netdev - Re: Linux, tcpdump and vlan

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Thu, 19 Jul 2007 14:41:31 +0100
From:	Stephen Hemminger <shemminger@...ux-foundation.org>
To:	Krzysztof Halasa <khc@...waw.pl>
Cc:	Patrick McHardy <kaber@...sh.net>,
	andrei radulescu-banu <iubica2@...oo.com>,
	linux-kernel@...r.kernel.org,
	Linux Netdev List <netdev@...r.kernel.org>
Subject: Re: Linux, tcpdump and vlan

On Thu, 19 Jul 2007 15:28:46 +0200
Krzysztof Halasa <khc@...waw.pl> wrote:

> Patrick McHardy <kaber@...sh.net> writes:
> 
> > Your suggestion of disabling VLAN acceleration in promiscous
> > mode sounds like a reasonable solution until then ..
> 
> From a user perspective:
> 
> I'm not sure promiscous mode is related to the problem.
> Tcpdump without promiscous mode makes perfect sense.
> 
> I don't know very well VLAN code internals, but I think
> the VLAN # is used for looking up the interface, so
> presenting the "original" packet on the trunk device
> would IMHO involve some skb cloning, and perhaps some
> ethtool option could probably control that.
> 
> Not sure about untagged frames vs. tagged frames with
> the default VLAN id - can the hardware at all differentiate
> between them?
> 
> 
> Or, perhaps it should be left (almost) as is - with "software"
> VLANs the traffic always goes through the master interface,
> but with "accelerated" mode it only goes through logical
> interfaces and doesn't show up on master? Probably with
> exception of invalid VLANs, which could be injected back to
> master (because no logical device exists)?


I don't claim to be a VLAN expert but there are really three cases
for handling tagged frames

1) non-accelerated device 
    * all frames show in promiscious mode
    * tag is part of the frame that shows up
       in tcpdump, and then gets stripped by the 8021q module.
2) rx tag stripping device
     * all frames show in promiscious mode
     * tag is in skb but NOT passed to tcpdump
3) rx vlan acceleration
     * only frames that for vlan's that are registered show up
        in promisicous mode
     * tag is in skb but NOT passed to tcpdump

Unfortunately, the tag is lost as part of the VLAN acceleration process
so it is not a simple matter of changing code in AF_PACKET receive
to restore the tag.
-
To unsubscribe from this list: send the line "unsubscribe netdev" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html