Nemesis is a command line, portable "human IP stack".  It can be useful for
easy injection of packet streams from simple shell scripts.  It supports
8 protocols (ARP, DNS, ICMP, IGMP, OSPF, RIP, TCP, UDP), and packets can be
injected on either Layer 2 or Layer 3.

* nemesis-tcp -v -S 192.168.1.1 -D 192.168.2.2 -fS -fA -y 22 -P foo
  send TCP packet (SYN/ACK) with payload from ascii file 'foo' to target's
  ssh port from 192.168.1.1 to 192.168.2.2. (-v allows a stdout visual of
  current injected packet)
       
* nemesis-icmp -S 10.10.10.3 -D 10.10.10.1 -G 10.10.10.3 -i 5
  send ICMP REDIRECT (network) packet from 10.10.10.3 to 10.10.10.1
  with preferred gateway as source address.  

* nemesis-arp -v -d ne0 -H 0:1:2:3:4:5 -S 10.11.30.5 -D 10.10.15.1
  send ARP packet through device 'ne0' from hardware source address
  00:01:02:03:04:05 with IP source address 10.11.30.5 to destination IP
  address 10.10.15.1 with broadcast destination hardware address.
  In other words, who-has the mac address of 10.10.15.1, tell 10.11.30.5 
  - assuming 00:01:02:03:04:05 is the source mac address of our 'ne0' device.

WWW:  http://www.packetninja.net/nemesis/