Sx – Fast, Modern, Easy-To-Use Network Scanner

sx is the command-line network scanner designed to follow the UNIX philosophy.

The goal of this project is to create the fastest network scanner with clean and simple code.

• 30x times faster than nmap

• ARP scan: Scan your local networks to detect live devices
• ICMP scan: Use advanced ICMP scanning techniques to detect live hosts and firewall rules
• TCP SYN scan: Traditional half-open scan to find open TCP ports
• TCP FIN / NULL / Xmas scans: Scan techniques to bypass some firewall rules
• Custom TCP scans with any TCP flags: Send whatever exotic packets you want and get a result with all the TCP flags set in the reply packet
• UDP scan: Scan UDP ports and get full ICMP replies to detect open ports or firewall rules
• Application scans:
  • SOCKS5 scan: Detect live SOCKS5 proxies by scanning ip range or list of ip/port pairs from a file
  • Docker scan: Detect open Docker daemons listening on TCP ports and get information about the docker node
  • Elasticsearch scan: Detect open Elasticsearch nodes and pull out cluster information with all index names
• Randomized iteration over IP addresses using finite cyclic multiplicative groups
• JSON output support: sx is designed specifically for convenient automatic processing of results

The simplest way is to download from GitHub Releases and place the executable file in your PATH.

Requirements:

• Go 1.15 or newer
• libpcap (already installed if you use wireshark)

From the root of the source tree, run:

go build

Here’s a quick examples showing how you can scan networks with sx.

ARP scan

Scan your local network and display the IP address, MAC address and associated hardware vendor of connected devices:

sx arp 192.168.0.1/24

sample output:

192.168.0.1          b0:be:76:40:05:8d    TP-LINK TECHNOLOGIES CO.,LTD.
192.168.0.111        80:c5:f2:0b:02:e3    AzureWave Technology Inc.
192.168.0.171        88:53:95:2d:3c:af    Apple, Inc.

with JSON output:

sx arp --json 192.168.0.1/24

sample output:

{"ip":"192.168.0.1","mac":"b0:be:76:40:05:8d","vendor":"TP-LINK TECHNOLOGIES CO.,LTD."}
{"ip":"192.168.0.111","mac":"80:c5:f2:0b:02:e3","vendor":"AzureWave Technology Inc."}
{"ip":"192.168.0.171","mac":"88:53:95:2d:3c:af","vendor":"Apple, Inc."}

wait 5 seconds before exiting to receive delayed reply packets, by default sx waits 300 milliseconds:

sx arp --exit-delay 5s 192.168.0.1/24

Live scan mode that rescans network every 10 seconds:

sx arp 192.168.0.1/24 --live 10s

TCP scan

Unlike nmap and other scanners that implicitly perform ARP requests to resolve IP addresses to MAC addresses before the actual scan, sx explicitly uses the ARP cache concept. ARP cache file is a simple text file containing JSON string on each line (JSONL file), which has the same JSON fields as the ARP scan JSON output described above. Scans of higher-level protocols like TCP and UDP read the ARP cache file from the stdin and then start the actual scan.

This not only simplifies the design of the program, but also speeds up the scanning process, since it is not necessary to perform an ARP scan every time.

Let’s assume that the actual ARP cache is in the arp.cache file. We can create it manually or use ARP scan as shown below:

sx arp 192.168.0.1/24 --json | tee arp.cache

Once we have the ARP cache file, we can run scans of higher-level protocols like TCP SYN scan:

cat arp.cache | sx tcp -p 1-65535 192.168.0.171

sample output:

192.168.0.171        22
192.168.0.171        443

In this case we find out that ports 22 and 443 are open.

scan with JSON output:

cat arp.cache | sx tcp  --json -p 1-65535 192.168.0.171

sample output:

{"scan":"tcpsyn","ip":"192.168.0.171","port":22}
{"scan":"tcpsyn","ip":"192.168.0.171","port":443}

scan multiple port ranges:

cat arp.cache | sx tcp -p 1-23,25-443 192.168.0.171

or individual ports:

cat arp.cache | sx tcp -p 22,443 192.168.0.171

scan ip/port pairs from a file with JSON output:

cat arp.cache | sx tcp --json -f ip_ports_file.jsonl

Each line of the input file is a json string, which must contain the ip and port fields.

sample input file:

{"ip":"10.0.1.1","port":1080}
{"ip":"10.0.2.2","port":1081}

It is possible to specify the ARP cache file using the -a or --arp-cache options:

sx tcp -a arp.cache -p 22,443 192.168.0.171

or stdin redirect:

sx tcp -p 22,443 192.168.0.171 < arp.cache

You can also use the tcp syn subcommand instead of the tcp:

cat arp.cache | sx tcp syn -p 22 192.168.0.171

tcp subcomand is just a shorthand for tcp syn subcommand unless --flags option is passed, see below.

TCP FIN scan

Most network scanners try to interpret results of the scan. For instance they say “this port is closed” instead of “I received a RST”. Sometimes they are right. Sometimes not. It’s easier for beginners, but when you know what you’re doing, you keep on trying to deduce what really happened from the program’s interpretation, especially for more advanced scan techniques.

sx tries to overcome those problems. It returns information about all reply packets for TCP FIN, NULL, Xmas and custom TCP scans. The information contains IP address, TCP port and all TCP flags set in the reply packet.

TCP FIN scan and its other variations (NULL and Xmas) exploit RFC793 Section 3.9:

SEGMENT ARRIVES

If the state is CLOSED (i.e., TCB does not exist) then

 all data in the incoming segment is discarded.  An incoming
 segment containing a RST is discarded.  An incoming segment not
 containing a RST causes a RST to be sent in response.  The
 acknowledgment and sequence field values are selected to make the
 reset sequence acceptable to the TCP that sent the offending
 segment.

so closed port should return packet with RST flag.

This section also states that:

If the state is LISTEN then

…

Any other control or text-bearing segment (not containing SYN) must have an ACK and thus would be discarded by the ACK processing. An incoming RST segment could not be valid, since it could not have been sent in response to anything sent by this incarnation of the connection. So you are unlikely to get here, but if you do, drop the segment, and return.

the main phrase here: drop the segment, and return. So an open port on most operating systems will drop the TCP packet containing any flags except SYN,ACK and RST.

Let’s scan some closed port with TCP FIN scan:

cat arp.cache | sx tcp fin --json -p 23 192.168.0.171

sample output:

{"scan":"tcpfin","ip":"192.168.0.171","port":23,"flags":"ar"}

flags field contains all TCP flags in the reply packet, where each letter represents one of the TCP flags:

• s – SYN flag
• a – ACK flag
• f – FIN flag
• r – RST flag
• p – PSH flag
• u – URG flag
• e – ECE flag
• c – CWR flag
• n – NS flag

In this case we find out that port 23 sent reply packet with ACK and RST flags set (typical response for a closed port according to the rfc793).

If we scan an open port, we get no response (unless the firewall is spoofing the responses).

Other types of TCP scans can be conducted by analogy.

TCP NULL scan:

cat arp.cache | sx tcp null --json -p 23 192.168.0.171

TCP Xmas scan:

cat arp.cache | sx tcp xmas --json -p 23 192.168.0.171

Custom TCP scans

It is possible to send TCP packets with custom TCP flags using --flags option.

Let’s send TCP packet with SYN, FIN and ACK flags set to fingerprint remote OS:

cat arp.cache | sx tcp --flags syn,fin,ack --json -p 23 192.168.0.171

Windows and MacOS will not respond to this packet, but Linux will send reply packet with RST flag.

Possible arguments to --flags option:

• syn – SYN flag
• ack – ACK flag
• fin – FIN flag
• rst – RST flag
• psh – PSH flag
• urg – URG flag
• ece – ECE flag
• cwr – CWR flag
• ns – NS flag

UDP scan

sx can help investigate open UDP ports. UDP scan exploits RFC1122 Section 4.1.3.1:

If a datagram arrives addressed to a UDP port for which there is no pending LISTEN call, UDP SHOULD send an ICMP Port Unreachable message.

Similar to TCP scans, sx returns information about all reply ICMP packets for UDP scan. The information contains IP address, ICMP packet type and code set in the reply packet.

For instance, to detect DNS server on host, run:

cat arp.cache | sx udp --json -p 53 192.168.0.171

sample output:

{"scan":"udp","ip":"192.168.0.171","icmp":{"type":3,"code":3}}

In this case we find out that host sent ICMP reply packet with Destination Unreachable type and Port Unreachable code (typical response for a closed port according to the rfc1122).

Firewalls typically set ICMP code distinct from Port Unreachanble and so can be easily detected.

Rate limiting

Sometimes you need to limit the speed at which generated packets are sent. This can be done with the --rate option.

For example, to limit the speed to 1 packet per 5 seconds:

cat arp.cache | sx tcp --rate 1/5s --json -p 22,80,443 192.168.0.171

Exclude subnets

Sometimes you need to exclude some ip addresses and subnets from scanning. This can be done with the --exclude option. It specifies a file with IPs or subnets in CIDR notation to exclude, one-per line.

For instance, to exclude RFC 1918 addresses, create a file ips.txt with the following contents:

10.0.0.0/8
172.16.0.0/16
192.168.0.0/16

You can also insert comments and blank lines:

# exclude RFC 1918 addresses
10.0.0.0/8 # comment 1
172.16.0.0/12 # comment 2
192.168.0.0/16 # comment 3

0.0.0.0/8 # used in initialization procedures (RFC 6890)

# exclude RFC 5735 addresses
127.0.0.0/8 # loopback address
192.0.0.0/24 # reserved block for IETF protocol assignments
224.0.0.0/4 # allocated for use in IPv4 multicast address assignments
240.0.0.0/4 # reserved for future use

# exclude Amazon network
3.0.0.0/8

# ip addresses are valid as well
1.1.1.1

and run a scan with --exclude ips.txt option.

Live LAN TCP SYN scanner

As an example of scan composition, you can combine ARP and TCP SYN scans to create live TCP port scanner that periodically scan whole LAN network.

Start live ARP scan and save results to arp.cache file:

sx arp 192.168.0.1/24 --live 10s --json | tee arp.cache

In another terminal start TCP SYN scan:

while true; do sx tcp -p 1-65535 -a arp.cache -f arp.cache; sleep 30; done

SOCKS5 scan

sx can detect live SOCKS5 proxies. To scan, you must specify an IP range or JSONL file with ip/port pairs.

For example, an IP range scan:

sx socks -p 1080 10.0.0.1/16

scan ip/port pairs from a file with JSON output:

sx socks --json -f ip_ports_file.jsonl 

Each line of the input file is a json string, which must contain the ip and port fields.

sample input file:

{"ip":"10.0.1.1","port":1080}
{"ip":"10.0.2.2","port":1081}

You can also specify a range of ports to scan:

sx socks -p 1080-4567 -f ips_file.jsonl

In this case only ip addresses will be taken from the file and the port field is no longer necessary.

Elasticsearch scan

Elasticsearch scan retrieves the cluster information and a list of all indexes along with aliases.

For example, an IP range scan:

sx elastic -p 9200 10.0.0.1/16

By default the scan uses the http protocol, to use the https protocol specify the --proto option:

sx elastic --proto https -p 9200 10.0.0.1/16

scan ip/port pairs from a file with JSON output:

sx elastic --json -f ip_ports_file.jsonl

Each line of the input file is a json string, which must contain the ip and port fields.

sample input file:

{"ip":"10.0.1.1","port":9200}
{"ip":"10.0.2.2","port":9201}

You can also specify a range of ports to scan:

sx elastic -p 9200-9267 -f ips_file.jsonl

In this case only ip addresses will be taken from the file and the port field is no longer necessary.

Usage help

sx help

• Network Security Assessment: Know Your Network 1st Edition by Chris McNab
• ICMP Usage in Scanning – The Complete Know-How by Ofir Arkin
• Transmission Control Protocol ( rfc793 )
• User Datagram Protocol ( rfc768 )
• Requirements for Internet Hosts — Communication Layers ( rfc1122 )
• SOCKS Protocol Version 5 ( rfc1928 )
• Internet Control Message Protocol ( rfc792 )

Contributions, issues and feature requests are welcome.

Logo is designed by mikhailtsoy.com