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VLAN tags) in ASCII format. Because VLAN tags are converted to ASCII with use of modulo, interface name is generated from right to left, starting with inner VLAN and prepended with dash characters to ensure a final length of IF_NAMESIZE bytes (16), e.g.: ----ens6f0.83.20 We avoid null bytes in circuit ID to ensure compatibility with DHCP servers that interpret null as string terminator.
Usage
-----
dhcp_user_xdp takes network interface, DHCP server IP and DHCP relay agent IP
as inputs and stores them in a map. Filters the incoming DHCP requests and inserts
option 82 in the DHCP request packets and overwrites the destination IP to that
of DHCP relay server IP.
Build instructions:
cd bpf-examples/dhcp-relay
make
Loading bpf program:
sudo ./dhcp_user_xdp -i <netif> -d <dhcp server IP> -s <dhcp relay agent IP>
where,
netif: Ingress network interface name
unloading program:
sudo ./dhcp_user_xdp -i <netif> -u
To run in SKB mode:
add option "-m skb" for both load and uload commands
Please beware that testing requires a physical NIC because we tail extend
packets due to complications with variable length DHCP options.
Enable forwarding and allow local address spoofing:
sudo echo 1 > /proc/sys/net/ipv4/ip_forward
sudo echo 1 > /proc/sys/net/ipv4/conf/all/accept_local
Verify using tcpdump:
sudo tcpdump -s 0 -i <netif> port 67 and port 68 -vvv
====== TEST SETUP ======
2 physical Linux servers connected back-to-back with a patch cable.
===== Client =====
Kernel: 5.4.0-72-generic
NIC model: Mellanox Technologies MT27800 Family [ConnectX-5]
NIC name: ens6f0
Preparation:
sudo ip link set ens6f0 up
sudo ip link add link ens6f0 name ens6f0.83 type vlan id 83
sudo ip link set ens6f0.83 up
sudo ip link add link ens6f0.83 name ens6f0.83.20 type vlan id 20
sudo ip link set ens6f0.83.20 up
Test:
sudo dhclient ens6f0.83.20
===== BNG =====
Kernel: 5.9.0-050900-lowlatency
NIC model: Mellanox Technologies MT27800 Family [ConnectX-5]
NIC name: ens6f0np0
Preparation:
sudo ip link set ens6f0np0 up
sudo ip link add link ens6f0np0 name ens6f0np0.83 type vlan id 83
sudo ip link set ens6f0np0.83 up
sudo ip link add link ens6f0np0.83 name ens6f0np0.83.20 type vlan id 20
sudo ip link set ens6f0np0.83.20 up
sudo ethtool -K ens6f0np0 rxvlan off
sudo echo 1 > /proc/sys/net/ipv4/conf/ens6f0np0.83.20/accept_local
sudo echo 1 > /proc/sys/net/ipv4/ip_forward
Test:
# Launch XDP program
sudo ./dhcp_user_xdp -i ens6f0np0 -d 192.168.4.20 -s 10.100.100.3
# tcpdump upstream interface to verify that packet is forwarded
sudo tcpdump -n -i eno1 port 67
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eno1, link-type EN10MB (Ethernet), capture size 262144 bytes
10:10:00.191071 IP 10.100.100.3.68 > 192.168.4.20.67: BOOTP/DHCP, Request from b8:59:9f:06:5a:98, length 300
10:10:03.114784 IP 10.100.100.3.68 > 192.168.4.20.67: BOOTP/DHCP, Request from b8:59:9f:06:5a:98, length 300
^C
2 packets captured
2 packets received by filter
0 packets dropped by kernel
# Open trace pipe to see debug messages from XDP program
sudo cat /sys/kernel/debug/tracing/trace_pipe
<idle>-0 [003] d.s2 11051597.318498: bpf_trace_printk: Ethertype 800
<idle>-0 [003] d.s2 11051597.318499: bpf_trace_printk: Found VLAN tag 83 at depth 0
<idle>-0 [003] d.s2 11051597.318500: bpf_trace_printk: Found VLAN tag 20 at depth 1
<idle>-0 [003] d.s2 11051597.318501: bpf_trace_printk: Parsing DHCP packet, opcode 1, hops 0
<idle>-0 [003] d.s2 11051597.318502: bpf_trace_printk: Broadcast packet received, opcode 1, hops 0
<idle>-0 [003] d.s2 11051597.318503: bpf_trace_printk: Got option code 53 at offset 290, hex 122
<idle>-0 [003] d.s2 11051597.318504: bpf_trace_printk: Got option code 12 at offset 293, hex 125
<idle>-0 [003] d.s2 11051597.318506: bpf_trace_printk: Got option code 55 at offset 304, hex 130
<idle>-0 [003] d.s2 11051597.318508: bpf_trace_printk: Got option code 255 at offset 319, hex 13f
<idle>-0 [003] d.s2 11051597.318509: bpf_trace_printk: Going to write DHCP option at offset 319
<idle>-0 [003] d.s2 11051597.318510: bpf_trace_printk: Wrote DHCP option at offset 290, returning XDP_PASS
^C