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FS#1592 - map.sh failing to do MTU:1460bytes #6655

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openwrt-bot opened this issue Jun 17, 2018 · 1 comment
Closed

FS#1592 - map.sh failing to do MTU:1460bytes #6655

openwrt-bot opened this issue Jun 17, 2018 · 1 comment
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@openwrt-bot
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tahiro86j:

  • Device problem occurs on
    Buffalo BHR-4GRV

  • Software versions of OpenWrt/LEDE release, packages, etc.
    Reboot (17.01.4, r3560-79f57e422d)

  • Steps to reproduce
    Somewhat relevant to the bug report on //map.sh// from a few days ago, I also discovered the following :

While the MTU of the IPv4-over-IPv6 tunnel created by that script may be set to 1460bytes in ///etc/config/network//, the actual tunnel will only achieve 1452bytes when tested with ping at DF=1.

FYI, the MAP-E service I am using (called "v6plus" provided by JPNE, AS2516) runs on the 1500byte-MTU for IPv6 and is known to support 1460bytes.
You may also see this on the page of YAMAHA, for their brand-new firmware released for RTX1210 router. (http://www.rtpro.yamaha.co.jp/RT/docs/v6plus/index.html - in Japanese)

BTW, the MTU of such tunnels being limited to some value that is 8bytes less than what is theoretically possible on Linux-based systems is something I've seen before and it has something to do with encapsulation limit option (which can be disabled).
If this were on Debian/Ubuntu I would do something like this - or else I get the 1452bytes with extra 8bytes subtracted from 1460bytes for MTU.

//pre-up ip tunnel add ${IFACE} mode ipip6 remote local encaplimit none//

Now, I tried to see how I could do the same on OpenWrt - and since I did not know how scripts brought up tunnels precisely, I began by seeing if any other implementations for OpenWrt using IPv4-over-IPv6 tunnel behaved any differently. I discovered that the //dslite.sh// in the package //ds-lite// installed on BHR-4GRV actually does the 1460bytes-MTU when tested with my server on AS9370 (Sakura Internet, with the MTU at 1500bytes for the L3, behaving as a simulated CGN of DS-Lite). I then tried to see if those two scripts did anything different with respect to creating tunnels. Unfortunately, I was not able to spot any notable differences (or maybe I should have gone further with the investigation).

I know this is most likely a simple fix, and maybe the most I need to get from this bug report is the info on what to look into rather than actually filing a bug report. Any helpful info is appreciated since I do intend to work on this by myself.

Thanks

@openwrt-bot
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dedeckeh:

Hi,

In commit https://git.openwrt.org/?p=openwrt/openwrt.git;a=commit;h=e4577d2e68ab8943375e719204a680cbbe3bd561 encaplimit has been made configurable for the map-e protocol. Setting encaplimit to ignore will not include the encapsulation limit option in the IPv4-over-IPv6 packets; remark this encapsulation limit option config is only available on master. It would be nice if you could give this a try

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