relay: Add AddressDisableIPv6 torrc option

This option controls if a tor relay will attempt address auto discovery and
thus ultimately publish an IPv6 ORPort in the descriptor.

Behavior is from proposal 312 section 3.2.6.

Closes #33245

Signed-off-by: David Goulet <dgoulet@torproject.org>

doc/man/tor.1.txt

@@ -2138,6 +2138,12 @@ is non-zero):
     binds to.  To bind to a different address, use the ORPort and
     OutboundBindAddress options.
 
+[[AddressDisableIPv6]] **AddressDisableIPv6** **0**|**1**::
+   By default, Tor will attempt to find the IPv6 of the relay if there is no
+   IPv4Only ORPort. If set, this options disable IPv6 auto discovery which
+   means no IPv6 address resolution, no IPv6 ORPorts, no IPv6 reachability
+   checks, and won't publish an IPv6 ORPort in its descriptor. (Default: 0)
+
 [[AssumeReachable]] **AssumeReachable** **0**|**1**::
     This option is used when bootstrapping a new Tor network. If set to 1,
     don't do self-reachability testing; just upload your server descriptor

src/app/config/config.c

@@ -315,6 +315,7 @@ static const config_var_t option_vars_[] = {
   VAR("AccountingRule",          STRING,   AccountingRule_option,  "max"),
   V(AccountingStart,             STRING,   NULL),
   V(Address,                     LINELIST, NULL),
+  V(AddressDisableIPv6,          BOOL,     "0"),
   OBSOLETE("AllowDotExit"),
   OBSOLETE("AllowInvalidNodes"),
   V(AllowNonRFC953Hostnames,     BOOL,     "0"),

src/app/config/or_options_st.h

@@ -75,6 +75,10 @@ struct or_options_t {
    * options is accepted as in IPv4 and IPv6. */
   struct config_line_t *Address;
 
+  /** Boolean: If set, disable IPv6 address resolution, IPv6 ORPorts, IPv6
+   * reachability checks, and publishing an IPv6 ORPort in its descriptor. */
+  int AddressDisableIPv6;
+
   char *PidFile; /**< Where to store PID of Tor process. */
 
   struct routerset_t *ExitNodes; /**< Structure containing nicknames, digests,

src/app/config/resolve_addr.c

@@ -632,6 +632,13 @@ find_my_address(const or_options_t *options, int family, int warn_severity,
   if (method_out) *method_out = NULL;
   if (hostname_out) *hostname_out = NULL;
 
+  /* If an IPv6 is requested, check if IPv6 address discovery is disabled and
+   * if so we always return a failure. It is done here so we don't populate
+   * the resolve cache or do any DNS resolution. */
+  if (family == AF_INET6 && options->AddressDisableIPv6) {
+    return false;
+  }
+
   /*
    * Step 1: Discover address by attempting 3 different methods consecutively.
    */

src/feature/relay/relay_find_addr.c

@@ -105,6 +105,13 @@ relay_find_addr_to_publish, (const or_options_t *options, int family,
 
   tor_addr_make_unspec(addr_out);
 
+  /* If an IPv6 is requested, check if IPv6 address discovery is disabled on
+   * this instance. If so, we return a failure. It is done here so we don't
+   * query the suggested cache that might be populated with an IPv6. */
+  if (family == AF_INET6 && options->AddressDisableIPv6) {
+    return false;
+  }
+
   /* First, check our resolved address cache. It should contain the address
    * we've discovered from the periodic relay event. */
   resolved_addr_get_last(family, addr_out);

src/test/test_config.c

@@ -1260,6 +1260,7 @@ get_interface_address6_failure(int severity, sa_family_t family,
   do {                                        \
     config_free_lines(options->Address);      \
     config_free_lines(options->ORPort_lines); \
+    options->AddressDisableIPv6 = 0;          \
     options->ORPort_set = 0;                  \
     tor_free(options->DirAuthorities);        \
     tor_free(hostname_out);                   \
@@ -1455,6 +1456,24 @@ test_config_find_my_address(void *arg)
   options = options_new();
   options_init(options);
 
+  /*
+   * Case 0:
+   *    AddressDisableIPv6 is set.
+   *
+   * Only run this if we are in the IPv6 test.
+   */
+  if (p->family == AF_INET6) {
+    options->AddressDisableIPv6 = 1;
+    /* Set a valid IPv6. However, the discovery should still fail. */
+    config_line_append(&options->Address, "Address", p->public_ip);
+    tor_addr_parse(&test_addr, p->public_ip);
+
+    retval = find_my_address(options, p->family, LOG_NOTICE, &resolved_addr,
+                             &method_used, &hostname_out);
+    VALIDATE_FOUND_ADDRESS(false, NULL, NULL);
+    CLEANUP_FOUND_ADDRESS;
+  }
+
   /*
    * Case 1:
    *    1. Address is a valid address.

src/test/test_relay.c

@@ -302,6 +302,8 @@ test_find_addr_to_publish(void *arg)
 
   (void) arg;
 
+  memset(&options, 0, sizeof(options));
+
   /* Populate our resolved cache with a valid IPv4 and IPv6. */
   family = tor_addr_parse(&ipv4_addr, "1.2.3.4");
   tt_int_op(family, OP_EQ, AF_INET);