[Dnsmasq-discuss] dnsmasq + nat(solved)

[richardvoigt at gmail.com]

On Mon, Jan 10, 2011 at 12:53 PM, Jan Seiffert <kaffeemonster at googlemail.com
> wrote:

> 2011/1/10 andu novac <novac.andu at gmail.com>:
> >> You're welcome.  However you would not say "nice crystal ball" if you
> saw
> >> the scratch marks it leaves on the furniture ;)
> >
> > Furniture is replaceable, I'd say it's worth it :)
> >
>
> But since your furniture may be of value...
> Someone already solved this quite nicely, look at the iptables manpage:
>

This is fantastic if you must control stuff centrally.  But it will result
in every outgoing packet getting fragmented.  Reducing the mtu on the client
avoids that.


>
> TCPMSS
>       This target allows to alter the MSS value of TCP SYN packets,
> to control the maximum size for that connection (usually  lim‐
>       iting  it  to your outgoing interface's MTU minus 40 for IPv4
> or 60 for IPv6, respectively).  Of course, it can only be used
>       in conjunction with -p tcp.  It is only valid in the mangle table.
>       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 configuration like:
>
>               iptables -t mangle -A FORWARD -p tcp --tcp-flags SYN,RST SYN
>                           -j TCPMSS --clamp-mss-to-pmtu
>
>       --set-mss value
>              Explicitly sets MSS option to specified value. If the
> MSS of the packet is already lower than value, it will  not  be
>              increased (from Linux 2.6.25 onwards) to avoid more
> problems with hosts relying on a proper MSS.
>
>       --clamp-mss-to-pmtu
>              Automatically  clamp  MSS  value  to  (path_MTU - 40 for
> IPv4; -60 for IPv6).  This may not function as desired where
>              asymmetric routes with differing path MTU exist — the
> kernel uses the path MTU which it would  use  to  send  packets
>              from  itself  to the source and destination IP
> addresses. Prior to Linux 2.6.25, only the path MTU to the destination
>              IP address was considered by this option; subsequent
> kernels also consider the path MTU to the source IP address.
>
>       These options are mutually exclusive
>
>
> Greetings
> Jan
>
> --
> Murphy's Law of Combat
> Rule #3: "Never forget that your weapon was manufactured by the
> lowest bidder"
>
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>
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