write to tun device with ip packet, bad format

[chengen]

i use:
tt.write(packet);
to send ip packet and than use tcp dump to capture the packet:
tcpdump -i tun8
it seems tcp dump can't recognize the packet.
here is my tun device setup:
tt = tuntap({
type: 'tun',
name: 'tun8',
mtu: 1500,
addr: '192.168.100.1',
dest: '192.168.100.2',
mask: '255.255.255.0',
ethtype_comp: 'none',
persist: true,
up: true,
running: true,
});

than i look into the code, and modify the code in
src/tuntap-itf/tuntap-itf-linux.inc.cc
while open the tun device, and add the flag as follows:

109 if(opts.mode == tuntap_itf_opts_t::MODE_TUN)
110 ifr.ifr_flags |= IFF_TUN;
111 else if(opts.mode == tuntap_itf_opts_t::MODE_TAP)
112 ifr.ifr_flags |= IFF_TAP;
113
114 //add by chengen
115 ifr.ifr_flags |= IFF_NO_PI;
116
117 MK_IOCTL(*fd, TUNSETIFF, &ifr)

line 115. and then it works. tcpdump can recognize the packet. is this a bug or i missed something on configuration?

[Dagrut]

I think that it's more a packet formatting issue than a code error here, since I never had this problem. You may be adding too much data with this flag.

Currently, the only non-data value added to the packet is the ethernet type (two bytes), as listed in src/ethertypes.itm, written on 4 byte (two of them will then be 0). The code was not made to handle more than this, and may crash when being close to the MTU (IFF_NO_PI seems to require more data : https://www.mjmwired.net/kernel/Documentation/networking/tuntap.txt ).

Try to read data from the interface and dump it as a hex string, to compare the code output and the wireshark output :-) .