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man iptables

       This  target  is  used to overcome criminally braindead ISPs or servers
       which block "ICMP Fragmentation Needed"  or  "ICMPv6  Packet  Too  Big"
       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.

       Workaround:  activate  this option and add a rule to your firewall con‐
       figuration like:

               iptables -t mangle -A FORWARD -p tcp --tcp-flags SYN,RST SYN
                           -j TCPMSS --clamp-mss-to-pmtu

man iptables

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.

       Workaround:  activate  this option and add a rule to your firewall con‐
       figuration like:

               iptables -t mangle -A FORWARD -p tcp --tcp-flags SYN,RST SYN
                           -j TCPMSS --clamp-mss-to-pmtu