> Problem the Linux server in (1) and (2) will not connect properly to
> certain mail servers (usually telcos). Also "telnet mailhost5.wcom.com.hk
> 25" does not work (it says: "Connecting to <ip address of the machine>",
> but doesn't - tcpdump reveals that it sends packets but does not receive
> any). The funny part is that in (1) it works properly from both, the
> CISCO router AND from the internal network. With (2) it works properly
> from the internal network.
> (3) and (4) work fine under all circumstances.
The other side is likely pmtu blackholed and blocking all ICMPs
(misconfigured firewall)
Set a mss of 1440 on the default route and flush the routing cache
afterwards
(ip route replace default .... advmss 1440 ; ip route flush)
Another possibility is ECN, make sure it is turned off. Would be again
a broken firewall on the other side. echo 0 > /proc/sys/net/ipv4/tcp_ecn
-Andi
P.S.: netdev is for network code development. your question would have been
more appropiate on linux-net.
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