netfilter buglog - Jun 2019 - [Bug 1346] New: REDIRECT tftp doesn't work with docker

https://bugzilla.netfilter.org/show_bug.cgi?id=1346

            Bug ID: 1346
           Summary: REDIRECT tftp doesn't work with docker
           Product: netfilter/iptables
           Version: unspecified
          Hardware: x86_64
                OS: Ubuntu
            Status: NEW
          Severity: enhancement
          Priority: P5
         Component: nf_conntrack
          Assignee: netfilter-buglog at lists.netfilter.org
          Reporter: maxim.kaskevich at gmail.com

REDIRECT target for tftp traffic doesn't work with tftp server launched
inside
docker container. Not sure it's iptables bug but same thing for http/https
works fine. 

How to reproduce
On host machine:

# modprobe nf_conntrack_tftp
# sudo iptables -t nat -A PREROUTING -m mac --mac-source <REMOTE DEVICE
MAC> -p
udp --dport 69 -j REDIRECT --to-port 6969
# echo TEST > /var/tftpboot/hello.txt
# docker run -p 6969:69/udp -v /var/tftpboot:/var/tftpboot pghalliday/tftp

On device with <REMOTE DEVICE MAC>:

# tftp  -g -r hello.txt <my ip>

Expect: file is downloaded
Actual results: "tftp: timeout"


System:
4.15.0-52-generic #56-Ubuntu SMP Tue Jun 4 22:49:08 UTC 2019 x86_64

Additional info:
- "conntrack -L" shows that packets from remote device are
"[UNREPLIED]"
- "tftp  -g -r hello.txt <my ip> 6969" works fine
- I checked on the same enviroment: if instead docker, run regular tftp (I used
tftpd-hpa) and configure it to port 6969, tftp command on remote device works
fine.

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https://bugzilla.netfilter.org/show_bug.cgi?id=1346

--- Comment #1 from Max <maxim.kaskevich at gmail.com> ---
Docker version 18.09.5, build e8ff056

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