Hi,
When the IPv6 default route is first autoconfigured, it seems the
advertised MSS doesn't get set properly.
I have computer A connected to an external interface with IPv4 and
tunneling IPv6 using 6to4. It runs radvd on an internal interface to
provide connectivity to my LAN. radvd.conf contains:
interface brint {
AdvLinkMTU 1420; # advertise tiny MTU due to tunnel
AdvSendAdvert on;
prefix 0:0:0:1::/64
{ Base6to4Interface brext; };
};
Computer B is on the LAN and autoconfigures an IPv6 address. However,
I have problems with some specific hosts [1]. They're probably doing
broken ICMP/PMTU filtering or something, but I've tracked down some
weirdness in the kernel:
computerA# /etc/init.d/radvd start
computerB# ifconfig eth0 down
computerB# ifconfig eth0 up
computerB# ip -6 route
2002:125f:205:1::/64 dev eth0 proto kernel metric 256 expires 2592312sec
mtu 1420 advmss 1360 hoplimit 4294967295
fe80::/64 dev eth0 metric 256 expires 21334361sec mtu 1420 advmss 1360
hoplimit 4294967295
ff00::/8 dev eth0 metric 256 expires 21334361sec mtu 1420 advmss 1360
hoplimit 4294967295
default via fe80::230:48ff:fe8b:5df4 dev eth0 proto kernel metric 1024
expires 1788sec mtu 1420 advmss 1440 hoplimit 64
At this point, I see the network problems. Note how the advmss in the
last line is not only larger than all the other lines, it's even
bigger than the MTU. I won't be able to download [1] until I manually
fix advmss, _or_ just restart radvd on the other computer:
computerA# /etc/init.d/radvd restart
computerB# ip -6 route
2002:125f:205:1::/64 dev eth0 proto kernel metric 256 expires 2592160sec
mtu 1420 advmss 1360 hoplimit 4294967295
fe80::/64 dev eth0 metric 256 expires 21334355sec mtu 1420 advmss 1360
hoplimit 4294967295
ff00::/8 dev eth0 metric 256 expires 21334355sec mtu 1420 advmss 1360
hoplimit 4294967295
default via fe80::230:48ff:fe8b:5df4 dev eth0 proto kernel metric 1024
expires 1798sec mtu 1420 advmss 1360 hoplimit 64
Now the advmss is better and everything works fine. It seems that if
it's being created for the first time, the advmss is not correctly set
based on the MTU, but if there's already a route when it gets the RA,
it reconfigures it correctly.
Computer A is 2.6.23.1 and computer B is 2.6.24-rc8, both x86_64.
Any suggestions or other debugging I can do?
-jim
[1] wget -6
http://releases.mozilla.org/pub/mozilla.org/addons/12/all-in-one_gestures-0.18.0-fx.xpi
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