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>

5 files changed, 116 insertions, 32 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);
 	}
 
diff --git a/passt.1 b/passt.1
index 059abd3..15c8338 100644
--- a/passt.1
+++ b/passt.1
@@ -160,7 +160,9 @@ once for IPv6).
 By default, assigned IPv4 and IPv6 addresses are taken from the host interfaces
 with the first default route, if any, for the corresponding IP version. If no
 default routes are available and there is any interface with any route for a
-given IP version, the first of these interfaces will be chosen instead.
+given IP version, the first of these interfaces will be chosen instead. If no
+such interface exists, the link-local address 169.254.2.1 is assigned for IPv4,
+and no additional address will be assigned for IPv6.
 
 .TP
 .BR \-n ", " \-\-netmask " " \fImask
@@ -187,7 +189,9 @@ first default route, if any, for the corresponding IP version. If the default
 route is a multipath one, the gateway is the first nexthop router returned by
 the kernel which has the highest weight in the set of paths. If no default
 routes are available and there is just one interface with any route, that
-interface will be chosen instead.
+interface will be chosen instead. If no such interface exists, the link-local
+address 169.254.2.2 is used for IPv4, and the link-local address fe80::1 is used
+for IPv6.
 
 Note: these addresses are also used as source address for packets directed to
 the guest or to the target namespace having a loopback or local source address,
@@ -202,7 +206,9 @@ Default is to use the interfaces specified by \fB--outbound-if4\fR and
 
 If no interfaces are given, the interface with the first default routes for each
 IP version is selected. If no default routes are available and there is just one
-interface with any route, that interface will be chosen instead.
+interface with any route, that interface will be chosen instead. If no such
+interface exists, host interfaces will be ignored for the purposes of assigning
+addresses and routes, and link-local addresses will be used instead.
 
 .TP
 .BR \-o ", " \-\-outbound " " \fIaddr
@@ -221,7 +227,8 @@ derive IPv4 addresses and routes.
 
 By default, the interface given by the default route is selected. If no default
 routes are available and there is just one interface with any route, that
-interface will be chosen instead.
+interface will be chosen instead. If no such interface exists, outbound sockets
+will not be bound to any specific interface.
 
 .TP
 .BR \-\-outbound-if6 " " \fIname
@@ -231,7 +238,8 @@ derive IPv6 addresses and routes.
 
 By default, the interface given by the default route is selected. If no default
 routes are available and there is just one interface with any route, that
-interface will be chosen instead.
+interface will be chosen instead. If no such interface exists, outbound sockets
+will not be bound to any specific interface.
 
 .TP
 .BR \-D ", " \-\-dns " " \fIaddr
@@ -503,6 +511,7 @@ Default is \fBnone\fR.
 .BR \-I ", " \-\-ns-ifname " " \fIname
 Name of tap interface to be created in target namespace.
 By default, the same interface name as the external, routable interface is used.
+If no such interface exists, the name \fItap0\fR will be used instead.
 
 .TP
 .BR \-t ", " \-\-tcp-ports " " \fIspec
@@ -1031,6 +1040,20 @@ If the sending window cannot be queried, it will always be announced as the
 current sending buffer size to guest or target namespace. This might affect
 throughput of TCP connections.
 
+.SS Local mode for disconnected setups
+
+If \fBpasst\fR and \fBpasta\fR fail to find a host interface with a configured
+address, other than loopback addresses, they will, obviously, not attempt to
+source addresses or routes from the host.
+
+In this case, unless configured otherwise, they will assign the IPv4 link-local
+address 169.254.2.1 to the guest or target namespace, and no IPv6 address. The
+notion of the guest or target namespace IPv6 address is derived from the first
+link-local address observed.
+
+Default gateways will be assigned as the link-local address 169.254.2.2 for
+IPv4, and as the link-local address fe80::1 for IPv6.
+
 .SH LIMITATIONS
 
 Currently, IGMP/MLD proxying (RFC 4605) and support for SCTP (RFC 4960) are not
diff --git a/passt.h b/passt.h
index 72c7f72..799ee50 100644
--- a/passt.h
+++ b/passt.h
@@ -202,10 +202,10 @@ struct ip6_ctx {
  * @our_tap_mac:	Pasta/passt's MAC on the tap link
  * @guest_mac:		MAC address of guest or namespace, seen or configured
  * @hash_secret:	128-bit secret for siphash functions
- * @ifi4:		Index of template interface for IPv4, 0 if IPv4 disabled
+ * @ifi4:		Template interface for IPv4, -1: none, 0: IPv4 disabled
  * @ip:			IPv4 configuration
  * @dns_search:		DNS search list
- * @ifi6:		Index of template interface for IPv6, 0 if IPv6 disabled
+ * @ifi6:		Template interface for IPv6, -1: none, 0: IPv6 disabled
  * @ip6:		IPv6 configuration
  * @pasta_ifn:		Name of namespace interface for pasta
  * @pasta_ifi:		Index of namespace interface for pasta
@@ -258,12 +258,12 @@ struct ctx {
 	unsigned char guest_mac[ETH_ALEN];
 	uint64_t hash_secret[2];
 
-	unsigned int ifi4;
+	int ifi4;
 	struct ip4_ctx ip4;
 
 	struct fqdn dns_search[MAXDNSRCH];
 
-	unsigned int ifi6;
+	int ifi6;
 	struct ip6_ctx ip6;
 
 	char pasta_ifn[IF_NAMESIZE];
diff --git a/pasta.c b/pasta.c
index a117704..96dacc3 100644
--- a/pasta.c
+++ b/pasta.c
@@ -369,8 +369,11 @@ void pasta_ns_conf(struct ctx *c)
 					  0, IFF_NOARP);
 
 			if (c->ip6.no_copy_addrs) {
-				rc = nl_addr_set(nl_sock_ns, c->pasta_ifi,
-						 AF_INET6, &c->ip6.addr, 64);
+				if (!IN6_IS_ADDR_UNSPECIFIED(&c->ip6.addr)) {
+					rc = nl_addr_set(nl_sock_ns,
+							 c->pasta_ifi, AF_INET6,
+							 &c->ip6.addr, 64);
+				}
 			} else {
 				rc = nl_addr_dup(nl_sock, c->ifi6,
 						 nl_sock_ns, c->pasta_ifi,
diff --git a/tap.c b/tap.c
index 14d9b3d..5347df4 100644
--- a/tap.c
+++ b/tap.c
@@ -803,6 +803,9 @@ resume:
 			if (IN6_IS_ADDR_UNSPECIFIED(&c->ip6.addr_seen)) {
 				c->ip6.addr_seen = *saddr;
 			}
+
+			if (IN6_IS_ADDR_UNSPECIFIED(&c->ip6.addr))
+				c->ip6.addr = *saddr;
 		} else if (!IN6_IS_ADDR_UNSPECIFIED(saddr)){
 			c->ip6.addr_seen = *saddr;
 		}