oam/knowledge base/nc.md

Netcat

• -N: close the network socket when finished; not available in nmap's netcat
• -l: bind to the port and listen for incoming connections (server mode)
• -n: do not resolve hostnames via DNS
• -p: specify the source port to use
• -t: use telnet negotiation
• -u: use UDP
• -v: set verbosity level; can be used several times
• -w=SECS: timeout for connects and final net reads, in seconds
• -z: zero-I/O mode, exit once connected

TL;DR

# Check ports on hosts.
nc -Nnvz 192.168.0.81 22-25
nc -Nvz host.name 443
nc -Nvz -u dns.server 123

# List hosts with a specific port open.
# But you might just want to use `nmap`.
parallel -j 0 "nc -Nnvz -w 2 192.168.0.{} 22 2>&1" ::: {2..254} \
| grep -v "timed out"

# Wait for a host to be up.
until nc -Nvz -w 3 pi.lan 22; do sleep 3; done

# Listen mode.
nc -l 5666

Sources

• How To use Netcat to establish and test TCP and UDP connections