netdev - Re: Linux, tcpdump and vlan

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Message-ID: <839334.9648.qm@web56605.mail.re3.yahoo.com>
Date:	Thu, 19 Jul 2007 11:20:43 -0700 (PDT)
From:	andrei radulescu-banu <iubica2@...oo.com>
To:	Ben Greear <greearb@...delatech.com>
Cc:	Patrick McHardy <kaber@...sh.net>,
	Stephen Hemminger <shemminger@...ux-foundation.org>,
	Krzysztof Halasa <khc@...waw.pl>, linux-kernel@...r.kernel.org,
	Linux Netdev List <netdev@...r.kernel.org>
Subject: Re: Linux, tcpdump and vlan

> [Ben] If tcpdump and/or bridging needs to disable the hw-accel, then it can 
explicitly do so by some API.  That is better than overloading
the promisc flag in my opinion.  

I guess I could be persuaded in the end. But let me still play devil advocate. The semantics of 'promiscuous', in my opinion, mean 'receive everything', including vlan.

> [Ben] This is especially true since promisc 
is not easily readable by user-space and things like tcpdump
cannot have full control of promisc (if a mac-vlan has the NIC in 
promisc mode, for instance, then tcpdump can never disable it.)

I agree with all the above. For example when you run 'ifconfig' during 'tcpdump', the interface does not have the promiscuous flag set!! 

This confused me for a while, until I realized that tcpdump's packet socket was using an obscure packet_dev_mc() API (af_packet.c) to get the interface in promiscuous mode. The reason for this is that packet_mc_add() implements a reference counted mechanism for promiscuous. So that:
- starting tcpdump instance 1 sets promiscuous mode
- starting tcpdump instance 2 bumps the ref count in packet_mc_add()
- killing tcpdump instance 1 bumps down the ref count, the interface stays promiscuous
- killing tcpdump instance 2 truly clear promiscuous mode.

The trick here is that when you kill tcpdump, the kernel clears the packet socket, and in process bumps down the ref count. Had tcpdump manually set/cleared the promisc flag, the interface would have stayed promisc after tcpdump was killed.

(The mac-vlan driver must have this corner problem as well. If a mac-vlan interface is disabled while tcpdump runs, it may yank promiscuousness from under tcpdump.)

So if you want to create an ethtool API to set vlan-promiscuous mode, one problem to grapple is that we need a similar mechanism to the above, so you can run two concurrent tcpdump's (or tcpdump while bridging vlans) and the vlan-promiscuous mode gets set correctly each time.  For tcpdump at least, the new ethtool API needs to be called from packet_mc_add().

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