[arch-general] Switching between Internet connections

Stefano Avallone stavallo at unina.it
Fri Dec 7 10:20:07 EST 2012

On Friday 07 December 2012 13:02:12 Sudaraka Wijesinghe wrote:
> Hello,
> My laptop is connected to a LAN with Internet connection and it gets the
> IP stuff (address, routing, dns) via dhcp using the default systemd
> scripts on boot. I also have a 3G modem (USB) which I have created the
> ppp scripts to connect based on the Wiki article
> https://wiki.archlinux.org/index.php/3G_and_GPRS_modems_with_pppd
> Both connections work fine, except after switching to modem from LAN
> some applications (pidgin, xchat, ect. which were running) still uses
> the DNS nameserver from previous connection (LAN) meaning they still
> trying to send DNS queries to LAN router (which is not reachable at that
> time). This gets fixed after that application is closed and reopened.
> Is this behavior normal or a bug?

If I understood it correctly, it is normal. The applications you mention 
establish TCP connections using the IP address available at the time you 
launch them as the IP source address. If you later "switch" interface to 
connect to the Internet (and hence change your IP address), that address is no 
longer available and hence the TCP connection is broken. You can only restart 
the application, so that a new TCP connection is established using the new IP 

There are solutions to such a problem (e.g., HIP - Host Identity Protocol), 
but they are not yet deployed.


> Thanks.
> P.S.
> To disconnect/connect from LAN I either use:
> > # systemctl stop dhcpcd at wlan0
> > # systemctl start dhcpcd at wlan0
> or
> > # dhcpcd -k wlan0
> > # dhcpcd wlan0
> To disconnect/connect I use poff/pon commands and the scripts are setup
> exactly as described on the Wiki.

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