2

I am considering a setup with a primary DHCP server providing "IP data" (IP address, subnet mask, DNS, …), and a proxyDHCP server providing only PXE boot options. As it happens, my proxyDHCP server does not only provide the PXE options, but also the IP data (hence it's not really a proxyDHCP).

Using Wireshark, I now observe:

  1. The client (PXE-ROM) sends an "extended" DHCPDISCOVER (asking for IP data and PXE options)
  2. The primary DHCP server sends a DHCPOFFER (with only IP data)
  3. The client does not accept the DHCPOFFER (because it’s missing the PXE options?)
  4. (Much later) The proxyDHCP server sends a DHCPOFFER (with IP data and PXE options)
  5. The client accepts the second DHCPOFFER (and PXE-boots successfully)

Now I am wondering:

  • Why does the client not accept the first DHCPOFFER? That would be mandatory for the intended primary-proxy setup to work – how else would the client get its IP data?
  • If it’s all a matter of requested and provided DHCP options: Which specific missing options make the client ignore the offer? Why does the server even bother sending a DHCPOFFER, when it knows that it cannot provide all options and that the client would not accept the DHCPOFFER anyway?

Client Request:

Ethernet II, Src: cc:cc:cc:cc:cc:cc (cc:cc:cc:cc:cc:cc), Dst: Broadcast (ff:ff:ff:ff:ff:ff) Internet Protocol Version 4, Src: 0.0.0.0, Dst: 255.255.255.255 User Datagram Protocol, Src Port: 68, Dst Port: 67 Dynamic Host Configuration Protocol (Discover) Message type: Boot Request (1) Hardware type: Ethernet (0x01) Hardware address length: 6 Hops: 0 Transaction ID: 0xcf952d8d Seconds elapsed: 0 Bootp flags: 0x8000, Broadcast flag (Broadcast) Client IP address: 0.0.0.0 Your (client) IP address: 0.0.0.0 Next server IP address: 0.0.0.0 Relay agent IP address: 0.0.0.0 Client MAC address: cc:cc:cc:cc:cc:cc (cc:cc:cc:cc:cc:cc) Client hardware address padding: 00000000000000000000 Server host name not given Boot file name not given Magic cookie: DHCP Option: (53) DHCP Message Type (Discover) Option: (57) Maximum DHCP Message Size Option: (55) Parameter Request List Length: 35 Parameter Request List Item: (1) Subnet Mask Parameter Request List Item: (2) Time Offset Parameter Request List Item: (3) Router Parameter Request List Item: (4) Time Server Parameter Request List Item: (5) Name Server Parameter Request List Item: (6) Domain Name Server Parameter Request List Item: (12) Host Name Parameter Request List Item: (13) Boot File Size Parameter Request List Item: (15) Domain Name Parameter Request List Item: (17) Root Path Parameter Request List Item: (18) Extensions Path Parameter Request List Item: (22) Maximum Datagram Reassembly Size Parameter Request List Item: (23) Default IP Time-to-Live Parameter Request List Item: (28) Broadcast Address Parameter Request List Item: (40) Network Information Service Domain Parameter Request List Item: (41) Network Information Service Servers Parameter Request List Item: (42) Network Time Protocol Servers Parameter Request List Item: (43) Vendor-Specific Information Parameter Request List Item: (50) Requested IP Address Parameter Request List Item: (51) IP Address Lease Time Parameter Request List Item: (54) DHCP Server Identifier Parameter Request List Item: (58) Renewal Time Value Parameter Request List Item: (59) Rebinding Time Value Parameter Request List Item: (60) Vendor class identifier Parameter Request List Item: (66) TFTP Server Name Parameter Request List Item: (67) Bootfile name Parameter Request List Item: (97) UUID/GUID-based Client Identifier Parameter Request List Item: (128) DOCSIS full security server IP [TODO] Parameter Request List Item: (129) PXE - undefined (vendor specific) Parameter Request List Item: (130) PXE - undefined (vendor specific) Parameter Request List Item: (131) PXE - undefined (vendor specific) Parameter Request List Item: (132) PXE - undefined (vendor specific) Parameter Request List Item: (133) PXE - undefined (vendor specific) Parameter Request List Item: (134) PXE - undefined (vendor specific) Parameter Request List Item: (135) PXE - undefined (vendor specific) Option: (97) UUID/GUID-based Client Identifier Option: (94) Client Network Device Interface Length: 3 Major Version: 3 Minor Version: 16 Option: (93) Client System Architecture Length: 2 Client System Architecture: EFI x64 (7) Option: (60) Vendor class identifier Length: 32 Vendor class identifier: PXEClient:Arch:00007:UNDI:003016 Option: (255) End

(Ignored) Response from primary DHCP server (172.17.34.30 / aa:aa:aa:aa:aa:aa):

Ethernet II, Src: aa:aa:aa:aa:aa:aa (aa:aa:aa:aa:aa:aa), Dst: Broadcast (ff:ff:ff:ff:ff:ff) Internet Protocol Version 4, Src: 172.17.34.30, Dst: 255.255.255.255 User Datagram Protocol, Src Port: 67, Dst Port: 68 Dynamic Host Configuration Protocol (Offer) Message type: Boot Reply (2) Hardware type: Ethernet (0x01) Hardware address length: 6 Hops: 0 Transaction ID: 0xcf952d8d Seconds elapsed: 0 Bootp flags: 0x8000, Broadcast flag (Broadcast) Client IP address: 0.0.0.0 Your (client) IP address: 172.17.34.1 Next server IP address: 172.17.34.30 Relay agent IP address: 0.0.0.0 Client MAC address: cc:cc:cc:cc:cc:cc (cc:cc:cc:cc:cc:cc) Client hardware address padding: 00000000000000000000 Server host name not given Boot file name not given Magic cookie: DHCP Option: (53) DHCP Message Type (Offer) Option: (54) DHCP Server Identifier (172.17.34.30) Option: (51) IP Address Lease Time Length: 4 IP Address Lease Time: (900s) 15 minutes Option: (58) Renewal Time Value Length: 4 Renewal Time Value: (450s) 7 minutes, 30 seconds Option: (59) Rebinding Time Value Length: 4 Rebinding Time Value: (787s) 13 minutes, 7 seconds Option: (28) Broadcast Address (172.17.63.255) Option: (1) Subnet Mask (255.255.224.0) Option: (255) End

(Accepted) Response from proxyDHCP server (172.17.61.1 / bb:bb:bb:bb:bb:bb):

Ethernet II, Src: bb:bb:bb:bb:bb:bb (bb:bb:bb:bb:bb:bb), Dst: Broadcast (ff:ff:ff:ff:ff:ff) Internet Protocol Version 4, Src: 172.17.61.1, Dst: 255.255.255.255 User Datagram Protocol, Src Port: 67, Dst Port: 68 Dynamic Host Configuration Protocol (Offer) Message type: Boot Reply (2) Hardware type: Ethernet (0x01) Hardware address length: 6 Hops: 0 Transaction ID: 0xcf952d8d Seconds elapsed: 0 Bootp flags: 0x8000, Broadcast flag (Broadcast) Client IP address: 0.0.0.0 Your (client) IP address: 172.17.61.61 Next server IP address: 172.17.61.1 Relay agent IP address: 0.0.0.0 Client MAC address: cc:cc:cc:cc:cc:cc (cc:cc:cc:cc:cc:cc) Client hardware address padding: 00000000000000000000 Server host name not given Boot file name not given Magic cookie: DHCP Option: (53) DHCP Message Type (Offer) Option: (54) DHCP Server Identifier (172.17.61.1) Option: (51) IP Address Lease Time Length: 4 IP Address Lease Time: (600s) 10 minutes Option: (67) Bootfile name Length: 13 Bootfile name: syslinux.efi Option: (58) Renewal Time Value Length: 4 Renewal Time Value: (300s) 5 minutes Option: (59) Rebinding Time Value Length: 4 Rebinding Time Value: (525s) 8 minutes, 45 seconds Option: (1) Subnet Mask (255.255.224.0) Option: (28) Broadcast Address (172.17.63.255) Option: (3) Router Length: 4 Router: 172.17.61.1 Option: (6) Domain Name Server Length: 4 Domain Name Server: 172.17.61.1 Option: (255) End

(MACs disguised)

Improve this question
2

2 Answers 2

Reset to default
2

The ignored offer provides:

Your (client) IP address: 172.17.34.1 << offered IP address Next server IP address: 172.17.34.30 << TFTP Server IP Boot file name **not given** << NBP name

which is an incomplete PXE offer (Boot filename is missing when "next server" was provided) this is erroneous and could lead the client to totally ignore the offer.

The accepted offer provides:

Your (client) IP address: 172.17.61.61 << offered IP address Next server IP address: 172.17.61.1 << TFTP Server IP Option: (67) Bootfile name -> syslinux.efi << NBP name

This is a complete PXE offer but also not quite right.

In both cases the offers were sent by a DHCP server there was not proxyDHCP here. In a DHCP Server - proxyDHCP scenario the DHCP server only provides IP info and related options and the proxyDHCP only PXE info

The PXE info (in the DHCP server or the proxyDHCP server) is either provided by "next server" and "file" fields of the DHCP offer (preferred method) "or" by DHCP options 66 and 67, but never by a combination of this 2 alternatives. Now you can see why the accepted offer is also not totally correct even when taken by the client.

Improve this answer
0

You should disable the dhcp configuration in proxy DHCP server and try if the pxe client is getting the IP from primary DHCP and boot from the proxy dhcp. When a pxe client boots, it will send a dhcp discovery with option 60. In your case the proxy server is providing the option 60. That's y you second dhcp is accepted by pxe client.

Improve this answer

You must log in to answer this question.

Start asking to get answers

Find the answer to your question by asking.

Ask question

Explore related questions

See similar questions with these tags.