gprs: Add a custom GPRS filter

Allow to inspect UDP messages and check for GPRS, NS, BSSGP
and then filter LLC frames. Parsing the vL datastructure with
the libpcap syntax is a pain. It could be done using BPF but
we do not want to use bpf asm to specify the entire ruleset.

I looked into using libepan/libwireshark but this has memory
issues and is painful too. So let's parse UDP, NS, BSSGP using
the info we already have. I tried a bit of editcap to generate
a bit of broken data. The length check might still be bad.

I used my crash_20100602.pcap file to count the LLC frames we
detect and compare that to wireshark it ended with the right
number.

  pcap add-filter gprs

can be used to enable the new filtering option after the OS
has received the packet.

Fixes: ONW#1314

1 files changed, 1 insertions, 0 deletions

diff --git a/include/osmo-pcap/osmo_pcap_client.h b/include/osmo-pcap/osmo_pcap_client.h
index 7aae7ca..30b55ee 100644
--- a/include/osmo-pcap/osmo_pcap_client.h
+++ b/include/osmo-pcap/osmo_pcap_client.h
@@ -36,6 +36,7 @@ struct osmo_pcap_client {
 	struct bpf_program bpf;
 	char   *filter_string;
 	int filter_itself;
+	int gprs_filtering;
 	struct osmo_fd fd;
 
 	char *srv_ip;