{"id":178,"date":"2018-01-17T00:26:30","date_gmt":"2018-01-17T00:26:30","guid":{"rendered":"https:\/\/www.kuncar.net\/blog\/?p=178"},"modified":"2018-01-17T00:28:14","modified_gmt":"2018-01-17T00:28:14","slug":"dhcp-option-82-and-dhcp-relay","status":"publish","type":"post","link":"https:\/\/www.kuncar.net\/blog\/2018\/dhcp-option-82-and-dhcp-relay\/","title":{"rendered":"DHCP option 82 and DHCP relay"},"content":{"rendered":"
\n

\"\"I was actually playing with DHCP relay and what I did find out is kind of interesting. For what is DHCP relay look for\u00a0rfc3046\u00a0<\/a>.<\/span><\/span>
\n In short it is when the client sends a DHCP a broadcast message (discover) some device \u00a0– usually the switch that is the client directly connected to – intercepts the packet and sends a unicast discover instead of it to remote server over L3. The advantage is that the amount of boadcasts in the networks drops and secondly the DHCP server can be in different subnet. This is great because you can now use centralised dhcp server for all networks.<\/span><\/p>\n

But this advantage of using the single DHCP brings one big problem – user identification. The unicast relay will use the appropriate IP (usually a gw or the subnet) but if you need more more information about the supplicant that is where option 82 of DHCP comes to play.<\/span>
\n In short the DHCP option 82 is here to convey some additional and more detailed user-related information to the DHCP server. The definition by ietf is very vague and the content of the fields is vendor-specific. What must be said is that option 82 has two\u00a0most widely used\u00a0sub-options: \u00a0\u201cagent circuit id\u201d sub-option and \u201cagent remote ID\u201d<\/span>
\n The basic structure of option 82 by ietf looks like this:<\/span><\/p>\n

\n
agent circuit id<\/span>\r\n SubOpt|Len|Circuit ID<\/span>\r\n +\u2014\u2014+\u2014\u2014+\u2014\u2014+\u2014\u2014+\u2014\u2014+\u2014\u2014+\u2014\u2014+\u2014\u2014+\u2013<\/span>\r\n | \u00a0\u00a01  \u00a0 |\u00a0 \u00a0n   | \u00a0c1\u00a0  |   c2\u00a0|\u00a0 \u00a0c3\u00a0 | \u00a0c4   |  c5  | \u00a0c6   | \u2026<\/span>\r\n +\u2014\u2014+\u2014\u2014+\u2014\u2014+\u2014\u2014+\u2014\u2014+\u2014\u2014+\u2014\u2014+\u2014\u2014+\u2013<\/span><\/pre>\n<\/blockquote>\n
and for the other sub-option it is defined this way<\/span><\/p>\n
\n
agent\u00a0remote id<\/span>\r\n SubOpt Len Agent Remote ID<\/span>\r\n +\u2014\u2014+\u2014\u2014+\u2014\u2014+\u2014\u2014+\u2014\u2014+\u2014\u2014+\u2014\u2014+\u2014\u2014+\u2013<\/span>\r\n |\u00a0 \u00a02\u00a0   |\u00a0\u00a0\u00a0r   \u00a0| \u00a0 r1\u00a0\u00a0|\u00a0\u00a0r2  \u00a0|\u00a0\u00a0r3  \u00a0|\u00a0\u00a0r4   \u00a0|\u00a0 r5   |\u00a0\u00a0r6   | \u2026<\/span>\r\n +\u2014\u2014+\u2014\u2014+\u2014\u2014+\u2014\u2014+\u2014\u2014+\u2014\u2014+\u2014\u2014+\u2014\u2014+\u2013<\/span><\/pre>\n<\/blockquote>\n
This seems like a pure theory with no real information inside so lets have a look at some real stuff.<\/span><\/p>\n
\n
Option: (t=82,l=18) Agent Information Option<\/span>\r\n Option: (82) Agent Information Option<\/span>\r\n Length: 18<\/span>\r\n Value: 01060004000A00000208000600E0FC490D22<\/span>\r\n Agent Circuit ID: 0004000A0000<\/span>\r\n Agent Remote ID: 000600E0FC490D22<\/span><\/pre>\n<\/blockquote>\n
This is a real capture from WireShark for Huawei switch S3900 with option 82 enabled. In this case while using standard, pre-defined values for the sub-options, sub-option 1 receives L2 port No. of the packet and vlan No.; whereas sub-option 2 receives MAC address of the packet device.<\/span>
\n In our case it is the received string deciphered like this:<\/span><\/p>\n
\n
01 06 00 04 000A 0000 02 08 00 06 00E0FC490D22<\/span>\r\n 01\u2013CIRCUIT_ID<\/span>\r\n 06\u2013length ( \u201c00 04 0005 0080\u2033 length)<\/span>\r\n 00\u2013CIRCUIT_ID type<\/span>\r\n 04\u2013length ( \u201c0005 0080\u2033 length)<\/span>\r\n 000A\u2013vlan No. (10 in dec.)<\/span>\r\n 0000\u2013global port No.<\/span>\r\n 02\u2013REMOTE_ID<\/span>\r\n 08\u2013length (\u201d00 06 00E0FC6AE332\u2033 length)<\/span>\r\n 00\u2013REMOTE_ID type<\/span>\r\n 06\u2013length<\/span>\r\n 00E0FC490D22\u2013system bridge MAC<\/span><\/pre>\n<\/blockquote>\n
In this case everything is seems to be quite clear. But there is a catch – the global port number. It\u2019s value is counted by this formula:<\/span><\/p>\n
\n
52*slotnumber (0) + present port index (0) = 0<\/span><\/pre>\n<\/blockquote>\n
Note that in huawei’s case:<\/span><\/p>\n