Firewall rule in before.rules for dhcp is wrong

[Joshua Stark]

The file delivered - /usr/share/ufw/iptables/before.rules
which is then copied to - /etc/ufw/before.rules

Delivered by Package:

# allow dhcp client to work
-A ufw-before-input -p udp --sport 67 --dport 68 -j ACCEPT

The ports for
--sport and --dport are swapped

Should be:

-A ufw-before-input -p udp --sport 68 --dport 67 -j ACCEPT

Package version found in:
  0.36-0ubuntu0.1

Note: ISC DHCP uses RAW sockets, which bypasses iptables anyway and doesn't drop the packets with the incorrect configuration. This has had me stumped for the last hour.

[Jamie Strandboge (jdstrand)]

Thank you for filing a bug.

The firewall policy is a combination of the default policy for each of 'incoming', 'outgoing' and 'routed' (forward) along with the policies shipped in before{,6}.rules, after{,6}.rules and whatever gets added to user{,6}.rules. Specifically, what is in before{,6}.rules is designed with default deny for incoming (and forward), default allow for outgoing and default accept for established connections. Considering that dhcp uses port 68/udp for the client and port 67/udp for the server, the shipped default policy allows:

* outgoing from this host port 68/udp to any port 67/udp (via default allow outgoing; eg, for dhcp request)
* incoming for established connection (via before.rules RELATED,ESTABLISHED; eg, dhcp reply from the server we connected to on port 67/udp)
* incoming from port 67/udp (via the before.rules you mentioned; eg, for a server responding to the broadcast)

I suspect that you've updated your default policy to deny to perform egress filtering so you need to add a corresponding 'ufw allow out to any port 67 proto udp comment "dhcp discover"' rule or similar.

[Jamie Strandboge (jdstrand)]

Marking as Invalid since the default firewall policy is working as intended.

[Joshua Stark (starkjs)]

Thanks Jamie,

Ah, cool, so that ufw config is when the install is a client.

I am having issues with the install as a DHCPv4 server.

I will revert the UFW changes I have made and add in a new /etc/ufw/application.d/dhcpd config to allow the install to run a DHCPv4 server

Thanks
Josh

PS. isc-dhcp-server when setup, by default is using "raw" sockets and thus the ufw rules are bypassed