Nftables rules change when network interfaces disappear

[Mikhail Morfikov <mmorfikov@xxxxxxxxx>]

I was trying to write some FW policy for VPN and I added the following rules to
my openvpn script:

    nft create chain ip nat force-vpn
    nft add rule ip nat POSTROUTING meta oif ${dev} counter jump force-vpn
    nft add rule ip nat force-vpn meta oif ${dev} counter snat ${ifconfig_local}

When the VPN connections is being established, the rules do their job, the ${dev} 
variable is properly resolved and nftables can live with it just fine:

# nft -a list table ip nat

table ip nat { # handle 55
...
        chain POSTROUTING { # handle 3
...
                oif "tun0" counter packets 0 bytes 0 jump force-vpn # handle 20
        }
...
        chain force-vpn { # handle 19
                oif "tun0" counter packets 0 bytes 0 snat to 172.27.100.20 # handle 21
        }
}

But when I close the VPN connection, something weird happens. The above rules 
now looks like this:

# nft -a list table ip nat

table ip nat { # handle 55
...
        chain POSTROUTING { # handle 3
...
                oif 61 counter packets 0 bytes 0 jump force-vpn # handle 20
        }
...
        chain force-vpn { # handle 19
                oif 61 counter packets 0 bytes 0 snat to 172.27.100.20 # handle 21
        }
}

So the output interface is now 61 and not "tun0" . My script doesn't do anything 
with the nftables rules when the VPN connection is closing. So the value of the 
output interface magically changed on its own.

The number is the one that can be found in the output of the `ip` command when 
the interface was created:

# ip addr show
...
61: tun0: ...
...

Is this a bug or is this intended behavior?

---
# nft -v
nftables v0.9.6 (Capital Idea #2)

# cat /proc/version
Linux version 5.8.5-amd64 (morfik@morfikownia) (gcc (Debian 10.2.0-5) 10.2.0, GNU ld (GNU Binutils for Debian) 2.35) #2 SMP PREEMPT Thu Aug 27 12:08:37 CEST 2020

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