Two taps, one IP?
Can you suggest any way for me to configure my Ubuntu host or Debian
qemu guests such that the host can access each webserver running in both
guests?
(I originally posted this to the qemu list, but I'm starting to think
it's not a qemu issue; rather, I think qemu is fine but I'm configuring
either my Debian guests or Ubuntu host wrong. I expect it's the sort of
thing that the right person will say "ah, obviously!" -- are you that
person?)
In particular, I'm able to access the webserver on one image just fine,
but not the other: wget fails with "Connecting to 172.20.0.3:80...
failed: No route to host."
Can you explain why and set me straight?
I have two Debian qemu images (0 and 1), identical in all respects
except that image0 and image1 are configured to use static IPs
172.20.0.2 and 172.20.0.3, respectively. I've launched both
simultaneously with the following commands:
sudo qemu -kernel-kqemu -net nic,macaddr=00:00:00:00:00:00 -net tap
image0.raw
sudo qemu -kernel-kqemu -net nic,macaddr=00:00:00:00:00:11 -net tap
image1.raw
This creates two tap interfaces (0 and 1) on the Ubuntu host, curiously
with the same IP:
tap0 Link encap:Ethernet HWaddr 00:ff:84:12:9d:72
inet addr:172.20.0.1 Bcast:172.20.255.255 Mask:255.255.0.0
inet6 addr: fe80::2ff:84ff:fe12:9d72/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:18 errors:0 dropped:0 overruns:0 frame:0
TX packets:36 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:500
RX bytes:1336 (1.3 KB) TX bytes:4704 (4.5 KB)
tap1 Link encap:Ethernet HWaddr 00:ff:af:9a:48:29
inet addr:172.20.0.1 Bcast:172.20.255.255 Mask:255.255.0.0
inet6 addr: fe80::2ff:afff:fe9a:4829/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:24 errors:0 dropped:0 overruns:0 frame:0
TX packets:34 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:500
RX bytes:1656 (1.6 KB) TX bytes:4664 (4.5 KB)
Furthermore, each image is configured with the following
/etc/network/interfaces:
auto lo
iface lo inet loopback
allow-hotplug eth0
iface eth0 inet static
address 172.20.0.2 <--- image1 has: address 172.20.0.3
netmask 255.255.0.0
gateway 172.20.0.1
The upshot is "wget http://172.20.0.2"; and "wget http://172.20.0.3"; each
work fine inside their respective VMs. But each is unable to wget the
other's webserver.
Furthermore, and most unusual, the host is able to wget image0's
webserver fine, but not image1. Specifically, the second wget fails as
follows:
david@SonOfLappy:/svn/staging$ wget http://172.20.0.3
--18:17:12-- http://172.20.0.3/
=> `index.html.1'
Connecting to 172.20.0.3:80... failed: No route to host.
david@SonOfLappy:/svn/staging$
The error message suggests some sort of routing problem, and the routing
table is:
david@SonOfLappy:/svn/staging$ route
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use
Iface
68.28.57.85 * 255.255.255.255 UH 0 0 0 ppp0
172.20.0.0 * 255.255.0.0 U 0 0 0 tap0
172.20.0.0 * 255.255.0.0 U 0 0 0 tap1
default * 0.0.0.0 U 0 0 0 ppp0
david@SonOfLappy:/svn/staging$
However, I'll admit I don't know much about the routing layer and thus
I'm not sure how to diagnose beyond that. But it seems very strange to
me to have two network interfaces with the same IP.
With this in mind, if I shut down image0, the tap0 interface goes away,
and now the wget to image1 works fine. Again, this is suggesting
there's some kind of conflict where the second tap interface is somehow
"blocked" by the first.
Anyway, that's as far as I can get. Can you suggest any way I can
configure the host's routing table in order to access image1? Or can I
manually create and share a single tap device? (I tried setting fd=tap0
on the second launch command, but this had no effect.)
(Incidentally, I also tried putting the second image onto a different
vlan by replacing adding "vlan=0" with "vlan=1" to each image's
respective launch command, but that had no effect -- the results were
identical.)
Any suggestions? Thanks for any tips you can provide!
-david
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