> The man page says:
> When the demand option is used, the interface IP addresses
> have already been set at the point when IPCP comes up. If
> pppd has not been able to negotiate the same addresses
> that it used to configure the interface (for example when
> the peer is an ISP that uses dynamic IP address assign-
> ment), pppd has to change the interface IP addresses to
> the negotiated addresses. This may disrupt existing con-
> nections, and the use of demand dialling with peers that
> do dynamic IP address assignment is not recommended.
>
> What is meant by `This may disrupt existing connections'? If a TCP
> connection assumes an IP, changing the interface will most
> certainly break the connection????
As I understand it (or not as the case may be) pppd by default was designed
up to maintain a single set of addresses. The problem comes when you've
already got traffic on your system pointing to the existing addresses.
[that first packet that triggered the demand dial for example] It's
pointing to an address that doesn't exist anymore and therefore is broken
(without kernel address change support - see below). New packets arriving
after the address change work as expected.
Telling pppd to accept address changes does work for me. Using versions
2.3.3 and above I've never seen a problem. But it makes sense that the
warning is still in place.
> Is there any way around this so that I can effectively use
> dynamic IP with demand dialing?
add these lines to /etc/ppp/options (or where ever you are specifying your
pppd options)
# override existing address with those negotiated by IPCP
ipcp-accept-remote
ipcp-accept-local
[ they are explained in man pppd ]
and these lines to one of your init scripts
# enable kernel support for dynamic address changes
echo "1" > /proc/sys/net/ipv4/ip_dynaddr
[ explained in linux/Documentation/networking/ip_dynaddr.txt if you have the
kernel sources installed. ]
> What code would need to be written to get this to work? I
> would like to write support for this - what is needed?
It generally works as is.
Hope this helps,
Lourdes
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