What does /dev/tcp do?

Bash supports read/write operations on a pseudo-device file /dev/tcp/[host]/[port] [1].

Writing to this special file makes bash open a tcp connection to host:port, and this feature may be used for some useful purposes, for example:

Query an NTP server

cat </dev/tcp/time.nist.gov/13

reads the time in Daytime Protocol from the NIST Internet Time Service server.

Fetch a web page

this script

exec 3<>/dev/tcp/www.google.com/80

echo -e "GET / HTTP/1.1\r\nhost: http://www.google.com\r\nConnection: close\r\n\r\n" >&3

cat <&3

(echo > /dev/tcp/$HOST/$PORT) >/dev/null 2>&1

result=$?

references:

https://andreafortuna.org/2021/03/06/some-useful-tips-about-dev-tcp/