NetworkManager

[Smoot Carl-Mitchell]

On Wed, 2008-10-08 at 16:00 -0300, Derek Broughton wrote:
> NoOp wrote:
> 
> > 
> > Set NM for "Roaming mode" and
> > sudo /etc/init.d/networking restart
> > cat /etc/resolv.conf
> > 
> > You will see that Network Manager wrote into the /etc/resolv.conf file
> > as shown in Leonard's output.
> 
> How?  Network manager doesn't get _touched_ when you do "networking
> restart" - which is how this whole discussion started, because Karl
> couldn't figure out why a restart had no effect.

I think it is pretty clear NetworkManager overwrites /etc/resolv.conf.
Do this:

smoot at smoot:~$ strings /usr/sbin/NetworkManager | grep Modified_by:
# Modified_by:  NetworkManager


NetworkManager is a dbus aware application and I believe dhclient is
also.  They communicate with each other via dbus.  A hint is the
argument to a running dhclient e.g.

smoot at smoot:~$ ps ax | grep dhclient
10565 ?        S      0:00 /sbin/dhclient -1
-lf /var/lib/dhcp3/dhclient.ath0.leases -pf /var/run/dhclient.ath0.pid
-q -e dhc_dbus=31 -d ath0

dhclient certainly gets the nameserver information from the DHCP server
and I will bet it passes the info back to NetworkManager which handles
the update of /etc/resolv.conf.

A bit more sleuthing with "strings" shows:

smoot at smoot:~$ strings /usr/sbin/NetworkManager | grep resolv.conf
nm_system_should_modify_resolv_conf
/etc/resolv.conf.tmp
/etc/resolv.conf
Could not update /etc/resolv.conf: %s
<WARN>  %s(): Could not write resolv.conf.  Error: '%s'
rewrite_resolv_conf
set_ip4_config_from_resolv_conf

You can also do an strace on a running NM dameon like this:

sudo strace -p <pid_of_NM> -e open -e rename

which will show the open of /etc/resolv.conf.tmp and the rename
to /etc/resolv.conf as you switch networks.

-- 
Smoot Carl-Mitchell
System/Network Architect
smoot at tic.com
+1 480 922 7313
cell: +1 602 421 9005