Using ICMP and SNMP to detect the dynamic IP address of your router

The following Delphi code uses ICMP and SNMP to detect the dynamic IP address of your router when it is connected to a WAN.

It first gets the IP address of www.borland.com and then attempts to do a trace route to it. The trace route is only allowed to travel at most 1 hop. In a corporate environment, some changes are required in order to detect the router for the corporate environment.

Since the first hop is definitely the router's IP address, it assumes it has gotten the gateway IP address.

It then uses the SNMP protocol to discover the dynamic IP address that the router has been assigned with.

var
  GatewayIP, BorlandIP: string;
  DynamicIP, origOid: string;
  I: Integer;
  SNMP: TIdSNMP;
  ICMP: TIdIcmpClient;
begin
  SNMP := TIdSNMP.Create(nil);
  ICMP := TIdIcmpClient.Create(nil);
  try
  // Step 1: Get the Gateway IP address
  // Assumes the Gateway is only 1 connection away from the machine
    BorlandIP := GStack.ResolveHost('www.borland.com');
    ICMP.Host := BorlandIP;
    ICMP.TTL := 1;
    ICMP.ReceiveTimeout := 5000;
    ICMP.Ping;

    GatewayIP := ICMP.ReplyStatus.FromIpAddress;

  // Step 2: Query the gateway using SNMP
  // This assumes the gateway is the immediate connection to the Internet
  // or leads to a xDSL modem running in bridging mode that connects to
  // the Internet

    SNMP.Community := 'public';
    SNMP.Host := GatewayIP;
    SNMP.Query.Clear;
    origOID := '1.3.6.1.2.1.4.20.1.1';
    SNMP.Query.MIBAdd(origOID, '');
    SNMP.Query.PDUType := PDUGetNextRequest;

    while SNMP.SendQuery do
      begin
        if Copy(SNMP.Reply.MIBOID[0], 1, Length(origOID)) <> origOID then
          Break;

  // For each result, check that it is not the Gateway IP
  // If it is not, assume that it is the Dynamic IP
  // If a gateway has multiple IP addresses assigned, the last
  // IP address will be considered to be the Dynamic IP
        for I := 0 to SNMP.Reply.ValueCount - 1 do
          if SNMP.Reply.Value[I] <> GatewayIP then
            DynamicIP := SNMP.Reply.Value[I];

        SNMP.Query.Clear;
        SNMP.Query.MIBAdd(SNMP.Reply.ValueOID[0], '');
        SNMP.Query.PDUType := PDUGetNextRequest;
      end;
  finally
    ICMP.Free;
    SNMP.Free;
  end;