Support SRV servers in failover

Adds a new failover API call fo_add_srv_server that allows the caller
to specify a server that is later resolved into a list of specific
servers using SRV requests.

Also adds a new failover option that specifies how often should the
servers resolved from SRV query considered valid until we need a
refresh.

The "real" servers to connect to are returned to the user as usual,
using the fo_resolve_service_{send,recv} calls.

Make SRV resolution work with c-ares 1.6

5 files changed, 551 insertions, 60 deletions

diff --git a/src/Makefile.am b/src/Makefile.am
index 92244f4a..22ac3581 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -252,6 +252,11 @@ SSSD_TOOLS_OBJ = \
 
 SSSD_RESOLV_OBJ = \
     resolv/async_resolv.c
+if BUILD_ARES_DATA
+    SSSD_RESOLV_OBJ += \
+	resolv/ares/ares_parse_srv_reply.c \
+	resolv/ares/ares_data.c
+endif
 
 SSSD_FAILOVER_OBJ = \
     providers/fail_over.c \
@@ -553,9 +558,7 @@ SSSD_RESOLV_TESTS_OBJ = \
     $(SSSD_RESOLV_OBJ)
 if BUILD_ARES_DATA
     SSSD_RESOLV_TESTS_OBJ += \
-	resolv/ares/ares_parse_srv_reply.c \
-	resolv/ares/ares_parse_txt_reply.c \
-	resolv/ares/ares_data.c
+	resolv/ares/ares_parse_txt_reply.c
 endif
 
 resolv_tests_SOURCES = \
@@ -565,7 +568,7 @@ resolv_tests_SOURCES = \
 resolv_tests_CFLAGS = \
     $(AM_CFLAGS) \
     $(CHECK_CFLAGS) \
-    -DBUILD_TXT_SRV
+    -DBUILD_TXT
 resolv_tests_LDADD = \
     $(SSSD_LIBS) \
     $(CHECK_LIBS) \
diff --git a/src/providers/fail_over.c b/src/providers/fail_over.c
index 02d913b9..231b9f08 100644
--- a/src/providers/fail_over.c
+++ b/src/providers/fail_over.c
@@ -5,8 +5,9 @@
 
    Authors:
         Martin Nagy <mnagy@redhat.com>
+        Jakub Hrozek <jhrozek@redhat.com>
 
-   Copyright (C) Red Hat, Inc 2009
+   Copyright (C) Red Hat, Inc 2010
 
    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
@@ -40,6 +41,14 @@
 
 #define DEFAULT_PORT_STATUS PORT_NEUTRAL
 #define DEFAULT_SERVER_STATUS SERVER_NAME_NOT_RESOLVED
+#define DEFAULT_SRV_STATUS SRV_NEUTRAL
+
+enum srv_lookup_status {
+    SRV_NEUTRAL,        /* We didn't try this SRV lookup yet */
+    SRV_RESOLVED,       /* This SRV lookup is resolved       */
+    SRV_NOT_RESOLVED,   /* Could not resolve this SRV lookup */
+    SRV_EXPIRED         /* Need to refresh the SRV query     */
+};
 
 struct fo_ctx {
     struct fo_service *service_list;
@@ -66,6 +75,7 @@ struct fo_server {
     void *user_data;
     int port;
     int port_status;
+    struct srv_data *srv_data;
     struct fo_service *service;
     struct timeval last_status_change;
     struct server_common *common;
@@ -86,6 +96,17 @@ struct server_common {
     struct timeval last_status_change;
 };
 
+struct srv_data {
+    char *domain;
+    char *proto;
+    char *srv;
+
+    struct fo_server *meta;
+
+    int srv_lookup_status;
+    struct timeval last_status_change;
+};
+
 struct resolve_service_request {
     struct resolve_service_request *prev;
     struct resolve_service_request *next;
@@ -115,6 +136,7 @@ fo_context_init(TALLOC_CTX *mem_ctx, struct fo_options *opts)
         return NULL;
     }
 
+    ctx->opts->srv_retry_timeout = opts->srv_retry_timeout;
     ctx->opts->retry_timeout = opts->retry_timeout;
     ctx->opts->family_order  = opts->family_order;
 
@@ -139,6 +161,23 @@ str_port_status(enum port_status status)
 }
 
 static const char *
+str_srv_data_status(enum srv_lookup_status status)
+{
+    switch (status) {
+    case SRV_NEUTRAL:
+        return "neutral";
+    case SRV_RESOLVED:
+        return "resolved";
+    case SRV_NOT_RESOLVED:
+        return "not resolved";
+    case SRV_EXPIRED:
+        return "expired";
+    }
+
+    return "unknown SRV lookup status";
+}
+
+static const char *
 str_server_status(enum server_status status)
 {
     switch (status) {
@@ -157,6 +196,105 @@ str_server_status(enum server_status status)
     return "unknown server status";
 }
 
+int fo_is_srv_lookup(struct fo_server *s)
+{
+    return s && s->srv_data;
+}
+
+static char *
+get_srv_query(TALLOC_CTX *mem_ctx, struct fo_server *server)
+{
+    char *query;
+
+    if (!fo_is_srv_lookup(server)) {
+        return NULL;
+    }
+
+    query = talloc_asprintf(mem_ctx, "_%s._%s.%s", server->srv_data->srv,
+                                                   server->srv_data->proto,
+                                                   server->srv_data->domain);
+    return query;
+}
+
+static struct fo_server *
+collapse_srv_lookup(struct fo_server *server)
+{
+    struct fo_server *tmp, *meta;
+
+    meta = server->srv_data->meta;
+    DEBUG(4, ("Need to refresh SRV lookup for domain %s\n", meta->srv_data->domain))
+
+    if (server != meta) {
+        while (server->prev && server->prev->srv_data == meta->srv_data) {
+            tmp = server->prev;
+            DLIST_REMOVE(server->service->server_list, tmp);
+            talloc_zfree(tmp);
+        }
+        while (server->next && server->next->srv_data == meta->srv_data) {
+            tmp = server->next;
+            DLIST_REMOVE(server->service->server_list, tmp);
+            talloc_zfree(tmp);
+        }
+
+        if (server == server->service->active_server) {
+            server->service->active_server = NULL;
+        }
+        if (server == server->service->last_tried_server) {
+            server->service->last_tried_server = meta;
+        }
+
+        /* add back the meta server to denote SRV lookup */
+        DLIST_ADD_AFTER(server->service->server_list, meta, server);
+        DLIST_REMOVE(server->service->server_list, server);
+        talloc_zfree(server);
+    }
+
+    meta->srv_data->srv_lookup_status = SRV_NEUTRAL;
+    meta->srv_data->last_status_change.tv_sec = 0;
+
+    return meta;
+}
+
+static enum srv_lookup_status
+get_srv_data_status(struct srv_data *data)
+{
+    struct timeval tv;
+    time_t timeout;
+
+    timeout = data->meta->service->ctx->opts->srv_retry_timeout;
+    gettimeofday(&tv, NULL);
+
+    if (timeout && STATUS_DIFF(data, tv) > timeout) {
+        switch(data->srv_lookup_status) {
+        case SRV_EXPIRED:
+        case SRV_NEUTRAL:
+            break;
+        case SRV_RESOLVED:
+            data->srv_lookup_status = SRV_EXPIRED;
+            data->last_status_change.tv_sec = 0;
+            break;
+        case SRV_NOT_RESOLVED:
+            data->srv_lookup_status = SRV_NEUTRAL;
+            data->last_status_change.tv_sec = 0;
+            break;
+        default:
+            DEBUG(1, ("Unknown state for SRV server!\n"));
+        }
+    }
+
+    return data->srv_lookup_status;
+}
+
+static void
+set_srv_data_status(struct srv_data *data, enum srv_lookup_status status)
+{
+    DEBUG(4, ("Marking SRV lookup of service '%s' as '%s'\n",
+              data->meta->service->name, str_srv_data_status(status)));
+
+    gettimeofday(&data->last_status_change, NULL);
+    data->srv_lookup_status = status;
+}
+
 /*
  * This function will return the status of the server. If the status was
  * last updated a long time ago, we will first reset the status.
@@ -359,22 +497,23 @@ create_server_common(TALLOC_CTX *mem_ctx, struct fo_ctx *ctx, const char *name)
 }
 
 int
-fo_add_server(struct fo_service *service, const char *name, int port,
-              void *user_data)
+fo_add_srv_server(struct fo_service *service, const char *srv,
+                  const char *domain, const char *proto, void *user_data)
 {
     struct fo_server *server;
-    int ret;
 
-    DEBUG(3, ("Adding new server '%s', to service '%s'\n",
-              name ? name : "(no name)", service->name));
+    DEBUG(3, ("Adding new SRV server in domain '%s', to service '%s'\n",
+              domain, service->name));
+
     DLIST_FOR_EACH(server, service->server_list) {
-        if (server->port != port || server->user_data != user_data)
+        if (server->user_data != user_data)
             continue;
-        if (name == NULL && server->common == NULL) {
-            return EEXIST;
-        } else if (name != NULL && server->common != NULL) {
-            if (!strcasecmp(name, server->common->name))
+
+        if (fo_is_srv_lookup(server)) {
+            if (strcasecmp(server->srv_data->domain, domain) == 0 &&
+                strcasecmp(server->srv_data->proto, proto) == 0) {
                 return EEXIST;
+            }
         }
     }
 
@@ -382,6 +521,42 @@ fo_add_server(struct fo_service *service, const char *name, int port,
     if (server == NULL)
         return ENOMEM;
 
+    server->user_data = user_data;
+    server->service = service;
+    server->port_status = DEFAULT_PORT_STATUS;
+
+    /* add the SRV-specific data */
+    server->srv_data = talloc_zero(service, struct srv_data);
+    if (server->srv_data == NULL)
+        return ENOMEM;
+
+    server->srv_data->domain = talloc_strdup(server->srv_data, domain);
+    server->srv_data->proto = talloc_strdup(server->srv_data, proto);
+    server->srv_data->srv = talloc_strdup(server->srv_data, srv);
+    if (server->srv_data->domain == NULL ||
+        server->srv_data->proto == NULL ||
+        server->srv_data->srv == NULL)
+        return ENOMEM;
+
+    server->srv_data->meta = server;
+    server->srv_data->srv_lookup_status = DEFAULT_SRV_STATUS;
+    server->srv_data->last_status_change.tv_sec = 0;
+
+    DLIST_ADD_END(service->server_list, server, struct fo_server *);
+    return EOK;
+}
+
+static struct fo_server *
+create_fo_server(struct fo_service *service, const char *name,
+                 int port, void *user_data)
+{
+    struct fo_server *server;
+    int ret;
+
+    server = talloc_zero(service, struct fo_server);
+    if (server == NULL)
+        return NULL;
+
     server->port = port;
     server->user_data = user_data;
     server->service = service;
@@ -393,14 +568,41 @@ fo_add_server(struct fo_service *service, const char *name, int port,
             server->common = create_server_common(server, service->ctx, name);
             if (server->common == NULL) {
                 talloc_free(server);
-                return ENOMEM;
+                return NULL;
             }
         } else if (ret != EOK) {
             talloc_free(server);
-            return ret;
+            return NULL;
         }
     }
 
+    return server;
+}
+
+int
+fo_add_server(struct fo_service *service, const char *name, int port,
+              void *user_data)
+{
+    struct fo_server *server;
+
+    DEBUG(3, ("Adding new server '%s', to service '%s'\n",
+              name ? name : "(no name)", service->name));
+    DLIST_FOR_EACH(server, service->server_list) {
+        if (server->port != port || server->user_data != user_data)
+            continue;
+        if (name == NULL && server->common == NULL) {
+            return EEXIST;
+        } else if (name != NULL && server->common != NULL) {
+            if (!strcasecmp(name, server->common->name))
+                return EEXIST;
+        }
+    }
+
+    server = create_fo_server(service, name, port, user_data);
+    if (!server) {
+        return ENOMEM;
+    }
+
     DLIST_ADD_END(service->server_list, server, struct fo_server *);
 
     return EOK;
@@ -489,9 +691,24 @@ set_lookup_hook(struct fo_server *server, struct tevent_req *req)
 
 struct resolve_service_state {
     struct fo_server *server;
+
+    struct resolv_ctx *resolv;
+    struct tevent_context *ev;
+    struct fo_ctx *fo_ctx;
 };
 
+
+static void fo_resolve_service_cont(struct tevent_req *subreq);
 static void fo_resolve_service_done(struct tevent_req *subreq);
+static bool fo_resolve_service_server(struct tevent_req *req);
+
+/* Forward declarations for SRV resolving */
+static struct tevent_req *
+resolve_srv_send(TALLOC_CTX *mem_ctx, struct tevent_context *ev,
+                    struct resolv_ctx *resolv, struct fo_ctx *ctx,
+                    struct fo_server *server);
+static int
+resolve_srv_recv(struct tevent_req *req, struct fo_server **server);
 
 struct tevent_req *
 fo_resolve_service_send(TALLOC_CTX *mem_ctx, struct tevent_context *ev,
@@ -509,83 +726,139 @@ fo_resolve_service_send(TALLOC_CTX *mem_ctx, struct tevent_context *ev,
     if (req == NULL)
         return NULL;
 
+    state->resolv = resolv;
+    state->ev = ev;
+    state->fo_ctx = ctx;
+
     ret = get_first_server_entity(service, &server);
     if (ret != EOK) {
         DEBUG(1, ("No available servers for service '%s'\n", service->name));
         goto done;
     }
 
+    if (fo_is_srv_lookup(server)) {
+        /* Don't know the server yet, must do a SRV lookup */
+        subreq = resolve_srv_send(state, ev, resolv,
+                                  ctx, server);
+        if (subreq == NULL) {
+            ret = ENOMEM;
+            goto done;
+        }
+
+        tevent_req_set_callback(subreq,
+                                fo_resolve_service_cont,
+                                req);
+        return req;
+    }
+
+    /* This is a regular server, just do hostname lookup */
     state->server = server;
+    if (fo_resolve_service_server(req)) {
+        tevent_req_post(req, ev);
+    }
 
-    if (server->common == NULL) {
-        /* This server doesn't have a name, we don't do name resolution. */
-        tevent_req_done(req);
+    ret = EOK;
+done:
+    if (ret != EOK) {
+        tevent_req_error(req, ret);
         tevent_req_post(req, ev);
-        return req;
     }
+    return req;
+}
 
-    switch (get_server_status(server)) {
+static void set_server_common_status(struct server_common *common,
+                                     enum server_status status);
+
+/* SRV resolving finished, see if we got server to work with */
+static void
+fo_resolve_service_cont(struct tevent_req *subreq)
+{
+    struct tevent_req *req = tevent_req_callback_data(subreq,
+                                                      struct tevent_req);
+    struct resolve_service_state *state = tevent_req_data(req,
+                                        struct resolve_service_state);
+    int ret;
+
+    ret = resolve_srv_recv(subreq, &state->server);
+    talloc_zfree(subreq);
+
+    if (ret) {
+        tevent_req_error(req, ret);
+        return;
+    }
+
+    fo_resolve_service_server(req);
+}
+
+static bool
+fo_resolve_service_server(struct tevent_req *req)
+{
+    struct resolve_service_state *state = tevent_req_data(req,
+                                        struct resolve_service_state);
+    struct tevent_req *subreq;
+    int ret;
+
+    switch (get_server_status(state->server)) {
     case SERVER_NAME_NOT_RESOLVED: /* Request name resolution. */
-        subreq = resolv_gethostbyname_send(server->common, ev, resolv,
-                                           server->common->name,
-                                           ctx->opts->family_order);
+        subreq = resolv_gethostbyname_send(state->server->common,
+                                           state->ev, state->resolv,
+                                           state->server->common->name,
+                                           state->fo_ctx->opts->family_order);
         if (subreq == NULL) {
-            ret = ENOMEM;
-            goto done;
+            tevent_req_error(req, ENOMEM);
+            return true;
         }
-        tevent_req_set_callback(subreq, fo_resolve_service_done, server->common);
-        fo_set_server_status(server, SERVER_RESOLVING_NAME);
+        tevent_req_set_callback(subreq, fo_resolve_service_done, req);
+        fo_set_server_status(state->server, SERVER_RESOLVING_NAME);
         /* FALLTHROUGH */
     case SERVER_RESOLVING_NAME:
         /* Name resolution is already under way. Just add ourselves into the
          * waiting queue so we get notified after the operation is finished. */
-        ret = set_lookup_hook(server, req);
-        if (ret != EOK)
-            goto done;
+        ret = set_lookup_hook(state->server, req);
+        if (ret != EOK) {
+            tevent_req_error(req, ret);
+            return true;
+        }
         break;
     default: /* The name is already resolved. Return immediately. */
         tevent_req_done(req);
-        tevent_req_post(req, ev);
-        break;
+        return true;
     }
 
-done:
-    if (ret != EOK) {
-        tevent_req_error(req, ret);
-        tevent_req_post(req, ev);
-    }
-    return req;
+    return false;
 }
 
-static void set_server_common_status(struct server_common *common,
-                                     enum server_status status);
-
 static void
 fo_resolve_service_done(struct tevent_req *subreq)
 {
+    struct tevent_req *req = tevent_req_callback_data(subreq,
+                                                      struct tevent_req);
+    struct resolve_service_state *state = tevent_req_data(req,
+                                                struct resolve_service_state);
+    struct server_common *common;
     int resolv_status;
     struct resolve_service_request *request;
-    struct server_common *common;
     int ret;
 
-    common = tevent_req_callback_data(subreq, struct server_common);
-
-    if (common->hostent != NULL) {
-        talloc_zfree(common->hostent);
+    if (state->server->common->hostent != NULL) {
+        talloc_zfree(state->server->common->hostent);
     }
 
-    ret = resolv_gethostbyname_recv(subreq, common,
-                                    &resolv_status, NULL, &common->hostent);
-    talloc_free(subreq);
+    ret = resolv_gethostbyname_recv(subreq, state->server->common,
+                                    &resolv_status, NULL,
+                                    &state->server->common->hostent);
+    talloc_zfree(subreq);
     if (ret != EOK) {
-        DEBUG(1, ("Failed to resolve server '%s': %s\n", common->name,
-              resolv_strerror(resolv_status)));
-        set_server_common_status(common, SERVER_NOT_WORKING);
+        DEBUG(1, ("Failed to resolve server '%s': %s\n",
+                  state->server->common->name,
+                  resolv_strerror(resolv_status)));
+        set_server_common_status(state->server->common, SERVER_NOT_WORKING);
     } else {
-        set_server_common_status(common, SERVER_NAME_RESOLVED);
+        set_server_common_status(state->server->common, SERVER_NAME_RESOLVED);
     }
 
     /* Take care of all requests for this server. */
+    common = state->server->common; /* state can disappear now */
     while ((request = common->request_list) != NULL) {
         DLIST_REMOVE(common->request_list, request);
         if (resolv_status) {
@@ -617,6 +890,187 @@ fo_resolve_service_recv(struct tevent_req *req, struct fo_server **server)
     return EOK;
 }
 
+/*******************************************************************
+ * Resolve the server to connect to using a SRV query.             *
+ *******************************************************************/
+
+static void resolve_srv_done(struct tevent_req *subreq);
+
+struct resolve_srv_state {
+    struct fo_server *meta;
+    struct fo_service *service;
+
+    struct fo_server *out;
+
+    struct resolv_ctx *resolv;
+    struct tevent_context *ev;
+    struct fo_ctx *fo_ctx;
+};
+
+static struct tevent_req *
+resolve_srv_send(TALLOC_CTX *mem_ctx, struct tevent_context *ev,
+                 struct resolv_ctx *resolv, struct fo_ctx *ctx,
+                 struct fo_server *server)
+{
+    int ret;
+    char *query;
+    struct tevent_req *req;
+    struct tevent_req *subreq;
+    struct resolve_srv_state *state;
+    int status;
+
+    req = tevent_req_create(mem_ctx, &state, struct resolve_srv_state);
+    if (req == NULL)
+        return NULL;
+
+    state->service = server->service;
+    state->ev = ev;
+    state->resolv = resolv;
+    state->fo_ctx = ctx;
+    state->meta = server;
+
+    status = get_srv_data_status(server->srv_data);
+    DEBUG(6, ("The status of SRV lookup is %s\n",
+              str_srv_data_status(status)));
+    switch(status) {
+    case SRV_EXPIRED: /* Need a refresh */
+        state->meta = collapse_srv_lookup(server);
+        /* FALLTHROUGH */
+    case SRV_NEUTRAL: /* Request SRV lookup */
+        query = get_srv_query(state, state->meta);
+        if (!query) {
+            ret = ENOMEM;
+            goto done;
+        }
+        DEBUG(4, ("Searching for servers via SRV query '%s'\n", query));
+
+        subreq = resolv_getsrv_send(state, ev, resolv, query);
+        if (subreq == NULL) {
+            ret = ENOMEM;
+            goto done;
+        }
+        tevent_req_set_callback(subreq, resolve_srv_done, req);
+        break;
+    case SRV_NOT_RESOLVED: /* query could not be resolved but don't retry yet */
+        ret = EIO;
+        goto done;
+    case SRV_RESOLVED:  /* The query is resolved and valid. Return. */
+        state->out = server;
+        tevent_req_done(req);
+        tevent_req_post(req, state->ev);
+        return req;
+    default:
+        DEBUG(1, ("Unexpected status %d for a SRV server\n", status));
+        ret = EIO;
+        break;
+    }
+
+done:
+    if (ret != EOK) {
+        tevent_req_error(req, ret);
+        tevent_req_post(req, ev);
+    }
+    return req;
+}
+
+static void
+resolve_srv_done(struct tevent_req *subreq)
+{
+    struct tevent_req *req = tevent_req_callback_data(subreq,
+                                                      struct tevent_req);
+    struct resolve_srv_state *state = tevent_req_data(req,
+                                                struct resolve_srv_state);
+    struct ares_srv_reply *reply_list;
+    struct ares_srv_reply *reply;
+    struct fo_server *server = NULL;
+    struct fo_server *srv_list = NULL;
+    int ret;
+    int resolv_status;
+
+    ret = resolv_getsrv_recv(state, subreq,
+                             &resolv_status, NULL, &reply_list);
+    talloc_free(subreq);
+    if (ret != EOK) {
+        DEBUG(1, ("SRV query failed %s\n",
+                  resolv_strerror(resolv_status)));
+        fo_set_port_status(state->meta, PORT_NOT_WORKING);
+        goto fail;
+    }
+
+    ret = resolv_sort_srv_reply(state, &reply_list);
+    if (ret != EOK) {
+        DEBUG(1, ("Could not sort the answers from DNS [%d]: %s\n",
+                  ret, strerror(ret)));
+        fo_set_port_status(state->meta, PORT_NOT_WORKING);
+        goto fail;
+    }
+
+    for (reply = reply_list; reply; reply = reply->next) {
+        ret = EOK;
+        DLIST_FOR_EACH(server, state->service->server_list) {
+            if (server->port == reply->port) {
+                ret = EEXIST;
+                break;
+            }
+        }
+        if (ret == EEXIST) continue;
+
+        server = create_fo_server(state->service, reply->host,
+                                  reply->port, state->meta->user_data);
+        if (!server) {
+            ret = ENOMEM;
+            goto fail;
+        }
+        server->srv_data = state->meta->srv_data;
+
+        DLIST_ADD_END(srv_list, server, struct fo_server *);
+        DEBUG(6, ("Inserted server '%s:%d' for service %s\n",
+                  server->common->name,
+                  server->port,
+                  state->service->name));
+    }
+
+    if (srv_list) {
+        DLIST_ADD_LIST_AFTER(state->service->server_list, state->meta,
+                             srv_list, struct fo_server *);
+
+        DLIST_REMOVE(state->service->server_list, state->meta);
+        if (state->service->last_tried_server == state->meta) {
+            state->service->last_tried_server = srv_list;
+        }
+
+        state->out = srv_list;
+        set_srv_data_status(state->meta->srv_data, SRV_RESOLVED);
+        tevent_req_done(req);
+        return;
+    } else {
+        ret = EIO;
+        goto fail;
+    }
+
+fail:
+    state->out = state->meta;
+    set_srv_data_status(state->meta->srv_data, SRV_NOT_RESOLVED);
+    tevent_req_error(req, ret);
+}
+
+static int
+resolve_srv_recv(struct tevent_req *req, struct fo_server **server)
+{
+    struct resolve_srv_state *state = tevent_req_data(req,
+                                                struct resolve_srv_state);
+
+    /* always return the server if asked for, otherwise the caller
+     * cannot mark it as faulty in case we return an error */
+    if (server) {
+        *server = state->out;
+    }
+
+    TEVENT_REQ_RETURN_ON_ERROR(req);
+
+    return EOK;
+}
+
 static void
 set_server_common_status(struct server_common *common,
                          enum server_status status)
@@ -667,6 +1121,10 @@ fo_get_server_port(struct fo_server *server)
 
 const char *fo_get_server_name(struct fo_server *server)
 {
+    if (!server->common && fo_is_srv_lookup(server)) {
+        return "SRV lookup meta-server";
+    }
+
     return server->common->name;
 }
 
diff --git a/src/providers/fail_over.h b/src/providers/fail_over.h
index ffcd0687..70e694fe 100644
--- a/src/providers/fail_over.h
+++ b/src/providers/fail_over.h
@@ -30,6 +30,9 @@
 
 #include "resolv/async_resolv.h"
 
+#define FO_PROTO_TCP "tcp"
+#define FO_PROTO_UDP "udp"
+
 /* Some forward declarations that don't have to do anything with fail over. */
 struct hostent;
 struct tevent_context;
@@ -60,10 +63,14 @@ struct fo_server;
  * duration in seconds of how long a server or port will be considered
  * non-working after being marked as such.
  *
+ * The 'srv_retry_timeout' member specifies how long a SRV lookup
+ * is considered valid until we ask the server again.
+ *
  * The family_order member specifies the order of address families to
  * try when looking up the service.
  */
 struct fo_options {
+    time_t srv_retry_timeout;
     time_t retry_timeout;
     enum restrict_family family_order;
 };
@@ -101,6 +108,13 @@ int fo_add_server(struct fo_service *service,
                   int port,
                   void *user_data);
 
+
+int fo_add_srv_server(struct fo_service *service,
+                      const char *srv,
+                      const char *domain,
+                      const char *proto,
+                      void *user_data);
+
 /*
  * Request the first server from the service's list of servers. It is only
  * considered if it is not marked as not working (or the retry interval already
diff --git a/src/resolv/async_resolv.c b/src/resolv/async_resolv.c
index fe3b3f6b..70d60e37 100644
--- a/src/resolv/async_resolv.c
+++ b/src/resolv/async_resolv.c
@@ -661,11 +661,6 @@ ares_gethostbyname_wakeup(struct tevent_req *subreq)
                        state->family, resolv_gethostbyname_done, req);
 }
 
-/* SRV and TXT parsing is not used anywhere in the code yet, so we disable it
- * for now
- */
-#ifdef BUILD_TXT_SRV
-
 /*
  * A simple helper function that will take an array of struct ares_srv_reply that
  * was allocated by malloc() in c-ares and copies it using talloc. The old one
@@ -869,6 +864,11 @@ ares_getsrv_wakeup(struct tevent_req *subreq)
                ns_c_in, ns_t_srv, resolv_getsrv_done, req);
 }
 
+/* TXT parsing is not used anywhere in the code yet, so we disable it
+ * for now
+ */
+#ifdef BUILD_TXT
+
 /*
  * A simple helper function that will take an array of struct txt_reply that
  * was allocated by malloc() in c-ares and copies it using talloc. The old one
diff --git a/src/util/dlinklist.h b/src/util/dlinklist.h
index be5ff914..f323de70 100644
--- a/src/util/dlinklist.h
+++ b/src/util/dlinklist.h
@@ -110,6 +110,22 @@ do { \
 		} \
 } while (0)
 
+/* insert all elements from list2 after the given element 'el' in the
+ * first list */
+#define DLIST_ADD_LIST_AFTER(list1, el, list2, type) \
+do { \
+                if (!(list1) || !(el) || !(list2)) { \
+                    DLIST_CONCATENATE(list1, list2, type); \
+                } else { \
+                    type tmp; \
+                    for (tmp = (list2); tmp->next; tmp = tmp->next) ; \
+                    (list2)->prev = (el); \
+                    tmp->next = (el)->next; \
+                    (el)->next = (list2); \
+                    if ((el)->next != NULL) (el)->next->prev = tmp; \
+    } \
+} while (0);
+
 #define DLIST_FOR_EACH(p, list) \
 	for ((p) = (list); (p) != NULL; (p) = (p)->next)