Difference in search domain resolution behavior: native resolver vs inet_res

[door]

Hi!

Not quite sure if this is a bug or feature request.

Describe the bug

There is a difference between inet_res and the native resolver regarding search domains.

Let's assume the resolv.conf file has a search domain defined: "search e", and we're querying a single-segment name.

The native resolver will first attempt a query for the name without a domain, then with the domain.
inet_res will only try queries with the domains.

To Reproduce
This matters when running inside a Docker container.
For example, let's say the host machine has "search e" in its resolv.conf. Docker will copy this into the container.

docker-compose.yml:

services:
  db:
    image: debian:13
    command: ["/bin/bash", "-c", "sleep 1d"]

  server:
    image: erlang:28
    command: ["/bin/bash", "-c", "sleep 1d"]

root@fbae68c3d16b:/# grep search /etc/resolv.conf 
search e

root@fbae68c3d16b:/# echo '{lookup,[dns]}.' > /inetrc

root@fbae68c3d16b:/# ERL_INETRC=/inetrc erl

1> inet_db:get_rc().
[{nameservers,{127,0,0,11}},
 {search,["e"]},
 {dnssec_ok,undefined},
 {resolv_conf,"/etc/resolv.conf"},
 {hosts_file,"/etc/hosts"},
 {lookup,[dns]}]

2> inet:gethostbyname("db").
{error,nxdomain}

3> inet_gethost_native:gethostbyname("db").
{ok,{hostent,"db",[],inet,4,[{172,18,0,2}]}}

In this example host and container OS version: debian 13 / x86_64

Expected behavior
inet_res will response with IP address.

Affected versions
I checked 27 and 28.

Additional context
I haven't been able to find a satisfactory workaround for this issue. For our tasks, we require inet_res. The search in resolv.conf is real.
Docker does not allow adding a dot "." to the search list: there's no way to add it, you can only define the entire list, and it ignores the dot in dns-search.
Adding a dot to inetrc or via inet_db:add_search/1 also doesn't work – inet_db reads resolv.conf and removes the dot, leaving only ["e"].

[RaimoNiskanen]

inet_res tries to implement what the man page resolv.conf(5) describes, which is:

search Search list for host-name lookup.
   By default, the search list contains one entry, the local
   domain name.  It is determined from the local hostname
   returned by gethostname(2); the local domain name is taken
   to be everything after the first '.'.  Finally, if the
   hostname does not contain a '.', the root domain is assumed
   as the local domain name.

   This may be changed by listing the desired domain search
   path following the search keyword with spaces or tabs
   separating the names.  Resolver queries having fewer than
   ndots dots (default is 1) in them will be attempted using
   each component of the search path in turn until a match is
   found.  For environments with multiple subdomains please
   read options ndots:n below to avoid man-in-the-middle
   attacks and unnecessary traffic for the root-dns-servers.
   Note that this process may be slow and will generate a lot
   of network traffic if the servers for the listed domains
   are not local, and that queries will time out if no server
   is available for one of the domains.

ndots:n
   Sets a threshold for the number of dots which must
   appear in a name given to res_query(3) (see
   resolver(3)) before an initial absolute query will
   be made.  The default for n is 1, meaning that if
   there are any dots in a name, the name will be tried
   first as an absolute name before any search list
   elements are appended to it.  The value for this
   option is silently capped to 15.

ndots is fixed to the default: 1.

This is an established behaviour that has been documented since the dawn of resolv.conf. So it is the same on for example FreeBSD and OpenBSD. I think it came from either the Solaris or the BSD Unix branch.

It seems the native resolver in your docker environment does not behave like that since it first tries an absolute lookup even if the name to look up has no dots, that is, it behaves as if ndots = 0.

inet_res will, if the host name to look up ends with a dot, regard the name as absolute, which should do what you want.

Setting the search list to contain the domain ".", or the domain "" should work, but to avoid that /etc/resolv.conf is re-read when changed one can disable that by setting the resolv.conf filename to "". But then the name server has to be set manually. The order in inetrc matters because the directive {resolv_conf,File} reads File and overrides already set nameservers and search domain, etc. This works for me as I think you want:

{lookup, [dns]}.
{resolv_conf, ""}.
{nameserver, {127,0,0,53}}.
{search, [".", "e"]}.

https://www.erlang.org/docs/28/apps/erts/inet_cfg