MavEtJu's Distorted View of the World - PXE

Strange PXE Booting

Posted on 2008-09-10 17:00:00, modified on 2008-09-11 12:00:00
Tags: Networking, FreeBSD, PXE, tftp

After being able to use PXE to boot up virtual machines in QEMU, I found an old computer with an old (1998 firmware) fxp ethernet card (Intel EtherExpress PRO/100 Ethernet) I thought "let's boot FreeBSD -current on it!"

That was easier said than done, because for some reason the ethernet card it was requesting a very strange path:

16:48:24.742828 IP 10.204.250.12.2071 > 10.204.250.2.69:  30 RRQ "pxebootM-^?" octet blksize 1456
        0x0000:  4500 003a 0006 0000 1411 9d06 0acc fa0c  E..:............
	0x0010:  0acc fa02 0817 0045 0026 a4f9 0001 7078  .......E.&....px
	0x0020:  6562 6f6f 74ff 006f 6374 6574 0062 6c6b  eboot..octet.blk
	0x0030:  7369 7a65 0031 3435 3600                 size.1456.

Where is that 0xff coming from? And how can I ever create a file like that?

Let's see if the DHCP answer is correct (with net/dhcpdump, also available from my website):

  TIME: 2008-09-11 10:48:23.169
    IP: 10.204.250.2 (00:0f:ea:2c:d5:18) > 255.255.255.255 (ff:ff:ff:ff:ff:ff)
    OP: 2 (BOOTPREPLY)
 HTYPE: 1 (Ethernet)
  HLEN: 6
  HOPS: 0
   XID: b45ceb89
  SECS: 1024
 FLAGS: 7f80
CIADDR: 0.0.0.0
YIADDR: 10.204.250.12
SIADDR: 10.204.250.2
GIADDR: 0.0.0.0
CHADDR: 00:02:b3:5c:eb:89:00:00:00:00:00:00:00:00:00:00
 SNAME: .
 FNAME: pxeboot.
OPTION:  53 (  1) DHCP message type         2 (DHCPOFFER)
OPTION:  54 (  4) Server identifier         10.204.250.2
OPTION:  51 (  4) IP address leasetime      600 (10m)
OPTION:   1 (  4) Subnet mask               255.255.255.0
OPTION:   3 (  4) Routers                   10.204.250.1
OPTION:  60 (  6) Vendor class identifier   Mavvie
OPTION:  67 (  7) Bootfile name             pxeboot

pxeboot, exactly what I expected. The ethernet card wasn't really helpful neither, it just said "TFTP: File not found" without specifying which file it was looking for. Maybe it happened because the option with the Bootfile name is the last one in the packet and it doesn't know how to handle it. Unfortunately this is 1998 firmware and I'm pretty sure that it isn't available from anywhere, let alone be able to update it...

So let's give it the file it wants. The shell I use nor the terminals I use actually make it possible for me to enter the ASCII character 255. So it's Perl to the rescue:

[/tftpboot] root@k7>perl -e 'link("pxeboot", "pxeboot\xff"); '
[/tftpboot] root@k7>ls -al
total 854
drwxr-xr-x   2 nobody  wheel     512 Sep 10 11:05 .
drwxr-xr-x  21 root    wheel     512 Aug  2 07:40 ..
-rw-r--r--   2 root    wheel  260097 Aug 27 21:20 pxeboot
-rw-r--r--   2 root    wheel  260097 Aug 27 21:20 pxeboot?

Oh... That's a hard-link. Oh well, as long as it works.

Did you see the blksize 1456? If you are using the net/freebsd-tftp port, you will send packets with that size instead of 512 bytes:

16:48:24.747635 IP 10.204.250.2.65265 > 10.204.250.12.2071: UDP, length 15
16:48:24.747821 IP 10.204.250.12.2071 > 10.204.250.2.65265: UDP, length 4
16:48:24.747961 IP 10.204.250.2.65265 > 10.204.250.12.2071: UDP, length 1460
16:48:24.748919 IP 10.204.250.12.2071 > 10.204.250.2.65265: UDP, length 4

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