NEVPNManager can't establish IPv6 connection on iPhoneX

Hello,

In the code I have utilised the NEVPNManager to establish an IKEv2 connection to my StrongSwan server. I have disabled IPv4 on the server to focus only on the IPv6 issues.

When I run the app under MacOS, it works perfectly fine. When I run the app under iPad (iOS) it also works perfectly fine. Both obtain an IPv6 from the server.

But when I run the app under my iPhone X, the IPv6 connection can't be established. I have updated the OS to iOS 15.1, but it didn't help. I have restarted the phone several times and made sure the DNS settings in Wifi connection is set to automatic.

When I do a sudo tcpdump icmp6 on the server I get this for iPad:

tcpdump: verbose output suppressed, use -v[v]... for full protocol decode
listening on eth0, link-type EN10MB (Ethernet), snapshot length 262144 bytes
 
08:46:45.036393 IP6 fe80::9400:ff:fef1:6bcb > fe80::1: ICMP6, neighbor solicitation, who has fe80::1, length 32
 
08:46:45.037036 IP6 fe80::1 > fe80::9400:ff:fef1:6bcb: ICMP6, neighbor advertisement, tgt is fe80::1, length 24

But when I run it under my iPhone X I get errors on tcpdump:

tcpdump: verbose output suppressed, use -v[v]... for full protocol decode
listening on eth0, link-type EN10MB (Ethernet), snapshot length 262144 bytes
 
08:49:36.594341 IP6 2a01:4f8:c17:1f2d::1 > one.one.one.one: ICMP6, destination unreachable, unreachable port, 2a01:4f8:c17:1f2d::1 udp port 61921, length 121
 
08:49:36.594693 IP6 2a01:4f8:c17:1f2d::1 > one.one.one.one: ICMP6, destination unreachable, unreachable port, 2a01:4f8:c17:1f2d::1 udp port 49461, length 155
 
08:49:36.605300 IP6 2a01:4f8:c17:1f2d::1 > one.one.one.one: ICMP6, destination unreachable, unreachable port, 2a01:4f8:c17:1f2d::1 udp port 63572, length 152
...
 

Does the iPhone has some sort of firewall enabled?

Below is my code. Why is the iPhone X affected, but not the iPad?

func connectIKEv2VPN() {
   self.vpnManager.loadFromPreferences(completionHandler: vpnLoadHandler)
}
 
func vpnLoadHandler(error: (Error?)) -> Void {
        let tkcs = MyKeychainService()
        let deviceService = DeviceService.shared
        var protcol: NEVPNProtocol
        let ikev2 = NEVPNProtocolIKEv2()
        ikev2.useExtendedAuthentication = true
        ikev2.localIdentifier = "My VPN"
        ikev2.remoteIdentifier = EnvFabric.shared.getUrlForCode(DeviceService.shared.getRetrievedServerCode())
        ikev2.authenticationMethod = NEVPNIKEAuthenticationMethod.none
        ikev2.deadPeerDetectionRate = NEVPNIKEv2DeadPeerDetectionRate.medium
        ikev2.ikeSecurityAssociationParameters.encryptionAlgorithm = .algorithmAES256GCM
        ikev2.ikeSecurityAssociationParameters.integrityAlgorithm = .SHA256
        ikev2.ikeSecurityAssociationParameters.diffieHellmanGroup = .group19
        ikev2.ikeSecurityAssociationParameters.lifetimeMinutes = 1440
        ikev2.childSecurityAssociationParameters.encryptionAlgorithm = .algorithmAES256GCM
        ikev2.childSecurityAssociationParameters.integrityAlgorithm = .SHA256
        ikev2.childSecurityAssociationParameters.diffieHellmanGroup = .group19
        ikev2.childSecurityAssociationParameters.lifetimeMinutes = 1440
        ikev2.disableMOBIKE = false
        protcol = ikev2
        protcol.disconnectOnSleep = false
        protcol.serverAddress = EnvFabric.shared.getUrlForCode(DeviceService.shared.getRetrievedServerCode())
        self.vpnManager.localizedDescription = DeviceService.shared.getRetrievedServerName()
        protcol.username = deviceService.getDeviceId()
        tkcs.save(key: "DEVICE_TOKEN", value: deviceService.getDeviceToken())
        protcol.passwordReference = tkcs.load(key: "DEVICE_TOKEN")
        self.vpnManager.protocolConfiguration = protcol
        self.vpnManager.onDemandRules = nil
        self.vpnManager.isOnDemandEnabled = false
        self.vpnManager.isEnabled = true   
        self.vpnManager.saveToPreferences(completionHandler: vpnSaveHandler)
}
 
func vpnSaveHandler(error: (Error?)) -> Void {
        if let error = error {
            if ((error as NSError).code == 4) {
                return
            }
        } else {
            do {
                    try self.vpnManager.connection.startVPNTunnel()
                }
            } catch let error {
                SwiftyBeaver.warning("Error starting VPN Connection", error.localizedDescription)
            }
        }
    }