[ipxe-devel] Option user-class violates RFC 3004.

[Mats Erik Andersson]

Hello there,

in the source file 'src/net/udp/dhcp.c', line 90,
the user class option is implemented to produce

77, 4, 'i', 'P', 'X', 'E'

which is in violation of RFC 3004, page 3. The option
in question is an opaque construct intending the possi-
bility of multiple suboptions, usually called instances.
Your code does not specify the total length of the pay-
load, it only states the length of the first and only
instance "iPXE". The correct form is thus

77, 5, 4, 'i', 'P', 'X', 'E'

The tcpdump of OpenSolaris' origin does not recognize
the error, but tcpdump from any *BSD or GNU/Linux will
show a recoverable error in instance #1.

Best regards
Mats Erik Andersson

[Christian Nilsson]

Post by Mats Erik Andersson
Hello there,
in the source file 'src/net/udp/dhcp.c', line 90,
the user class option is implemented to produce
77, 4, 'i', 'P', 'X', 'E'
which is in violation of RFC 3004, page 3. The option
in question is an opaque construct intending the possi-
bility of multiple suboptions, usually called instances.
Your code does not specify the total length of the pay-
load, it only states the length of the first and only
instance "iPXE". The correct form is thus
77, 5, 4, 'i', 'P', 'X', 'E'
The tcpdump of OpenSolaris' origin does not recognize
the error, but tcpdump from any *BSD or GNU/Linux will
show a recoverable error in instance #1.

Hej,
There already is documentation regarding this, please see
https://ipxe.org/cfg/user-class#notes
Unformated version:

RFC 3004 defines the DHCP user class as a set of length-value tuples,
but iPXE treats it as a string.1) You can choose to manually construct
a value which conforms to RFC 3004 using the set command. For example:

# Length 4, value 'i' (0x69), 'P' (0x50), 'X' (0x48), 'E' (0x45)
set user-class:hex 04:69:50:48:45

1) This is a long-standing bug both in iPXE and in the reference DHCP
server implementation (ISC dhcpd). There is a substantial amount of
documentation which suggests checking option 77 for the value “iPXE”
to identify iPXE clients, and using “set user-class <text>” to specify
a custom user class. To avoid breaking existing setups, iPXE will
continue to provide option 77 as a plain string for DHCPv4.