[bug #58571] DNS host list is global even for multiple netifs

[yuanjianmin]

URL:
  <https://savannah.nongnu.org/bugs/?58571>

                 Summary: DNS host list is global even for multiple netifs
                 Project: lwIP - A Lightweight TCP/IP stack
            Submitted by: ashesman
            Submitted on: Sun 14 Jun 2020 11:03:31 PM UTC
                Category: DNS
                Severity: 3 - Normal
              Item Group: Change Request
                  Status: None
                 Privacy: Public
             Assigned to: None
             Open/Closed: Open
         Discussion Lock: Any
         Planned Release: None
            lwIP version: 2.1.1

    _______________________________________________________

Details:

I have stumbled across a problem today where DNS resolution was failing when
using multiple netifs.  I tracked it down to the use of a global dns_servers
array that is updated by the last netif that gets given a DNS address, even if
that netif has no internet connection.  Other netifs can then not perform DNS
resolution even though they have a valid internet connection.

Could the dns_servers array not be part of netif so that the correct dns
server address can be fetched from the current default netif?  Other sending
activities default to using the default netif.

Use case:

In this instance there is a DHCP capable ethernet interface and a PPP wifi
interface.  Both of these interfaces set the dns_servers array while
negotiating IP address.  Depending on which netif sets the dns_servers array
last sets the IP address used for DNS resolution.

Typically Ethernet is set as the default netif and has a valid internet
connection.  It comes up first and sets a DNS  address given by DHCP.

The PPP interface is always up, even if the wifi module has no link, which it
often doesn't.  It comes up second setting dns_servers to the PPP given
values, overwriting the ethernet dns address.

If I try to use DNS resolution on ethernet, it uses the PPP dns_server values
which of course fails.  If I unplug ethernet, plug it back in.  dns_servers
are updated by the Ethernets DHCP, then then DNS resolution can be used.




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[yuanjianmin]

Follow-up Comment #1, bug #58571 (project lwip):

The list of DNS servers is global in other OSes, too. Why does your netif add
DNS servers without being connected to the internet at all?

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[yuanjianmin]

Follow-up Comment #2, bug #58571 (project lwip):

The netif is a PPP connection to a u-Blox W152 WiFi module.  When the PPP
connection is established to the module, it appears to negotiate a DNS address
and set it.  The DNS address is actually just the IP address of the module on
the PPP connection.  However, the module has no internet connection to the
outside world until you send it commands (on a UDP port over the PPP link) to
tell it to make the WiFi link.  Additionally the module is also a Bluetooth
device which is controlled over the same UDP port.  Sometimes we wish to only
use it for Bluetooth and never enable the WiFi.

So, to lwIP, the PPP connection appears as a valid outside world connection,
which is fair enough, but it is not.  In fact, while the WiFi link is not
enabled, the netif is set to link down.

But a DNS resolution on another netif still uses the DNS address supplied by
the PPP connection even though it is in link down state.  Because it was the
last netif to call dns_setserver.  But the DNS address is the local address of
the module on the PPP link, not an outside world address.

The other situation is when you have two netifs (e.g. ethernet and wifi), both
have gateways that have a preferred DNS address which is supplied upon DHCP
resolution, maybe even offering a local cache or service.  Should the
preferred DNS address not be used for each netif rather than just whatever the
last supplied was?

There appears to be no hooks or way to manage the receive of DNS addresses to
manage them yourself.  Auto assigning by DHCP can be disabled, but not by PPP.
  The received DNS address is not stored anywhere for later access, except
overwrites the global array (in dns_setserver).

Without deep investigation, it seems to me that if dns_setserver took a netif
pointer and all reads of DNS addresses were through dns_getserver (rather than
direct array access) and had a netif pointer, and these functions could be
hooked or overridden.  Then you could manually manage your own DNS addresses.

In fact, it seems I have no option but to make this change in my situation?

Furthermore, there appears to be no way to manually set a DNS address without
risk of it being overwritten by DHCP or PPP.  

Hope that helps explain.

Cheers

Ashley


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[yuanjianmin]

Follow-up Comment #3, bug #58571 (project lwip):

Hi,

I am with Ashley here. Having per netif DNS server storage and using DNS
servers of the current default interface would be very nice.

I had a similar setup with wired Ethernet (default) and PPP (GPRS backup) and
we had to write a DNS server switch outside lwIP. Basically interface UP hooks
to copy negotiated DNS servers from DHCP and/or PPP outside lwIP to keep track
of them, and something very ugly to enforce using the DNS servers list from
the current default interface. I can't remember the outcome exactly but I was
not very happy writing it :-)

Today's DNS servers handling in OSes is MUCH MORE complicated than just
overwriting /etc/resolv.conf or equivalent. With systemd-networkd or
NetworkManager, and last time I had to deal with that on Windows with VPN
clients it was much better than a naive overwrite of the previous DNS servers
list.

Sylvain

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[yuanjianmin]

Follow-up Comment #4, bug #58571 (project lwip):

Windows allows manually or automatically setting two DNS servers per network
interface.  No idea what linux/unix does.

Cheers

Ashley

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[yuanjianmin]

Follow-up Comment #5, bug #58571 (project lwip):

I have made what I think is a simple solution to this issue by allowing (as an
option) separate DNS servers per netif.  Minimal code changes were required.
When the option is turned off, there are no actual code changes outside of
dns.c/h.  When turned on the changes to external modules are minor.  The
changes to dns.c are also minor.

I have tested the code and would like to submit a patch for review so that
this change may be considered for inclusion in lwIP or at least be a reference
for other people who come up against this situation.

What is the process for submitting this patch?

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[yuanjianmin]

Follow-up Comment #6, bug #58571 (project lwip):

> What is the process for submitting this patch?

Just attach the patch file to this bug (see bottom of this page). Ideally,
you'd provide a 'git format-patch -1 HEAD' like patch to have one commit from
our current git master HEAD.

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[yuanjianmin]

Follow-up Comment #7, bug #58571 (project lwip):

I have attached the patch file below.  The patch is for STABLE-2.1.2 release.

Sorry, I have not been allowed time to spend on implementing the patch on the
current master head.

The patch does not include any changes that may be required to default
projects, tests or lwipopts.h sample files.  LWIP_DNS_SERVER_PER_NETIF must be
set to 1 in lwipopts.h for this patch to be effective, otherwise code should
compile pretty much as it did before the patch.

I am interested in hearing thoughts or anything I may have messed up.  I am
not overly familiar with lwIP so  may have easily missed an important detail
or use case.

Regards

Ashley
 

(file #49370)
    _______________________________________________________

Additional Item Attachment:

File name: dnsserverpernetif.patch        Size:11 KB
    <https://savannah.nongnu.org/file/dnsserverpernetif.patch?file_id=49370>



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[yuanjianmin]

Follow-up Comment #8, bug #58571 (project lwip):

As I've stumbled into exactly the same problem, I've taken the liberty to take
the patch posted earlier by Ashley and update it to the latest lwIP master. It
should now apply easily, with no errors or rejects. I've also allowed myself
to improve it slightly:
- I've renamed the option from LWIP_DNS_SERVER_PER_NETIF to
LWIP_DNS_SERVERS_PER_NETIF, as this seems more correct
- I've added the new option along with a (rather long) description to opt.h
- I've modified the few remaining dns_{get,set}server() calls which were left
by the original patch (if I remember correctly - one in dns_init(), one in
dhcp6.c and a few in nd6.c)
- couple of minor things

Please let me know what can be done to merge this patch, as I would really
prefer NOT to keep working with a forked submodule (; I'm open to suggestions
how to further improve it or even change it into some other method for solving
this not-so-common issue.

(file #50107)
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Additional Item Attachment:

File name: 0001-Introduce-LWIP_DNS_SERVERS_PER_NETIF-option.patch Size:14 KB
   
<https://file.savannah.nongnu.org/file/0001-Introduce-LWIP_DNS_SERVERS_PER_NETIF-option.patch?file_id=50107>



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[yuanjianmin]

Follow-up Comment #9, bug #58571 (project lwip):

Too many spaces between the option name and default value in opt.h (;

I'm uploading updated version.

(file #50109)
    _______________________________________________________

Additional Item Attachment:

File name: 0001-Introduce-LWIP_DNS_SERVERS_PER_NETIF-option.patch Size:14 KB
   
<https://file.savannah.nongnu.org/file/0001-Introduce-LWIP_DNS_SERVERS_PER_NETIF-option.patch?file_id=50109>



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