UDP truncation

[Guus Sliepen]

On Fri, May 20, 2016 at 10:09:27PM +0300, Vladimir Kudrya wrote:

> Could you help me with UDP truncation problem?
> I have three hosts, two on broadband connection, one on 4G (lower MTU).
> Judging by tincd logs, MTU discovery works, for example on 4G host:
> 
>     tincd[4598]: Got MTU probe length 1363
> 
> TCP inside the VPN network works fine, but UDP is truncated. By testing with
> iperf and dumping traffic I get this:
> 
> Between broadband hosts:
>     20:45:04.259936 IP (tos 0x0, ttl 64, id 52068, offset 0, flags [+],
> proto UDP (17), length 1444)
>         10.2.0.1.5001 > 10.2.0.3.54616: UDP, bad length 1470 > 1416

Tinc will fragment UDP packets that are larger than the discovered path
MTU and that don't have the DF bit set. Could it be that you have a
firewall rule somewhere that is blocking fragments?

-- 
Met vriendelijke groet / with kind regards,
     Guus Sliepen <guus at tinc-vpn.org>
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