06.03.2019 5:46, Goran Mekić wrote: > 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)!)
As shown, nc generates UDP datagramm without (optional) checksum. > 192.168.5.94.36465 > 255.255.255.255.10024: UDP, length 8 > 0x0000: 4500 0024 708a 0000 4011 0000 c0a8 055e > E..$p...@......^ > 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 Here tcpdump dump does not complain on missing checksum. This means Android sends checksummed datagramms. Perhaps, receiver discards datagramms without checksum. > So what am I doing wrong and how can I send UDP broadcast from either script > of C code? Thank you! You should try ncat from security/nmap instead of nc. _______________________________________________ freebsd-net@freebsd.org mailing list https://lists.freebsd.org/mailman/listinfo/freebsd-net To unsubscribe, send any mail to "freebsd-net-unsubscr...@freebsd.org"
