[PATCH v2] treewide: Introduce 'local mode' for disconnected setups

[PATCH v2] treewide: Introduce 'local mode' for disconnected setups

[Stefano Brivio]

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>
---
v2:
 - drop fixed link-local address for IPv6
 - change addresses to be reminiscent of libslirp's default choices
 - add man page changes and commit message
 - fix several things around, from testing (checked with several
   --map-guest-addr and --map-host-loopback combinations etc.)

 conf.c  | 92 +++++++++++++++++++++++++++++++++++++++++++++------------
 passt.1 | 33 +++++++++++++++++----
 passt.h |  8 ++---
 pasta.c |  7 +++--
 tap.c   |  3 ++
 5 files changed, 113 insertions(+), 30 deletions(-)

diff --git a/conf.c b/conf.c
index 86566db..551c2b0 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;
 		}
 	}
@@ -677,6 +691,19 @@ static unsigned int conf_ip4(unsigned int ifi, struct ip4_ctx *ip4)
 	return ifi;
 }
 
+/**
+ * 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)
@@ -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;
 	}
 
@@ -726,6 +753,17 @@ static unsigned int conf_ip6(unsigned int ifi, struct ip6_ctx *ip6)
 	return ifi;
 }
 
+/**
+ * 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
@@ -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;
@@ -1735,8 +1775,20 @@ void conf(struct ctx *c, int argc, char **argv)
 		c->ifi6 = conf_ip6(ifi6, &c->ip6);
 	if ((!c->ifi4 && !c->ifi6) ||
 	    (*c->ip4.ifname_out && !c->ifi4) ||
-	    (*c->ip6.ifname_out && !c->ifi6))
-		die("External interface not usable");
+	    (*c->ip6.ifname_out && !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 +1892,8 @@ 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)
diff --git a/passt.1 b/passt.1
index f084978..f5cf8a4 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
@@ -188,7 +190,9 @@ 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,
@@ -203,7 +207,9 @@ 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
@@ -222,7 +228,8 @@ 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
@@ -232,7 +239,8 @@ 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
@@ -504,6 +512,7 @@ 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
@@ -1032,6 +1041,20 @@ 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;
 		}
-- 
@@ -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;
 		}
-- 
2.43.0

Re: [PATCH v2] treewide: Introduce 'local mode' for disconnected setups

[David Gibson]

[-- Attachment #1: Type: text/plain, Size: 17011 bytes --]

On Mon, Nov 25, 2024 at 12:50:37AM +0100, Stefano Brivio wrote:
> 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.

This looks pretty good to me.  It's simpler than I feared, and I don't
think it will tie our hands much in terms of future fancier handling
(netlink monitors, no-template-interface mode etc.).  I'm pretty good
to go ahead with this soon.

I do have a few thoughts on some details which might be polished, see
below.

> 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.

ifi4 and ifi6 are currently kind of overloaded.  A few places that
actually need to deal with the host side plumbing use them for the
interface index.  Most places use these only to tell if IPv4/IPv6 is
enabled (for the guest).

This patch is disconnecting whether we have IPv4/IPv6 enabled from
whether there exists a suitable external interface.  So, I think it
makes sense to disconnect the variables as well, rather than
complexifying the encoding into ifi4/ifi6.

I'd suggest instead that we introduce new boolean ip[46]_enabled
variables, and change all the places that check ifi4/ifi6 only for
that reason to use those instead.  That can be a preiminary patch.
Then we keep the encoding of ifi4/ifi6 as is.

I think that will make things slightly cleaner and not a great deal
harder for this initial version of a "local" setting.  It will also
extend more naturally if we have future modes that can interact with
multiple host side interfaces or other more complex scenarios.

> 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>
> ---
> v2:
>  - drop fixed link-local address for IPv6
>  - change addresses to be reminiscent of libslirp's default choices
>  - add man page changes and commit message
>  - fix several things around, from testing (checked with several
>    --map-guest-addr and --map-host-loopback combinations etc.)
> 
>  conf.c  | 92 +++++++++++++++++++++++++++++++++++++++++++++------------
>  passt.1 | 33 +++++++++++++++++----
>  passt.h |  8 ++---
>  pasta.c |  7 +++--
>  tap.c   |  3 ++
>  5 files changed, 113 insertions(+), 30 deletions(-)
> 
> diff --git a/conf.c b/conf.c
> index 86566db..551c2b0 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";

Not sure I like this as a default name, though I guess it's fine.  It
kind of exposes the underlying mechanisms we're using in a way that's
not really necessary.  Maybe just call it "pasta" or "pasta0"?

> +
>  /**
>   * 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;
>  		}
>  	}
> @@ -677,6 +691,19 @@ static unsigned int conf_ip4(unsigned int ifi, struct ip4_ctx *ip4)
>  	return ifi;
>  }
>  
> +/**
> + * 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)
> @@ -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;
>  	}
>  
> @@ -726,6 +753,17 @@ static unsigned int conf_ip6(unsigned int ifi, struct ip6_ctx *ip6)
>  	return ifi;
>  }
>  
> +/**
> + * 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
> @@ -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;
> @@ -1735,8 +1775,20 @@ void conf(struct ctx *c, int argc, char **argv)
>  		c->ifi6 = conf_ip6(ifi6, &c->ip6);
>  	if ((!c->ifi4 && !c->ifi6) ||
>  	    (*c->ip4.ifname_out && !c->ifi4) ||
> -	    (*c->ip6.ifname_out && !c->ifi6))
> -		die("External interface not usable");

I guess a question is if the host has IPv4 but no IPv6 connectivity,
do we want to disable IPv6 in the guest, or configure local-mode IPv6?
That is something we can probably change without too much danger later
on.

> +	    (*c->ip6.ifname_out && !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 +1892,8 @@ 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)
> diff --git a/passt.1 b/passt.1
> index f084978..f5cf8a4 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
> @@ -188,7 +190,9 @@ 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,
> @@ -203,7 +207,9 @@ 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
> @@ -222,7 +228,8 @@ 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
> @@ -232,7 +239,8 @@ 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
> @@ -504,6 +512,7 @@ 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
> @@ -1032,6 +1041,20 @@ 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;

Hmm.. not entirely sure if this is a good idea, or whether we should
instead keep the assigned address blank as an indication that we never
assigned something, the guest just picked for itself.

>  		} else if (!IN6_IS_ADDR_UNSPECIFIED(saddr)){
>  			c->ip6.addr_seen = *saddr;
>  		}

-- 
David Gibson (he or they)	| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you, not the other way
				| around.
http://www.ozlabs.org/~dgibson

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Re: [PATCH v2] treewide: Introduce 'local mode' for disconnected setups

[Stefano Brivio]

On Tue, 26 Nov 2024 15:01:06 +1100
David Gibson <david@gibson.dropbear.id.au> wrote:

> On Mon, Nov 25, 2024 at 12:50:37AM +0100, Stefano Brivio wrote:
> > 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.  
> 
> This looks pretty good to me.  It's simpler than I feared, and I don't
> think it will tie our hands much in terms of future fancier handling
> (netlink monitors, no-template-interface mode etc.).  I'm pretty good
> to go ahead with this soon.
> 
> I do have a few thoughts on some details which might be polished, see
> below.
> 
> > 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.  
> 
> ifi4 and ifi6 are currently kind of overloaded.  A few places that
> actually need to deal with the host side plumbing use them for the
> interface index.  Most places use these only to tell if IPv4/IPv6 is
> enabled (for the guest).
> 
> This patch is disconnecting whether we have IPv4/IPv6 enabled from
> whether there exists a suitable external interface.  So, I think it
> makes sense to disconnect the variables as well, rather than
> complexifying the encoding into ifi4/ifi6.
> 
> I'd suggest instead that we introduce new boolean ip[46]_enabled
> variables, and change all the places that check ifi4/ifi6 only for
> that reason to use those instead.  That can be a preiminary patch.
> Then we keep the encoding of ifi4/ifi6 as is.

That turns this patch into a series, which is exactly what I'm trying
to avoid, for no particular advantage. As you say below: "slightly
cleaner".

I think we should be responsible toward distribution maintainers
(including myself) who might be trying to fix a pressing issue with a
_simple_ backport.

> I think that will make things slightly cleaner and not a great deal
> harder for this initial version of a "local" setting.  It will also
> extend more naturally if we have future modes that can interact with
> multiple host side interfaces or other more complex scenarios.

That can be changed later as needed.

> > 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>
> > ---
> > v2:
> >  - drop fixed link-local address for IPv6
> >  - change addresses to be reminiscent of libslirp's default choices
> >  - add man page changes and commit message
> >  - fix several things around, from testing (checked with several
> >    --map-guest-addr and --map-host-loopback combinations etc.)
> > 
> >  conf.c  | 92 +++++++++++++++++++++++++++++++++++++++++++++------------
> >  passt.1 | 33 +++++++++++++++++----
> >  passt.h |  8 ++---
> >  pasta.c |  7 +++--
> >  tap.c   |  3 ++
> >  5 files changed, 113 insertions(+), 30 deletions(-)
> > 
> > diff --git a/conf.c b/conf.c
> > index 86566db..551c2b0 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";  
> 
> Not sure I like this as a default name, though I guess it's fine.  It
> kind of exposes the underlying mechanisms we're using in a way that's
> not really necessary.  Maybe just call it "pasta" or "pasta0"?

That's the default in slirp4netns, and if we have a slight chance of
being slightly more compatible, I'd pick that over proving a point or
being correct.

> > +
> >  /**
> >   * 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;
> >  		}
> >  	}
> > @@ -677,6 +691,19 @@ static unsigned int conf_ip4(unsigned int ifi, struct ip4_ctx *ip4)
> >  	return ifi;
> >  }
> >  
> > +/**
> > + * 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)
> > @@ -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;
> >  	}
> >  
> > @@ -726,6 +753,17 @@ static unsigned int conf_ip6(unsigned int ifi, struct ip6_ctx *ip6)
> >  	return ifi;
> >  }
> >  
> > +/**
> > + * 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
> > @@ -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;
> > @@ -1735,8 +1775,20 @@ void conf(struct ctx *c, int argc, char **argv)
> >  		c->ifi6 = conf_ip6(ifi6, &c->ip6);
> >  	if ((!c->ifi4 && !c->ifi6) ||
> >  	    (*c->ip4.ifname_out && !c->ifi4) ||
> > -	    (*c->ip6.ifname_out && !c->ifi6))
> > -		die("External interface not usable");  
> 
> I guess a question is if the host has IPv4 but no IPv6 connectivity,
> do we want to disable IPv6 in the guest, or configure local-mode IPv6?
> That is something we can probably change without too much danger later
> on.

I think we want to disable it by default, because the whole point here
is about permanently or temporarily disconnected setups, not what IP
versions are enabled on the host. If IPv4 is there, it's connected.

> > +	    (*c->ip6.ifname_out && !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 +1892,8 @@ 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)
> > diff --git a/passt.1 b/passt.1
> > index f084978..f5cf8a4 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
> > @@ -188,7 +190,9 @@ 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,
> > @@ -203,7 +207,9 @@ 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
> > @@ -222,7 +228,8 @@ 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
> > @@ -232,7 +239,8 @@ 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
> > @@ -504,6 +512,7 @@ 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
> > @@ -1032,6 +1041,20 @@ 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;  
> 
> Hmm.. not entirely sure if this is a good idea, or whether we should
> instead keep the assigned address blank as an indication that we never
> assigned something, the guest just picked for itself.

If you keep this blank, mapping mechanisms don't work as intended
anymore.

In v1, this was consistent with IPv4 as we always had a fixed address,
fe80::2. But we don't need to invent a particular address to assign to
the guest for IPv6, so we want to effectively behave as if we assigned
whatever the guest already picked.

> >  		} else if (!IN6_IS_ADDR_UNSPECIFIED(saddr)){
> >  			c->ip6.addr_seen = *saddr;
> >  		}  

-- 
Stefano

Re: [PATCH v2] treewide: Introduce 'local mode' for disconnected setups

[David Gibson]

[-- Attachment #1: Type: text/plain, Size: 20425 bytes --]

On Tue, Nov 26, 2024 at 05:39:39AM +0100, Stefano Brivio wrote:
> On Tue, 26 Nov 2024 15:01:06 +1100
> David Gibson <david@gibson.dropbear.id.au> wrote:
> 
> > On Mon, Nov 25, 2024 at 12:50:37AM +0100, Stefano Brivio wrote:
> > > 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.  
> > 
> > This looks pretty good to me.  It's simpler than I feared, and I don't
> > think it will tie our hands much in terms of future fancier handling
> > (netlink monitors, no-template-interface mode etc.).  I'm pretty good
> > to go ahead with this soon.
> > 
> > I do have a few thoughts on some details which might be polished, see
> > below.
> > 
> > > 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.  
> > 
> > ifi4 and ifi6 are currently kind of overloaded.  A few places that
> > actually need to deal with the host side plumbing use them for the
> > interface index.  Most places use these only to tell if IPv4/IPv6 is
> > enabled (for the guest).
> > 
> > This patch is disconnecting whether we have IPv4/IPv6 enabled from
> > whether there exists a suitable external interface.  So, I think it
> > makes sense to disconnect the variables as well, rather than
> > complexifying the encoding into ifi4/ifi6.
> > 
> > I'd suggest instead that we introduce new boolean ip[46]_enabled
> > variables, and change all the places that check ifi4/ifi6 only for
> > that reason to use those instead.  That can be a preiminary patch.
> > Then we keep the encoding of ifi4/ifi6 as is.
> 
> That turns this patch into a series, which is exactly what I'm trying
> to avoid, for no particular advantage. As you say below: "slightly
> cleaner".
> 
> I think we should be responsible toward distribution maintainers
> (including myself) who might be trying to fix a pressing issue with a
> _simple_ backport.

Ah, yeah, I forgot about the complexity it would add for backporting.
Ok fair enough, we can fix this up later.

> > I think that will make things slightly cleaner and not a great deal
> > harder for this initial version of a "local" setting.  It will also
> > extend more naturally if we have future modes that can interact with
> > multiple host side interfaces or other more complex scenarios.
> 
> That can be changed later as needed.
> 
> > > 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>
> > > ---
> > > v2:
> > >  - drop fixed link-local address for IPv6
> > >  - change addresses to be reminiscent of libslirp's default choices
> > >  - add man page changes and commit message
> > >  - fix several things around, from testing (checked with several
> > >    --map-guest-addr and --map-host-loopback combinations etc.)
> > > 
> > >  conf.c  | 92 +++++++++++++++++++++++++++++++++++++++++++++------------
> > >  passt.1 | 33 +++++++++++++++++----
> > >  passt.h |  8 ++---
> > >  pasta.c |  7 +++--
> > >  tap.c   |  3 ++
> > >  5 files changed, 113 insertions(+), 30 deletions(-)
> > > 
> > > diff --git a/conf.c b/conf.c
> > > index 86566db..551c2b0 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";  
> > 
> > Not sure I like this as a default name, though I guess it's fine.  It
> > kind of exposes the underlying mechanisms we're using in a way that's
> > not really necessary.  Maybe just call it "pasta" or "pasta0"?
> 
> That's the default in slirp4netns, and if we have a slight chance of
> being slightly more compatible, I'd pick that over proving a point or
> being correct.

Ok, that seems a good enough reason for "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;
> > >  		}
> > >  	}
> > > @@ -677,6 +691,19 @@ static unsigned int conf_ip4(unsigned int ifi, struct ip4_ctx *ip4)
> > >  	return ifi;
> > >  }
> > >  
> > > +/**
> > > + * 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)
> > > @@ -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;
> > >  	}
> > >  
> > > @@ -726,6 +753,17 @@ static unsigned int conf_ip6(unsigned int ifi, struct ip6_ctx *ip6)
> > >  	return ifi;
> > >  }
> > >  
> > > +/**
> > > + * 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
> > > @@ -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;
> > > @@ -1735,8 +1775,20 @@ void conf(struct ctx *c, int argc, char **argv)
> > >  		c->ifi6 = conf_ip6(ifi6, &c->ip6);
> > >  	if ((!c->ifi4 && !c->ifi6) ||
> > >  	    (*c->ip4.ifname_out && !c->ifi4) ||
> > > -	    (*c->ip6.ifname_out && !c->ifi6))
> > > -		die("External interface not usable");  
> > 
> > I guess a question is if the host has IPv4 but no IPv6 connectivity,
> > do we want to disable IPv6 in the guest, or configure local-mode IPv6?
> > That is something we can probably change without too much danger later
> > on.
> 
> I think we want to disable it by default, because the whole point here
> is about permanently or temporarily disconnected setups, not what IP
> versions are enabled on the host. If IPv4 is there, it's connected.

Ok, that's reasonable.  I don't think it should be too bad to change
it later if we want to.

> > > +	    (*c->ip6.ifname_out && !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 +1892,8 @@ 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)
> > > diff --git a/passt.1 b/passt.1
> > > index f084978..f5cf8a4 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
> > > @@ -188,7 +190,9 @@ 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,
> > > @@ -203,7 +207,9 @@ 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
> > > @@ -222,7 +228,8 @@ 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
> > > @@ -232,7 +239,8 @@ 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
> > > @@ -504,6 +512,7 @@ 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
> > > @@ -1032,6 +1041,20 @@ 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;  
> > 
> > Hmm.. not entirely sure if this is a good idea, or whether we should
> > instead keep the assigned address blank as an indication that we never
> > assigned something, the guest just picked for itself.
> 
> If you keep this blank, mapping mechanisms don't work as intended
> anymore.

Ah, --map-guest-addr specifically.

> In v1, this was consistent with IPv4 as we always had a fixed address,
> fe80::2. But we don't need to invent a particular address to assign to
> the guest for IPv6, so we want to effectively behave as if we assigned
> whatever the guest already picked.

Ok.  Good enough for the time being, anyway.


So, comments considered

Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

> 
> > >  		} else if (!IN6_IS_ADDR_UNSPECIFIED(saddr)){
> > >  			c->ip6.addr_seen = *saddr;
> > >  		}  
> 

-- 
David Gibson (he or they)	| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you, not the other way
				| around.
http://www.ozlabs.org/~dgibson

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