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https://github.com/robertdavidgraham/masscan

TCP port scanner, spews SYN packets asynchronously, scanning entire Internet in under 5 minutes.
https://github.com/robertdavidgraham/masscan

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TCP port scanner, spews SYN packets asynchronously, scanning entire Internet in under 5 minutes.

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[![unittests](https://github.com/robertdavidgraham/masscan/actions/workflows/unittests.yml/badge.svg?branch=master)](https://github.com/robertdavidgraham/masscan/actions/workflows/unittests.yml/?branch=master)



# MASSCAN: Mass IP port scanner



This is an Internet-scale port scanner. It can scan the entire Internet

in under 5 minutes, transmitting 10 million packets per second,

from a single machine.



Its usage (parameters, output) is similar to `nmap`, the most famous port scanner.

When in doubt, try one of those features -- features that support widespread

scanning of many machines are supported, while in-depth scanning of single

machines aren't.



Internally, it uses asynchronous transmission, similar to port scanners

like  `scanrand`, `unicornscan`, and `ZMap`. It's more flexible, allowing

arbitrary port and address ranges.



NOTE: masscan uses its own **ad hoc TCP/IP stack**. Anything other than

simple port scans may cause conflict with the local TCP/IP stack. This means you 

need to use either the `--src-ip` option to run from a different IP address, or

use `--src-port` to configure which source ports masscan uses, then also

configure the internal firewall (like `pf` or `iptables`) to firewall those ports

from the rest of the operating system.



This tool is free, but consider contributing money to its development:

Bitcoin wallet address: 1MASSCANaHUiyTtR3bJ2sLGuMw5kDBaj4T



# Building



On Debian/Ubuntu, it goes something like the following. It doesn't

really have any dependencies other than a C compiler (such as `gcc`

or `clang`).



	sudo apt-get --assume-yes install git make gcc

	git clone https://github.com/robertdavidgraham/masscan

	cd masscan

	make



This puts the program in the `masscan/bin` subdirectory. 

To install it (on Linux) run:



    make install



The source consists of a lot of small files, so building goes a lot faster

by using the multi-threaded build. This requires more than 2gigs on a 

Raspberry Pi (and breaks), so you might use a smaller number, like `-j4` rather than

all possible threads.



	make -j



While Linux is the primary target platform, the code runs well on many other

systems (Windows, macOS, etc.). Here's some additional build info:



  * Windows w/ Visual Studio: use the VS10 project

  * Windows w/ MinGW: just type `make`

  * Windows w/ cygwin: won't work

  * Mac OS X /w XCode: use the XCode4 project

  * Mac OS X /w cmdline: just type `make`

  * FreeBSD: type `gmake`

  * other: try just compiling all the files together, `cc src/*.c -o bin/masscan`



On macOS, the x86 binaries seem to work just as fast under ARM emulation.



# Usage



Usage is similar to `nmap`. To scan a network segment for some ports:



	# masscan -p80,8000-8100 10.0.0.0/8 2603:3001:2d00:da00::/112



This will:

* scan the `10.x.x.x` subnet, and `2603:3001:2d00:da00::x` subnets

* scans port 80 and the range 8000 to 8100, or 102 ports total, on both subnets

* print output to `` that can be redirected to a file



To see the complete list of options, use the `--echo` feature. This

dumps the current configuration and exits. This output can be used as input back

into the program:



	# masscan -p80,8000-8100 10.0.0.0/8 2603:3001:2d00:da00::/112 --echo > xxx.conf

	# masscan -c xxx.conf --rate 1000



## Banner checking



Masscan can do more than just detect whether ports are open. It can also

complete the TCP connection and interaction with the application at that

port in order to grab simple "banner" information.



Masscan supports banner checking on the following protocols:

  * FTP

  * HTTP

  * IMAP4

  * memcached

  * POP3

  * SMTP

  * SSH

  * SSL

  * SMBv1

  * SMBv2

  * Telnet

  * RDP

  * VNC



The problem with this is that masscan contains its own TCP/IP stack

separate from the system you run it on. When the local system receives

a SYN-ACK from the probed target, it responds with a RST packet that kills

the connection before masscan can grab the banner.



The easiest way to prevent this is to assign masscan a separate IP

address. This would look like one of the following examples:



	# masscan 10.0.0.0/8 -p80 --banners --source-ip 192.168.1.200

      # masscan 2a00:1450:4007:810::/112 -p80 --banners --source-ip 2603:3001:2d00:da00:91d7:b54:b498:859d



The address you choose has to be on the local subnet and not otherwise

be used by another system. Masscan will warn you that you've made a

mistake, but you might've messed up the other machine's communications

for several minutes, so be careful.



In some cases, such as WiFi, this isn't possible. In those cases, you can

firewall the port that masscan uses. This prevents the local TCP/IP stack

from seeing the packet, but masscan still sees it since it bypasses the

local stack. For Linux, this would look like:



	# iptables -A INPUT -p tcp --dport 61000 -j DROP

	# masscan 10.0.0.0/8 -p80 --banners --source-port 61000



You probably want to pick ports that don't conflict with ports Linux might otherwise

choose for source-ports. You can see the range Linux uses, and reconfigure

that range, by looking in the file:



    /proc/sys/net/ipv4/ip_local_port_range



On the latest version of Kali Linux (2018-August), that range is  32768  to  60999, so

you should choose ports either below 32768 or 61000 and above.



Setting an `iptables` rule only lasts until the next reboot. You need to lookup how to

save the configuration depending upon your distro, such as using `iptables-save` 

and/or `iptables-persistent`.



On Mac OS X and BSD, there are similar steps. To find out the ranges to avoid,

use a command like the following:



    # sysctl net.inet.ip.portrange.first net.inet.ip.portrange.last



On FreeBSD and older MacOS, use an `ipfw` command: 



	# sudo ipfw add 1 deny tcp from any to any 40000 in

	# masscan 10.0.0.0/8 -p80 --banners --source-port 40000



On newer MacOS and OpenBSD, use the `pf` packet-filter utility. 

Edit the file `/etc/pf.conf` to add a line like the following:



    block in proto tcp from any to any port 40000:40015

    

Then to enable the firewall, run the command:

    

    # pfctl -E    



If the firewall is already running, then either reboot or reload the rules

with the following command:



    # pfctl -f /etc/pf.conf



Windows doesn't respond with RST packets, so neither of these techniques

are necessary. However, masscan is still designed to work best using its

own IP address, so you should run that way when possible, even when it is

not strictly necessary.



The same thing is needed for other checks, such as the `--heartbleed` check,

which is just a form of banner checking.



## How to scan the entire Internet



While useful for smaller, internal networks, the program is really designed

with the entire Internet in mind. It might look something like this:



	# masscan 0.0.0.0/0 -p0-65535



Scanning the entire Internet is bad. For one thing, parts of the Internet react

badly to being scanned. For another thing, some sites track scans and add you

to a ban list, which will get you firewalled from useful parts of the Internet.

Therefore, you want to exclude a lot of ranges. To blacklist or exclude ranges,

you want to use the following syntax:



	# masscan 0.0.0.0/0 -p0-65535 --excludefile exclude.txt



This just prints the results to the command-line. You probably want them

saved to a file instead. Therefore, you want something like:



	# masscan 0.0.0.0/0 -p0-65535 -oX scan.xml



This saves the results in an XML file, allowing you to easily dump the

results in a database or something.



But, this only goes at the default rate of 100 packets/second, which will

take forever to scan the Internet. You need to speed it up as so:



	# masscan 0.0.0.0/0 -p0-65535 --max-rate 100000



This increases the rate to 100,000 packets/second, which will scan the

entire Internet (minus excludes) in about 10 hours per port (or 655,360 hours

if scanning all ports).



The thing to notice about this command-line is that these are all `nmap`

compatible options. In addition, "invisible" options compatible with `nmap`

are also set for you: `-sS -Pn -n --randomize-hosts --send-eth`. Likewise,

the format of the XML file is inspired by `nmap`. There are, of course, a

lot of differences, because the *asynchronous* nature of the program

leads to a fundamentally different approach to the problem.



The above command-line is a bit cumbersome. Instead of putting everything

on the command-line, it can be stored in a file instead. The above settings

would look like this:



	# My Scan

	rate =  100000.00

	output-format = xml

	output-status = all

	output-filename = scan.xml

	ports = 0-65535

	range = 0.0.0.0-255.255.255.255

	excludefile = exclude.txt



To use this configuration file, use the `-c`:



	# masscan -c myscan.conf



This also makes things easier when you repeat a scan.



By default, masscan first loads the configuration file 

`/etc/masscan/masscan.conf`. Any later configuration parameters override what's

in this default configuration file. That's where I put my "excludefile" 

parameter so that I don't ever forget it. It just works automatically.



## Getting output



By default, masscan produces fairly large text files, but it's easy 

to convert them into any other format. There are five supported output formats:



1. xml:  Just use the parameter `-oX `. 

	Or, use the parameters `--output-format xml` and `--output-filename `.



2. binary: This is the masscan builtin format. It produces much smaller files so that

when I scan the Internet my disk doesn't fill up. They need to be parsed,

though. The command-line option `--readscan` will read binary scan files.

Using `--readscan` with the `-oX` option will produce an XML version of the 

results file.



3. grepable: This is an implementation of the Nmap -oG

output that can be easily parsed by command-line tools. Just use the

parameter `-oG `. Or, use the parameters `--output-format grepable` and

`--output-filename `.



4. json: This saves the results in JSON format. Just use the

parameter `-oJ `. Or, use the parameters `--output-format json` and

`--output-filename `.



5. list: This is a simple list with one host and port pair 

per line. Just use the parameter `-oL `. Or, use the parameters 

`--output-format list` and `--output-filename `. The format is:



	```

	      

	open tcp 80 XXX.XXX.XXX.XXX 1390380064

	```	



## Comparison with Nmap



Where reasonable, every effort has been taken to make the program familiar

to `nmap` users, even though it's fundamentally different. Masscan is tuned

for wide range scanning of a lot of machines, whereas nmap is designed for

intensive scanning of a single machine or a small range.



Two important differences are:



* no default ports to scan, you must specify `-p `

* target hosts are IP addresses or simple ranges, not DNS names, nor 

  the funky subnet ranges `nmap` can use (like `10.0.0-255.0-255`).



You can think of `masscan` as having the following settings permanently

enabled:

* `-sS`: this does SYN scan only (currently, will change in the future)

* `-Pn`: doesn't ping hosts first, which is fundamental to the async operation

* `-n`: no DNS resolution happens

* `--randomize-hosts`: scan completely randomized, always, you can't change this

* `--send-eth`: sends using raw `libpcap`



If you want a list of additional `nmap` compatible settings, use the following

command:



	# masscan --nmap



## Transmit rate (IMPORTANT!!)



This program spews out packets very fast. On Windows, or from VMs,

it can do 300,000 packets/second. On Linux (no virtualization) it'll

do 1.6 million packets-per-second. That's fast enough to melt most networks.



Note that it'll only melt your own network. It randomizes the target

IP addresses so that it shouldn't overwhelm any distant network.



By default, the rate is set to 100 packets/second. To increase the rate to

a million use something like `--rate 1000000`.



When scanning the IPv4 Internet, you'll be scanning lots of subnets,

so even though there's a high rate of packets going out, each

target subnet will receive a small rate of incoming packets.



However, with IPv6 scanning, you'll tend to focus on a single

target subnet with billions of addresses. Thus, your default

behavior will overwhelm the target network. Networks often

crash under the load that masscan can generate.



# Design



This section describes the major design issues of the program.



## Code Layout



The file `main.c` contains the `main()` function, as you'd expect. It also

contains the `transmit_thread()` and `receive_thread()` functions. These

functions have been deliberately flattened and heavily commented so that you

can read the design of the program simply by stepping line-by-line through

each of these.



## Asynchronous



This is an *asynchronous* design. In other words, it is to `nmap` what

the `nginx` web-server is to `Apache`. It has separate transmit and receive

threads that are largely independent from each other. It's the same sort of

design found in `scanrand`, `unicornscan`, and `ZMap`.



Because it's asynchronous, it runs as fast as the underlying packet transmit

allows.



## Randomization



A key difference between Masscan and other scanners is the way it randomizes

targets.



The fundamental principle is to have a single index variable that starts at

zero and is incremented by one for every probe. In C code, this is expressed

as:



    for (i = 0; i < range; i++) {

        scan(i);

    }



We have to translate the index into an IP address. Let's say that you want to

scan all "private" IP addresses. That would be the table of ranges like:

    

    192.168.0.0/16

    10.0.0.0/8

    172.16.0.0/12



In this example, the first 64k indexes are appended to 192.168.x.x to form

the target address. Then, the next 16-million are appended to 10.x.x.x.

The remaining indexes in the range are applied to 172.16.x.x.



In this example, we only have three ranges. When scanning the entire Internet,

we have in practice more than 100 ranges. That's because you have to blacklist

or exclude a lot of sub-ranges. This chops up the desired range into hundreds

of smaller ranges.



This leads to one of the slowest parts of the code. We transmit 10 million

packets per second and have to convert an index variable to an IP address

for each and every probe. We solve this by doing a "binary search" in a small

amount of memory. At this packet rate, cache efficiencies start to dominate

over algorithm efficiencies. There are a lot of more efficient techniques in

theory, but they all require so much memory as to be slower in practice.



We call the function that translates from an index into an IP address

the `pick()` function. In use, it looks like:



    for (i = 0; i < range; i++) {

        ip = pick(addresses, i);

        scan(ip);

    }



Masscan supports not only IP address ranges, but also port ranges. This means

we need to pick from the index variable both an IP address and a port. This

is fairly straightforward:



    range = ip_count * port_count;

    for (i = 0; i < range; i++) {

        ip   = pick(addresses, i / port_count);

        port = pick(ports,     i % port_count);

        scan(ip, port);

    }



This leads to another expensive part of the code. The division/modulus

instructions are around 90 clock cycles, or 30 nanoseconds, on x86 CPUs. When

transmitting at a rate of 10 million packets/second, we have only

100 nanoseconds per packet. I see no way to optimize this any better. Luckily,

though, two such operations can be executed simultaneously, so doing two 

of these, as shown above, is no more expensive than doing one.



There are actually some easy optimizations for the above performance problems,

but they all rely upon `i++`, the fact that the index variable increases one

by one through the scan. Actually, we need to randomize this variable. We

need to randomize the order of IP addresses that we scan or we'll blast the

heck out of target networks that aren't built for this level of speed. We 

need to spread our traffic evenly over the target.



The way we randomize is simply by encrypting the index variable. By definition,

encryption is random and creates a 1-to-1 mapping between the original index

variable and the output. This means that while we linearly go through the

range, the output IP addresses are completely random. In code, this looks like:



    range = ip_count * port_count;

    for (i = 0; i < range; i++) {

        x = encrypt(i);

        ip   = pick(addresses, x / port_count);

        port = pick(ports,     x % port_count);

        scan(ip, port);

    }



This also has a major cost. Since the range is an unpredictable size instead

of a nice even power of 2, we can't use cheap binary techniques like

AND (&) and XOR (^). Instead, we have to use expensive operations like 

MODULUS (%). In my current benchmarks, it's taking 40 nanoseconds to

encrypt the variable.



This architecture allows for lots of cool features. For example, it supports

"shards". You can set up 5 machines each doing a fifth of the scan or

`range / shard_count`. Shards can be multiple machines, or simply multiple

network adapters on the same machine, or even (if you want) multiple IP

source addresses on the same network adapter.



Or, you can use a 'seed' or 'key' to the encryption function, so that you get

a different order each time you scan, like `x = encrypt(seed, i)`.



We can also pause the scan by exiting out of the program, and simply

remembering the current value of `i`, and restart it later. I do that a lot

during development. I see something going wrong with my Internet scan, so

I hit  to stop the scan, then restart it after I've fixed the bug.



Another feature is retransmits/retries. Packets sometimes get dropped on the

Internet, so you can send two packets back-to-back. However, something that

drops one packet may drop the immediately following packet. Therefore, you

want to send the copy about 1 second apart. This is simple. We already have

a 'rate' variable, which is the number of packets-per-second rate we are

transmitting at, so the retransmit function is simply to use `i + rate`

as the index. One of these days I'm going to do a study of the Internet,

and differentiate "back-to-back", "1 second", "10 second", and "1 minute"

retransmits this way in order to see if there is any difference in what

gets dropped.



## C10 Scalability



The asynchronous technique is known as a solution to the "c10k problem".

Masscan is designed for the next level of scalability, the "C10M problem".



The C10M solution is to bypass the kernel. There are three primary kernel

bypasses in Masscan:

* custom network driver

* user-mode TCP stack

* user-mode synchronization



Masscan can use the PF_RING DNA driver. This driver DMAs packets directly

from user-mode memory to the network driver with zero kernel involvement.

That allows software, even with a slow CPU, to transmit packets at the maximum

rate the hardware allows. If you put 8 10-gbps network cards in a computer,

this means it could transmit at 100-million packets/second.



Masscan has its own built-in TCP stack for grabbing banners from TCP

connections. This means it can easily support 10 million concurrent TCP

connections, assuming of course that the computer has enough memory.



Masscan has no "mutex". Modern mutexes (aka. futexes) are mostly user-mode,

but they have two problems. The first problem is that they cause cache-lines

to bounce quickly back-and-forth between CPUs. The second is that when there

is contention, they'll do a system call into the kernel, which kills

performance. A mutex on the fast path of a program severely limits scalability.

Instead, Masscan uses "rings" to synchronize things, such as when the

user-mode TCP stack in the receive thread needs to transmit a packet without

interfering with the transmit thread.



## Portability



The code runs well on Linux, Windows, and Mac OS X. All the important bits are

in standard C (C90). Therefore, it compiles on Visual Studio with Microsoft's

compiler, the Clang/LLVM compiler on Mac OS X, and GCC on Linux.



Windows and Macs aren't tuned for packet transmit, and get only about 300,000

packets-per-second, whereas Linux can do 1,500,000 packets/second. That's

probably faster than you want anyway.



## Safe code



A bounty is offered for vulnerabilities, see the VULNINFO.md file for more

information.



This project uses safe functions like `safe_strcpy()` instead of unsafe functions

like `strcpy()`.



This project has automated unit regression tests (`make regress`).



## Compatibility



A lot of effort has gone into making the input/output look like `nmap`, which

everyone who does port scans is (or should be) familiar with.



## IPv6 and IPv4 coexistence



Masscan supports IPv6, but there is no special mode, both are supported

at the same time. (There is no `-6` option -- it's always available).



In any example you see of masscan usage,

simply put an IPv6 address where you see an IPv4 address. You can include

IPv4 and IPv6 addresses simultaneously in the same scan. Output includes

the appropriate address at the same location, with no special marking.



Just remember that IPv6 address space is really big. You probably don't want to scan

for big ranges, except maybe the first 64k addresses of a subnet that were assigned

via DHCPv6.



Instead, you'll probably want to scan large lists of addresses stored

in a file (`--include-file filename.txt`) that you got from other sources.

Like everywhere else, this file can contain lists of both IPv4 and IPv6 addresses.

The test file I use contains 8 million addresses. Files of that size need a couple

extra seconds to be read on startup (masscan sorts the addresses and removes

duplicates before scanning).



Remember that masscan contains its own network stack. Thus, the local machine

you run masscan from does not need to be IPv6 enabled -- though the local

network needs to be able to route IPv6 packets.



## PF_RING



To get beyond 2 million packets/second, you need an Intel 10-gbps Ethernet

adapter and a special driver known as ["PF_RING ZC" from ntop](http://www.ntop.org/products/packet-capture/pf_ring/pf_ring-zc-zero-copy/). Masscan doesn't need to be rebuilt in order to use PF_RING. To use PF_RING,

you need to build the following components:



  * `libpfring.so` (installed in /usr/lib/libpfring.so)

  * `pf_ring.ko` (their kernel driver)

  * `ixgbe.ko` (their version of the Intel 10-gbps Ethernet driver)



You don't need to build their version of `libpcap.so`.



When Masscan detects that an adapter is named something like `zc:enp1s0` instead

of something like `enp1s0`, it'll automatically switch to PF_RING ZC mode.



A more detail discussion can be found in **PoC||GTFO 0x15**.



## Regression testing



The project contains a built-in unit test:



    $ make test

    bin/masscan --selftest

    selftest: success!



This tests a lot of tricky bits of the code. You should do this after building.



## Performance testing



To test performance, run something like the following to a throw-away address,

to avoid overloading your local router:



    $ bin/masscan 0.0.0.0/4 -p80 --rate 100000000 --router-mac 66-55-44-33-22-11



The bogus `--router-mac` keeps packets on the local network segments so that

they won't go out to the Internet.



You can also test in "offline" mode, which is how fast the program runs

without the transmit overhead:



    $ bin/masscan 0.0.0.0/4 -p80 --rate 100000000 --offline

    

This second benchmark shows roughly how fast the program would run if it were

using PF_RING, which has near zero overhead.



By the way, the randomization algorithm makes heavy use of "integer arithmetic",

a chronically slow operation on CPUs. Modern CPUs have doubled the speed

at which they perform this calculation, making `masscan` much faster.



# Authors



This tool created by Robert Graham:

email: [email protected]

twitter: @ErrataRob



# License



Copyright (c) 2013 Robert David Graham



This program is free software: you can redistribute it and/or modify

it under the terms of the GNU Affero General Public License as published by

the Free Software Foundation, version 3 of the License.



This program is distributed in the hope that it will be useful,

but WITHOUT ANY WARRANTY; without even the implied warranty of

MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the

GNU Affero General Public License for more details.



You should have received a copy of the GNU Affero General Public License

along with this program.  If not, see .