dhcp (dhclient)

• install dhclient (n.b. deprecated)

pacman -S dhclient

• start system service

systemctl enable dhclient@eno1.service
systemctl start  dhclient@eno1.service

• dhclient overwrites /etc/resolv.conf which is a problem if using vpn, etc.
• see arch forum - dhclient overwrites resolv.conf even when resolvconf is installed
• I can't get this to work, resolvconf processes it in the wrong order
• this could be fixed by hardcoding or just ignoring DNS altogether (I get my DNS server from mullvad or 1.1.1.1 anyways)
• patch from arch forum in /etc/dhclient-enter-hooks

	# if [ -f /etc/resolv.conf ]; then
	#     chown --reference=/etc/resolv.conf $new_resolv_conf
	#     chmod --reference=/etc/resolv.conf $new_resolv_conf
	# fi
	# mv -f $new_resolv_conf /etc/resolv.conf

        # use resolvconf
        cat $new_resolv_conf | /usr/bin/resolvconf -a $interface
        rm $new_resolv_conf

• use unbound to prevent DNS servers being in the wrong order

operation not permitted

• error journalctl -u dhclient@eno1.service

Jun 11 16:22:03 neko dhclient[687]: send_packet: Operation not permitted
Jun 11 16:22:03 neko dhclient[687]: dhclient.c:2996: Failed to send 300 byte long packet over fallback interface.

• see linuxquestions - dhcpd complains "Failed to send 300 byte long packet over fallback interface."
• when it works check tcpdump output (pacman -S tcpdump)

sudo tcpdump > out.txt

• note that bootps is usually port 67 and bootpc is usually port 68

02:58:43.767759 IP neko.bootpc > 255.255.255.255.bootps: BOOTP/DHCP, Request from b0:25:aa:44:bd:c2 (oui Unknown), length 300
02:58:43.770219 ARP, Request who-has res388d-128-61-95-198.res.gatech.edu (Broadcast) tell _gateway, length 46
02:58:43.773852 IP _gateway.bootps > neko.bootpc: BOOTP/DHCP, Reply, length 300
02:58:43.774738 IP _gateway.bootps > neko.bootpc: BOOTP/DHCP, Reply, length 300

• firewall issue, try using built-in kernel packet filter, front end iptables
• arch wiki - iptables
• arch wiki - ebtables
• arch wiki - nftables
• open bootps and bootpc ports for DHCP

iptables -A OUTPUT -p udp --sport 1024:65535 --dport 67 -j ACCEPT
iptables -A OUTPUT -p udp --sport 68 --dport 67 -j ACCEPT

iptables -A INPUT -p udp --sport 1024:65535 --dport 68 -j ACCEPT
iptables -A INPUT -p udp --sport 67 --dport 68 -j ACCEPT

• list

ebtables-nft --list

• to undo run

iptables -F
ebtables-nft -F

• firewall created by mullvad
• solution: use split tunnel to exclude dhclient from the vpn, see mullvad - How to use the Mullvad CLI
• edit /etc/dhclient-exit-hooks

# exclude dhclient from vpn, also from firewall
mullvad split-tunnel pid add "$(pgrep --oldest dhclient)"