tun/tap interface cannot reach the internet

Question

I am trying to set up a NAT: a tun/tap interface tap0, with IP masquerading.

But i cannot reach the internet through tap0 when the default route is through it. Can you help me troubleshoot please?

These are the commands i have run, where 192.168.A.B is a placeholder for the address of tap0:

ip tuntap add mode tap tap0
ip addr add 192.168.A.B/24 dev tap0
ifconfig tap0 192.168.A.B up
ip route add default via 192.168.A.B
ip link set tap0 up`

And these are my iptables rules, with some of them specific to another user named someuser, while i was root when i was doing my tests (so those with owner UID match someuser are not relevant) :

[root@localhost ~]# iptables -L
Chain INPUT (policy ACCEPT)
target prot opt source destination

Chain FORWARD (policy ACCEPT)
target prot opt source destination

Chain OUTPUT (policy ACCEPT)
target prot opt source destination
ACCEPT tcp -- anywhere localhost owner UID match someuser tcp dpt:krb524
ACCEPT udp -- anywhere localhost owner UID match someuser udp dpt:krb524
ACCEPT tcp -- anywhere localhost owner UID match someuser tcp dpt:upnotifyp
ACCEPT udp -- anywhere localhost owner UID match someuser udp dpt:upnotifyp
REJECT all -- anywhere anywhere owner UID match someuser reject-with icmp-port-unreachable

[root@localhost ~]# iptables -L -t nat
Chain PREROUTING (policy ACCEPT)
target prot opt source destination

Chain INPUT (policy ACCEPT)
target prot opt source destination

Chain OUTPUT (policy ACCEPT)
target prot opt source destination

Chain POSTROUTING (policy ACCEPT)
target prot opt source destination
MASQUERADE all -- 192.168.A.0/24 anywhere
MASQUERADE all -- anywhere anywhere

At this point, when i ping imdb.com , the host is unreachable. Are the MASQUERADE iptables rules to blame?

This is the output of ip route, where eth0 is the real interface with address 192.168.X.Y and my gateway is 192.168.X.Z

[root@localhost ~]# ip route
default via 192.168.A.B dev tap0 linkdown
default via 192.168.X.Z dev eth0 proto dhcp src 192.168.X.Y metric 100
127.0.0.0/8 dev lo proto kernel scope link src 127.0.0.1 metric 30
192.168.X.0/24 dev eth0 proto kernel scope link src 192.168.X.Y metric 100
192.168.A.0/24 dev tap0 proto kernel scope link src 192.168.A.B metric 350 linkdown

Even though it says "linkdown", tap0 seems up:

[root@localhost ~]# ifconfig
eth0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> ...
...
tap0: flags=4099<UP,BROADCAST,MULTICAST> mtu 1500
inet 192.168.A.B netmask [255.255.255.0](https://255.255.255.0)  broadcast 192.168.A.255

ip addr show shows this for tap0:

tap0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc fq_codel state DOWN group default qlen 1000

and this for eth0

<BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UNKNOWN group default qlen 1000

Now, if i do the following, the ping works

ip route delete default via 192.168.A.B

Just to say that everything works through my real gateway directly.

Something that i can try: creating a bridge through nmcli/nmtui, between tap0 and eth0. Would it fix it and how to do that?