[Bug] tcpreplay stop to generate unique-ip with --unique-ip option after some cycles

[Polexy]

Describe the bug
I have pcap with only one udp packet.
I playing it like:

tcpreplay -i ens7 -p 30000 -K -l 0 --unique-ip my.pcap

after ~ 11 minutes / 16 million cycles i see tcpreplay start to generate 2 same packets, after 38 million packets tcpreplay start to generate 4 same packets and etc.

12:58:15.045217 IP 7.190.152.209.1947 > 12.67.16.142.51802: UDP, length 22
12:58:15.045250 IP 7.190.152.209.1947 > 12.67.16.142.51802: UDP, length 22
12:58:15.045284 IP 7.190.152.208.1947 > 12.67.16.143.51802: UDP, length 22
12:58:15.045316 IP 7.190.152.208.1947 > 12.67.16.143.51802: UDP, length 22

i take pcap with 2 different packets and tried to play it on same speed(30k pps) and i see this situation start after ~38 million packets ~22min.
so, this bug reproduced after ~ 16-17 million cycles.

To Reproduce
Steps to reproduce the behavior:

1. Find/Create pcap with one udp packet
2. Play this pcap. (For example tcpreplay -i ens7 -p 30000 -K -l 0 --unique-ip my.pcap)
3. After ~16-17 million cyclyes on speed 30000 pps ~10-12min - look to packets(i used tcpdump)

Expected behavior
I thought tcpreplay can generate ~2 milliard unique packets. ( (255^4)/2 -because every cycle: dst.ip +1, src.ip -1)

System :
Ubuntu 16.04.6 LTS
tcpreplay version: 4.3.1 (build git:v4.3.1)
Copyright 2013-2018 by Fred Klassen - AppNeta
Copyright 2000-2012 by Aaron Turner
The entire Tcpreplay Suite is licensed under the GPLv3
Cache file supported: 04
Not compiled with libdnet.
Compiled against libpcap: 1.7.4
64 bit packet counters: enabled
Verbose printing via tcpdump: enabled
Packet editing: disabled
Fragroute engine: disabled
Default injection method: PF_PACKET send()
Optional injection method: netmap

[fklassen]

Thanks for the detailed report. It was very helpful in reproducing the issue. Fixed in PR #600