Properly parse MAC Address

Question

When I use tools such as snmp-walk or snmp-get to query an OID with a return type of MacAddress, It'll always parse the data as a HexString and display it properly. Even when they don't have the MIBs loaded it'll still works.

bash#snmpwalk -v 2c -c public 10.1.2.3 1.3.6.1.4.1.14179.2.2.1.1
    SNMPv2-SMI::enterprises.14179.2.2.1.1.1.16.189.24.206.212.64 = Hex-STRING: 10 BD 18 CE D4 40 
    SNMPv2-SMI::enterprises.14179.2.2.1.1.1.100.233.80.151.114.192 = Hex-STRING: 64 E9 50 97 72 C0

However, I can't seem to get the same result from Lextm.SharpSnmpLib (11.2.0). Data types of MacAddress don't get decoded correctly and it's a manual process to convert it to a proper MAC.

public void WalkTable()
    {
    const string baseOid = "1.3.6.1.4.1.14179.2.2.1.1"; //The entire table
    const string community = "public";

    var ep = new IPEndPoint(IPAddress.Parse("10.1.2.3"), 161);
    var results = new List<Variable>();
    Messenger.Walk(VersionCode.V2, ep, new OctetString(community), new ObjectIdentifier(baseOid), results, 60000, WalkMode.WithinSubtree);

    foreach(var v in results)
    Console.WriteLine(v.Data.ToString());
    }

Am I doing something wrong or is this just how the library works?

"Even when they don't have the MIBs loaded it'll still work" is not true. NET-SNMP does ship with some MIB documents, http://net-snmp.sourceforge.net/wiki/index.php/TUT:Using_and_loading_MIBS — Lex Li, Jul 08 '19 at 23:54
I've updated my post to include the results from snmp-walk. Yes, net-snmp does contain a base set of MIBs but it doesn't have the one for the OID in question. Since 'MacAddress' is defined in SNMPv2-TC (which is loaded) is that why snmp-walk is decoding it correctly? I didn't know the data type was transmitted back in the get-response packet, I thought it needed the MIB to know. — Brian Mitchell, Jul 09 '19 at 00:59
The OCTET STRING type is passed back. Applications that display this OCTET STRING, in the absence of a TC, usually have rules whether to display in ASCII or Hex, eg. if all ASCII, display in ASCII, else display in hex. Maybe snmpwalk has it too? — Gambit Support, Jul 11 '19 at 17:55

score 2 · Accepted Answer · answered Aug 02 '19 at 19:28

You are outputting the MAC Address as ASCII instead of Hex. Here's a quick method I put together that will detect non-ascii characters and output as hex if any are found.

public void WalkTable()
    {
    const string baseOid = "1.3.6.1.4.1.14179.2.2.1.1"; //The entire table
    const string community = "public";

    var ep = new IPEndPoint(IPAddress.Parse("10.1.2.3"), 161);
    var results = new List<Variable>();
    Messenger.Walk(VersionCode.V2, ep, new OctetString(community), new ObjectIdentifier(baseOid), results, 60000, WalkMode.WithinSubtree);

    foreach(var v in results)
        //If the result is an OctetString, check for ascii, otherwise use ToString()
        Console.WriteLine(v.Data.TypeCode.ToString()=="OctetString" ? DecodeOctetString(v.Data.ToBytes()) : v.Data.ToString())
    }
}

public string DecodeOctetString(byte[] raw)
{
    //First 2 bytes are the Type, so remove them
    byte[] bytes = new byte[raw.Length - 2];
    Array.Copy(raw, 2, bytes, 0, bytes.Length);

    //Check if there are any non-ascii characters
    bool ascii = true;
    foreach (char c in Encoding.UTF8.GetString(bytes))
    {
        if (c >= 128)
        {
            ascii = false;
        }
    }

    //If it's all ascii, return as ascii, else convert to hex
    return ascii ? Encoding.ASCII.GetString(bytes) : BitConverter.ToString(bytes);
}

this solution works but its changes all other octet strings like System Name and change it to hex (there is any other solution to segregate macAddress and something like System Name)? — Ramin Azali, Aug 01 '22 at 12:25

Properly parse MAC Address

1 Answers1