How do I get a list of all subdomains of a domain?

The hint (using axfr) only works if the NS you're querying (ns1.foo.example in your example) is configured to allow AXFR requests from the IP you're using; this is unlikely, unless your IP is configured as a secondary for the domain in question.

Basically, there's no easy way to do it if you're not allowed to use axfr. This is intentional, so the only way around it would be via brute force (i.e. dig a.example.com, dig b.example.com, ...), which I can't recommend, as it could be viewed as a denial of service attack.

1. dig example.com soa
2. dig @ns.SOA.example example.com axfr

You can only do this if you are connecting to a DNS server for the domain -and- AXFR is enabled for your IP address. This is the mechanism that secondary systems use to load a zone from the primary. In the old days, this was not restricted, but due to security concerns, most primary name servers have a whitelist of: secondary name servers + a couple special systems.

If the nameserver you are using allows this then you can use dig or nslookup.

For example:

#nslookup

>ls example.com

NOTE: because nslookup is being deprecated for dig and other newere tools, some versions of nslookup do not support "ls", most notably macOS X's bundled version.

In Windows nslookup the command is

ls -d example.com > outfile.txt

which stores the subdomain list in outfile.txt

few domains these days allow this

If the DNS server is configured properly, you won't be able to get the entire domain. If for some reason is allows zone transfers from any host, you'll have to send it the correct packet to make that request. I suspect that's what the dig statement you included does.