ngrep strives to provide most of GNU grep’s common features, applying them to the network layer. ngrep is a pcap-aware tool that will allow you to specify extended regular or hexadecimal expressions to match against data payloads of packets. It currently recognizes IPv4/6, TCP, UDP, ICMPv4/6, IGMP and Raw across Ethernet, PPP, SLIP, FDDI, Token Ring and null interfaces, and understands BPF filter logic in the same fashion as more common packet sniffing tools, such as tcpdump and snoop.

Basic Usage :

• ngrep -d any port 25

  Monitor all activity crossing source or destination port 25 (SMTP).

• ngrep -d any ‘error’ port syslog

  Monitor any network-based syslog traffic for  word “error”. See all errors on your network

• ngrep -wi -d any ‘user|pass’ port 21

  Monitor any traffic crossing source or destination port 21 - replace user/pass with what you need , and it will monitor  them
  

You can download it from here