Enable Jumbo Frames Causes External Hostname Problem

Well, its not a naming issue, since you can reach the webserver and it has the same DDNS name.

Are you using https or http to access the NAS? Maybe try both, just to help isolate.

One challenge is that just because jumbo frames are enabled doesn't necessarily mean that packets > 1500 bytes are being used.
Maybe load up wireshark on the PC and take a trace while you are accessing both the webserver and the NAS. Most routers don't do jumbo frames, and certainly your ISP won't. Using the ddns name means that your traffic will be going through your router.

With DynDNS the IP in use is the one assigned to your router by your ISP. Your Router is then set to port forward to your NAS. So if you do a ping jumbo frame test you want to do it to the "external" IP of your router or to the dyndns name you have assigned. Most likely your router is not set to respond to pings though so the test won't work.

I am interested in your final answer though. I have one PC in my house that will not access the NAS using the dyndns name. The others work fine. I don't understand what is special about that system. It doesn't matter because it's not a laptop so it always uses the internal name, but I do wonder why it won't work. What error do you get? In my case the system does get an authentication challenge so it's reaching the NAS but after entering the password the browser stays blank. I've futzed with browser caches and what not but I just can't figure it out. I don't see where you've said exactly what error you get. Could you describe it?

steve

StephenB wrote:

Well, its not a naming issue, since you can reach the webserver and it has the same DDNS name. Are you using https or http to access the NAS? Maybe try both, just to help isolate. One challenge is that just because jumbo frames are enabled doesn't necessarily mean that packets > 1500 bytes are being used. Maybe load up wireshark on the PC and take a trace while you are accessing both the webserver and the NAS. Most routers don't do jumbo frames, and certainly your ISP won't. Using the ddns name means that your traffic will be going through your router.

I've tried both http/https but neither work when the router points to the NAS. Pinging the external hostname with packets larger than 1500 bytes obviously fails since the WAN MTU is 1500...

The NAS and PC are both on a Jumbo frames supported switch and not connected directly to the router. File transfers and shares both work regardless of whether or not jumbo frames is enabled on one or both. If I point the router to a spare PC (with jumbo frames) running a webserver on the same switch, I can use the external hostname without a problem, but that's not the case with the NAS.

dsm1212 wrote:

With DynDNS the IP in use is the one assigned to your router by your ISP. Your Router is then set to port forward to your NAS. So if you do a ping jumbo frame test you want to do it to the "external" IP of your router or to the dyndns name you have assigned. Most likely your router is not set to respond to pings though so the test won't work. I am interested in your final answer though. I have one PC in my house that will not access the NAS using the dyndns name. The others work fine. I don't understand what is special about that system. It doesn't matter because it's not a laptop so it always uses the internal name, but I do wonder why it won't work. What error do you get? In my case the system does get an authentication challenge so it's reaching the NAS but after entering the password the browser stays blank. I've futzed with browser caches and what not but I just can't figure it out. I don't see where you've said exactly what error you get. Could you describe it? steve

Based on my testing no client on the LAN can access the NAS using the external hostname, if jumbo frames are enabled on the client and NAS, so this isn't isolated to a single client. The problem disappears if I run the webserver on a jumbo frame enabled spare computer instead of the NAS so there is something weird going on with the NAS.

It presumably also fails if you use the external IP address? Also, I am thinking that the computer webserver test was also using the external host name?

Can you try ping 192.168.1.1 -f -l 8972 Also ping xxx.xxx.xxx.xxx -f -l 8972 where xxx.xxx.xxx.xxx is your current external IP address.

BTW, just to amplify something both dsm1212 and chirpa said. When you use the external IP address/host name from within your home network, the traffic still stays within your home network. It will flow upstream to your router, which should then see it's own IP address, and apply its inbound forwarding rules - which will redirect the traffic back downstream to the NAS. My router will respond to ping and tracert to the external IP address [from inside the LAN] even if it is configured to block incoming ping - since the ping is actually going outbound.

Also, though your ethernet switch will normally bypass the router if it knows the layer 2 mac address, if you use the external IP address the router cannot be bypassed. Saying that a different way - if you use the NAS IP address, the traffic never touches your router. But if you use the external IP address, it takes a different path and will go through it. I am thinking that your router doesn't support Jumbo frames, and that may be part of the explanation here.

As I said I have one system that doesn't work either and I now see what's special. I think it's because that's the one system I set to use jumbo frames along with my NAS. There was a second system of my kids that has jumbo frames enabled and I tried that one and sure enough it has the same problem. I have a switched network internally so the path from system to NAS is clear and 9k packets work. However my cisco e4200 router seems to not support jumbo frames at all. Pinging the internal and external IP with anything larger than a 1500 byte packet gets no response. I had erroneously thought the router's switch supported jumbo frames, but it doesn't and the way I have things switched fortunately means it mostly doesn't matter. But, for some reason if a PC which supports jumbos tries to talk to the NAS that supports jumbos through the router it doesn't work. This is pretty much what the OP is saying too I think. I do get the initial Auth challenge which means something is getting through, possibly the initial http hit is small enough, but then nothing. I'm wondering if we should expect this to work or if a networking guru would say of course that won't work. I work with a few guys who might know, I'll ask them when I get a chance.

steve

When you are sending a jumbo packet via the external address, the ethernet packet is addressed to the router (having the router's mac address as the destination). So any pure layer-2 devices cannot tell that it is really for the NAS, and they will direct it to the router. This includes the built-in switch in the router itself. As far as that hardware is concerned, the destination is the router itself. So the packet is delivered to the router software, which then re-sends it back downstream to the NAS.

Not only does the router hardware need to support JF, the router software does as well.

This only applies to packets that are actually bigger than 1500 bytes, other packets should be delivered normally.

If you have a managed ethernet switch in between, you might be able to set it up to fragment JF packets on the switch-router link.

While I think I understand why jumbo frames can't be delivered, I am a bit puzzled as to why the PC and NAS IP stacks are trying to send them off-net in the first place (that is to the external IP). From their point of view the connection is being made over the internet, and as far as I can see, they shouldn't be attempting to use JF on that path. You might ask your friends about that.

What happens if you are really off-net (trying to access the NAS over the internet)?

Outside my house I have no problem from any system or device. The two pc's in question are both desktops though so I haven't taken then off site. Would be interesting to try to access it from another private site where you have jumbos working. PM me and I'll give you a link to connect to my nas if you have that setup. Not that I really need to fix this, but are there home routers that do support jumbos? I'm reading that all cisco/linksys home wifi routers do not.

steve

Pinging the internal IP of the router and the external IP of the router both fail with -f -l 8972. I'm not really surprised by this at all. If I ping my external address from within the LAN it will respond to the ICMP, even though I've asked it not to. The ping fails when outside of the LAN, as expected.

I'm not convinced that the router is the problem and I'll try and explain this again :) If I run a webserver on a spare PC with Jumbo Frames enabled, on the same switch, same router (and configure the port forward to the spare computer instead of the NAS), then I can use the external address on the LAN with jumbo frame enabled clients. The same configuration fails on jumbo frame enabled clients when I host the webserver with the NAS with jumbo frames enabled. However(!), when I am outside of the LAN, at the office for example, then the external address will work with jumbo frames enabled on the NAS, but when I get home then it fails on the LAN.

As far as the NAS goes, I see the same behavior. I don't have a webserver on a pc to try.

steve