a question about firewalls (or whatever else that might cause packet drop)

[Matej Kosik <5764c029b688c1c0d24a2e97cd764f@gmail.com>]

Hi,

I am experiencing some deterministic packet drop:
- when I tcpreplay on "lo" some pcap (0.pcap) file,
  that traffic does not reach listening applications
- when I change source IP address from whatever it was to, e.g.,
  10.0.10.6, 10.0.10.7 etc,
  then when I try to replay the modified pcap file (1.pcap),
  that traffic does reache applications.

I would like to find out the cause of this.
The only thing which could be causing thing I was aware of was
"iptables". However, when I apt-get removed it, nothing changed.

What else should I check?
Which other programs might cause dropping of packets?
Are there any logs with dropped packets?
Or is there a way to enable such logs?

Thank you in advance for any clues.