I had to change
clampmss no
to
clampmss 1400
in /etc/shorewall/shorewall.conf
yeehaw.
#
# 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.
#
# You may also set this option to a numeric value in which case Shorewall will
# set up a rule to modify the MSS value in SYN packets to the value that
# you specify.
#
CLAMPMSS=1400
