It is possible to commit an illegal nat configuration, in this example the translation address is not specified on commit and it renders an exception.
This example is from a newly installed iso.
Welcome to VyOS - vyos ttyS0 vyos login: vyos Password: Linux vyos 4.19.139-amd64-vyos #1 SMP Sat Aug 15 09:30:29 UTC 2020 x86_64 The programs included with the Debian GNU/Linux system are free software; the exact distribution terms for each program are described in the individual files in /usr/share/doc/*/copyright. Debian GNU/Linux comes with ABSOLUTELY NO WARRANTY, to the extent permitted by applicable law. vyos@vyos:~$ conf [edit] vyos@vyos# set nat source rule 100 outbound-interface eth0 [edit] vyos@vyos# commit [ nat ] VyOS had an issue completing a command. We are sorry that you encountered a problem while using VyOS. There are a few things you can do to help us (and yourself): - Make sure you are running the latest version of the code available at https://downloads.vyos.io/rolling/current/amd64/vyos-rolling-latest.iso - Consult the forum to see how to handle this issue https://forum.vyos.io - Join our community on slack where our users exchange help and advice https://vyos.slack.com When reporting problems, please include as much information as possible: - do not obfuscate any data (feel free to contact us privately if your business policy requires it) - and include all the information presented below Report Time: 2020-08-19 16:58:25 Image Version: VyOS 1.3-rolling-202008190118 Release Train: equuleus Built by: [email protected] Built on: Wed 19 Aug 2020 01:18 UTC Build UUID: bb4ab508-0e69-48c4-ba73-f559c811a083 Build Commit ID: 9cb142f438773c Architecture: x86_64 Boot via: installed image System type: KVM guest Hardware vendor: QEMU Hardware model: Standard PC (i440FX + PIIX, 1996) Hardware S/N: Hardware UUID: Unknown Traceback (most recent call last): File "/usr/libexec/vyos/conf_mode/nat.py", line 271, in <module> apply(c) File "/usr/libexec/vyos/conf_mode/nat.py", line 259, in apply cmd(f'{iptables_nat_config}') File "/usr/lib/python3/dist-packages/vyos/util.py", line 179, in cmd raise OSError(code, feedback) PermissionError: [Errno 1] failed to run command: /tmp/vyos-nat-rules.nft returned: exit code: 1 noteworthy: cmd '/tmp/vyos-nat-rules.nft' returned (out): returned (err): /tmp/vyos-nat-rules.nft:41:61-67: Error: syntax error, unexpected comment add rule ip nat POSTROUTING oifname "eth0" counter snat to comment "DST-NAT-100" ^^^^^^^ [[nat]] failed Commit failed [edit] vyos@vyos#