resolve-tool: provide resolvconf(8) compatibility

This turns resolve-tool into a multi-call binary. When invoked as
"resolvconf" it provides minimal compatibility with the resolvconf(8)
tool of various distributions (and FreeBSD as it appears).

This new interface understands to varying degrees features of the two
major implementations of resolvconf(8): Debian's original one and
"openresolv". Specifically:

Fully supported:

        -a -d (supported by all implementations)
        -f    (introduced by openresolv)

Somewhat supported:

        -x    (introduced by openresolv, mapped to a '~.' domain entry)

Unsupported and ignored:

        -m -p (introduced by openresolv, not really necessary for us)

Unsupported and resulting in failure:
        -u    (supported by all other implementations)
        -I -i -l -R -r -v -V
              (all introduced by openresolv)
        --enable-updates --disable-updates --updates-are-enabled
              (specific to Debian's implementation)

Of course, resolvconf(8) is a tool with multiple backends, in our
implementation systemd-resolved is the only backend.

Fixes: #7202

meson.build

@@ -1632,6 +1632,10 @@ if conf.get('ENABLE_RESOLVE') == 1
                          install_rpath : rootlibexecdir,
                          install : true)
         public_programs += [exe]
+
+        meson.add_install_script(meson_make_symlink,
+                         join_paths(bindir, 'systemd-resolve'),
+                         join_paths(rootsbindir, 'resolvconf'))
 endif
 
 if conf.get('ENABLE_LOGIND') == 1

src/resolve/meson.build

@@ -80,7 +80,12 @@ systemd_resolved_sources = files('''
         resolved-etc-hosts.c
 '''.split())
 
-systemd_resolve_sources = files('resolve-tool.c')
+systemd_resolve_sources = files('''
+         resolvconf-compat.c
+         resolvconf-compat.h
+         resolve-tool.c
+         resolve-tool.h
+'''.split())
 
 ############################################################
 

src/resolve/resolvconf-compat.c

@@ -0,0 +1,312 @@
+/* SPDX-License-Identifier: LGPL-2.1+ */
+
+#include <getopt.h>
+#include <net/if.h>
+
+#include "alloc-util.h"
+#include "def.h"
+#include "dns-domain.h"
+#include "extract-word.h"
+#include "fileio.h"
+#include "parse-util.h"
+#include "resolvconf-compat.h"
+#include "resolve-tool.h"
+#include "resolved-def.h"
+#include "string-util.h"
+#include "strv.h"
+
+static void resolvconf_help(void) {
+        printf("%1$s -a INTERFACE < FILE\n"
+               "%1$s -d INTERFACE\n"
+               "\n"
+               "Register DNS server and domain configuration with systemd-resolved.\n\n"
+               "  -h --help     Show this help\n"
+               "     --version  Show package version\n"
+               "  -a            Register per-interface DNS server and domain data\n"
+               "  -d            Unregister per-interface DNS server and domain data\n"
+               "  -f            Ignore if specified interface does not exist\n"
+               "  -x            Send DNS traffic preferably over this interface\n"
+               "\n"
+               "This is a compatibility alias for the systemd-resolve(1) tool, providing native\n"
+               "command line compatibility with the resolvconf(8) tool of various Linux\n"
+               "distributions and BSD systems. Some options supported by other implementations\n"
+               "are not supported and are ignored: -m, -p. Various options supported by other\n"
+               "implementations are not supported and will cause the invocation to fail: -u,\n"
+               "-I, -i, -l, -R, -r, -v, -V, --enable-updates, --disable-updates,\n"
+               "--updates-are-enabled.\n"
+               , program_invocation_short_name);
+}
+
+static int parse_nameserver(const char *string) {
+        int r;
+
+        assert(string);
+
+        for (;;) {
+                _cleanup_free_ char *word = NULL;
+                struct in_addr_data data, *n;
+                int ifindex = 0;
+
+                r = extract_first_word(&string, &word, NULL, 0);
+                if (r < 0)
+                        return r;
+                if (r == 0)
+                        break;
+
+                r = in_addr_ifindex_from_string_auto(word, &data.family, &data.address, &ifindex);
+                if (r < 0)
+                        return log_error_errno(r, "Failed to parse name server '%s': %m", word);
+
+                if (ifindex > 0 && ifindex != arg_ifindex) {
+                        log_error("Name server interface '%s' does not match selected interface: %m", word);
+                        return -EINVAL;
+                }
+
+                /* Some superficial filtering */
+                if (in_addr_is_null(data.family, &data.address))
+                        continue;
+                if (data.family == AF_INET && data.address.in.s_addr == htobe32(INADDR_DNS_STUB)) /* resolved's own stub? */
+                        continue;
+
+                n = reallocarray(arg_set_dns, arg_n_set_dns + 1, sizeof(struct in_addr_data));
+                if (!n)
+                        return log_oom();
+                arg_set_dns = n;
+
+                arg_set_dns[arg_n_set_dns++] = data;
+        }
+
+        return 0;
+}
+
+static int parse_search_domain(const char *string) {
+        int r;
+
+        assert(string);
+
+        for (;;) {
+                _cleanup_free_ char *word = NULL;
+
+                r = extract_first_word(&string, &word, NULL, EXTRACT_QUOTES);
+                if (r < 0)
+                        return r;
+                if (r == 0)
+                        break;
+
+                r = dns_name_is_valid(word);
+                if (r < 0)
+                        return log_error_errno(r, "Failed to validate specified domain '%s': %m", word);
+                if (r == 0) {
+                        log_error("Domain not valid: %s", word);
+                        return -EINVAL;
+                }
+
+                if (strv_push(&arg_set_domain, word) < 0)
+                        return log_oom();
+
+                word = NULL;
+        }
+
+        return 0;
+}
+
+int resolvconf_parse_argv(int argc, char *argv[]) {
+
+        enum {
+                ARG_VERSION = 0x100,
+                ARG_ENABLE_UPDATES,
+                ARG_DISABLE_UPDATES,
+                ARG_UPDATES_ARE_ENABLED,
+        };
+
+        static const struct option options[] = {
+                { "help",                no_argument, NULL, 'h'                     },
+                { "version",             no_argument, NULL, ARG_VERSION             },
+
+                /* The following are specific to Debian's original resolvconf */
+                { "enable-updates",      no_argument, NULL, ARG_ENABLE_UPDATES      },
+                { "disable-updates",     no_argument, NULL, ARG_DISABLE_UPDATES     },
+                { "updates-are-enabled", no_argument, NULL, ARG_UPDATES_ARE_ENABLED },
+                {}
+        };
+
+
+        enum {
+                TYPE_REGULAR,
+                TYPE_PRIVATE,   /* -p: Not supported, treated identically to TYPE_REGULAR */
+                TYPE_EXCLUSIVE, /* -x */
+        } type = TYPE_REGULAR;
+
+        const char *dot, *iface;
+        int c, r;
+
+        assert(argc >= 0);
+        assert(argv);
+
+        /* openresolv checks these environment variables */
+        if (getenv("IF_EXCLUSIVE"))
+                type = TYPE_EXCLUSIVE;
+        if (getenv("IF_PRIVATE"))
+                type = TYPE_PRIVATE; /* not actually supported */
+
+        arg_mode = _MODE_INVALID;
+
+        while ((c = getopt_long(argc, argv, "hadxpfm:uIi:l:Rr:vV", options, NULL)) >= 0)
+                switch(c) {
+
+                case 'h':
+                        resolvconf_help();
+                        return 0; /* done */;
+
+                case ARG_VERSION:
+                        return version();
+
+                /* -a and -d is what everybody can agree on */
+                case 'a':
+                        arg_mode = MODE_SET_LINK;
+                        break;
+
+                case 'd':
+                        arg_mode = MODE_REVERT_LINK;
+                        break;
+
+                /* The exclusive/private/force stuff is an openresolv invention, we support in some skewed way */
+                case 'x':
+                        type = TYPE_EXCLUSIVE;
+                        break;
+
+                case 'p':
+                        type = TYPE_PRIVATE; /* not actually supported */
+                        break;
+
+                case 'f':
+                        arg_ifindex_permissive = true;
+                        break;
+
+                /* The metrics stuff is an openresolv invention we ignore (and don't really need) */
+                case 'm':
+                        log_debug("Switch -%c ignored.", c);
+                        break;
+
+                /* Everybody else can agree on the existance of -u but we don't support it. */
+                case 'u':
+
+                /* The following options are openresolv inventions we don't support. */
+                case 'I':
+                case 'i':
+                case 'l':
+                case 'R':
+                case 'r':
+                case 'v':
+                case 'V':
+                        log_error("Switch -%c not supported.", c);
+                        return -EINVAL;
+
+                /* The Debian resolvconf commands we don't support. */
+                case ARG_ENABLE_UPDATES:
+                        log_error("Switch --enable-updates not supported.");
+                        return -EINVAL;
+                case ARG_DISABLE_UPDATES:
+                        log_error("Switch --disable-updates not supported.");
+                        return -EINVAL;
+                case ARG_UPDATES_ARE_ENABLED:
+                        log_error("Switch --updates-are-enabled not supported.");
+                        return -EINVAL;
+
+                case '?':
+                        return -EINVAL;
+
+                default:
+                        assert_not_reached("Unhandled option");
+                }
+
+        if (arg_mode == _MODE_INVALID) {
+                log_error("Expected either -a or -d on the command line.");
+                return -EINVAL;
+        }
+
+        if (optind+1 != argc) {
+                log_error("Expected interface name as argument.");
+                return -EINVAL;
+        }
+
+        dot = strchr(argv[optind], '.');
+        if (dot) {
+                iface = strndupa(argv[optind], dot - argv[optind]);
+                log_debug("Ignoring protocol specifier '%s'.", dot + 1);
+        } else
+                iface = argv[optind];
+        optind++;
+
+        if (parse_ifindex(iface, &arg_ifindex) < 0) {
+                int ifi;
+
+                ifi = if_nametoindex(iface);
+                if (ifi <= 0) {
+                        if (errno == ENODEV && arg_ifindex_permissive) {
+                                log_debug("Interface '%s' not found, but -f specified, ignoring.", iface);
+                                return 0; /* done */
+                        }
+
+                        return log_error_errno(errno, "Unknown interface '%s': %m", iface);
+                }
+
+                arg_ifindex = ifi;
+        }
+
+        if (arg_mode == MODE_SET_LINK) {
+                unsigned n = 0;
+
+                for (;;) {
+                        _cleanup_free_ char *line = NULL;
+                        const char *a, *l;
+
+                        r = read_line(stdin, LONG_LINE_MAX, &line);
+                        if (r < 0)
+                                return log_error_errno(r, "Failed to read from stdin: %m");
+                        if (r == 0)
+                                break;
+
+                        n++;
+
+                        l = strstrip(line);
+                        if (IN_SET(*l, '#', ';', 0))
+                                continue;
+
+                        a = first_word(l, "nameserver");
+                        if (a) {
+                                (void) parse_nameserver(a);
+                                continue;
+                        }
+
+                        a = first_word(l, "domain");
+                        if (!a)
+                                a = first_word(l, "search");
+                        if (a) {
+                                (void) parse_search_domain(a);
+                                continue;
+                        }
+
+                        log_syntax(NULL, LOG_DEBUG, "stdin", n, 0, "Ignoring resolv.conf line: %s", l);
+                }
+
+                if (type == TYPE_EXCLUSIVE) {
+
+                        /* If -x mode is selected, let's preferably route non-suffixed lookups to this interface. This
+                         * somewhat matches the original -x behaviour */
+
+                        r = strv_extend(&arg_set_domain, "~.");
+                        if (r < 0)
+                                return log_oom();
+
+                } else if (type == TYPE_PRIVATE)
+                        log_debug("Private DNS server data not supported, ignoring.");
+
+                if (arg_n_set_dns == 0) {
+                        log_error("No DNS servers specified, refusing operation.");
+                        return -EINVAL;
+                }
+        }
+
+        return 1; /* work to do */
+}

src/resolve/resolvconf-compat.h

@@ -0,0 +1,3 @@
+/* SPDX-License-Identifier: LGPL-2.1+ */
+
+int resolvconf_parse_argv(int argc, char *argv[]);