[Snort-users] ERSPAN

Carter Waxman (cwaxman) cwaxman at ...589...
Tue Apr 1 09:13:57 EDT 2014


Hello,

It appears most of the packets in "notdecoded.pcap" are malformed (in particular, the IP->GRE->IP data), and the output you are seeing is from GRE-encapsulated non-IP data (at least this is how Wireshark and Snort interpret it). The packets that are malformed are simply being dropped by Snort.

I can't speak for your network, but somehow the length fields in the outer IP headers are smaller than they should be(ex. 87 —the length without the GRE and inner ethernet headers — instead of 115 for the first packet), which is why these packets are being rejected.

-Carter

From: "Russ Combs (rucombs)" <rucombs at ...589...<mailto:rucombs at ...589...>>
Date: Monday, March 31, 2014 11:40 AM
To: Fernando Cardoso <fcardoso at ...14432...<mailto:fcardoso at ...14432...>>, "snort-users at lists.sourceforge.net<mailto:snort-users at lists.sourceforge.net>" <snort-users at lists.sourceforge.net<mailto:snort-users at ...973...et>>
Subject: Re: [Snort-users] ERSPAN

Can you send a pcap?
________________________________
From: Fernando Cardoso [fcardoso at ...14432...<mailto:fcardoso at ...14432...>]
Sent: Friday, March 28, 2014 11:00 AM
To: snort-users at lists.sourceforge.net<mailto:snort-users at ...3783...net>
Subject: [Snort-users] ERSPAN

Hello,

I'm using  Snort version 2.9.6.0 GRE (Build 47) on a Ubuntu Server to sniff ERSPAN traffic.
Snort output show me entire packet of many different vlans but the source address and destination is the same configured on my switch session.
Sniffing example running snort:
snort -X -i eth1
=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+

03/28-11:37:15.569789 10.199.11.1 -> 10.200.10.10
GRE TTL:255 TOS:0x0 ID:900 IpLen:20 DgmLen:84 DF
0x0000: 00 50 56 91 06 B7 54 7F EE 96 AC 7C 08 00 45 00  .PV...T....|..E.
0x0010: 00 54 03 84 40 00 FF 2F 65 02 0A C7 C7 01 0A 64  .T.. at ...846.../e......d
0x0020: 36 C8 10 00 88 BE 32 4E CB 44 12 6B 00 01 00 01  6.....2N.D.k....
0x0030: 00 00 02 0A BD 00 00 00 02 0A BE 00 00 00 89 03  ................
0x0040: 40 20 00 B0 D1 34 32 31 00 50 56 91 72 E3 81 00  @ ...421.PV.r...
0x0050: 02 6B 08 00 45 00 00 28 67 D8 40 00 40 06 E8 6A  .k..E..(g. at ...4294.....@...16759...
0x0060: 0A FC 13 05 BA DF 11 AD 1F 90 C6 6E 81 51 5B D9  ...........n.Q[.
0x0070: 6E 90 0F 3E 50 10 00 F2 83 5D 00 00 00 00 00 00  n..>P....]......
                              ..
Where 10.199.11.1 is my source and 10.200.10.10 is my destination in my session configuration

When I use tools like tshark and gulp I can see the right source and dest not only source and dest from GRE.

My switch is a nexus 5k and my config is something like this:
session 1
---------------
type              : erspan-source
state             : up
erspan-id         : 1
vrf-name          : default
destination-ip    : 10.200.10.10
ip-ttl            : 255
ip-dscp           : 0
origin-ip         : 10.199.11.1 (global)
source intf       :
    rx            :
    tx            :
    both          :
source VLANs      :
    rx            : 10,50,100-150


My question is, can snort show the ip adress dest and source from decapsulated erspan like tshark and gulp?


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