Cope with fragmented UDP packets.

Fixes #275.

Use this as the systemd unit file to run as a service:

[Unit]
Description=Dougal Network Packet Capture
After=network.target remote-fs.target nss-lookup.target

[Service]
ExecStart=/srv/dougal/software/sbin/packet-capture.sh
ExecStop=/bin/kill -s QUIT $MAINPID
Restart=always
User=root
Group=users
Environment=PATH=/usr/bin:/usr/sbin:/usr/local/bin
Environment=INS_HOST=172.31.10.254
WorkingDirectory=/srv/dougal/software/var/
SyslogIdentifier=dougal.pcap

[Install]
WantedBy=multi-user.target

sbin/packet-capture.sh

@@ -16,7 +16,12 @@ OUTPATH="$OUTDIR/$OUTNAME"
 # 30000/UDP: Navigation system headers
 # Not all inputs will be present in all systems.
 #
-EXPR="udp and (port 4461 or port 4462 or port 30000)"
+# NOTE: $INS_HOST must be defined and point to the
+# navigation server. The reason we don't use a port
+# filter for this data is because that doesn't work
+# with fragmented UDP packets.
+#
+EXPR="udp and (port 4461 or port 4462 or src host $INS_HOST)"
 
 if [[ ! -d "$OUTDIR" ]]; then
     mkdir "$OUTDIR"