[Dnsmasq-discuss] host in subnet LAN cannot resolve name in outer LAN

Petr Menšík pemensik at redhat.com
Tue Jun 15 22:17:33 UTC 2021


Hi,

interesting problem. If you want to be able to resolve mDNS .local on
network 46, I doubt that can be archieved using dnsmasq directly.

.local domain is for multicast resolution on local network. That means
rpi machines are on different local network than newton. local domain
should not be used for unicast DNS, because it would clash. The result
would be inconsistent.

Have you tried main router, whether it can serve names of dhcp clients?
Many routers use dnsmasq inside, they might be able to resolve newton

I think you might be able to misuse systemd-resolved on home52 to
provide resolution for mdns. Enable MDNS for wlan0 interface in
resolved.conf.

Then add to dnsmasq.conf:

server=/local/127.0.0.53

Because systemd-resolved talks DNS even to names on different protocol,
it might be able to resolve. I would consider it a wrong hack. Instead,
try to offer some domain from router. For example 46.lan on router,
52.lan on home52. If router has also dnsmasq, it might work. Add to
dnsmasq.conf:

# update with actual router IP.
server=/46.lan/192.168.46.1

but that would work only when router also maintains names. I think
ubiquity and routerboards can do that. This way would be more reliable,
than any hacks reusing local domain.

Hope that helps,

Petr

On 6/9/21 11:48 PM, mjbmon wrote:
> Hello,
>
> I am trying to build a subnet where dnsmasq is the DHCP/DNS server. This
> seems like a very common case but I cannot find any information on
> solving
> the problem. TL;DR the hosts in the subnet LAN cannot resolve hosts in
> the
> outer LAN, although they can ping them by ip and of course also resolve
> names in the internet at large.
>
> The top level network is 192.168.46.0/24, consisting of a black box
> router
> connected to the internet at large and connected to local hosts via WIFI.
> Under this are three hosts (more, but these are the relevant ones).
> Newton
> is a laptop running Linux Mint, home46 is a Raspberry Pi running
> Raspbian,
> and home52 is another Rpi which is the head node for the subnet, which is
> 192.168.52.0/24. This subnet is implemented with ethernet.
>
> Newton, home46, home52 all connect to the router via wifi. So far this
> is a very standard home network and these hosts can ping each other using
> names such as newton.local, home46.local, and home52.local. I know
> very well
> that .local is a terrible choice for a TLD, but the router is in
> charge of
> this. Changing out the router is a different topic for a different time.
> I believe that this subnet uses mDNS to map between names and
> ip addresses on the subnet.
>
>
>    router
>      |
>  --------------------   (192.168.46.0/24, this is .local)
>  |         |        |
> newton   home46     |
>                     | (wlan0)
>                  home52
>                     | (eth0)
>                     |
>           ----------------------   (192.168.46.0/24, this is .52.lan)
>           |         |          |
>         rpi0       rpi1       rpi2
>
> home52 is the interesting host. It runs dnsmasq to provide DHCP and DNS
> services to rpi0, rpi1, and rpi2 on the interface eth0. There are
> iptables
> rules which route traffic between wlan0 and eth0 on home52.
> Everything works and rpi0, rpi1, rpi2 can all ping each other by name
> as well as hosts in the internet at large, e.g. www.google.com.
> rpi0,rpi1,rpi2
> can also ping hosts newton and home46, but ONLY by ip address, not by
> name. If I "ping newton.local" from rpi2 I get the following in the
> dnsmasq log on home52, from systemctl status dnsmasq:
>
> home52 dnsmasq[851]: query[SOA] local from 192.168.52.100
> home52 dnsmasq[851]: forwarded local to 8.8.4.4
> home52 dnsmasq[851]: forwarded local to 8.8.8.8
> home52 dnsmasq[851]: forwarded local to 192.168.46.1
>
> and of course all of this is wrong.
>
> So how can I get dnsmasq to serve hosts in the subnet with names from
> the outer net? Note: I want the hosts in the subnet to be zeroconf --
> they must not know anything about the outer net. Suggestions about
> modifying
> their /etc/hosts file are not useful and in any case the ip addresses
> in the outer subnet are subject to change after rebooting, since they
> are all provided by DHCP from the router.
>
> ------------------------
>
> dnsmasq.conf:
>
>  # Use interface eth0
>  interface=eth0
>
>  # Explicitly specify the address to listen on
>  listen-address=192.168.52.1
>
>  # Bind to the interface to make sure we aren't sending things elsewhere
>  bind-interfaces
>
>  # don't forward unqualified names (e.g. myserver)
>  domain-needed
>
>  # won't forward some non-routed addresses
>  bogus-priv
>
>  # won't forward requests for the intranet subdomain
>  local=/52.lan/
>
>  # append the domain (below) to all hosts
>  domain=52.lan
>
>  # Assign IP addresses between 192.168.52.50 and 192.168.52.150
>  # with a 12 hour lease time
>  dhcp-range=192.168.52.50,192.168.52.150,12h
>
>  # Forward DNS requests to the local DNS and then Google DNS
>  server=192.168.52.1
>  server=8.8.8.8
>  server=8.8.4.4
>
>  # Use the /etc/ethers file to specify static mappings
>  # read-ethers
>
>  # log DNS queries, for debugging
>  log-queries
>
>
> -------------------
>
>  /etc/hosts file
>
> 127.0.0.1       localhost
> ::1             localhost ip6-localhost ip6-loopback
> ff02::1         ip6-allnodes
> ff02::2         ip6-allrouters
>
> 127.0.1.1 home52
>
> 192.168.52.1 home52.52.lan
>
>
> -----------------
>
>  commands to set up iptables
> sysctl -w net.ipv4.ip_forward=1
> iptables -t nat -A POSTROUTING -o wlan0 -j MASQUERADE
> iptables -A FORWARD -i eth0 -o wlan0 -j ACCEPT
> iptables -A FORWARD -i wlan0 -o eth0 -j ACCEPT \
>     -m state --state ESTABLISHED,RELATED
>
>
>
>
>
>
>
>
>
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