nss-myhostname: always resolve the host name "gateway" to the local default gateway

This is useful inside of containers or local networks to intrdouce a
stable name of the default gateway host (in case of containers usually
the host, in case of LANs usually local router).

.gitignore

@@ -203,6 +203,7 @@
 /test-list
 /test-unaligned
 /test-locale-util
+/test-local-addresses
 /test-log
 /test-login
 /test-login-shared

Makefile.am

@@ -2782,6 +2782,7 @@ tests += \
 	test-bus-gvariant \
 	test-event \
 	test-rtnl \
+	test-local-addresses \
 	test-resolve
 
 bin_PROGRAMS += \
@@ -2940,6 +2941,13 @@ test_rtnl_LDADD = \
 	libsystemd-internal.la \
 	libsystemd-shared.la
 
+test_local_addresses_SOURCES = \
+	src/libsystemd/sd-rtnl/test-local-addresses.c
+
+test_local_addresses_LDADD = \
+	libsystemd-internal.la \
+	libsystemd-shared.la
+
 test_resolve_SOURCES = \
 	src/libsystemd/sd-resolve/test-resolve.c
 

src/libsystemd/sd-rtnl/local-addresses.c

@@ -30,14 +30,19 @@ static int address_compare(const void *_a, const void *_b) {
 
         /* Order lowest scope first, IPv4 before IPv6, lowest interface index first */
 
+        if (a->family == AF_INET && b->family == AF_INET6)
+                return -1;
+        if (a->family == AF_INET6 && b->family == AF_INET)
+                return 1;
+
         if (a->scope < b->scope)
                 return -1;
         if (a->scope > b->scope)
                 return 1;
 
-        if (a->family == AF_INET && b->family == AF_INET6)
+        if (a->metric < b->metric)
                 return -1;
-        if (a->family == AF_INET6 && b->family == AF_INET)
+        if (a->metric > b->metric)
                 return 1;
 
         if (a->ifindex < b->ifindex)
@@ -105,7 +110,7 @@ int local_addresses(sd_rtnl *context, int ifindex, struct local_address **ret) {
                 if (flags & IFA_F_DEPRECATED)
                         continue;
 
-                if (!GREEDY_REALLOC(list, n_allocated, n_list+1))
+                if (!GREEDY_REALLOC0(list, n_allocated, n_list+1))
                         return -ENOMEM;
 
                 a = list + n_list;
@@ -150,7 +155,111 @@ int local_addresses(sd_rtnl *context, int ifindex, struct local_address **ret) {
                 n_list++;
         };
 
-        if (n_list)
+        if (n_list > 0)
+                qsort(list, n_list, sizeof(struct local_address), address_compare);
+
+        *ret = list;
+        list = NULL;
+
+        return (int) n_list;
+}
+
+int local_gateways(sd_rtnl *context, int ifindex, struct local_address **ret) {
+        _cleanup_rtnl_message_unref_ sd_rtnl_message *req = NULL, *reply = NULL;
+        _cleanup_rtnl_unref_ sd_rtnl *rtnl = NULL;
+        _cleanup_free_ struct local_address *list = NULL;
+        sd_rtnl_message *m = NULL;
+        size_t n_list = 0, n_allocated = 0;
+        int r;
+
+        assert(ret);
+
+        if (context)
+                rtnl = sd_rtnl_ref(context);
+        else {
+                r = sd_rtnl_open(&rtnl, 0);
+                if (r < 0)
+                        return r;
+        }
+
+        r = sd_rtnl_message_new_route(rtnl, &req, RTM_GETROUTE, AF_UNSPEC, RTPROT_UNSPEC);
+        if (r < 0)
+                return r;
+
+        r = sd_rtnl_message_request_dump(req, true);
+        if (r < 0)
+                return r;
+
+        r = sd_rtnl_call(rtnl, req, 0, &reply);
+        if (r < 0)
+                return r;
+
+        for (m = reply; m; m = sd_rtnl_message_next(m)) {
+                struct local_address *a;
+                uint16_t type;
+                unsigned char dst_len;
+                uint32_t ifi;
+
+                r = sd_rtnl_message_get_errno(m);
+                if (r < 0)
+                        return r;
+
+                r = sd_rtnl_message_get_type(m, &type);
+                if (r < 0)
+                        return r;
+
+                if (type != RTM_NEWROUTE)
+                        continue;
+
+                r = sd_rtnl_message_route_get_dst_len(m, &dst_len);
+                if (r < 0)
+                        return r;
+
+                /* We only care for default routes */
+                if (dst_len != 0)
+                        continue;
+
+                r = sd_rtnl_message_read_u32(m, RTA_OIF, &ifi);
+                if (r < 0)
+                        return r;
+
+                if (ifindex > 0 && (int) ifi != ifindex)
+                        continue;
+
+                if (!GREEDY_REALLOC0(list, n_allocated, n_list + 1))
+                        return -ENOMEM;
+
+                a = list + n_list;
+
+                r = sd_rtnl_message_route_get_family(m, &a->family);
+                if (r < 0)
+                        return r;
+
+                switch (a->family) {
+                case AF_INET:
+                        r = sd_rtnl_message_read_in_addr(m, RTA_GATEWAY, &a->address.in);
+                        if (r < 0)
+                                continue;
+
+                        break;
+                case AF_INET6:
+                        r = sd_rtnl_message_read_in6_addr(m, RTA_GATEWAY, &a->address.in6);
+                        if (r < 0)
+                                continue;
+
+                        break;
+                default:
+                        continue;
+                }
+
+                sd_rtnl_message_read_u32(m, RTA_PRIORITY, &a->metric);
+
+                a->ifindex = ifi;
+                n_list++;
+
+        }
+
+        if (n_list > 0)
                 qsort(list, n_list, sizeof(struct local_address), address_compare);
 
         *ret = list;

src/libsystemd/sd-rtnl/local-addresses.h

@@ -32,7 +32,10 @@
 struct local_address {
         int family, ifindex;
         unsigned char scope;
+        uint32_t metric;
         union in_addr_union address;
 };
 
 int local_addresses(sd_rtnl *rtnl, int ifindex, struct local_address **ret);
+
+int local_gateways(sd_rtnl *rtnl, int ifindex, struct local_address **ret);

src/libsystemd/sd-rtnl/rtnl-message.c

@@ -143,6 +143,21 @@ int sd_rtnl_message_route_get_family(sd_rtnl_message *m, int *family) {
         return 0;
 }
 
+int sd_rtnl_message_route_get_dst_len(sd_rtnl_message *m, unsigned char *dst_len) {
+        struct rtmsg *rtm;
+
+        assert_return(m, -EINVAL);
+        assert_return(m->hdr, -EINVAL);
+        assert_return(rtnl_message_type_is_route(m->hdr->nlmsg_type), -EINVAL);
+        assert_return(dst_len, -EINVAL);
+
+        rtm = NLMSG_DATA(m->hdr);
+
+        *dst_len = rtm->rtm_dst_len;
+
+        return 0;
+}
+
 int sd_rtnl_message_new_route(sd_rtnl *rtnl, sd_rtnl_message **ret,
                               uint16_t nlmsg_type, int rtm_family,
                               unsigned char rtm_protocol) {

src/libsystemd/sd-rtnl/test-local-addresses.c

@@ -0,0 +1,58 @@
+/*-*- Mode: C; c-basic-offset: 8; indent-tabs-mode: nil -*-*/
+
+/***
+  This file is part of systemd.
+
+  Copyright 2014 Lennart Poettering
+
+  systemd is free software; you can redistribute it and/or modify it
+  under the terms of the GNU Lesser General Public License as published by
+  the Free Software Foundation; either version 2.1 of the License, or
+  (at your option) any later version.
+
+  systemd is distributed in the hope that it will be useful, but
+  WITHOUT ANY WARRANTY; without even the implied warranty of
+  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+  Lesser General Public License for more details.
+
+  You should have received a copy of the GNU Lesser General Public License
+  along with systemd; If not, see <http://www.gnu.org/licenses/>.
+***/
+
+#include "in-addr-util.h"
+#include "local-addresses.h"
+#include "af-list.h"
+
+static void print_local_addresses(struct local_address *a, unsigned n) {
+        unsigned i;
+
+        for (i = 0; i < n; i++) {
+                _cleanup_free_ char *b = NULL;
+
+                assert_se(in_addr_to_string(a[i].family, &a[i].address, &b) >= 0);
+                printf("%s if%i scope=%i metric=%u address=%s\n", af_to_name(a[i].family), a[i].ifindex, a[i].scope, a[i].metric, b);
+        }
+}
+
+int main(int argc, char *argv[]) {
+        struct local_address *a;
+        int n;
+
+        a = NULL;
+        n = local_addresses(NULL, 0, &a);
+        assert_se(n >= 0);
+
+        printf("Local Addresses:\n");
+        print_local_addresses(a, (unsigned) n);
+        free(a);
+
+        a = NULL;
+        n = local_gateways(NULL, 0, &a);
+        assert_se(n >= 0);
+
+        printf("Local Gateways:\n");
+        print_local_addresses(a, (unsigned) n);
+        free(a);
+
+        return 0;
+}

src/nss-myhostname/nss-myhostname.c

@@ -77,6 +77,18 @@ enum nss_status _nss_myhostname_gethostbyname4_r(
 
                 canonical = "localhost";
                 local_address_ipv4 = htonl(INADDR_LOOPBACK);
+
+        } else if (streq(name, "gateway")) {
+
+                n_addresses = local_gateways(NULL, 0, &addresses);
+                if (n_addresses <= 0) {
+                        *errnop = ENOENT;
+                        *h_errnop = HOST_NOT_FOUND;
+                        return NSS_STATUS_NOTFOUND;
+                }
+
+                canonical = "gateway";
+
         } else {
                 hn = gethostname_malloc();
                 if (!hn) {
@@ -314,7 +326,7 @@ enum nss_status _nss_myhostname_gethostbyname3_r(
         _cleanup_free_ struct local_address *addresses = NULL;
         const char *canonical, *additional = NULL;
         _cleanup_free_ char *hn = NULL;
-        uint32_t local_address_ipv4;
+        uint32_t local_address_ipv4 = 0;
         int n_addresses = 0;
 
         assert(name);
@@ -335,6 +347,18 @@ enum nss_status _nss_myhostname_gethostbyname3_r(
         if (is_localhost(name)) {
                 canonical = "localhost";
                 local_address_ipv4 = htonl(INADDR_LOOPBACK);
+
+        } else if (streq(name, "gateway")) {
+
+                n_addresses = local_gateways(NULL, af, &addresses);
+                if (n_addresses <= 0) {
+                        *errnop = ENOENT;
+                        *h_errnop = HOST_NOT_FOUND;
+                        return NSS_STATUS_NOTFOUND;
+                }
+
+                canonical = "gateway";
+
         } else {
                 hn = gethostname_malloc();
                 if (!hn) {
@@ -349,7 +373,7 @@ enum nss_status _nss_myhostname_gethostbyname3_r(
                         return NSS_STATUS_NOTFOUND;
                 }
 
-                n_addresses = local_addresses(NULL, 0, &addresses);
+                n_addresses = local_addresses(NULL, af, &addresses);
                 if (n_addresses < 0)
                         n_addresses = 0;
 
@@ -426,15 +450,41 @@ enum nss_status _nss_myhostname_gethostbyaddr2_r(
         }
 
         n_addresses = local_addresses(NULL, 0, &addresses);
-        if (n_addresses < 0)
-                n_addresses = 0;
+        if (n_addresses > 0) {
+                for (a = addresses, n = 0; (int) n < n_addresses; n++, a++) {
+                        if (af != a->family)
+                                continue;
 
-        for (a = addresses, n = 0; (int) n < n_addresses; n++, a++) {
-                if (af != a->family)
-                        continue;
+                        if (memcmp(addr, &a->address, FAMILY_ADDRESS_SIZE(af)) == 0) {
 
-                if (memcmp(addr, &a->address, FAMILY_ADDRESS_SIZE(af)) == 0)
-                        goto found;
+                                hn = gethostname_malloc();
+                                if (!hn) {
+                                        *errnop = ENOMEM;
+                                        *h_errnop = NO_RECOVERY;
+                                        return NSS_STATUS_TRYAGAIN;
+                                }
+
+                                canonical = hn;
+                                goto found;
+                        }
+                }
+        }
+
+        free(addresses);
+        addresses = NULL;
+
+        n_addresses = local_gateways(NULL, 0, &addresses);
+        if (n_addresses > 0) {
+                for (a = addresses, n = 0; (int) n < n_addresses; n++, a++) {
+                        if (af != a->family)
+                                continue;
+
+                        if (memcmp(addr, &a->address, FAMILY_ADDRESS_SIZE(af)) == 0) {
+
+                                canonical = "gateway";
+                                goto found;
+                        }
+                }
         }
 
         *errnop = ENOENT;
@@ -443,16 +493,6 @@ enum nss_status _nss_myhostname_gethostbyaddr2_r(
         return NSS_STATUS_NOTFOUND;
 
 found:
-        if (!canonical) {
-                hn = gethostname_malloc();
-                if (!hn) {
-                        *errnop = ENOMEM;
-                        *h_errnop = NO_RECOVERY;
-                        return NSS_STATUS_TRYAGAIN;
-                }
-
-                canonical = hn;
-        }
 
         return fill_in_hostent(
                         canonical, additional,

src/systemd/sd-rtnl.h

@@ -104,6 +104,7 @@ int sd_rtnl_message_link_get_type(sd_rtnl_message *m, unsigned *type);
 int sd_rtnl_message_route_set_dst_prefixlen(sd_rtnl_message *m, unsigned char prefixlen);
 int sd_rtnl_message_route_set_scope(sd_rtnl_message *m, unsigned char scope);
 int sd_rtnl_message_route_get_family(sd_rtnl_message *m, int *family);
+int sd_rtnl_message_route_get_dst_len(sd_rtnl_message *m, unsigned char *dst_len);
 
 int sd_rtnl_message_neigh_get_family(sd_rtnl_message *m, int *family);
 int sd_rtnl_message_neigh_get_ifindex(sd_rtnl_message *m, int *family);