treewide: Introduce 'local mode' for disconnected setups

There are setups where no host interface is available or configured
at all, intentionally or not, temporarily or not, but users expect
(Podman) containers to run in any case as they did with slirp4netns,
and we're now getting reports that we broke such setups at a rather
alarming rate.

To this end, if we don't find any usable host interface, instead of
exiting:

- for IPv4, use 169.254.2.1 as guest/container address and 169.254.2.2
  as default gateway

- for IPv6, don't assign any address (forcibly disable DHCPv6), and
  use the *first* link-local address we observe to represent the
  guest/container. Advertise fe80::1 as default gateway

- use 'tap0' as default interface name for pasta

Change ifi4 and ifi6 in struct ctx to int and accept a special -1
value meaning that no host interface was selected, but the IP family
is enabled. The fact that the kernel uses unsigned int values for
those is not an issue as 1. one can't create so many interfaces
anyway and 2. we otherwise handle those values transparently.

Fix a botched conditional in conf_print() to actually skip printing
DHCPv6 information if DHCPv6 is disabled (and skip printing NDP
information if NDP is disabled).

Link: https://github.com/containers/podman/issues/24614
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

1 files changed, 76 insertions, 21 deletions

diff --git a/conf.c b/conf.c
index d342c8a..c6bffc4 100644
--- a/conf.c
+++ b/conf.c
@@ -48,6 +48,20 @@
 
 #define NETNS_RUN_DIR	"/run/netns"
 
+#define IP4_LL_GUEST_ADDR	(struct in_addr){ htonl_constant(0xa9fe0201) }
+				/* 169.254.2.1, libslirp default: 10.0.2.1 */
+
+#define IP4_LL_GUEST_GW		(struct in_addr){ htonl_constant(0xa9fe0202) }
+				/* 169.254.2.2, libslirp default: 10.0.2.2 */
+
+#define IP4_LL_PREFIX_LEN	16
+
+#define IP6_LL_GUEST_GW		(struct in6_addr)			\
+				{{{ 0xfe, 0x80, 0, 0, 0, 0, 0, 0,	\
+				       0, 0, 0, 0, 0, 0, 0, 0x01 }}}
+
+const char *pasta_default_ifn = "tap0";
+
 /**
  * next_chunk - Return the next piece of a string delimited by a character
  * @s:		String to search
@@ -631,7 +645,7 @@ static unsigned int conf_ip4(unsigned int ifi, struct ip4_ctx *ip4)
 		ifi = nl_get_ext_if(nl_sock, AF_INET);
 
 	if (!ifi) {
-		info("Couldn't pick external interface: disabling IPv4");
+		debug("Failed to detect external interface for IPv4");
 		return 0;
 	}
 
@@ -639,8 +653,8 @@ static unsigned int conf_ip4(unsigned int ifi, struct ip4_ctx *ip4)
 		int rc = nl_route_get_def(nl_sock, ifi, AF_INET,
 					  &ip4->guest_gw);
 		if (rc < 0) {
-			err("Couldn't discover IPv4 gateway address: %s",
-			    strerror(-rc));
+			debug("Couldn't discover IPv4 gateway address: %s",
+			      strerror(-rc));
 			return 0;
 		}
 	}
@@ -649,8 +663,8 @@ static unsigned int conf_ip4(unsigned int ifi, struct ip4_ctx *ip4)
 		int rc = nl_addr_get(nl_sock, ifi, AF_INET,
 				     &ip4->addr, &ip4->prefix_len, NULL);
 		if (rc < 0) {
-			err("Couldn't discover IPv4 address: %s",
-			    strerror(-rc));
+			debug("Couldn't discover IPv4 address: %s",
+			      strerror(-rc));
 			return 0;
 		}
 	}
@@ -678,6 +692,19 @@ static unsigned int conf_ip4(unsigned int ifi, struct ip4_ctx *ip4)
 }
 
 /**
+ * conf_ip4_local() - Configure IPv4 addresses and attributes for local mode
+ * @ip4:	IPv4 context (will be written)
+ */
+static void conf_ip4_local(struct ip4_ctx *ip4)
+{
+	ip4->addr_seen = ip4->addr = IP4_LL_GUEST_ADDR;
+	ip4->our_tap_addr = ip4->guest_gw = IP4_LL_GUEST_GW;
+	ip4->prefix_len = IP4_LL_PREFIX_LEN;
+
+	ip4->no_copy_addrs = ip4->no_copy_routes = true;
+}
+
+/**
  * conf_ip6() - Verify or detect IPv6 support, get relevant addresses
  * @ifi:	Host interface to attempt (0 to determine one)
  * @ip6:	IPv6 context (will be written)
@@ -693,15 +720,15 @@ static unsigned int conf_ip6(unsigned int ifi, struct ip6_ctx *ip6)
 		ifi = nl_get_ext_if(nl_sock, AF_INET6);
 
 	if (!ifi) {
-		info("Couldn't pick external interface: disabling IPv6");
+		debug("Failed to detect external interface for IPv6");
 		return 0;
 	}
 
 	if (IN6_IS_ADDR_UNSPECIFIED(&ip6->guest_gw)) {
 		rc = nl_route_get_def(nl_sock, ifi, AF_INET6, &ip6->guest_gw);
 		if (rc < 0) {
-			err("Couldn't discover IPv6 gateway address: %s",
-			    strerror(-rc));
+			debug("Couldn't discover IPv6 gateway address: %s",
+			      strerror(-rc));
 			return 0;
 		}
 	}
@@ -710,7 +737,7 @@ static unsigned int conf_ip6(unsigned int ifi, struct ip6_ctx *ip6)
 			 IN6_IS_ADDR_UNSPECIFIED(&ip6->addr) ? &ip6->addr : NULL,
 			 &prefix_len, &ip6->our_tap_ll);
 	if (rc < 0) {
-		err("Couldn't discover IPv6 address: %s", strerror(-rc));
+		debug("Couldn't discover IPv6 address: %s", strerror(-rc));
 		return 0;
 	}
 
@@ -727,6 +754,17 @@ static unsigned int conf_ip6(unsigned int ifi, struct ip6_ctx *ip6)
 }
 
 /**
+ * conf_ip6_local() - Configure IPv6 addresses and attributes for local mode
+ * @ip6:	IPv6 context (will be written)
+ */
+static void conf_ip6_local(struct ip6_ctx *ip6)
+{
+	ip6->our_tap_ll = ip6->guest_gw = IP6_LL_GUEST_GW;
+
+	ip6->no_copy_addrs = ip6->no_copy_routes = true;
+}
+
+/**
  * usage() - Print usage, exit with given status code
  * @name:	Executable name
  * @f:		Stream to print usage info to
@@ -948,12 +986,14 @@ static void conf_print(const struct ctx *c)
 	char bufmac[ETH_ADDRSTRLEN], ifn[IFNAMSIZ];
 	int i;
 
-	info("Template interface: %s%s%s%s%s",
-	     c->ifi4 ? if_indextoname(c->ifi4, ifn) : "",
-	     c->ifi4 ? " (IPv4)" : "",
-	     (c->ifi4 && c->ifi6) ? ", " : "",
-	     c->ifi6 ? if_indextoname(c->ifi6, ifn) : "",
-	     c->ifi6 ? " (IPv6)" : "");
+	if (c->ifi4 > 0 || c->ifi6 > 0) {
+		info("Template interface: %s%s%s%s%s",
+		     c->ifi4 > 0 ? if_indextoname(c->ifi4, ifn) : "",
+		     c->ifi4 > 0 ? " (IPv4)" : "",
+		     (c->ifi4 && c->ifi6) ? ", " : "",
+		     c->ifi6 > 0 ? if_indextoname(c->ifi6, ifn) : "",
+		     c->ifi6 > 0 ? " (IPv6)" : "");
+	}
 
 	if (*c->ip4.ifname_out || *c->ip6.ifname_out) {
 		info("Outbound interface: %s%s%s%s%s",
@@ -1024,9 +1064,9 @@ static void conf_print(const struct ctx *c)
 
 		if (!c->no_ndp && !c->no_dhcpv6)
 			info("NDP/DHCPv6:");
-		else if (!c->no_ndp)
-			info("DHCPv6:");
 		else if (!c->no_dhcpv6)
+			info("DHCPv6:");
+		else if (!c->no_ndp)
 			info("NDP:");
 		else
 			goto dns6;
@@ -1733,10 +1773,23 @@ void conf(struct ctx *c, int argc, char **argv)
 		c->ifi4 = conf_ip4(ifi4, &c->ip4);
 	if (!v4_only)
 		c->ifi6 = conf_ip6(ifi6, &c->ip6);
-	if ((!c->ifi4 && !c->ifi6) ||
-	    (*c->ip4.ifname_out && !c->ifi4) ||
+	if ((*c->ip4.ifname_out && !c->ifi4) ||
 	    (*c->ip6.ifname_out && !c->ifi6))
 		die("External interface not usable");
+	if (!c->ifi4 && !c->ifi6) {
+		info("No external interface as template, switch to local mode");
+
+		conf_ip4_local(&c->ip4);
+		c->ifi4 = -1;
+
+		conf_ip6_local(&c->ip6);
+		c->ifi6 = -1;
+
+		if (!*c->pasta_ifn) {
+			strncpy(c->pasta_ifn, pasta_default_ifn,
+				sizeof(c->pasta_ifn) - 1);
+		}
+	}
 
 	if (c->ifi4 && !no_map_gw &&
 	    IN4_IS_ADDR_UNSPECIFIED(&c->ip4.map_host_loopback))
@@ -1840,6 +1893,8 @@ void conf(struct ctx *c, int argc, char **argv)
 	if (!c->ifi6) {
 		c->no_ndp = 1;
 		c->no_dhcpv6 = 1;
+	} else if (IN6_IS_ADDR_UNSPECIFIED(&c->ip6.addr)) {
+		c->no_dhcpv6 = 1;
 	}
 
 	if (!c->mtu)
@@ -1848,9 +1903,9 @@ void conf(struct ctx *c, int argc, char **argv)
 	get_dns(c);
 
 	if (!*c->pasta_ifn) {
-		if (c->ifi4)
+		if (c->ifi4 > 0)
 			if_indextoname(c->ifi4, c->pasta_ifn);
-		else
+		else if (c->ifi6 > 0)
 			if_indextoname(c->ifi6, c->pasta_ifn);
 	}