[GTALUG] [Bulk] Re: Rogers and MTU size

David Collier-Brown davec-b at rogers.com
Mon Jan 5 00:21:45 UTC 2015


On 01/04/2015 03:30 PM, James Knott wrote:
> On 01/04/2015 01:59 PM, Kevin Cozens wrote:
>> On 15-01-04 12:55 PM, James Knott wrote:
>>> One thing I noticed recently was that my firewall, running Linux, was
>>> getting a MTU of 576 bytes, instead of the normal 1500 bytes.  This was
>>> being set by the Rogers DHCP server (option 26).
>> [snip]
>>> I have advised Rogers of this and the person I was speaking to agreed it
>>> should be 1500.  Let's see if they fix it.
>> I am on Rogers and just checked my MTU setting after reading James'
>> email. The ifconfig output reports an MTU of 1500. I didn't change any
>> settings for DHCP. Rogers may have already fixed the problem.
>>
> I just tried some more testing with my notebook computer.  It uses the
> KDE network manager and appears to ignore the DHCP MTU value.  However,
> I can only ping with up to 1472 bytes when connected to the cable
> modem.  I get the same when I tether to my smart phone (Rogers), but get
> the full 1500 to a local computer on my network.  I do not use DHCP
> option 26 on my DHCP server.  It also doesn't appear to be used when
> tethered to my smart phone.
>
>
>
> ---
> GTALUG Talk Mailing List - talk at gtalug.org
> http://gtalug.org/mailman/listinfo/talk
> $ ping -s 1500 acm.org
> PING acm.org (64.238.147.76) 1500(1528) bytes of data.
> 1508 bytes from ACMWEB16.acm.org (64.238.147.76): icmp_seq=1 ttl=49 time=42.5 ms
> 1508 bytes from ACMWEB16.acm.org (64.238.147.76): icmp_seq=2 ttl=49 time=44.1 ms
> 1508 bytes from ACMWEB16.acm.org (64.238.147.76): icmp_seq=3 ttl=49 time=44.5 ms
> 1508 bytes from ACMWEB16.acm.org (64.238.147.76): icmp_seq=4 ttl=49 time=44.2 ms
> 1508 bytes from ACMWEB16.acm.org (64.238.147.76): icmp_seq=5 ttl=49 time=52.9 ms
> 1508 bytes from ACMWEB16.acm.org (64.238.147.76): icmp_seq=6 ttl=49 time=44.1 ms
> ^C
> --- acm.org ping statistics ---
> 6 packets transmitted, 6 received, 0% packet loss, time 5007ms
> rtt min/avg/max/mdev = 42.576/45.430/52.995/3.445 ms
>

 From a different rogers subnet, I get a default of 56, successes with 
1500-byte pings to the router and to the outside world, but failures 
internally talking to rogers DNS servers

First, the dns server:
$ cat /etc/resolv.conf
# Generated by NetworkManager
nameserver 64.71.255.204
nameserver 64.71.255.198

$ ping -s 1500 64.71.255.204
PING 64.71.255.204 (64.71.255.204) 1500(1528) bytes of data.
^C
--- 64.71.255.204 ping statistics ---
19 packets transmitted, 0 received, 100% packet loss, time 17999ms


$ ping 64.71.255.204
PING 64.71.255.204 (64.71.255.204) 56(84) bytes of data.
64 bytes from 64.71.255.204: icmp_seq=1 ttl=58 time=7.63 ms
64 bytes from 64.71.255.204: icmp_seq=2 ttl=58 time=7.35 ms
^C
--- 64.71.255.204 ping statistics ---
2 packets transmitted, 2 received, 0% packet loss, time 1002ms
rtt min/avg/max/mdev = 7.350/7.492/7.635/0.166 ms


Second, ACM.org
$ ping -s 1500 acm.org
PING acm.org (64.238.147.76) 1500(1528) bytes of data.
1508 bytes from ACMWEB16.acm.org (64.238.147.76): icmp_seq=1 ttl=49 
time=42.5 ms
1508 bytes from ACMWEB16.acm.org (64.238.147.76): icmp_seq=2 ttl=49 
time=44.1 ms
1508 bytes from ACMWEB16.acm.org (64.238.147.76): icmp_seq=3 ttl=49 
time=44.5 ms
1508 bytes from ACMWEB16.acm.org (64.238.147.76): icmp_seq=4 ttl=49 
time=44.2 ms
1508 bytes from ACMWEB16.acm.org (64.238.147.76): icmp_seq=5 ttl=49 
time=52.9 ms
1508 bytes from ACMWEB16.acm.org (64.238.147.76): icmp_seq=6 ttl=49 
time=44.1 ms
^C
--- acm.org ping statistics ---
6 packets transmitted, 6 received, 0% packet loss, time 5007ms
rtt min/avg/max/mdev = 42.576/45.430/52.995/3.445 ms

--dave

-- 
David Collier-Brown,         | Always do right. This will gratify
System Programmer and Author | some people and astonish the rest
davecb at spamcop.net           |                      -- Mark Twain



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