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[Weaver]

The letmecheck.it MTU tester website (I Googled "online MTU test") works ok when accessed from my iPad’s WLAN IPv4 (which is public/routable) and it comes back with the expected result that it can’t get in past my firewall.

However, there’s no firewall on the 4G i/f of my IPad, and that is IPv4-only, which is as I recall a good thing since the site used to go bonkers if you have IPv6 available or try and quote an IPv6 address to it. But when I try it it just hangs. If I close my web page in my browser and go back to the site, then it seems completely hung and no page appears, as if the browser is getting no replies at all maybe. No idea what’s going on.

[burakkucat]

I'm not surprised.

This online MTU test is in BETA. Please let us know, using our contact form, if you find any irregularities.

Also, take a look at the date of the "Latest News" --

Latest News
We want to shout it out...
5th Januari 2013

We are here! LetMeCheck.it is your buddy on the web to help test and check common IP functions.

It is essentially a dead site.

[Weaver]

It used to work provided you not upset it by mentioning IPv6. And when it worked it was incredibly useful.

Does anyone know if there’s an alternative similar tool?

[digbey]

The site letmecheck.it is still alive but I get this warning message from Firefox..

The certificate for this site is not valid.
Because this connection is not secure, information (such as passwords or credit cards) will not be securely sent to this site and may be intercepted or seen by others.
We suggest you don't enter personal information into this site or avoid using this site.​

In the interests of experiment, I ignored all the dire warnings and proceeded to the site and was able to check my MTU.

[Chrysalis]

It used to work provided you not upset it by mentioning IPv6. And when it worked it was incredibly useful.

Does anyone know if there’s an alternative similar tool?

cloudflare have one but I wont post it as its also broken.

Its good old command line testing I am afraid.

Actually there is the old speedguide tool which works.

speedguide.net, than hover over broadband, then over broadband tools, and select tcp-ip analyzer.

It will report mtu, mss and the window size. (it says rwin, but might be the congestion window)

[Weaver]

That’s a brilliant tip. I have the usual tools on my iPad: ping / traceroute and many more, and I could IPv4-ping my wife’s iPad successfully going out via 4G and back in through the main DSL WAN i/f because the firewall knows to let my 4G IP in, that being one of a number of special inbound holes.

Code: [Select]

TCP options = 020405b4010303060101080a359068bd0000000004020000
MTU = 1500
MTU is fully optimized for broadband.
MSS = 1460
Maximum useful data in each packet = 1448, which is less than MSS because of Timestamps, or other TCP/IP options used.
Default TCP Receive Window (RWIN) = 131712
RWIN Scaling (RFC1323) = 6 bits (scale factor: 2^6=64)
Unscaled TCP Receive Window = 2058

RWIN is not multiple of MSS. If your OS supports setting RWIN directly, consider changing it to a multiple of MSS for optimum performance.
Other RWIN values that might work well with your current MTU/MSS:
64240  (up to 2 Mbit lines, depending on latency. MSS * 44)
128480 (1-5 Mbit lines, depending on latency. MSS * 44 * 2)
256960 (2-20 Mbit lines, depending on latency. MSS * 44 * 2^2)
513920 (20-100 Mbit lines, depending on latency. MSS * 44 * 2^3)
1027840 (80+ Mbit lines up to Gigabit connections, depending on latency. MSS * 44 * 2^4)
bandwidth * delay product (Note this is not a speed test):

Your current TCP Window limits you to: 527 Mbps (~53 Megabytes/s) @ 200ms latency
Your current TCP Window limits you to: 211 Mbps (~21 Megabytes/s) @ 500ms latency
Note: You can expect up to ~90% of this throughput considering line overhead.
MTU Discovery (RFC1191) = ON
Time to live left = 53 hops
TTL value is ok.
Timestamps (RFC1323) = ON
Note: Timestamps add 12 bytes to the TCP header of each packet, reducing the space available for useful data.
Selective Acknowledgements (RFC2018) = ON
IP type of service field (RFC1349) = 00000000 (0)

How can the MSS for IPv4 be 1460? Don’t we also have to take off the 12 bytes for TCP timestamps as it mentions? So going down to 1448?

And who’s to say that it’s IPv4? Could be 1428 for IPv6 + timestamps, no? So I don’t understand the point about optimising the RWIN in that case - would there be two different RWIN values for IPv4 vs IPv6 or would you just end up optimising TCP for one IP version and completely messing up the other if you get it wrong?

[Chrysalis]

I would ignore the RWIN tips, the site was brought up back in the XP days before we had auto tuning, now its mostly just useful for diagnosing the MTU/MSS configuration.

[Weaver]

> How can the MSS for IPv4 be 1460? Don’t we also have to take off the 12 bytes for TCP timestamps as it mentions? So going down to 1448?

Does anyone know if this is true or not?

[Chrysalis]

Its true.

[kitz]

The site letmecheck.it is still alive but I get this warning message from Firefox..

Its working for me here 

[Chrysalis]

Just found another site that reports MTU.  Albeit also one that is actually maintained.

https://browserleaks.com/ip

[Alex Atkin UK]

Just found another site that reports MTU.  Albeit also one that is actually maintained.

https://browserleaks.com/ip

I wonder how that DNS leak test works, as it found all the DNS servers for WAN links I have established but I'm not using any of them as I have full resolution on the router.

[Chrysalis]

I wonder how that DNS leak test works, as it found all the DNS servers for WAN links I have established but I'm not using any of them as I have full resolution on the router.

It doesnt grab ip's from your config if thats what you thinking.

e.g. I am using cloudflare 1.1.1.2 via DoH and the ipv6 variant, but the ip's cloudflare use to access authoritive servers are actual different ip's and those are what appear in the leak test.

They probably making queries to dns servers they control and can see from logs the ip's of the resolver doing the query.

This is my result.

Code: [Select]

Test Results Found 7 Servers, 1 ISP, 1 Location
Your DNS Servers
IP Address : ISP : Location :
172.70.88.95 Cloudflare, Inc. United Kingdom, London
172.70.88.147 Cloudflare, Inc. United Kingdom, London
172.70.161.7 Cloudflare, Inc. United Kingdom, London
172.70.161.35 Cloudflare, Inc. United Kingdom, London
2400:cb00:373:1024::ac46:a107 Cloudflare, Inc. United Kingdom, London
2400:cb00:373:1024::ac46:a123 Cloudflare, Inc. United Kingdom, London
2400:cb00:377:1024::ac46:585f Cloudflare, Inc. United Kingdom, London
So my guess is you think you not using them but you actually are.  If you think its wrong I suggest trying other dnsleak tests.

[jelv]

I can't make any sense of what it's telling me!

Test Results   Found 7 Servers, 1 ISP, 2 Locations
Your DNS Servers   
IP Address :   ISP :   Location :
90.155.53.33    Andrews & Arnold Ltd   United Kingdom, London
90.155.53.34    Andrews & Arnold Ltd   United Kingdom, London
90.155.62.53    Andrews & Arnold Ltd   United Kingdom, Bracknell
90.155.62.54    Andrews & Arnold Ltd   United Kingdom, Bracknell
2001:8b0:0:53::33    Andrews & Arnold Ltd   United Kingdom, Bracknell
2001:8b0:0:62::53    Andrews & Arnold Ltd   United Kingdom, Bracknell
2001:8b0:0:62::54    Andrews & Arnold Ltd   United Kingdom, Bracknell

Which is pretty strange as I left A&A ages ago and am now with Zen!

From ipconfig /all:

   DNS Servers . . . . . . . . . . . : 2a02:8011:xxxx:xxxx::1
                                       2606:4700:4700::1111
                                       192.168.178.1
                                       1.1.1.1

And from the fritz box:

DNS servers used       212.23.3.100
                       212.23.6.100 (currently used for standard queries)
                       2a02:8010:1::212:23:3:100

[Chrysalis]

more investigation needed, if it bothers you try another dnsleak test from another site, many VPN sites have one, or do some sniffing, but yeah at first glance looks odd, I dont know what to say as for me I get the expected result.

try here.

https://www.dnsleaktest.com

I am assuming 192.168.178.1 and the masked out ipv6 are your fritz box.

Which would indicate you should get a mix of Zen and Cloudflare DNS servers back.

There is a contact email at the bottom of the browserleaks site as well.