TL-SG108E V6: Wrong Source IP in DHCP DISCOVER messages
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When the switch is configured with DHCP enabled, it uses the wrong source IP in DHCP DISCOVER messages.
Per DHCP RFC, the source address should be 0.0.0.0, but it is in fact 192.168.0.1.
This causes issues with DHCP server running on linux with rp_filter active if using a different network than 192.168.0.0/24 as local address range.
If rp_filter is active (/proc/sys/net/ipv4/conf/all/rp_filter = 1 and/or /proc/sys/net/ipv4/conf/${interface}/rp_filter = 1), the packets will be dropped before reaching the DHCP server because the source IP (192.168.0.1) is invalid for this certain interface.
One fix is to disable rp_filter for the given interfaces (and also "all"), but a better option would be a firmware which sets the source IP to 0.0.0.0 for the DHCP DISCOVER packets...
Here is the sentence from the DHCP RFC 2131 Section 4.1 :
DHCP messages broadcast by a client prior to that client obtaining its IP address must have the source address field in the IP header set to 0.
Thanks for your response, but no, the DHCP client of the switch does not follow the standard. See attached pictures showing the DHCP DISCOVER packet of the switch:
This is the wrong behavior:
And this is the correct behavior (like every other DHCP client does):
After a firmware update to the latest firmware, I saw this packet once, but after that, it falls back to the wrong behavior.
I can make it work with my DHCP server, but the behavior of the DHCP client is still wrong and I can only make it run by lowering the security level (by disabling rp_filter).
