config: Make clients tell dual-stack exits they prefer IPv6

Make Tor clients tell dual-stack exits that they prefer IPv6
connections.

This change is equivalent to setting the PreferIPv6 flag on
SOCKSPorts (and most other listener ports). Tor Browser has
been setting this flag for some time, and we want to remove a
client distinguisher at exits.

Also update the man page, putting all the IP flags in their
non-default forms.

Closes ticket 32637.

5 files changed, 20 insertions, 9 deletions

diff --git a/changes/ticket32637 b/changes/ticket32637
new file mode 100644
index 0000000000..253f445c89
--- /dev/null
+++ b/changes/ticket32637
@@ -0,0 +1,6 @@
+  o Minor features (IPv6, client):
+    - Make Tor clients tell dual-stack exits that they prefer IPv6
+      connections. This change is equivalent to setting the PreferIPv6 flag
+      on SOCKSPorts (and most other listener ports). Tor Browser has been
+      setting this flag for some time, and we want to remove a client
+      distinguisher at exits. Closes ticket 32637.
diff --git a/doc/tor.1.txt b/doc/tor.1.txt
index d1583c3dc7..c84eba56e8 100644
--- a/doc/tor.1.txt
+++ b/doc/tor.1.txt
@@ -1755,14 +1755,16 @@ The following options are useful only for clients (that is, if
     Other recognized __flags__ for a SocksPort are:
     **NoIPv4Traffic**;;
         Tell exits to not connect to IPv4 addresses in response to SOCKS
-        requests on this connection.
-    **IPv6Traffic**;;
-        Tell exits to allow IPv6 addresses in response to SOCKS requests on
-        this connection, so long as SOCKS5 is in use.  (SOCKS4 can't handle
-        IPv6.)
-    **PreferIPv6**;;
+        requests on this connection. (Allowing IPv4 is the default.)
+    **NoIPv6Traffic**;;
+        Tell exits to not connect to IPv6 addresses in response to SOCKS
+        requests on this connection. This option is only relevant when SOCKS5
+        is in use, because SOCKS4 can't handle IPv6.  (Allowing IPv6 is the
+        default.)
+    **NoPreferIPv6**;;
         Tells exits that, if a host has both an IPv4 and an IPv6 address,
-        we would prefer to connect to it via IPv6. (IPv4 is the default.)
+        we would prefer to connect to it via IPv4. (IPv6 is the default in
+        recent versions of Tor.)
     **NoDNSRequest**;;
         Do not ask exits to resolve DNS addresses in SOCKS5 requests. Tor will
         connect to IPv4 addresses, IPv6 addresses (if IPv6Traffic is set) and
diff --git a/src/app/config/config.c b/src/app/config/config.c
index bc65977b20..bbf984ad08 100644
--- a/src/app/config/config.c
+++ b/src/app/config/config.c
@@ -5886,6 +5886,7 @@ port_cfg_new(size_t namelen)
   port_cfg_t *cfg = tor_malloc_zero(sizeof(port_cfg_t) + namelen + 1);
   cfg->entry_cfg.ipv4_traffic = 1;
   cfg->entry_cfg.ipv6_traffic = 1;
+  cfg->entry_cfg.prefer_ipv6 = 1;
   cfg->entry_cfg.dns_request = 1;
   cfg->entry_cfg.onion_traffic = 1;
   cfg->entry_cfg.prefer_ipv6_virtaddr = 1;
@@ -6133,7 +6134,7 @@ port_parse_config(smartlist_t *out,
     /* This must be kept in sync with port_cfg_new's defaults */
     int no_listen = 0, no_advertise = 0, all_addrs = 0,
       bind_ipv4_only = 0, bind_ipv6_only = 0,
-      ipv4_traffic = 1, ipv6_traffic = 1, prefer_ipv6 = 0, dns_request = 1,
+      ipv4_traffic = 1, ipv6_traffic = 1, prefer_ipv6 = 1, dns_request = 1,
       onion_traffic = 1,
       cache_ipv4 = 0, use_cached_ipv4 = 0,
       cache_ipv6 = 0, use_cached_ipv6 = 0,
diff --git a/src/core/mainloop/connection.c b/src/core/mainloop/connection.c
index f0ebce9bbb..223428a8f6 100644
--- a/src/core/mainloop/connection.c
+++ b/src/core/mainloop/connection.c
@@ -1516,7 +1516,7 @@ connection_listener_new(const struct sockaddr *listensockaddr,
   if (type != CONN_TYPE_AP_LISTENER) {
     lis_conn->entry_cfg.ipv4_traffic = 1;
     lis_conn->entry_cfg.ipv6_traffic = 1;
-    lis_conn->entry_cfg.prefer_ipv6 = 0;
+    lis_conn->entry_cfg.prefer_ipv6 = 1;
   }
 
   if (connection_add(conn) < 0) { /* no space, forget it */
diff --git a/src/test/test_config.c b/src/test/test_config.c
index c23d04ceb8..8473d80f18 100644
--- a/src/test/test_config.c
+++ b/src/test/test_config.c
@@ -4158,6 +4158,8 @@ test_config_parse_port_config__ports__ports_given(void *data)
   /* Test entry port defaults as initialised in port_parse_config */
   tt_int_op(port_cfg->entry_cfg.dns_request, OP_EQ, 1);
   tt_int_op(port_cfg->entry_cfg.ipv4_traffic, OP_EQ, 1);
+  tt_int_op(port_cfg->entry_cfg.ipv6_traffic, OP_EQ, 1);
+  tt_int_op(port_cfg->entry_cfg.prefer_ipv6, OP_EQ, 1);
   tt_int_op(port_cfg->entry_cfg.onion_traffic, OP_EQ, 1);
   tt_int_op(port_cfg->entry_cfg.cache_ipv4_answers, OP_EQ, 0);
   tt_int_op(port_cfg->entry_cfg.prefer_ipv6_virtaddr, OP_EQ, 1);