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BT Business Hub 5 - IP Fragmentation

pcollinson
Member

I've continued to persevere with the BT Business Hub 5 (AKA HH5) but I've been wondering for some time why my local DNS server sometimes fails to find IP addresses, or took so long to do so that browsers timed out waiting.

 

The DNS server is hosted on a machine nominated as the DMZ machine by the hub. There is a single wire connecting the BT hub to my server.. and I've set the MTU on the server to the magic 1488 that the hub seems to want.

 

Well, what I think is happening is that the BT Router seems NOT to pass on all packets to the DMZ machine, which it ought to, but it blocks 'fragmented UDP packets'. The log shows things like:

 

IN: BLOCK [5] Fragmented packet (IP XX.XX.XX.XX-​>II.II.II.II fragment_offset=1480)

 

XX is an external DNS server, II is my internal one.

 

So my DNS server is askng a question and the answer is being junked. This problem is getting worse as more people install DNSSEC and the DNS server expects to be able to use EDNS to get and send longer responses.

 

Why is the Hub doing this? Is it incapable of packet re-assembly?  Isn't that supposed to be a fundamental part of IP? Does it think that fragmented packets is someone attempting to break into my network? Is there anyone in BT who understands what is happening? It seems to impact on other uses too - VPNs and gaming stations that use UDP.

 

This is serious stuff, this business wants to run its own DNS so that it can manage its networks efficiently, I am sure we are not alone. If BT cannot provide a service which supports the single underlying system on which we all depend, then it's not really fit for purpose. 

 

OK - I could ditch the BT Hub and install a Vigor - which I am close to doing - but I hope that a moan here might do something, although to borrow a phrase from the past - getting helpful answers out of BT is like kicking a dead whale up a beach.

 

For the record the hub is

BT Business Hub 5 (Type A) | Software version 4.7.5.1.83.8.230 | Last updated 12/07/16

4 REPLIES 4

Woldstone
Member

Good luck! I feel your pain. I don't think BT really gives a hoot about these things. 

Teeafit
Member

[I have a gut feeling that my problem is somewhat related to this topic — if it isn't, perhaps a moderator will advise me, or re-direct this to a new topic.]

 

My business is audio/visual, and I frequently have to upload MP4 video files for clients to comment on and approve.  My first new hub (BTHub5-M8GW) persistently gave problems when the upload reached 24-25MB, with the connection dropping out (Broadband light turning red).  It would then automatically re-establish contact, but the upload had failed.

I borrowed my son's Hub5 (-TNH5), and the files uploaded OK (although that might have been a one-off).  Using that as evidence, a faulty hub was diagnosed and I was sent a replacement Hub5A (-3TRH), but the same thing is repeatedly happening.

Yesterday I used my son's broadband at his house (same exchange, slightly shorter copper to the same cabinet), and the files uploaded with no problem.

This is with 3 different MP4 files (25.9MB, 37.4 and 98.3), and 3 Hub5s.  I have tried uploading to 2 destinations — my drop box and my FTP website.  I have sent from my Desktop machine by LAN, and my Laptop by LAN and WiFi.  Very rarely the smallest file will get through, but the others always seem to kill the connection.  The laptop and WiFi will work fine on my son's connection.  The conclusion I drew from this is that there is something in my connection that 'gets indigestion' at 25MB, and sulks.

I had a long (and not upleasant) session to the phone helpdesk yesterday, and the agent (and other colleagues) were baffled.  Final suggestion: — that the imbalance between my download speed (11MB) and upload (1MB) was confusing the connection, and the Hub5 wasn't coping.  It was recommended that I get a third-party router, and a refund was given on my account to cover this.

Before I do that, I would like to run that past this forum.  Looking at my stats for the past couple of days there are MANY entries saying <<IN: BLOCK [9] Packet invalid in connection (Invalid tcp flags for current tcp state: TCP [217.199.169.90]:80-?>[86.146.89.66]:53634 on ppp3)>>.  Is this likely to be significant?

 

Any suggestions appreciated.

orthohin
Grand Master

Also thinking same 🙂

 

Teeafit
Member

BT solved the problem — "We'll credit you £75 on your account... go out and buy a different router".  This is the second time this has happened to me, following a 2-Wire hub problem back in the all-copper days.

 

I now have a TP-Link Archer VR600, which not only uploads ANYthing, but also gives me 14MB where the Hub 5s could only manage 9MB.

 

WHY do BT accept this as a satisfactory way of working?