Problems accessing with non-1500 mtu connection

Ben Cooksley bcooksley at
Wed May 1 08:48:35 BST 2019

On Wed, May 1, 2019 at 7:38 AM Fabian Bläse <fabian at> wrote:
> Hi there,

Hi Fabian,

> I'm having trouble accessing with internet connections with a smaller than 1500 MTU.
> This affects IPv6 as well as IPv4.
> It looks like ICMP(v6) Packet too big message are sent by the routers but are ignored by your servers.
> pings with payload exceeding maximum packet size (1500 ping payload) on a 1500 mtu connections work (local fragmentation seems to be working).
> However when trying the same on a 1492 MTU connection (or smaller), fragmented pings get sent, however I only receive the second (smaller) fragment. The first packet is too big to fit the connection, therefore a Packet too big icmp message is sent (which seems to be ignored).
> I've tried various different connections (with routers from completely different nets issuing those icmp errors) with no success.
> I've attached a tcpdump from a router sitting right behind the machine issuing those icmp errors.
> I would appreciate if you could take a look into this.

I've investigated this issue, and it appears that our Web Application
Firewall service provider's systems block abnormally sized pings (with
IPv4 at least)
Therefore it would seem we'll need to take a different tack to
investigating this issue.

Would you mind describing the access issues you're having with, and confirming whether this issue also affects
sites such as, and If it does,
can you confirm that isn't affected?

> Fabian

Ben Cooksley
KDE Sysadmin

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