fix: don't panic if no suitable IPv6 src_addr is found

examples/ping.rs

@@ -187,7 +187,7 @@ fn main() {
                         remote_addr
                     );
                     icmp_repr.emit(
-                        &iface.get_source_address_ipv6(&address).unwrap(),
+                        &iface.get_source_address_ipv6(&address),
                         &address,
                         &mut icmp_packet,
                         &device_caps.checksum,
@@ -221,7 +221,7 @@ fn main() {
                     let icmp_packet = Icmpv6Packet::new_checked(&payload).unwrap();
                     let icmp_repr = Icmpv6Repr::parse(
                         &address,
-                        &iface.get_source_address_ipv6(&address).unwrap(),
+                        &iface.get_source_address_ipv6(&address),
                         &icmp_packet,
                         &device_caps.checksum,
                     )

src/iface/interface/ipv6.rs

@@ -21,8 +21,13 @@ impl Default for HopByHopResponse<'_> {
 impl InterfaceInner {
     /// Return the IPv6 address that is a candidate source address for the given destination
     /// address, based on RFC 6724.
+    ///
+    /// # Panics
+    /// This function panics if the destination address is unspecified.
     #[allow(unused)]
-    pub(crate) fn get_source_address_ipv6(&self, dst_addr: &Ipv6Address) -> Option<Ipv6Address> {
+    pub(crate) fn get_source_address_ipv6(&self, dst_addr: &Ipv6Address) -> Ipv6Address {
+        assert!(!dst_addr.is_unspecified());
+
         // See RFC 6724 Section 4: Candidate source address
         fn is_candidate_source_address(dst_addr: &Ipv6Address, src_addr: &Ipv6Address) -> bool {
             // For all multicast and link-local destination addresses, the candidate address MUST
@@ -39,10 +44,10 @@ impl InterfaceInner {
                 return false;
             }
 
-            // Loopback addresses and multicast address can not be in the candidate source address
+            // Unspecified addresses and multicast address can not be in the candidate source address
             // list. Except when the destination multicast address has a link-local scope, then the
             // source address can also be link-local multicast.
-            if src_addr.is_loopback() || src_addr.is_multicast() {
+            if src_addr.is_unspecified() || src_addr.is_multicast() {
                 return false;
             }
 
@@ -67,18 +72,28 @@ impl InterfaceInner {
             bits as usize
         }
 
-        // Get the first address that is a candidate address.
+        // If the destination address is a loopback address, or when there are no IPv6 addresses in
+        // the interface, then the loopback address is the only candidate source address.
+        if dst_addr.is_loopback()
+            || self
+                .ip_addrs
+                .iter()
+                .filter(|a| matches!(a, IpCidr::Ipv6(_)))
+                .count()
+                == 0
+        {
+            return Ipv6Address::LOOPBACK;
+        }
+
         let mut candidate = self
             .ip_addrs
             .iter()
-            .filter_map(|a| match a {
+            .find_map(|a| match a {
                 #[cfg(feature = "proto-ipv4")]
                 IpCidr::Ipv4(_) => None,
-                #[cfg(feature = "proto-ipv6")]
                 IpCidr::Ipv6(a) => Some(a),
             })
-            .find(|a| is_candidate_source_address(dst_addr, &a.address()))
-            .unwrap();
+            .unwrap(); // NOTE: we check above that there is at least one IPv6 address.
 
         for addr in self.ip_addrs.iter().filter_map(|a| match a {
             #[cfg(feature = "proto-ipv4")]
@@ -116,7 +131,7 @@ impl InterfaceInner {
             }
         }
 
-        Some(candidate.address())
+        candidate.address()
     }
 
     /// Determine if the given `Ipv6Address` is the solicited node
@@ -436,11 +451,8 @@ impl InterfaceInner {
 
         let src_addr = if src_addr.is_unicast() {
             src_addr
-        } else if let Some(addr) = self.get_source_address_ipv6(&dst_addr) {
-            addr
         } else {
-            net_debug!("no suitable source address found");
-            return None;
+            self.get_source_address_ipv6(&dst_addr)
         };
 
         let ipv6_reply_repr = Ipv6Repr {

src/iface/interface/mod.rs

@@ -328,7 +328,7 @@ impl Interface {
     /// Get an address from the interface that could be used as source address. The selection is
     /// based on RFC6724.
     #[cfg(feature = "proto-ipv6")]
-    pub fn get_source_address_ipv6(&self, dst_addr: &Ipv6Address) -> Option<Ipv6Address> {
+    pub fn get_source_address_ipv6(&self, dst_addr: &Ipv6Address) -> Ipv6Address {
         self.inner.get_source_address_ipv6(dst_addr)
     }
 
@@ -728,7 +728,7 @@ impl InterfaceInner {
             #[cfg(feature = "proto-ipv4")]
             IpAddress::Ipv4(addr) => self.get_source_address_ipv4(addr).map(|a| a.into()),
             #[cfg(feature = "proto-ipv6")]
-            IpAddress::Ipv6(addr) => self.get_source_address_ipv6(addr).map(|a| a.into()),
+            IpAddress::Ipv6(addr) => Some(self.get_source_address_ipv6(addr).into()),
         }
     }