empty if no error occurred while scanning the host.

This is either a system error constant (such as ECONNREFUSED), or the string unknown (unknown error code). If the --receive option is active and the received data does not match the specified regular expresion, and no other error has occured, the column contains no match.

%E

The numeric error code corresponding to the %e error message, or zero if no error occurred. Negativ error numbers are returned for internal errors (such as a failed match against the --receive regular expression).

%n

The host name corresponding to the scanned IP address (based on a DNS lookup). Note that this slows down reporting a lot, in general. For this reason, it is not recommended to use %n together with --style unsorted.

%N

A verbatim ASCII LF (newline) character.

%r

The time when the information was gathered, measured in seconds since the scanning started.

%t

The time when the information was gathered, in local time.

%T

Same as %t, but in UTC (also known as GMT).

%%

A verbatim percent sign (%).

The default value for the --output option is %T\t%a\t%e\t%b, where \t denotes an ASCII HTAB character.

The --style or -S option supports the following arguments:

sort-by-host

The output is sorted by the IP address of the scanned host. (This is the default.)

unsorted

The output is not sorted and appears in the order the hosts responded.

Caution: Do not use this style together with an --output argument which includes %n, and do not pipe the output of doscan to a process which cannot read its standard input quickly. Output is performed synchronously, and if it is delayed, this might impact the scanning activity.

In all cases except unsorted, output is delayed just before the termination of the program.

Examples

doscan --banner 100 --port 13 192.0.2.1

Prints the time on the host 192.0.2.1 (if it runs a daytime server).

doscan --banner 100 --receive '(.*)\n$' --port 22 192.0.2.0/24

Scan for SSH servers and record the banners (usually containing version information about the SSH server).

doscan --banner 200 --receive '(.*?)\r?\n$' --port 25 192.0.2.0/24

Scan for SMTP servers and record their greeting messages. Works for FTP as well, with --port 21 instead of --port 25.

doscan --banner 2000 --send 'GET / HTTP/1.0\r\n\r\n' \

--receive '.*?\nServer: *([^\r\n]*) *\r?\n.*$' \

--port 80 192.0.2.0/24

Scan for HTTP servers and record their version strings.

doscan --protocol http_proxy --port 3128 \

--send GET --receive 80 192.0.2.0/24

Scan for open proxies on TCP port 3128, using the GET HTTP request method. Try to connect back to port 80 on the scanning host.

It is recommended that you use port 80 for the listening port if you scan using GET requests. For CONNECT requests, port 443 should be used (see below). Some administrators might restrict CONNECT to TCP port 443 (or filter it for the GET request method), so these choices give best results.

doscan --protocol http_proxy --port 8080 \

--send CONNECT --receive 443 192.0.2.0/24

Scan for open proxies on TCP port 8080, using the CONNECT HTTP request method. Try to connect back to port 443 on the scanning host.

Caveats

The most important option for tuning is --connections. Increasing this option can greatly increase scanning performance. However, there a two caveats: Many connections require many sockets, and your system might not support so many of them. Furthermore, a large number of parallel connections generates significant numbers of packets, and a high CPU load, which can both lead to spurious connection failures (false negatives).

To increase the number of connections your system can process, you usually have to raise the corresponding ulimit value in your shell, which requires root privileges. For example, in bash(1), you can invoke

ulimit -n 10030

to raise the descriptor limit to 10030. You can then pass --connections 10000 to doscan. (Some file descriptors are not used for scanning, but have to be open nonetheless, and count towards the ulimit -n limit.)

On Linux-based systems, you might have to adjust some sysctl values which control system-wide descriptor limits. Refer to sysctl.conf(5), the Documentation directory in the Linux source tree, or the source code itself for details.

Note, however, that if you increase the number of parallel connections beyond a certain value, you will lose some hosts, that is they will not be reported even though they are running a service on the scanned port. Therefore, you should watch both network and CPU utilization to detect bottlenecks. Although the random scatter technique employed by doscan tries to split the load across your whole network, this obviously fails if the next hop cannot bear the traffic.