[Dnsmasq-discuss] using DHCP to set clients' MTU

[Grant Coady]

On Fri, 12 Sep 2008 13:53:28 +0200, Jan 'RedBully' Seiffert <redbully at cc.hs-owl.de> wrote:

...
>I think 1430 is one of those "optimised values", and 1458 maybe still to big for 
>pppoa, but i'm not sure. Did you tried 1454 & 1452? I think it's something with 
>145[0-9] from what i read. Or is BT adding another encapsulation like L2TP?

>From my firewall script:

#-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
# maximum mss or mtu
# ```````````````````
# Policy: the firewall controls this value as it is required when ethernet
#  access to ADSL modem is used, clamping MSS here removes the need to
#  modify any localnet machines' MTU setting.
#
# Information: http://www.cisco.com/warp/public/794/router_mtu.html states
#  baseline (lowest) MSS value is = 1360, highest is 1452 for PPPoE, add
#  40 for matching MTU values.
#
# Check you have rp-pppoe.conf 'CLAMPMSS=no'
#
# Leave MAX_MSS empty to perform path mtu discovery, this doesn't work with
#  ISPs that drop ICMP traffic -- refer `man iptables`.
#
# MAX_MSS="1452" # maximum MSS (ref: cisco)
# MAX_MSS="1360" # minimum MSS (ref: cisco)
MAX_MSS="1412" # default MSS (ref: rp-pppoe default)
# MAX_MSS=""    # use PMTU discovery
#
...
        # clamp MTU for new TCP connections to world
        if [ -n "$MAX_MSS" ]
        then # use preset
                iptables -A OUTPUT -p tcp --tcp-flags SYN,RST SYN \
                                -o $X_WORLD -j TCPMSS --set-mss $MAX_MSS
        else # use path discovery
                iptables -A OUTPUT -p tcp --tcp-flags SYN,RST SYN \
                                -o $X_WORLD -j TCPMSS --clamp-mss-to-pmtu
        fi
...

Grant.