Tomas Carnecky
2010-Jan-31 23:34 UTC
[Bridge] Bug when using bridge + ipv6 link local addresses
While trying to make distcc compatible with IPv6, I came across this..
uhm, bug..?
My setup: I use a bridge as my primary network interface. This is so I
can brigde my KVM VMs with the physical network. But most of the time
the bridge (vmnet) contains only the physical interface (eth0):
$ brctl show vmnet
bridge name bridge id STP enabled interfaces
vmnet 8000.0016d3ce57d9 no eth0
The network configuration:
$ ifconfig
eth0 Link encap:Ethernet HWaddr 00:16:d3:ce:57:d9
inet6 addr: fe80::216:d3ff:fece:57d9/64 Scope:Link
vmnet Link encap:Ethernet HWaddr 00:16:d3:ce:57:d9
inet addr:192.168.0.82 Bcast:192.168.0.255 Mask:255.255.255.0
inet6 addr: 2001:xxxx:xxxx:0:216:d3ff:fece:57d9/64 Scope:Global
inet6 addr: fe80::216:d3ff:fece:57d9/64 Scope:Link
As you can see the IPv6 link local address is assigned to both vmnet and
eth0. But I can ping both interfaces (ping6 fe80..%vmnet and %eth0) just
fine.
I was able to create a small application which reproduces the bug. In
short, a server listens on ::, a client connects to a link-local address
(fe80..%eth0) and starts writing data, while the server reads it from
the socket. After a while both processes block in read/write
respectively and I see the following:
$ netstat -ntp
tcp6 0 53188 fe80::...:45398 fe80::....:8080 ESTABLISHED
tcp6 0 0 fe80::....:8080 fe80::...:45398 ESTABLISHED
The send-queue of the writing application has data in it, but somehow
the reading side can't read it. When I change the client to connect to
fe80..%vmnet (or ::1 or the global scope address) the test application
(and distcc) work as expected.
The test application I use is attached. You should change the address
the client is trying to connect to (line 63, replace it with your real
link local address and append the scope id of either the bridge or the
network interface attached to the bridge). This is the output on my box:
child: wrote 4096 bytes
child: wrote 4096 bytes
child: wrote 4096 bytes
child: wrote 4096 bytes
child: wrote 4096 bytes
child: wrote 4096 bytes
child: wrote 4096 bytes
child: wrote 4096 bytes
child: wrote 4096 bytes
child: wrote 4096 bytes
child: wrote 4096 bytes
child: wrote 4096 bytes
hostname: fe80::216:d3ff:fece:57d9%eth0
parent: read 4096 bytes
parent: read 4096 bytes
parent: read 4096 bytes
parent: read 4096 bytes
parent: read 4076 bytes
The child blocks while writing between byte 49152 and 53248 (which
matches the contents of the send-queue), and the server blocks sometime
after reading 20460 bytes.
I am willing to test patches, even experimental ones.
tom
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