MASSCAN: Mass IP port scanner

This is the fastest Internet port scanner. It can scan the entire Internet in under 6 minutes, transmitting 10 million packets per second.

It produces results similar to nmap, the most famous port scanner. Internally, it operates more like scanrand, unicornscan, and ZMap, using asynchronous transmission. The major difference is that it's faster than these other scanners. In addition, it's more flexible, allowing arbitrary address ranges and port ranges.

Building

On Debian/Ubuntu, it goes something like this:

$ git clone https://github.com/robertdavidgraham/masscan
$ cd masscan
$ sudo apt-get install libpcap-dev
$ make

This puts the program in the masscan/bin subdirectory. You'll have to manually copy it to something like /usr/local/bin if you want to install it elsewhere on the system.

While Linux is the primary target platform, the code runs well on many other systems. Here's some additional build info:

• Windows w/ Visual Studio: use the VS10 project
• Windows w/ MingGW: 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: I don't know, don't care

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 DNA" from http://www.ntop.org/products/pf_ring/. 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 dna0 instead of something like eth0, it'll automatically switch to PF_RING mode.

Regression testing

The project contains a built-in self-test:

$ make regress
bin/masscan --regress
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:

$ 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.

Usage

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

# masscan -p80,8000-8100 10.0.0.0/8

This will:

• scan the 10.x.x.x subnet, all 16 million addresses
• scans port 80 and the range 8000 to 8100, or 102 addresses total
• print output to <stdout> 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 --echo > xxx.conf
# masscan -c xxx.conf --rate 1000

How to scan the entire Internet

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

The are five primary formats for output.

1. xml: The default option also prodces fairly large files, but is easy to import into anything. Just use the parameter -oX <filename>. Or, use the parameters --output-format xml and --output-filename <filename>.

2. binary: This is the masscan builtin format. This 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 a XML version of the results file.

3. grepable: This is an implementation of the Nmap -oG output and can be easily parsed by command-line tools. Just use the parameter -oG <filename>. Or, use the parameters --output-format grepable and --output-filename <filename>.

4. json: This saves the results in a json format. Just use the parameter -oJ <filename>. Or, use the parameters --output-format json and --output-filename <filename>.

5. list: This is a simple list with one host and port pair per line. Just use the parameter -oL <filename>. Or, use the parameters --output-format list and --output-filename <filename>. The format is:

   <port state> <protocol> <port number> <IP address> <POSIX timestamp>  
   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. Two important differences are:

• no default ports to scan, you must specify -p <ports>
• 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
• --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.

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
172.16.0.0/20

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 setup 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. Mutexes 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). It therefore 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 strcpy_s() 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.

Authors

This tool created by Robert Graham: email: robert_david_graham@yahoo.com twitter: @ErrataRob