Comment 0 for bug 1829264

https://github.com/CanonicalLtd/netplan/blob/ea47bd4e1c5319b1a624ea61c843ee96e7c128da/src/networkd.c#L208

The separator used to add multiple ARP IP targets for the networkd renderer is ",".

Unfortunately, networkd doesn't allow that, but rather expects whitespace:

(in config_parse_arp_ip_target_address():)
[...]
        for (;;) {
                [...]
                r = extract_first_word(&rvalue, &n, NULL, 0);

extract_first_word's third parameter is const char *separators; where NULL is handled like so (src/basic/extract-word.c:37):

        if (!separators)
                separators = WHITESPACE;

Finally:

src/basic/string-util.h:#define WHITESPACE " \t\n\r"

So; it doesn't handle commas for the list, just whitespace; so let's give it whitespace please :)