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$ apt-cache search packet generator
bittwist - libpcap-based Ethernet packet generator
filtergen - packet filter generator for various firewall systems
mz - versatile packet creation and network traffic generation tool
packeth - Ethernet packet generator
python-scapy - Packet generator/sniffer and network scanner/discovery

hping

+1 for hping

hping3 is a network tool able to send custom TCP/IP packets and to dis‐
play target replies like ping program does with ICMP replies. hping3
handle fragmentation, arbitrary packets body and size and can be used
in order to transfer files encapsulated under supported protocols.
Using hping3 you are able to perform at least the following stuff:

- Test firewall rules
- Advanced port scanning
- Test net performance using different protocols,
packet size, TOS (type of service) and fragmentation.
- Path MTU discovery
- Transferring files between even really fascist firewall
rules.
- Traceroute-like under different protocols.
- Firewalk-like usage.
- Remote OS fingerprinting.
- TCP/IP stack auditing.
- A lot of others.

It's also a good didactic tool to learn TCP/IP