Running a CentOS 7.3 router with DHCP address from my ISP, I would sometimes get an IP address, sometimes not. Sometimes the IP address would get assigned but then fail to renew, causing seemingly random loss of connectivity. Rebooting would sometimes fix it, sometimes not. Very annoying.
Studying the problem over time, tcpdump revealed that the UDP checksum on outgoing packets from dhclient would periodically have bad checksums. Not empty, just incorrect. Why this happens only sometimes and not all the time is beyond me, it's probably just a stupid bug in dhclient but I found the following workaround:
firewall-cmd --permanent --direct --add-rule ipv4 mangle POSTROUTING 0 -p udp -m udp --dport 68 -j CHECKSUM --checksum-fill
This will recalculate the UDP checksum on all outgoing UDP/68 traffic and not touch anything else. Reload firewalld with "systemctl reload firewalld" or reboot for the change to take effect, then verify with tcpdump.
tcpdump -i ens160 -nn -vv port 68(Substitute ens160 with your external interface, obviously)
It's also possible to activate the change by repeating the firewall-cmd without the "--permanent" clause ofcourse, but I usually just reboot to make absolutely sure I didn't break anything while experimenting -- this may not be feasible in your environment BUT on the other hand, if you are reading this then chances are you are already experiencing random service interruptions so.... You decide.
