iftop: display bandwidth usage on an interface by host:

Synopsis: iftop -h | [-npblNBP] [-i interface] [-f filter code]
  [-F net/mask] [-G net6/mask6]

 -h                  display this message
 -n                  don't do hostname lookups
 -N                  don't convert port numbers to services
 -p                  run in promiscuous mode (show traffic between other
                     hosts on the same network segment)
 -b                  don't display a bar graph of traffic
 -B                  Display bandwidth in bytes
 -i interface        listen on named interface
 -f filter code      use filter code to select packets to count
                    (default: none, but only IP packets are counted)
 -F net/mask         show traffic flows in/out of IPv4 network
 -G net6/mask6       show traffic flows in/out of IPv6 network
 -l                  display and count link-local IPv6 traffic (default: off)
 -P                  show ports as well as hosts
 -m limit            sets the upper limit for the bandwidth scale
 -c config file      specifies an alternative configuration file
 -t                  use text interface without ncurses

 Sorting orders:
 -o 2s                Sort by first column (2s traffic average)
 -o 10s               Sort by second column (10s traffic average) [default]
 -o 40s               Sort by third column (40s traffic average)
 -o source            Sort by source address
 -o destination       Sort by destination address

 The following options are only available in combination with -t
 -s num              print one single text output afer num seconds, then quit
 -L num              number of lines to print


=> 代表发送
<= 代表接收


iftop           # 默认是监控第一块网卡的流量
iftop -i eth1   # 监控eth1
iftop -n        # 直接显示IP, 不进行DNS反解析
iftop -N        # 直接显示连接埠编号, 不显示服务名称
iftop -F or  # 显示某个网段进出封包流量

iftop -i eth1 -m 500m  # 设定尺度500m