NEXCOM IFA 1610 - Manual (Page 19)

Copyright © 2014 NEXCOM International Co., Ltd. All Rights Reserved. IFA 3610/IFA 2610/IFA 1610 User Manual
Chapter 1: The System Menu
16
The MTU Size
While the vast majority of the ISPs uses a standard value of 1500 bytes, in some circumstances the standard MTU
size results too high. If that happens, some strange network behaviours will be noticed, like e.g., downloads which
always stop after a while or connections which will not work at all.
If the ISP does not use a standard MTU size, it is easy to discover the correct one, by sending special ICMP packets
with a specific value, that can be lowered until no errors are encountered: At theist point, the MTU size is correct and
this value should be entered in the configuration options.
In order to send the icmp packets do the following:
Log in to the EFW and choose a host which can be actually reached (e.g., the ISP’s DNS, which should always be
reachable) and ping that host with the following command:
ping -c1 -M do -s 1460 <host> (please refer to the ping(8) manpage for more info).
If the MTU size 1460 is correct, ping replies like the following one are received:
PING 10.10.10.10 (10.10.10.10) 1460(1488) bytes of data.
1468 bytes from 10.10.10.10: icmp_seq=1 ttl=49 time=75.2 ms
If however the current MTU size is still too big for packets of the size 1460, an error message like this will appear:
PING 10.10.10.10 (62.116.64.82) 1461(1489) bytes of data.
ping: sendmsg: Message too long
Retry with different packet sizes (i.e., the value after the -s option), until the correct size has found and no error is
displayed. The value shown within brackets in the ping command’s output is the MTU size. In this example the output
is 1460(1488), therefore 1488 is the value to select for the MTU size.
An MTU value lower than 1500 may cause problems also in the OpenVPN setup and require to adjust some setting
there.
1.2.5 5/8 - Configure DNS resolver
This step allows to define up to two IP addresses for the DNS server, unless they are assigned automatically: In this case,
no configuration option can be set and it is safe to move to the next one. If only one DNS server should be used, the
same IP address must be entered twice. The IP address(es) of the DNS must be accessible from the IFA 3610/IFA 2610/IFA
1610 appliances, otherwise URL and domain resolution will not work.
See also: Changes to the RED interface, i.e., the uplink, and the DNS server can be modified later,
separately from the other network configuration:
Uplink Editor
Menubar
► Network ► Interfaces ► [edit uplink]