I have been trying to knock up a Qt program to write and read UDP packets and spent a day and half before eventually working out it's nothing to do with my Qt programming skills, but my desktop seemingly stopping all UDP packets arriving.

To give more details:

My desktop is 192.168.0.18 and my laptop is 192.168.0.19. I can send out packets from my desktop to an embedded board which reads the packet fine and returns the requisite data. Wireshark shows the returned packet is fine, but my desktop will not see it. So, I tried sending from my laptop to my desktop using my Qt program and it did not work. I eventually discovered netcap and started playing with that, finally working out that netcap would send from desktop to laptop and laptop's netcap would display packets, but sending from laptop to desktop, again, even though wireshark shows these packets being sent to the desktop (and indeed, the desktop is running wireshark), netcap on the desktop is receiving no packets.

I have no firewall switched on, so it can't be that, NTP works as far as I can see, so some UDP packets are getting in I think.

Can anyone think what on earth might be stopping other UDP packets or think of anything else I can try to track down the issue?

Many thanks!

What do you get when you do:

iptables -L
Chain INPUT (policy DROP)
target prot opt source destination
Ifw all -- anywhere anywhere
net2fw all -- anywhere anywhere
ACCEPT all -- anywhere anywhere
Reject all -- anywhere anywhere
LOG all -- anywhere anywhere LOG level info prefix "Shorewall:INPUT:REJECT:"
reject all -- anywhere anywhere [goto]

Chain FORWARD (policy DROP)
target prot opt source destination
Reject all -- anywhere anywhere
LOG all -- anywhere anywhere LOG level info prefix "Shorewall:FORWARD:REJECT:"
reject all -- anywhere anywhere [goto]

Chain OUTPUT (policy DROP)
target prot opt source destination
fw2net all -- anywhere anywhere
ACCEPT all -- anywhere anywhere
Reject all -- anywhere anywhere
LOG all -- anywhere anywhere LOG level info prefix "Shorewall:OUTPUT:REJECT:"
reject all -- anywhere anywhere [goto]

Chain Broadcast (2 references)
target prot opt source destination
DROP all -- anywhere anywhere ADDRTYPE match dst-type BROADCAST
DROP all -- anywhere anywhere ADDRTYPE match dst-type MULTICAST
DROP all -- anywhere anywhere ADDRTYPE match dst-type ANYCAST
DROP all -- anywhere base-address.mcast.net/4

Chain Drop (1 references)
target prot opt source destination
all -- anywhere anywhere
reject tcp -- anywhere anywhere tcp dpt:auth /* Auth */
Broadcast all -- anywhere anywhere
ACCEPT icmp -- anywhere anywhere icmp fragmentation-needed /* Needed ICMP types */
ACCEPT icmp -- anywhere anywhere icmp time-exceeded /* Needed ICMP types */
Invalid all -- anywhere anywhere
DROP udp -- anywhere anywhere multiport dports loc-srv,microsoft-ds /* SMB */
DROP udp -- anywhere anywhere udp dpts:netbios-ns:netbios-ssn /* SMB */
DROP udp -- anywhere anywhere udp spt:netbios-ns dpts:1024:65535 /* SMB */
DROP tcp -- anywhere anywhere multiport dports loc-srv,netbios-ssn,microsoft-ds /* SMB */
DROP udp -- anywhere anywhere udp dpt:1900 /* UPnP */
NotSyn tcp -- anywhere anywhere
DROP udp -- anywhere anywhere udp spt:domain /* Late DNS Replies */

Chain Ifw (1 references)
target prot opt source destination
RETURN all -- anywhere anywhere match-set ifw_wl src
DROP all -- anywhere anywhere match-set ifw_bl src
IFWLOG all -- anywhere anywhere ctstate INVALID,NEWpsd weight-threshold: 10 delay-threshold: 10000 lo-ports-weight: 2 hi-ports-weight: 1 IFWLOG prefix 'SCAN'

Chain Invalid (2 references)
target prot opt source destination
DROP all -- anywhere anywhere ctstate INVALID

Chain NotSyn (2 references)
target prot opt source destination
DROP tcp -- anywhere anywhere tcp flags:!FIN,SYN,RST,ACK/SYN

Chain Reject (3 references)
target prot opt source destination
all -- anywhere anywhere
reject tcp -- anywhere anywhere tcp dpt:auth /* Auth */
Broadcast all -- anywhere anywhere
ACCEPT icmp -- anywhere anywhere icmp fragmentation-needed /* Needed ICMP types */
ACCEPT icmp -- anywhere anywhere icmp time-exceeded /* Needed ICMP types */
Invalid all -- anywhere anywhere
reject udp -- anywhere anywhere multiport dports loc-srv,microsoft-ds /* SMB */
reject udp -- anywhere anywhere udp dpts:netbios-ns:netbios-ssn /* SMB */
reject udp -- anywhere anywhere udp spt:netbios-ns dpts:1024:65535 /* SMB */
reject tcp -- anywhere anywhere multiport dports loc-srv,netbios-ssn,microsoft-ds /* SMB */
DROP udp -- anywhere anywhere udp dpt:1900 /* UPnP */
NotSyn tcp -- anywhere anywhere
DROP udp -- anywhere anywhere udp spt:domain /* Late DNS Replies */

Chain dynamic (1 references)
target prot opt source destination

Chain fw2net (1 references)
target prot opt source destination
ACCEPT all -- anywhere anywhere ctstate RELATED,ESTABLISHED
ACCEPT all -- anywhere anywhere

Chain logdrop (0 references)
target prot opt source destination
DROP all -- anywhere anywhere

Chain logreject (0 references)
target prot opt source destination
reject all -- anywhere anywhere

Chain net2fw (1 references)
target prot opt source destination
dynamic all -- anywhere anywhere ctstate INVALID,NEW,UNTRACKED
ACCEPT all -- anywhere anywhere ctstate RELATED,ESTABLISHED
Drop all -- anywhere anywhere
LOG all -- anywhere anywhere LOG level info prefix "Shorewall:net2fwROP:"
DROP all -- anywhere anywhere

Chain reject (10 references)
target prot opt source destination
DROP all -- anywhere anywhere ADDRTYPE match src-type BROADCAST
DROP all -- base-address.mcast.net/4 anywhere
DROP igmp -- anywhere anywhere
REJECT tcp -- anywhere anywhere reject-with tcp-reset
REJECT udp -- anywhere anywhere reject-with icmp-port-unreachable
REJECT icmp -- anywhere anywhere reject-with icmp-host-unreachable
REJECT all -- anywhere anywhere reject-with icmp-host-prohibited

Chain sfilter (0 references)
target prot opt source destination
LOG all -- anywhere anywhere LOG level info prefix "Shorewall:sfilterROP:"
DROP all -- anywhere anywhere

Chain shorewall (0 references)
target prot opt source destination

well you do have a firewall active assuming this iptables output is from your desktop and I don't see any rule to allow udp packages coming in from local network, you have one rule that allows traffic from Internet on any port and protocol but nothing beyond that.

In any case, having that firewall up just add a rule to allow udp traffic to the ports you need from 192.168.0/24 network

Thanks for that. Will investigate further.