Re: PMTU, MSS and "fragmentation needed" problem with linux?

[Florian Attenberger]

from my shorewall.conf.
-----------------
#
# MSS CLAMPING
#
# Set this variable to "Yes" or "yes" if you want the TCP "Clamp MSS to 
PMTU"
# option. This option is most commonly required when your internet
# interface is some variant of PPP (PPTP or PPPoE). Your kernel must
# have CONFIG_IP_NF_TARGET_TCPMSS set.
#
# [From the kernel help:
#
#    This option adds a `TCPMSS' target, which allows you to alter the
#    MSS value of TCP SYN packets, to control the maximum size for that
#    connection (usually limiting it to your outgoing interface's MTU
#    minus 40).
#
#    This is used to overcome criminally braindead ISPs or servers which
#    block ICMP Fragmentation Needed packets.  The symptoms of this
#    problem are that everything works fine from your Linux
#    firewall/router, but machines behind it can never exchange large
#    packets:
#        1) Web browsers connect, then hang with no data received.
#    2) Small mail works fine, but large emails hang.
#    3) ssh works fine, but scp hangs after initial handshaking.
# ]
#
# If left blank, or set to "No" or "no", the option is not enabled.
#
CLAMPMSS=1412
--------------------

see also:
http://en.tldp.org/HOWTO/IP-Masquerade-HOWTO/mtu-issues.html
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