Strongswan with letsencrypt certificates (IKEv2-EAP)

Question

I am configuring Strongswan server for VPN clients to access internal network (EAP-IKEv2). I set it up successfully using self-signed server certificates and it works for clients using Mac OS X, Windows 7 and Windows 10 after adding ca.crt to the clients' Root CA's as trusted.

Now I would like to switch to Letsencrypt certificates which should be trusted without any additional client-side configuration and unfortunately I cannot get it working for some reason.

Server: Strongswan version 5.6.2 on Ubuntu 18.04. Client: Mac OS X 10.14.2 / Ubuntu 18.04 / Windows 7 / Windows 10

Mac OS X VPN error I receive is: The VPN server did not respond. Just for comparison with self-signed certificates without adding ca.crt to Mac OS I would receive User Authentication failed.

When I copy server.crt to the client it says This certificate is valid.

I also tried to set IP Security (IPsec) for DST ROOT CA X3 certificate to Always trust and any other Letsencrypt related CA certificates in Mac OS but that didn't help.

I also tried to enforce Always Trust for server.crt cert but still no luck.

I tested all of the mentioned OSes (Linux using strongswan network-manager applet) and it did not work.

Because I could not get any reasonable debug logs from Mac OS and Windows I set up Strongswan client on other server using Ubuntu without network-manager applet. It started working after copying DST_Root_CA_X3.pem certificate from /etc/ssl/certs to /etc/ipsec.d/cacerts on the client.

I have 3 questions:

How to get any debug logging from Mac OS native VPN client?
Does Strongswan VPN even work with Letsencrypt certificates? What could be the problem here?
Do you recommend any alternative that might be working? Racoon, Openswan? Let me leave OpenVPN as plan B.

Below you can find all the details.

Thank you for helping me out. I would appreciate any input.

Server

$ certbot certonly --rsa-key-size 2048 --standalone --agree-tos --no-eff-email --email [email protected] -d vpn.company.com

$ cp /etc/letsencrypt/live/vpn.company.com/fullchain.pem /etc/ipsec.d/certs/server.crt
$ cp /etc/letsencrypt/live/vpn.company.com/privkey.pem /etc/ipsec.d/private/server.key

I know that strongswan reads only first certificate found in server.crt. Still it does not work with second chain cert removed. It also does not work if I try to add chain.pem to /etc/ipsec.d/cacerts or any other CA certs from /etc/ssl/certs which is understood because CAcerts on server does not have impact on client auth.

I also tested converting certificates to DER and PEM formats.

PKI verify

$ ipsec pki --verify --in /etc/ipsec.d/certs/server.crt
no issuer certificate found for "CN=vpn.company.com"
  issuer is "C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3"
  using trusted certificate "CN=vpn.company.com"
certificate trusted, lifetimes valid

Certificate details

$ openssl x509 -in certs/server.crt -noout -text
Certificate:
    Data:
        Version: 3 (0x2)
        Serial Number:
            03:50:51:[...]
    Signature Algorithm: sha256WithRSAEncryption
        Issuer: C = US, O = Let's Encrypt, CN = Let's Encrypt Authority X3
        Validity
            Not Before: Mar  1 13:40:42 2019 GMT
            Not After : May 30 13:40:42 2019 GMT
        Subject: CN = vpn.company.com
        Subject Public Key Info:
            Public Key Algorithm: rsaEncryption
                Public-Key: (2048 bit)
                Modulus:
                    00:e3:a8:ea:8e:[...]
                Exponent: 65537 (0x10001)
        X509v3 extensions:
            X509v3 Key Usage: critical
                Digital Signature, Key Encipherment
            X509v3 Extended Key Usage:
                TLS Web Server Authentication, TLS Web Client Authentication
            X509v3 Basic Constraints: critical
                CA:FALSE
            X509v3 Subject Key Identifier:
                EC:6A:[...]
            X509v3 Authority Key Identifier:
                keyid:A8:4A:6A:[...]

            Authority Information Access:
                OCSP - URI:http://ocsp.int-x3.letsencrypt.org
                CA Issuers - URI:http://cert.int-x3.letsencrypt.org/

            X509v3 Subject Alternative Name:
                DNS:vpn.company.com
            X509v3 Certificate Policies:
                Policy: 2.23.140.1.2.1
                Policy: 1.3.6.1.4.1.44947.1.1.1
                  CPS: http://cps.letsencrypt.org

            CT Precertificate SCTs:
                Signed Certificate Timestamp:
                    Version   : v1 (0x0)
                    Log ID    : 74:7E:DA:[...]
                    Timestamp : Mar  1 14:40:42.419 2019 GMT
                    Extensions: none
                    Signature : ecdsa-with-SHA256
                                30:45:02:[...]
                Signed Certificate Timestamp:
                    Version   : v1 (0x0)
                    Log ID    : 29:3C:51:[...]
                    Timestamp : Mar  1 14:40:42.499 2019 GMT
                    Extensions: none
                    Signature : ecdsa-with-SHA256
                                30:46:02:[...]
    Signature Algorithm: sha256WithRSAEncryption
         8e:da:a3:[...]

ipsec.conf


config setup
  charondebug="dmn 1, mgr 1, ike 1, chd 1, job 1, cfg 1, knl 1, net 1, asn 1, enc 1, lib 1, esp 1, tls 1, tnc 1, imc 1, imv 1, pts 1"
    uniqueids=no

conn ikev2-vpn
    auto=add
    compress=no
    type=tunnel
    keyexchange=ikev2
    ike=aes256-sha1-modp1024
    esp=aes256-sha1
    fragmentation=no
    forceencaps=yes
    dpdaction=clear
    dpddelay=300s
    rekey=no
    left=%any
    [email protected]
  leftauth=pubkey
    leftcert=server.crt
    leftsendcert=always
    leftsubnet=0.0.0.0/0
  leftfirewall=yes
    right=%any
    rightid=%any
    rightauth=eap-mschapv2
    rightsourceip=10.255.255.0/24
    rightdns=1.1.1.1
    rightsendcert=never
    eap_identity=%identity

ipsec.secrets

vpn.company.com : RSA server.key
user %any% : EAP "user_password"

Strongswan server log

ipsec[11918]: Starting strongSwan 5.6.2 IPsec [starter]...
ipsec_starter[11918]: Starting strongSwan 5.6.2 IPsec [starter]...
charon[11943]: 00[DMN] Starting IKE charon daemon (strongSwan 5.6.2, Linux 4.15.0-45-generic, x86_64)
charon[11943]: 00[CFG] PKCS11 module '<name>' lacks library path
charon[11943]: 00[CFG] disabling load-tester plugin, not configured
charon[11943]: 00[LIB] plugin 'load-tester': failed to load - load_tester_plugin_create returned NULL
charon[11943]: 00[NET] could not open socket: Address family not supported by protocol
charon[11943]: 00[NET] could not open IPv6 socket, IPv6 disabled
charon[11943]: 00[KNL] received netlink error: Address family not supported by protocol (97)
charon[11943]: 00[KNL] unable to create IPv6 routing table rule
charon[11943]: 00[CFG] dnscert plugin is disabled
charon[11943]: 00[CFG] ipseckey plugin is disabled
charon[11943]: 00[CFG] attr-sql plugin: database URI not set
charon[11943]: 00[CFG] loading ca certificates from '/etc/ipsec.d/cacerts'
charon[11943]: 00[CFG] loading aa certificates from '/etc/ipsec.d/aacerts'
charon[11943]: 00[CFG] loading ocsp signer certificates from '/etc/ipsec.d/ocspcerts'
charon[11943]: 00[CFG] loading attribute certificates from '/etc/ipsec.d/acerts'
charon[11943]: 00[CFG] loading crls from '/etc/ipsec.d/crls'
charon[11943]: 00[CFG] loading secrets from '/etc/ipsec.secrets'
charon[11943]: 00[CFG]   loaded RSA private key from '/etc/ipsec.d/private/server.key'
charon[11943]: 00[CFG]   loaded EAP secret for USERNAME_HERE %any%
charon[11943]: 00[CFG] sql plugin: database URI not set
charon[11943]: 00[CFG] opening triplet file /etc/ipsec.d/triplets.dat failed: No such file or directory
charon[11943]: 00[CFG] eap-simaka-sql database URI missing
charon[11943]: 00[CFG] loaded 0 RADIUS server configurations
charon[11943]: 00[CFG] HA config misses local/remote address
charon[11943]: 00[CFG] no threshold configured for systime-fix, disabled
charon[11943]: 00[CFG] coupling file path unspecified
charon[11943]: 00[LIB] loaded plugins: charon test-vectors unbound ldap pkcs11 tpm aesni aes rc2 sha2 sha1 md4 md5 mgf1 rdrand random nonce x509 revocation constraints acert pubkey pkcs1 pkcs7 pkcs8 pkcs12 pgp dnskey sshkey dnscert ipseckey pem openssl gcrypt af-alg fips-prf gmp curve25519 agent chapoly xcbc cmac hmac ctr ccm gcm ntru bliss curl soup mysql sqlite attr kernel-netlink resolve socket-default connmark farp stroke updown eap-identity eap-sim eap-sim-pcsc eap-aka eap-aka-3gpp2 eap-simaka-pseudonym eap-simaka-reauth eap-md5 eap-gtc eap-mschapv2 eap-dynamic eap-radius eap-tls eap-ttls eap-peap eap-tnc xauth-generic xauth-eap xauth-pam xauth-noauth tnc-tnccs tnccs-20 tnccs-11 tnccs-dynamic dhcp whitelist lookip error-notify certexpire led radattr addrblock unity counters
charon[11943]: 00[LIB] dropped capabilities, running as uid 0, gid 0
charon[11943]: 00[JOB] spawning 16 worker threads
ipsec[11918]: charon (11943) started after 40 ms
ipsec_starter[11918]: charon (11943) started after 40 ms
charon[11943]: 06[CFG] received stroke: add connection 'ikev2-vpn'
charon[11943]: 06[CFG] adding virtual IP address pool 10.255.255.0/24
charon[11943]: 06[CFG]   loaded certificate "CN=vpn.company.com" from 'server.crt'
charon[11943]: 06[CFG] added configuration 'ikev2-vpn'
charon[11943]: 08[NET] received packet: from CLIENT_IP_HERE[44709] to SERVER_IP_HERE[500] (604 bytes)
charon[11943]: 08[ENC] parsed IKE_SA_INIT request 0 [ SA KE No N(REDIR_SUP) N(NATD_S_IP) N(NATD_D_IP) N(FRAG_SUP) ]
charon[11943]: 08[IKE] CLIENT_IP_HERE is initiating an IKE_SA
charon[11943]: 08[IKE] CLIENT_IP_HERE is initiating an IKE_SA
charon[11943]: 08[IKE] remote host is behind NAT
charon[11943]: 08[ENC] generating IKE_SA_INIT response 0 [ SA KE No N(NATD_S_IP) N(NATD_D_IP) N(MULT_AUTH) ]
charon[11943]: 08[NET] sending packet: from SERVER_IP_HERE[500] to CLIENT_IP_HERE[44709] (440 bytes)
charon[11943]: 09[NET] received packet: from CLIENT_IP_HERE[44710] to SERVER_IP_HERE[4500] (512 bytes)
charon[11943]: 09[ENC] unknown attribute type (25)
charon[11943]: 09[ENC] parsed IKE_AUTH request 1 [ IDi N(INIT_CONTACT) N(MOBIKE_SUP) IDr CPRQ(ADDR DHCP DNS MASK ADDR6 DHCP6 DNS6 (25)) N(ESP_TFC_PAD_N) N(NON_FIRST_FRAG) SA TSi TSr ]
charon[11943]: 09[CFG] looking for peer configs matching SERVER_IP_HERE[vpn.company.com]...CLIENT_IP_HERE[CLIENT_LOCAL_IP_HERE]
charon[11943]: 09[CFG] selected peer config 'ikev2-vpn'
charon[11943]: 09[IKE] initiating EAP_IDENTITY method (id 0x00)
charon[11943]: 09[IKE] received ESP_TFC_PADDING_NOT_SUPPORTED, not using ESPv3 TFC padding
charon[11943]: 09[IKE] peer supports MOBIKE
charon[11943]: 09[IKE] authentication of 'vpn.company.com' (myself) with RSA signature successful
charon[11943]: 09[IKE] sending end entity cert "CN=vpn.company.com"
charon[11943]: 09[ENC] generating IKE_AUTH response 1 [ IDr CERT AUTH EAP/REQ/ID ]
charon[11943]: 09[NET] sending packet: from SERVER_IP_HERE[4500] to CLIENT_IP_HERE[44710] (1744 bytes)
charon[11943]: 10[NET] received packet: from CLIENT_IP_HERE[44710] to SERVER_IP_HERE[4500] (512 bytes)
charon[11943]: 10[ENC] unknown attribute type (25)
ipsec[11918]: 00[DMN] Starting IKE charon daemon (strongSwan 5.6.2, Linux 4.15.0-45-generic, x86_64)
ipsec[11918]: 00[CFG] PKCS11 module '<name>' lacks library path
ipsec[11918]: 00[CFG] disabling load-tester plugin, not configured
ipsec[11918]: 00[LIB] plugin 'load-tester': failed to load - load_tester_plugin_create returned NULL
ipsec[11918]: 00[NET] could not open socket: Address family not supported by protocol
ipsec[11918]: 00[NET] could not open IPv6 socket, IPv6 disabled
ipsec[11918]: 00[KNL] received netlink error: Address family not supported by protocol (97)
ipsec[11918]: 00[KNL] unable to create IPv6 routing table rule
ipsec[11918]: 00[CFG] dnscert plugin is disabled
ipsec[11918]: 00[CFG] ipseckey plugin is disabled
ipsec[11918]: 00[CFG] attr-sql plugin: database URI not set
ipsec[11918]: 00[CFG] loading ca certificates from '/etc/ipsec.d/cacerts'
ipsec[11918]: 00[CFG] loading aa certificates from '/etc/ipsec.d/aacerts'
ipsec[11918]: 00[CFG] loading ocsp signer certificates from '/etc/ipsec.d/ocspcerts'
ipsec[11918]: 00[CFG] loading attribute certificates from '/etc/ipsec.d/acerts'
ipsec[11918]: 00[CFG] loading crls from '/etc/ipsec.d/crls'
ipsec[11918]: 00[CFG] loading secrets from '/etc/ipsec.secrets'
ipsec[11918]: 00[CFG]   loaded RSA private key from '/etc/ipsec.d/private/server.key'
ipsec[11918]: 00[CFG]   loaded EAP secret for USERNAME_HERE %any%
ipsec[11918]: 00[CFG] sql plugin: database URI not set
ipsec[11918]: 00[CFG] opening triplet file /etc/ipsec.d/triplets.dat failed: No such file or directory
ipsec[11918]: 00[CFG] eap-simaka-sql database URI missing
ipsec[11918]: 00[CFG] loaded 0 RADIUS server configurations
ipsec[11918]: 00[CFG] HA config misses local/remote address
ipsec[11918]: 00[CFG] no threshold configured for systime-fix, disabled
ipsec[11918]: 00[CFG] coupling file path unspecified
charon[11943]: 10[ENC] parsed IKE_AUTH request 1 [ IDi N(INIT_CONTACT) N(MOBIKE_SUP) IDr CPRQ(ADDR DHCP DNS MASK ADDR6 DHCP6 DNS6 (25)) N(ESP_TFC_PAD_N) N(NON_FIRST_FRAG) SA TSi TSr ]
ipsec[11918]: 00[LIB] loaded plugins: charon test-vectors unbound ldap pkcs11 tpm aesni aes rc2 sha2 sha1 md4 md5 mgf1 rdrand random nonce x509 revocation constraints acert pubkey pkcs1 pkcs7 pkcs8 pkcs12 pgp dnskey sshkey dnscert ipseckey pem openssl gcrypt af-alg fips-prf gmp curve25519 agent chapoly xcbc cmac hmac ctr ccm gcm ntru bliss curl soup mysql sqlite attr kernel-netlink resolve socket-default connmark farp stroke updown eap-identity eap-sim eap-sim-pcsc eap-aka eap-aka-3gpp2 eap-simaka-pseudonym eap-simaka-reauth eap-md5 eap-gtc eap-mschapv2 eap-dynamic eap-radius eap-tls eap-ttls eap-peap eap-tnc xauth-generic xauth-eap xauth-pam xauth-noauth tnc-tnccs tnccs-20 tnccs-11 tnccs-dynamic dhcp whitelist lookip error-notify certexpire led radattr addrblock unity counters
ipsec[11918]: 00[LIB] dropped capabilities, running as uid 0, gid 0
ipsec[11918]: 00[JOB] spawning 16 worker threads
ipsec[11918]: 06[CFG] received stroke: add connection 'ikev2-vpn'
ipsec[11918]: 06[CFG] adding virtual IP address pool 10.255.255.0/24
ipsec[11918]: 06[CFG]   loaded certificate "CN=vpn.company.com" from 'server.crt'
ipsec[11918]: 06[CFG] added configuration 'ikev2-vpn'
ipsec[11918]: 08[NET] received packet: from CLIENT_IP_HERE[44709] to SERVER_IP_HERE[500] (604 bytes)
ipsec[11918]: 08[ENC] parsed IKE_SA_INIT request 0 [ SA KE No N(REDIR_SUP) N(NATD_S_IP) N(NATD_D_IP) N(FRAG_SUP) ]
ipsec[11918]: 08[IKE] CLIENT_IP_HERE is initiating an IKE_SA
ipsec[11918]: 08[IKE] remote host is behind NAT
ipsec[11918]: 08[ENC] generating IKE_SA_INIT response 0 [ SA KE No N(NATD_S_IP) N(NATD_D_IP) N(MULT_AUTH) ]
ipsec[11918]: 08[NET] sending packet: from SERVER_IP_HERE[500] to CLIENT_IP_HERE[44709] (440 bytes)
ipsec[11918]: 09[NET] received packet: from CLIENT_IP_HERE[44710] to SERVER_IP_HERE[4500] (512 bytes)
ipsec[11918]: 09[ENC] unknown attribute type (25)
ipsec[11918]: 09[ENC] parsed IKE_AUTH request 1 [ IDi N(INIT_CONTACT) N(MOBIKE_SUP) IDr CPRQ(ADDR DHCP DNS MASK ADDR6 DHCP6 DNS6 (25)) N(ESP_TFC_PAD_N) N(NON_FIRST_FRAG) SA TSi TSr ]
ipsec[11918]: 09[CFG] looking for peer configs matching SERVER_IP_HERE[vpn.company.com]...CLIENT_IP_HERE[CLIENT_LOCAL_IP_HERE]
ipsec[11918]: 09[CFG] selected peer config 'ikev2-vpn'
ipsec[11918]: 09[IKE] initiating EAP_IDENTITY method (id 0x00)
charon[11943]: 10[IKE] received retransmit of request with ID 1, retransmitting response
ipsec[11918]: 09[IKE] received ESP_TFC_PADDING_NOT_SUPPORTED, not using ESPv3 TFC padding
ipsec[11918]: 09[IKE] peer supports MOBIKE
ipsec[11918]: 09[IKE] authentication of 'vpn.company.com' (myself) with RSA signature successful
ipsec[11918]: 09[IKE] sending end entity cert "CN=vpn.company.com"
ipsec[11918]: 09[ENC] generating IKE_AUTH response 1 [ IDr CERT AUTH EAP/REQ/ID ]
ipsec[11918]: 09[NET] sending packet: from SERVER_IP_HERE[4500] to CLIENT_IP_HERE[44710] (1744 bytes)
ipsec[11918]: 10[NET] received packet: from CLIENT_IP_HERE[44710] to SERVER_IP_HERE[4500] (512 bytes)
ipsec[11918]: 10[ENC] unknown attribute type (25)
charon[11943]: 10[NET] sending packet: from SERVER_IP_HERE[4500] to CLIENT_IP_HERE[44710] (1744 bytes)
charon[11943]: 11[NET] received packet: from CLIENT_IP_HERE[44710] to SERVER_IP_HERE[4500] (512 bytes)
charon[11943]: 11[ENC] unknown attribute type (25)
charon[11943]: 11[ENC] parsed IKE_AUTH request 1 [ IDi N(INIT_CONTACT) N(MOBIKE_SUP) IDr CPRQ(ADDR DHCP DNS MASK ADDR6 DHCP6 DNS6 (25)) N(ESP_TFC_PAD_N) N(NON_FIRST_FRAG) SA TSi TSr ]
charon[11943]: 11[IKE] received retransmit of request with ID 1, retransmitting response
charon[11943]: 11[NET] sending packet: from SERVER_IP_HERE[4500] to CLIENT_IP_HERE[44710] (1744 bytes)
charon[11943]: 12[NET] received packet: from CLIENT_IP_HERE[44710] to SERVER_IP_HERE[4500] (512 bytes)
charon[11943]: 12[ENC] unknown attribute type (25)
charon[11943]: 12[ENC] parsed IKE_AUTH request 1 [ IDi N(INIT_CONTACT) N(MOBIKE_SUP) IDr CPRQ(ADDR DHCP DNS MASK ADDR6 DHCP6 DNS6 (25)) N(ESP_TFC_PAD_N) N(NON_FIRST_FRAG) SA TSi TSr ]
charon[11943]: 12[IKE] received retransmit of request with ID 1, retransmitting response
charon[11943]: 12[NET] sending packet: from SERVER_IP_HERE[4500] to CLIENT_IP_HERE[44710] (1744 bytes)

EDIT: Mac OS started working after enabling fragmentation. Unfortunately Windows 10 ends up with error. Server log while connecting from Windows 10:

charon[12236]: 06[ENC] generating IKE_SA_INIT response 0 [ SA KE No N(NATD_S_IP) N(NATD_D_IP) N(FRAG_SUP) N(MULT_AUTH) ]
charon[12236]: 06[NET] sending packet: from SERVER_IP_HERE[500] to CLIENT_IP_HERE[44742] (320 bytes)
charon[12236]: 09[NET] received packet: from CLIENT_IP_HERE[44743] to SERVER_IP_HERE[4500] (576 bytes)
charon[12236]: 09[ENC] parsed IKE_AUTH request 1 [ EF(1/2) ]
charon[12236]: 09[ENC] received fragment #1 of 2, waiting for complete IKE message
charon[12236]: 07[NET] received packet: from CLIENT_IP_HERE[44743] to SERVER_IP_HERE[4500] (368 bytes)
charon[12236]: 07[ENC] parsed IKE_AUTH request 1 [ EF(2/2) ]
charon[12236]: 07[ENC] received fragment #2 of 2, reassembling fragmented IKE message
charon[12236]: 07[ENC] parsed IKE_AUTH request 1 [ IDi CERTREQ CPRQ(ADDR DNS NBNS SRV) SA TSi TSr ]
charon[12236]: 07[IKE] received 27 cert requests for an unknown ca
charon[12236]: 07[CFG] looking for peer configs matching SERVER_IP_HERE[%any]...CLIENT_IP_HERE[CLIENT_LOCAL_IP_HERE]
charon[12236]: 07[CFG] selected peer config 'ikev2-vpn'
charon[12236]: 07[IKE] initiating EAP_IDENTITY method (id 0x00)
charon[12236]: 07[IKE] authentication of 'vpn.autouncle.com' (myself) with RSA signature successful
charon[12236]: 07[IKE] sending end entity cert "CN=vpn.autouncle.com"
charon[12236]: 07[ENC] generating IKE_AUTH response 1 [ IDr CERT AUTH EAP/REQ/ID ]
charon[12236]: 07[ENC] splitting IKE message with length of 1740 bytes into 2 fragments
charon[12236]: 07[ENC] generating IKE_AUTH response 1 [ EF(1/2) ]
charon[12236]: 07[ENC] generating IKE_AUTH response 1 [ EF(2/2) ]
charon[12236]: 07[NET] sending packet: from SERVER_IP_HERE[4500] to CLIENT_IP_HERE[44743] (1248 bytes)
charon[12236]: 07[NET] sending packet: from SERVER_IP_HERE[4500] to CLIENT_IP_HERE[44743] (560 bytes)

Now I would like to switch to Letsencrypt certificates which should be trusted without any additional client-side configuration Why do you assume that? What is the question now? It does work with a strongswan client, so you know LE certs work with IPSec. Do you want to know why it doesn't work using MacOS and Windows? — Lenniey, Mar 4, 2019 at 16:30
Hi @Lenniey. Thank you for your comment. Regarding your concerns: 1. I assume that Letsencrypt certificates are trusted by default because Root CA for Letsencrypt is already added to the system. Any certificate signed by Root CA which is trusted will be trusted too. 2. My question is why my configuration isn't working with Mac OS and Windows and/or how to debug it to get my answer. Note that strongswan client works only if I add certain CA certs to the strongswan configuration. It does not use system-wide cacerts for some reason. — ahes, Mar 4, 2019 at 16:48
Windows 10 error is "Can't connect VPN_NAME_HERE. The network connection between your computer and the VPN server could not be established because the remote server is not responsing. This could be because one of the network devices (e.g, firewalls, NAT, routers, etc) between your computer and the remote server is not configured to allow VPN connections. Please contact your Administrator of your service provider to determine which device may be causing the problem." — ahes, Mar 5, 2019 at 9:07

ecdsa · Accepted Answer · 2019-03-05 07:52:36Z

3

Could be an IP fragmentation issue. Due to the certificate the IKE_AUTH response is larger than the MTU (1744 bytes):

charon[11943]: 09[ENC] generating IKE_AUTH response 1 [ IDr CERT AUTH EAP/REQ/ID ]
charon[11943]: 09[NET] sending packet: from SERVER_IP_HERE[4500] to CLIENT_IP_HERE[44710] (1744 bytes)

So this gets fragmented into multiple IP fragments. Some routers drop these and the client might not receive the complete packet.

Fortunately, the client supports IKEv2 fragmentation (FRAG_SUP notify):

charon[11943]: 08[NET] received packet: from CLIENT_IP_HERE[44709] to SERVER_IP_HERE[500] (604 bytes)
charon[11943]: 08[ENC] parsed IKE_SA_INIT request 0 [ SA KE No N(REDIR_SUP) N(NATD_S_IP) N(NATD_D_IP) N(FRAG_SUP) ]

So please try enabling IKEv2 fragmentation on the server, that is, enable the fragmentation option, or remove it as it's enabled by default.

answered Mar 5, 2019 at 7:52

ecdsa

4,10316 silver badges29 bronze badges

That was excellent analysis! It started working in Mac OS X. Thank you! Windows 10 started to show different error: "IKE authentication credentials are unacceptable".
– ahes
Mar 5, 2019 at 9:13
Server log while connecting from Windows 10: (added at the end of my original question)
– ahes
Mar 5, 2019 at 9:22
1

What did you configure as hostname/IP address on the Windows client? Does the Windows client actually have the required CA certificate(s) installed. Maybe it only has the root CA (DST Root CA X3) installed, then make sure to install the intermediate CA certificate (Let's Encrypt Authority X3) on the server so it is sent along with the server certificate.
– ecdsa
Mar 5, 2019 at 13:45
Man, you are truly amazing. That actually worked. It was a matter of adding chain.pem from letsencrypt to /etc/ipsec.d/cacerts on the stronsgwan server. Sweet! Thank you for helping. I hope it helps other people too.
– ahes
Mar 5, 2019 at 16:10
1

Unfortunately, it might not always be possible to fix this. One option is to try to reduce the size of the server certificate e.g. by using ECDSA instead of RSA. And sending the intermediate CA certificates is probably not possible (i.e. you'd have to install them on the clients) as the Let's Encrypt root and intermediate CA are not ECDSA yet. However, due to the certificate requests the Windows client sends, even the request might exceed the MTU in some situations. So clients without support for IKEv2 fragmentation can definitely be a problem (as are ISPs that drop IP fragments, of course).
– ecdsa
Mar 5, 2019 at 18:11

| Show 1 more comment

fds · Accepted Answer · 2019-03-12 02:55:41Z

You do want Letsencrypt's intermediate cert installed in addition to your leaf cert! Put that chain.pem back in ipsec.d/cacerts

What you want to see in your strongswan logs is it sending them both:

charon: 07[IKE] sending end entity cert "CN=vpn.example.com" charon: 07[IKE] sending issuer cert "C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3"

On Windows, I highly recommend creating and configuring your VPN entries through PowerShell, which allows you to tweak parameters well beyond what the GUI allows. For example,

Add-VpnConnection -Name "My VPN" -ServerAddress vpn.example.com -TunnelType IKEv2 -AuthenticationMethod EAP -EncryptionLevel Maximum -RememberCredential:$True -SplitTunnel:$False -PassThru
Set-VpnConnectionIPsecConfiguration -ConnectionName "My VPN" -AuthenticationTransformConstants SHA256128 -CipherTransformConstants AES256 -EncryptionMethod AES256 -IntegrityCheckMethod SHA256 -DHGroup Group14 -PfsGroup Pfs2048 -PassThru -Force

Documentation here: Add-VpnConnection Set-VpnConnectionIPsecConfiguration

Check in the Windows Event Log if something is still amiss. It definitely can work fine with a strongswan/letsencrypt cert setup.

copying sudo cp /etc/letsencrypt/live/domain.tld/chain.pem /etc/ipsec.d/cacerts solved the issue for me! Had the same issue: Self signed certificated worked but the one from letsencrypt didn't (source). — urbanSoft, Sep 30, 2021 at 20:08

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Strongswan with letsencrypt certificates (IKEv2-EAP)

Server

PKI verify

Certificate details

ipsec.conf

ipsec.secrets

Strongswan server log

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Strongswan with letsencrypt certificates (IKEv2-EAP)

Server

PKI verify

Certificate details

ipsec.conf

ipsec.secrets

Strongswan server log

2 Answers 2

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