RE: Writing resolv.conf issues with broken symlink

[Matthew Starr <mstarr hedonline com>]

> -----Original Message-----
> From: Thomas Haller [mailto:thaller redhat com]
> Sent: Friday, June 01, 2018 1:14 AM
>
> On Thu, 2018-05-31 at 21:18 +0000, Matthew Starr wrote:
> > > -----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?
>
> What you say, makes sense.
>
> I also thought it should be like that, and the man page explains it that way
> (well, I was involved writing the manual page with this understanding).
>
> https://cgit.freedesktop.org/NetworkManager/NetworkManager/tree/src/
> dns/nm-dns-manager.c?id=nm-1-8#n737
> Seems `realpath` only returns the destination file, if the path actually exists...
>
> I think this behavior is a bug, maybe it should be fixed, but it's a bit
> problematic to change at this point... I will follow up.
>
>
> In the meantime, can you not just symlink /etc/resolv.conf to
> /var/run/NetworkManager/resolv.conf ?
> That anyway seems the better solution to me.

Thank you for making this change.

As a work around I am just running "touch /tmp/resolv.conf" early in the init process before NetworkManager 
is launched.  I was not aware of the /var/run/NetworkManager/resolv.conf file at the time I did my 
workaround, but it is good to know it is there as a backup.

Best regards,
Matthew Starr

> > > 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