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[solved] How to find an unused TAP device ?
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toralf
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Joined: 01 Feb 2004
Posts: 2717
Location: Hamburg/Germany

PostPosted: Fri Dec 28, 2012 8:54 pm    Post subject: [solved] How to find an unused TAP device ? Reply with quote

With kernel 3.8. I just realized that all pre-defined TAP devices are RUNNING even if no virtual linux system is using it. Now I'm wondering whether this is a new feature of the upcoming kernel, a bug of the old - and how I can avoid that.
Background : When I start a user mode linux instance, I currently grep for the next free tap device in this way :
Code:
for t in $(ifconfig | grep "^tap" | cut -f1 -d:)
do
   ethtool $t | grep -q 'Link detected: no'
   if [[ $? -eq 0 ]]; then
      NET="tuntap,$t"
      break
   fi
done
but this doesn't work now anymore.

Here is the diff in dmesg
Code:
$ diff 3.7.1 3.8.0-rc1+ | grep UP
< br0: flags=4355<UP,BROADCAST,PROMISC,MULTICAST>  mtu 1500
> br0: flags=4419<UP,BROADCAST,RUNNING,PROMISC,MULTICAST>  mtu 1500
< tap0: flags=4099<UP,BROADCAST,MULTICAST>  mtu 1500
> tap0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
and my config
Code:
$ head -n 20 /etc/conf.d/net
# /etc/conf.d/net
#

#       KVM
#
bridge_br0="tap0 tap1 tap2 tap3"
config_br0="192.168.0.254/8"
brctl_br0="setfd 0
sethello 10
stp off"
rc_net_br0_need="net.tap0 net.tap1 net.tap2 net.tap3"

config_tap0="null"
tuntap_tap0="tap"
tunctl_tap0="-u <me>"
...


Last edited by toralf on Mon Jan 28, 2013 6:39 pm; edited 1 time in total
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Rexilion
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Joined: 17 Mar 2009
Posts: 1044

PostPosted: Sat Jan 26, 2013 11:37 am    Post subject: Reply with quote

That's a regression, not a feature. The kernel has an explicit goal to consistently (across versions) provide a reliable interface to the user.

I suggest you do a git bisect to speed up the debug. Perhaps:

Code:
git bisect start HEAD v3.7 -- drivers/net/tun.c


Looks like you are in for a treat =) * points down *

Quote:
gebruiker@Delta:~/Documenten/Ronald/kernel$ git rev-list --oneline v3.7..v3.8-rc3 -- drivers/net/tun.c
9fdc6be tuntap: dont use a private kmem_cache
d32649d tuntap: fix sparse warning
76fe458 tuntap: reset network header before calling skb_get_rxhash()
4008e97 tuntap: fix ambigious multiqueue API
4997442 tuntap: dont use skb after netif_rx_ni(skb)
a676847 tun: allow setting ethernet addresss while running
b3943ae tun: correctly report an error in tun_flow_init()
5d09710 tun: only queue packets on device
eb0fb36 tuntap: attach queue 0 before registering netdevice
3872baf tun: put correct method name in a debug message.
36fe8c09 vtun: fix typos.
9ce99cf tun: change tun_get_iff() prototype.
c260b77 net: Allow userns root to control tun and tap devices
149d36f tun: report orphan frags errors to zero copy callback
96442e42 tuntap: choose the txq based on rxq
cde8b15 tuntap: add ioctl to attach or detach a file form tuntap device
c8d68e6 tuntap: multiqueue support
6e914fc tuntap: RCUify dereferencing between tun_struct and tun_file
54f968d tuntap: move socket to tun_file
1e58833 tuntap: log the unsigned informaiton with %u
6a328d8 cgroup: net_cls: Rework update socket logic
fd9a08a cgroup: net_cls: Pass in task to sock_update_classid()

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toralf
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PostPosted: Mon Jan 28, 2013 6:39 pm    Post subject: Reply with quote

fixed in git tree
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