Today we will look at an interesting feature to use linux TCP and UDP port checking process.
Below two virtual device map provided by Linux.
1. How to use /dev/tcp and /dev/udp vitual paths ?
### Checking DNS 53 port on my local system. $ echo > /dev/tcp/localhost/53 && echo GOOD || echo "NOT Good" -bash: connect: Connection refused -bash: /dev/tcp/localhost/53: Connection refused NOT Good ### Checking ssh 22 port on my local system $ echo > /dev/tcp/localhost/22 && echo GOOD || echo "NOT GOOD" GOOD $
Similarly we can use it to verify UDP port connections.
$ echo > /dev/udp/127.0.0.1/22 && echo GOOD || echo "Not Good" GOOD
2. Why i see all UDP connections good and TCP connection rejected ?
UDP or User DataGram Protocol is a connectionless protocol, it does not wait for reply packet, hence all packets on UDP is successful.
Hence the tip is to use TCP vitual path i.e. /dev/tcp/ to verify if the port is open and receiving connections.
$ echo > /dev/udp/127.0.0.1/19 && echo GOOD || echo "Not Good" GOOD $ echo > /dev/udp/127.0.0.1/18 && echo GOOD || echo "Not Good" GOOD $ echo > /dev/tcp/127.0.0.1/18 && echo GOOD || echo "Not Good" -bash: connect: Connection refused -bash: /dev/tcp/127.0.0.1/18: Connection refused Not Good