使用自定义数据包解析PCAPNG（如何检查UnknowLinkType的数据包辅助数据）

I'm trying to parse pcapng file with gopacket package. It works well, but sometimes i meet custom (modified) packets and i need to detect them.

I know for sure that this custom packets has encapsulation type 148, that can't be recognized by wireshark or gopacket library. Here's the example of the packet:

Custom packet in Wireshark

When i'm try to analyze pcapng file with gopacket, i can check packet AncillaryData from packet.Metadata() for LinkTypeEthernet or other types. But for custom packets packet.Metadata().AncillaryData[0] returns LinkTypeUnknow.

Here's an example of code:

f, err := os.Open("sample.pcapng")
if err != nil {
    return err
}
defer f.Close()

r, err := pcapgo.NewNgReader(f, pcapgo.DefaultNgReaderOptions)
if err != nil {
    return err
}

packetSource := gopacket.NewPacketSource(r, layers.LayerTypeEthernet)
for packet := range packetSource.Packets() {

   if packet.Metadata().AncillaryData[0] == layers.LinkTypeEthernet {
     // It works!
   }

}

But this code throws an error: undefined layers.LinkTypeUnknown

if packet.Metadata().AncillaryData[0] == layers.LinkTypeUnknown {
    // Doesn't work
}

So, how can i check ancillary data for LinkTypeUnknown or value 148? Or, please, tell me another way to recognize this custom packets.