resolver: sysresolve, names resolved by the system routines

The command keeps the C library's resolution which a configuration
without nserver had before the system's name servers became the
default: no hosts file imported. nserver lines, if any, are still
there for dnspr to forward to. Of sysresolve and fakeresolve the last
given applies.

Tests: sysdns, with sysresolve dnspr has no hosts file names and a
name is resolved through the system routines.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

doc/html/man5/3proxy.cfg.5.html

@@ -498,6 +498,15 @@ user), Reply-Message. Use <b>authcache</b> to speedup
 authentication. RADIUS feature is currently
 experimental.</p>
 
+<p style="margin-left:9%; margin-top: 1em"><b>sysresolve</b>
+<br>
+Names are resolved by the system routines for name
+resolution (the C library), as without <b>nserver</b> before
+the system's name servers became the default; the hosts file
+is not imported. <b>nserver</b> lines, if any, are still used
+by <b>dnspr</b> to forward queries. Of <b>sysresolve</b> and
+<b>fakeresolve</b> the last one given applies.</p>
+
 <p style="margin-left:9%; margin-top: 1em"><b>nserver</b>
 <i>&lt;ipaddr&gt;</i>[:<i>port</i>][/<i>tcp</i>] <br>
 Nameserver to use for name resolutions; up to 3, asked in
@@ -507,7 +516,8 @@ has to be enclosed in square brackets: <br>
 <b>nserver 1.1.1.1</b> <br>
 <b>nserver [2001:4860:4860::8844]</b> <br>
 <b>nserver [2001:4860:4860::8844]:5353/tcp</b> <br>
-If none is specified (and there is no <b>fakeresolve</b>),
+If none is specified (and there is no <b>fakeresolve</b> or
+<b>sysresolve</b>),
 the system's name servers are used: up to 3 from
 <i>/etc/resolv.conf</i>, on Windows from the registry (those
 set for the system, those set for an interface, then those

man/3proxy.cfg.5

@@ -555,7 +555,7 @@ An IPv6 address has to be enclosed in square brackets:
 .br
  \fBnserver [2001:4860:4860::8844]:5353/tcp\fR
 .br
- If none is specified (and there is no \fBfakeresolve\fR), the system's name
+ If none is specified (and there is no \fBfakeresolve\fR or \fBsysresolve\fR), the system's name
 servers are used: up to 3 from \fI/etc/resolv.conf\fR, on Windows from the
 registry (those set for the system, those set for an interface, then those
 DHCP gave). Names in the hosts file (\fI/etc/hosts\fR, on Windows
@@ -600,6 +600,15 @@ are never consulted. A static record takes precedence over the hosts file. If 0.
 is used as a hostaddr host will never resolve, it can be used to
 blacklist something.
 
+.br
+.B sysresolve
+.br
+ Names are resolved by the system routines for name resolution (the C
+library), as without \fBnserver\fR before the system's name servers became
+the default; the hosts file is not imported. \fBnserver\fR lines, if any, are
+still used by \fBdnspr\fR to forward queries. Of \fBsysresolve\fR and
+\fBfakeresolve\fR the last one given applies.
+
 .br
 .B fakeresolve
 .br

src/conf.c

@@ -741,6 +741,8 @@ static int h_flushusers(int argc, unsigned char **argv){
    force (the process has one list) once it is committed */
 static struct nserver cfgns[MAXNSERVERS];
 static int cfgnsn = 0;
+/* sysresolve: names resolved by the system routines */
+static int cfgsysresolve = 0;
 
 static int h_nserver(int argc, unsigned char **argv){
   char *str;
@@ -764,13 +766,13 @@ static int h_nserver(int argc, unsigned char **argv){
 
 /* The configuration's name servers, or the system's in their place */
 void confdnscommit(struct extparam *c){
-	sysdnsapply(c, cfgns, cfgnsn);
+	sysdnsapply(c, cfgns, cfgnsn, cfgsysresolve);
 }
 
 /* Whether names would be resolved slowly: by the C library, or by name
    servers, the system's taken by default, without a cache */
 static int noresolver(void){
-	if(!conf->resolvfunc && !sysdnscount()) return 1;
+	if(cfgsysresolve || (!conf->resolvfunc && !sysdnscount())) return 1;
 	return (!conf->resolvfunc || conf->resolvfunc == myresolver) && !dns_table.poolsize;
 }
 
@@ -790,6 +792,12 @@ static int h_authnserver(int argc, unsigned char **argv){
 
 static int h_fakeresolve(int argc, unsigned char **argv){
 	conf->resolvfunc = fakeresolver;
+	cfgsysresolve = 0;
+	return 0;
+}
+
+static int h_sysresolve(int argc, unsigned char **argv){
+	cfgsysresolve = 1;
 	return 0;
 }
 
@@ -2146,6 +2154,7 @@ struct commands commandhandlers[]={
 	{NULL, "flush", h_flush, 1, 1},
 	{NULL, "nserver", h_nserver, 2, 2},
 	{NULL, "fakeresolve", h_fakeresolve, 1, 1},
+	{NULL, "sysresolve", h_sysresolve, 1, 1},
 	{NULL, "nscache", h_nscache, 2, 2},
 	{NULL, "nscache6", h_nscache6, 2, 2},
 	{NULL, "nsrecord", h_nsrecord, 3, 3},
@@ -2534,6 +2543,7 @@ void confbegin(void){
 	char * args[] = {"auth", "iponly", NULL};
 
 	cfgnsn = 0;
+	cfgsysresolve = 0;
 
 	h_auth(2, (unsigned char **)args);
 }

src/proxy.h

@@ -435,7 +435,7 @@ int nscacheresolv(int af, unsigned char *name, unsigned char *value, uint32_t *t
 void setnservers(const struct nserver *ns, int n);
 void sysdnsinit(void);
 int sysdnscount(void);
-void sysdnsapply(struct extparam *c, const struct nserver *ns, int n);
+void sysdnsapply(struct extparam *c, const struct nserver *ns, int n, int sysresolve);
 void sysdnscheck(void);
 void confdnscommit(struct extparam *c);
 uint32_t udpresolve(int af, unsigned char * name, unsigned char * value, uint32_t *retttl, struct clientparam* param, int makeauth);

src/resolve.c

@@ -585,9 +585,16 @@ int sysdnscount(void){
 
 /* A configuration committed: the name servers of its nserver lines, else,
    with nothing else to resolve names, the system's and its hosts file, read
-   again. Until then those in force stay. */
+   again. Until then those in force stay. With sysresolve the system
-void sysdnsapply(struct extparam *c, const struct nserver *ns, int n){
+   routines resolve names, nserver lines left to dnspr. */
+void sysdnsapply(struct extparam *c, const struct nserver *ns, int n, int sysresolve){
     sysdns.active = 0;
+    if(sysresolve){
+	c->resolvfunc = NULL;
+	setnservers(ns, n);
+	hostsremove();
+	return;
+    }
     if(n){
 	setnservers(ns, n);
 	hostsremove();

tests/cases/sysdns.py

@@ -1,6 +1,6 @@
 """The system's name servers and hosts file as the default resolver: with
 no nserver in a configuration, names in the hosts file are answered from
-it, through the name cache; with nserver lines, the file is not used."""
+it; with nserver lines, or sysresolve (the system routines), it is not."""
 
 import os
 import re
@@ -81,3 +81,26 @@ def run(t):
     wait(t, other, "localhost")
     t.ne(["127.0.0.1"], t.dns_query(other, "localhost") or [],
          "with nserver, the hosts file is not used")
+
+    # sysresolve: the system routines resolve names, and nothing is imported
+    # for dnspr to answer from
+    sysport, proxy, srv = t.free_port(), t.free_port(), t.free_port()
+    t.start("sysdns_sysresolve", f"""
+        log
+        flush
+        sysresolve
+        auth iponly
+        allow *
+        dnspr -p{sysport}
+        proxy -p{proxy}
+        flush
+        auth iponly
+        allow *
+        http echo * /echo**
+        httpsrv -p{srv}
+    """, ports=[proxy, srv])
+    wait(t, sysport, "localhost")
+    t.ne(["127.0.0.1"], t.dns_query(sysport, "localhost") or [],
+         "with sysresolve, dnspr has no hosts file names")
+    t.eq(200, t.http(f"http://localhost:{srv}/echo", proxy=f"127.0.0.1:{proxy}").status,
+         "and names resolve through the system routines")