RE: Writing resolv.conf issues with broken symlink

[Matthew Starr <mstarr hedonline com>]

> -----Original Message-----
> From: Thomas Haller [mailto:thaller redhat com]
> Sent: Thursday, May 31, 2018 3:35 PM
>
> On Thu, 2018-05-31 at 19:02 +0000, Matthew Starr wrote:
> > I am building NetworkManager 1.8.0 using buildroot with SysV init on
> > an ARM embedded device.  Buildroot has the /etc/resolv.conf file as a
> > symlink that points to /tmp/resolv.conf.  When I use NetworkManager
> > with this resolv.conf symlink configuration, DNS lookups do no work
> > because NetworkManger is not writing to the resolv.conf file or the
> > file pointed to by the symlink.
> >
> > I tried updating the NetworkManager.conf file to include "rc-
> > manager=file" since the man page states that NetworkManager will then
> > follow the symlink and write the contents of the resolv.conf there.
> > Unfortunately this does not work when the destination file the symlink
> > points to does not exist.  In this situation the symlink points to a
> > file in /tmp which is a tmpfs file system (RAM based) and wiped clean
> > on each boot to help support  a read-only root file system or doing
> > minimal writes to the root file system.
> >
> > Is there a specific reason NetworkManager doesn't create the file the
> > /etc/resolv.conf symlink points to if it doesn't exist?  If there
> > isn't a reason, could NetworkManager be updated to create the file
> > pointed to by the /etc/resolv.conf symlink?
>
> Hi,
>
> with "file" mode, NetworkManager should follow the symlink and write the
> file (if it doesn't exist). It wouldn't create the necessary parent directories,
> but since you have /tmp/resolv.conf, this shouldn't apply.
>
> Odd, if that doesn't work. Would need some investigation. Does the logfile
> say anything with level=TRACE?
>
> You could also have /etc/resolv.conf as symlink to
> /var/run/NetworkManager/resolv.conf and leave "symlink" setting.

I actually had that wrong in the original email.  What I was seeing where it wouldn't write the 
/etc/resolv.conf file was with the "symlink" setting.  I see that this is the expected action when 
/etc/resolv.conf is a symlink.

The issue I am seeing with the "file" setting is that instead of following the broken symlink at 
/etc/resolv.conf and writing the file at where the symlink is pointing (/tmp/resolv.conf), Network Manager is 
deleting the symlink and writing a new /etc/resolv.conf regular file.  Wouldn't it make more sense.  Based on 
what the man page states, I was expecting NetworkManager to follow the symlink and make a new file there.  
The man page says "NetworkManager will write /etc/resolv.conf as file. If it finds a symlink, it will follow 
the symlink and update the target instead."

In case it helps, here are some of the dns trace messages when NetworkManager was updating the DNS:
daemon.debug NetworkManager[311]: <debug> [1527801019.9945] dns-mgr: update-dns: updating resolv.conf
daemon.debug NetworkManager[311]: <trace> [1527801019.9948] dns-mgr: config:      100 best    v4 mlan0        
    : 208.67.222.222 208.67.222.220 4.2.2.2 
daemon.debug NetworkManager[311]: <trace> [1527801020.0115] dns-mgr: update-resolv-conf: write to 
/etc/resolv.conf succeeded (rc-manager=file)
daemon.debug NetworkManager[311]: <trace> [1527801020.0126] dns-mgr: update-resolv-conf: write internal file 
/etc/resolv.conf succeeded (rc-manager=file)
daemon.debug NetworkManager[311]: <trace> [1527801020.0135] dns-mgr: current configuration: [{'nameservers': 
<['208.67.222.222', '208.67.222.220', '4.2.2.2']>, 'interface': <'mlan0'>, 'priority': <100>, 'vpn': <false>}]
daemon.debug NetworkManager[311]: <debug> [1527801020.0632] dns-mgr: (device_ip4_config_changed): queueing 
DNS updates (1)

Best regards,
Matthew Starr