resolve: expose dns_server_address_valid()

src/shared/resolve-util.c

@@ -28,3 +28,16 @@ static const char* const dns_over_tls_mode_table[_DNS_OVER_TLS_MODE_MAX] = {
         [DNS_OVER_TLS_YES] = "yes",
 };
 DEFINE_STRING_TABLE_LOOKUP_WITH_BOOLEAN(dns_over_tls_mode, DnsOverTlsMode, DNS_OVER_TLS_YES);
+
+bool dns_server_address_valid(int family, const union in_addr_union *sa) {
+
+        /* Refuses the 0 IP addresses as well as 127.0.0.53 (which is our own DNS stub) */
+
+        if (in_addr_is_null(family, sa))
+                return false;
+
+        if (family == AF_INET && sa->in.s_addr == htobe32(INADDR_DNS_STUB))
+                return false;
+
+        return true;
+}

src/shared/resolve-util.h

@@ -2,8 +2,12 @@
 #pragma once
 
 #include "conf-parser.h"
+#include "in-addr-util.h"
 #include "macro.h"
 
+/* 127.0.0.53 in native endian */
+#define INADDR_DNS_STUB ((in_addr_t) 0x7f000035U)
+
 typedef enum ResolveSupport ResolveSupport;
 typedef enum DnssecMode DnssecMode;
 typedef enum DnsOverTlsMode DnsOverTlsMode;
@@ -61,3 +65,5 @@ DnssecMode dnssec_mode_from_string(const char *s) _pure_;
 
 const char* dns_over_tls_mode_to_string(DnsOverTlsMode p) _const_;
 DnsOverTlsMode dns_over_tls_mode_from_string(const char *s) _pure_;
+
+bool dns_server_address_valid(int family, const union in_addr_union *sa);