While looking into why the Zebra unix socket defaults to 0666, I noticed that the following syntax is supported since 2003:

unix:[user=uid,][group=gid,][umask=mask,]file=path

I first noticed it in the YAZ news, then found it in the code, and then got it working in Zebra 2.2.3.

https://github.com/indexdata/yaz/blob/master/NEWS#LL3562C2-L3562C52

https://github.com/indexdata/yaz/blob/master/src/unix.c#L435

I was thinking that https://software.indexdata.com/yaz/doc/server.vhosts.html might be a good place for the syntax to be documented?