shared(wg): default to 1400 if network CIDR is IPv6

Second fix for #102

shared/src/types.rs

@@ -385,9 +385,9 @@ pub struct NetworkOpt {
     /// If not set, innernet will auto-select based on availability.
     pub backend: Backend,
 
-    #[structopt(long, default_value = "1420")]
-    /// Specify the desired MTU for your interface.
-    pub mtu: u32,
+    #[structopt(long)]
+    /// Specify the desired MTU for your interface (default: 1420 for IPv4 and 1400 for IPv6).
+    pub mtu: Option<u32>,
 }
 
 #[derive(Debug, Clone, Deserialize, Serialize, PartialEq)]

shared/src/wg.rs

@@ -85,7 +85,7 @@ pub fn up(
         .set_private_key(wgctrl::Key::from_base64(&private_key).unwrap())
         .apply(interface, network.backend)?;
     set_addr(interface, address)?;
-    set_up(interface, network.mtu)?;
+    set_up(interface, network.mtu.unwrap_or_else(|| if address.is_ipv4() { 1420 } else { 1400 }))?;
     if !network.no_routing {
         add_route(interface, address)?;
     }