Passing Interface Name to Event Script

Simon Hobson dhcp1 at
Sat Nov 14 16:52:58 UTC 2009

Rick Solotke wrote:

>I have dhcpd running on a router with several network interfaces, 
>all of which belong to the same subnet.  Because all interfaces 
>belong to the same subnet, the Linux routing table is ambiguous (an 
>address on the subnet is reachable via more than one interface).

Hmm, interesting concept. You do realise that this fundamentally 
violates the most basic rules of IP addressing don't you ? I'm 
curious as to the reasons for needing this.

>  Consequently, once a DHCP transaction completes, the router and the 
>new client are unable to communicate until a static route is added 
>to the router's routing table.


>This route specifies the specific IP address of the client and the 
>interface on which that address is accessible (e.g. route add 
> dev eth0).  If I manually enter the route at the 
>command line, the router and client are then able to communicate 
>I'd like to automate this process, so that as soon as the DHCP 
>transaction occurs, the static route is automatically added.  The 
>dhcpd COMMIT event seems to be the right place to do this, and I am 
>able to make the IP address available to the script.  However, I see 
>no way to make the interface name available to the script.  Without 
>the interface name, the script is unable to add the static route.
>Any suggestions?

Parse the logs ?

>dhcpd obviously knows which interface the DHCP transaction occurred on

Don't rely on that !

Yes, when responding via broadcast, the server should respond using 
the same physical interface that the request came in by, but when 
responding by unicast I believe it simply sends a packet via the 
kernel. In real terms this isn't a problem - if the host route is 
lost, then the client presumably loses connectivity and being unable 
to renew the DHCP lease until it's nearly run out and the client 
reverts to broadcasts is likely to be the lease of your worries.

Simon Hobson

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author Gladys Hobson. Novels - poetry - short stories - ideal as
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