Connections to remote host

Show only sockets talking to a specific peer host/port, both ends matched

lsof -nP -i @<host>:<port>

More lsof commands

Binaries and libs of a PID

lsof -p <pid> -a -d txt,mem

Show only mapped executable and shared libs; -a ANDs the filters, great after a deploy
Count open fds of a PID

lsof -nP -p <pid> | wc -l

Quick fd count to spot leaks vs ulimit -n; faster: ls /proc/<pid>/fd | wc -l
List open files by process

lsof -p $(pgrep nginx) | head -30

Show all files (sockets, regular, pipes) opened by nginx
Who is using a port

lsof -i :8080

Find which process is listening on port 8080
List all network connections

lsof -i -n -P

Show all open network connections without DNS resolution