dhclient dry-run - need to know the def gateway without installing the def route

[Sten Carlsen]

Just a thought:

Did you consider to let the DHCP server send the correct gateway? Would
that solve the issue?

On 08/04/14 15.03, alessandro macuz wrote:
> 2014-04-07 22:46 GMT+02:00 Sten Carlsen <stenc at s-carlsen.dk
> <mailto:stenc at s-carlsen.dk>>:
>
>
>     On 07/04/14 22.23, alessandro macuz wrote:
>>     2014-04-07 21:15 GMT+02:00 Simon Hobson <dhcp1 at thehobsons.co.uk
>>     <mailto:dhcp1 at thehobsons.co.uk>>:
>>
>>         alessandro macuz <alessandro.macuz at gmail.com
>>         <mailto:alessandro.macuz at gmail.com>> wrote:
>>
>>         > I could deduce the gateway from the DHCP server IP address
>>         because normally they coincide but that would be my assumption.
>>
>>         You've heard the expression that "assume" makes an "ass" out
>>         of "u" and "me", well your assumption is badly wrong. In home
>>         and small networks it is likely to be true (not mine though).
>>         But once you get to business networks then its more likely
>>         that the DHCP server is not at the same IP address as the router.
>>
>>         However, I'm still not quite sure why you are doing things
>>         the long way round. What's specifically wrong with using the
>>         route that comes with the lease ? Or more to the point,
>>         what's wrong with dealing with it when you get it ? And BTW -
>>         have you considered the possibility of getting different
>>         answers to your two queries ?
>>
>>
>>     Sorry maybe I should have added more details but there is only
>>     one defaut gateway on the network of that interface and hence the
>>     dry-run returns always the same IP address. As to the second
>>     reply it will configure the interface.
>>
>>     Why I want to run a dry-run? Because eventually I need to install
>>     some routes pointing to the only gateway on the network. On the
>>     system there is another interface that gets the IP dinamically
>>     and two default routes are not OK
>     I suggest you look at: /sbin/dhclient-scripts
>
>     It contains the following code, I think a small modification to
>     this will do your job:
>     ~~~~~~~~~~~~~~~~~~~~~
>     add_default_gateway() {
>         router="${1}"
>
>         if is_router_reachable ${router} ; then
>             metric=""
>             if [ $# -gt 1 ] && [ ${2} -gt 0 ]; then
>                 metric="metric ${2}"
>             fi
>             ip -4 route replace default via ${router} dev ${interface}
>     ${metric}
>             if [ $? -ne 0 ]; then
>                 logmessage "failed to create default route: ${router}
>     dev ${interface} ${metric}"
>                 return 1
>             else
>                 return 0
>             fi
>         fi
>
>         return 1
>     }
>
>
>
> Thanks Sten,
>
> I don't see such add_default_gateway() function in my Ubuntu 12.04.4
> but I do see
>
>             for router in $new_routers; do
>                 if [ "$new_subnet_mask" = "255.255.255.255" ]; then
>                     # point-to-point connection => set explicit route
>                     ip -4 route add ${router} dev $interface
> >/dev/null 2>&1
>                 fi
>
>                 # set default route
>                 ip -4 route add default via ${router} dev ${interface} \
>                     ${metric_arg} >/dev/null 2>&1
>             done
>
> when $reason is set to BOUND or RENEW or REBIND or REBOOT or TIMEOUT.
>
> Eventually I realized I have access to the DHCP server (openwrt) and
> the trick may be not to send the default route and at the same time to
> use the option 121 (client must explicitly request it)
>
> http://lists.thekelleys.org.uk/pipermail/dnsmasq-discuss/2006q2/000818.html
>
> I'll give it a try.
>
> Alex
>
>
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-- 
Best regards

Sten Carlsen

No improvements come from shouting:

       "MALE BOVINE MANURE!!!" 

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