EDIT: The bad solution is to unblock UDP port 5353 but the port has to be source port, not destination port. (--sport flag) See the now modified rules. The issue is that this is very insecure (see this stackexchange question and comments) but obviously better than no firewall at all because at least I’m blocking TCP traffic.

The proper solution (other than using glibc and installing nss-mdns package) is to open a port with netcat (nc) in the background (using &) and then listen with dig on that port using the -b flag.

port="42069"
nc -l -p "$port" > /dev/null || exit 1 &
dig somehostname.local @224.0.0.241 -p 5353 -b "0.0.0.0#${port}"

Then we need to remember to kill the background process. The DNS reply will now be sent to port 42069, so we can just open it with this iptables rule:

-A INPUT -p udp -m udp --dport 42069 -j ACCEPT

---->END OF EDIT.

I want to setup iptables firewall but if I do that, it blocks multicast DNS which I need. I am using command

dig "somehostname.local" @224.0.0.251 -p 5353

to get the IP through mDNS and these are my iptables rules (from superuser.com):

*filter

# drop forwarded traffic. you only need it of you are running a router
:FORWARD DROP [0:0]

# Accept all outgoing traffic
:OUTPUT ACCEPT [623107326:1392470726908]


# Block all incoming traffic, all protocols (tcp, udp, icmp, ...) everything.
# This is the base rule we can define exceptions from.
:INPUT DROP [11486:513044]

# do not block already running connections (important for outgoing)
-A INPUT -m state --state RELATED,ESTABLISHED -j ACCEPT

# do not block localhost
-A INPUT -i lo -j ACCEPT

# do not block icmp for ping and network diagnostics. Remove if you do not want this
# note that -p icmp has no effect on ipv6, so we need an extra ipv6 rule
-4 -A INPUT -p icmp -j ACCEPT
-6 -A INPUT -p ipv6-icmp -j ACCEPT

# allow some incoming ports for services that should be public available
# -A INPUT -p tcp -m tcp --dport 22 -j ACCEPT
# -A INPUT -p udp -m udp --dport 5353 -j ACCEPT # does not help
-A OUTPUT -p udp -m udp --sport 5353 -j ACCEPT # SOLVES THE ISSUE BUT IS INSECURE - not recommended


# commit changes
COMMIT

Any help is welcome :)

  • just_another_person@lemmy.world
    2·
    3 days ago

    It’s blocking the querying as well? It’s hard to discern where the issue is.

    Mdns is udp on port 5353, and your rules don’t seem to block outgoing udp, so your dig should work if enabled or not.

        • TMP_NKcYUEoM7kXg4qYe@lemmy.worldOP
          1·
          2 days ago

          It’s solved now. Basically what’s happening is that I ask a multicast address on UDP port 5353 and get a response from different IP because the original IP was multicast. So my firewall blocks the reply, because it really isn’t a reply like downloading a webpage. I solved it by filtering based on the source port. Meaning the reply has source port 5353 but on my machine it arrives at some random UDP port so I cannot really filter based on the destination port.

          solution

          -A OUTPUT -p udp -m udp --sport 5353 -j ACCEPT

          Thanks for your help!