UDP broadcast

[Goran Mekić]

Hello,

I have a audio mixer which is controleable over network via android app.
The discovery is done by sending broadcast UDP message "/info\0\0\0" to
255.255.255.255 (checked by tcpdump on the router). I thought I can do
the same with:

printf "/info\0\0\0" | nc -4u -w 0 255.255.255.255 10024

But I never get the reply. This is what tcpdump sees:

tcpdump -nnSX -v 'src 192.168.5.80 or dst 255.255.255.255'
tcpdump: listening on re0, link-type EN10MB (Ethernet), capture size
262144 bytes
23:41:25.595663 IP (tos 0x0, ttl 64, id 28810, offset 0, flags [none],
proto UDP (17), length 36, bad cksum 0 (->4439)!)
    192.168.5.94.36465 > 255.255.255.255.10024: UDP, length 8
		0x0000:   4500 0024 708a 0000 4011 0000 c0a8 055e  E..$p... at ......^
		0x0010:  f  fff ffff 8e71 2728 0010 c627 2f69 6e66  .....q'(...'/inf
		0x0020:  6f00    0000                                o...

I never get any reply, but when I try the same with the android app while sniffing on the router:


tcpdump -nnSX -v -i bridge0 'dst 255.255.255.255 or src 192.168.5.80'
tcpdump: listening on bridge0, link-type EN10MB (Ethernet), capture size 262144 bytes
23:43:52.165821 IP (tos 0x0, ttl 64, id 20743, offset 0, flags [DF], proto UDP (17), length 36)
    192.168.5.79.57477 > 255.255.255.255.10024: UDP, length 8
	0x0000:  4500 0024 5107 4000 4011 23cb c0a8 054f  E..$Q. at .@.#....O
	0x0010:  ffff ffff e085 2728 0010 2559 2f69 6e66  ......'(..%Y/inf
	0x0020:  6f00 0000                                o...
23:43:52.171565 IP (tos 0x0, ttl 128, id 8, offset 0, flags [none], proto UDP (17), length 84)
    192.168.5.80.10024 > 192.168.5.79.57477: UDP, length 56
	0x0000:  4500 0054 0008 0000 8011 aea1 c0a8 0550  E..T...........P
	0x0010:  c0a8 054f 2728 e085 0040 2677 2f69 6e66  ...O'(...@&w/inf
	0x0020:  6f00 0000 2c73 7373 7300 0000 5630 2e30  o...,ssss...V0.0
	0x0030:  3400 0000 5852 3138 2d32 372d 3031 2d34  4...XR18-27-01-4
	0x0040:  3700 0000 5852 3138 0000 0000 312e 3039  7...XR18....1.09
	0x0050:  0000 0000                                ....

So what am I doing wrong and how can I send UDP broadcast from either script of C code? Thank you!

Regards,
meka
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