[CentOS] tftpd server S not responding

Thu Mar 29 11:21:45 UTC 2018
Steven Tardy <sjt5atra at gmail.com>

A STATEFUL firewall with “ip any any” can and will still block asymmetric
communications due to the firewall keeping track of state (hence tha name
stateful firewall).

Tcpdump on your servers /other/ NICs and you’ll see the tftp traffic
leaving your server on some other NIC (probably on with the default route).

The upstream firewall will then block the tftp response if it never saw the
tftp request (due to asymmetry).