/etc/network/interfaces: sysctl in as "up" command doesn't work!

[Carlos Rodrigues <carlos.efr@mail.telepac.pt>]

Hi!

I need proxy_arp enabled on an ethernet interface, so I have the 
following line in /etc/network/interfaces:

    "up sysctl -w net.ipv4.conf.eth0.proxy_arp=1"

When the interface is brought up (on boot, or manually with "ifup 
eth0"), sysctl gets executed and its output shows that proxy_arp has 
been set to 1 (adding another "up" to read the value confirms this). 
However, after ifup returns, proxy_arp is disabled again.

Between the "up" directives getting executed, and ifup returning, 
Debian's "interfaces" mechanism is resetting values to their defaults 
somehow.

Does anybody know what is happening here, and how I can work around it?

--
Carlos Rodrigues

PS: running Debian Sarge