Access to server behind firewall work only for MTU < 1420

Hi all. I am having problem accessing to my home server. If I try to connect to it from the network where I am connected now it does not work, unless I set up my server with an MTU < 1420 (this is the MTU of the router where I am connected now). It does not work only from here. My friends are all able to connect to it. I think it is a problem of packing/unpacking the packets since if the page to retrieve is small it works whichever is the MTU I set on the server.
Furthermore, if I disable the nat, I am able to retrieve the Router OS web page without any problems.

The configuration is the following:
Minirouter board with router os 2.9.51

  • ADSL modem connected to Ethernet 1 in PPPOE (MTU & MRU 1480)
  • Server connected to Ethernet 2 (MTU 1500, ip 192.168.0.1)
  • Other thinks connected to other ports
    Nat configuration is the following (very simple):
    0 chain=srcnat action=masquerade src-address=192.168.0.0/24
    1 chain=srcnat action=masquerade src-address=192.168.1.0/24
    2 chain=dstnat action=dst-nat to-addresses=192.168.0.1 to-ports=80
    in-interface=ADSL dst-port=80 protocol=tcp
    3 chain=dstnat action=dst-nat to-addresses=192.168.0.1 to-ports=80
    in-interface=ADSL dst-port=80 protocol=udp

I think there is for sure a problem with the router I am connected now (may be it rejects packet bigger than its MTU/MRU), but what I do not understand is why when I disable dst-nat (ie. with the Router OS web) page it works, but if I dst-nat to my home server it does not work.

Thanks!

Alberto

are you blocking ICMP packets ?

I think the ICMP packets are not blocked (what do you inted by blocked?). If I ping the router it answers.
However, I noticed that if I ping from the network where I am connected now, ping packets are blocked (I am even not able to ping whichever site). If I ping from another network it responds properly.

It is quite strange because I have another server connected on a different ADSL with MTU set to 1500 (on LAN) that I am able to connect without problems from here.

I think I should set something on my router…