so, we've scratching our heads today, facing some MTU issues on our
shiny new libre-mesh network,
and after a few hours debugging, tcpdumping and discussing, we came to
* the invented src-addr in the bmxOut tunnel makes it impossible for
hops along the path to return "Packet Too Big" to the originating node.
So, if a particular link has a smaller MTU than the first link (say, a
VPN is involved), the packet will be silently dropped.
A wants to send a packet to a network announced by D; so it creates a
tunnel with D as destination, and a "D-derived" fake address as src,
that matches the bmxIn "catchall" tunnel peer-addr in D
Then sends a 1460 + 40 = 1500 bytes packet through that tunnel,
B cannot push that packet to C, then tries to send back a ICMPv6 PTB...
but the src-addr it finds in the encapsulation is not A, but instead the
"D-derived" fake addr. Then, the ICMPv6 PTB is lost and A can never find
out about the smaller MTU.
This fundamentally breaks PMTUD which is a bad idea in IPv6
to avoid this there are 3 options:
* set mtu=1280 on every bmxOut tunnel (yuck! :( )
* probe before establishing each tunnel, with the real src-addr so that
PMTUD can happen correctly, until it reaches the desired endpoint node.
Then, use discovered PMTU for new tunnel. (*downside*: this will only
work as long as path doesn't change to pass through some thinner link.
In that case, PMTU will not be rediscovered, and packets will be dropped
* use the current bmxIn "catchall" tunnels only for sending special bmx6
control packets, that ask for a symmetric tunnel.
1) A sends to D (2001:db8::D) a packet (encapsulated with a fake
src-addr, to be catched by the catchall @ D) with content "I'm A and
this is my real address 2001:db8::A; please make a dedicated tunnel for me"
2) D gets that packet and creates a tunnel with "peer 2001:db8::A", then
sends back an ACK to A, again using "A-derived" fake-addr as src
3) A gets the ACK and creates the tunnel with "peer 2001:db8::D"
*** now both ends have a symmetric tunnel between them, with real src
and dest address ***
4) A finally sends the real payload through the symmetric tunnel, this
payload (may be bigger than 1280, say... 1450) will be encapsulated with
the real src-addr of A, so if any node in the path needs to send back a
Packet-too-big, will be able to, and PMTUD will happen correctly.
(at the cost of a full RTT latency before the first payload packet, but
with a reasonable tunnel expire time as it has currently, that shouldn't
back in April, i remember we discussed the idea of symmetric tunnels,
and you brought up this "control connection" idea which i'm simply
But that discussion was in another context, more like a 'feature', and
finally didn't really solve the idea we had originally,
yet, this PMTUD issue was not taken into account AFAIR, so the
"symmetric tunnel" idea now becomes more like a bugfix (i.e. don't
create PMTUD blackholes)
what do you think?
Again. No SSH, no web, no ping.
Let's do a copy for high availability. Isaac said to do it in his server. I also can do it in one of our servers. I'm sure that all of you have others servers.
Now we can do a copy, but if you want to have it distributed servers we can make it available from several sharing same IP with BGP in internet.
We can do a lot of things, but not let it down... is a bad image for the project.
After a day of debugging with Al here in garraf and some our spent on fixing
make files, and make address generation simpler and human understandable libre-
mesh finally compile, you can find the images ready for testing on torvic in
See you soon
P.S. We did realized after some work that there is some unmerged Guii work in
the repo and also that there will be some conflict on merging, so please do the
merge toghether so we can decide what to keep and what to not keep
As we can read in this thread  dnsmasq doesn't permit to specify a custom
mtu for ipv6, this is needed on out setup to avoid batman-adv level
fragmentation, so the actual solution in dnsmasq doesn't work because it
requires to put a lower mtu on specified interface and we cannot ( and don't
want to do this )
Gui can you ask to dnsmasq developer to add this option so dnsmasq can work
good also on our setup ?
P.S. With al we have debuged some mtu iussue and committed fixes
I don't remember well if finally we got to have libre-mesh enough stable for
use/testing, or if we are still experiencing random strange problems, do we
have some news after hackmeeting ?
The other day I was in a talk about guifi in madrid, and also talked a little
about libre-mesh, the result is that people of guifi madrid are very interested
in using libre-mesh, because it seems it is more adapt to their network
topology ( at moment they use point to multipoint and seems this is affecting
negativley the growing rate of the netowork )
BTW from what i eared they have mostly ubiquity devices, with just one
ethernet port that with libre-mesh will be needed both for mesh and client
access, so stable working vlan is now a must ( disabling mesh or client access
on eth0 to cope with tplink switch bugs is not acceptable anymore ) so let's
see what we can do ;)
Moreover we are trying to organize a little hackerasmus session during xmas
vacation, hopefully we can advance with the firmware ;)