Manual:Interface/SSTP

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Applies to RouterOS: v5, v6+


Summary

Standards: SSTP specification
Package: ppp


Secure Socket Tunneling Protocol (SSTP) transports a PPP tunnel over a TLS channel. The use of TLS over TCP port 443 allows SSTP to pass through virtually all firewalls and proxy servers.


SSTP connection mechanism

  • TCP connection is established from client to server (by default on port 443);
  • SSL validates server certificate. If certificate is valid connection is established otherwise connection is torn down. (But see note below)
  • The client sends SSTP control packets within the HTTPS session which establishes the SSTP state machine on both sides.
  • PPP negotiation over SSTP. Client authenticates to the server and binds IP addresses to SSTP interface
  • SSTP tunnel is now established and packet encapsulation can begin.

Note: Starting from v5.0beta2 SSTP does not require certificates to operate and can use any available authentication type. This feature will work only between two MikroTik routers, as it is not in accordance with Microsoft standard. Otherwise to establish secure tunnels mschap authentication and client/server certificates from the same chain should be used. Read more>>


Currently, SSTP clients exist in Windows Vista, Windows 7, Windows 8, Linux and RouterOS.

Note: While connecting to SSTP server, Windows does CRL (certificate revocation list) checking on server certificate which can introduce a significant delay to complete a connection or even prevent the user from accessing the SSTP server at all if Windows is unable to access CRL distribution point! Custom generated CA which does not include CRLs can be used to minimize connection delays and certificate costs (signed certificates with known CA usually are not for free), but this custom CA must be imported into each Windows client individually. It is possible to disable CRL check in Windows registry, but it is supported only by Windows Server 2008 and Windows 7 http://support.microsoft.com/kb/947054


Certificates

Note: Starting from RouterOS v6rc10 SSTP respects CRL


To set up a secure SSTP tunnel, certificates are required. On the server, authentication is done only by username and password, but on the client - the server is authenticated using a server certificate. It is also used by the client to cryptographically bind SSL and PPP authentication, meaning - the clients sends a special value over SSTP connection to the server, this value is derived from the key data that is generated during PPP authentication and server certificate, this allows the server to check if both channels are secure.

If SSTP clients are Windows PCs then only way to set up a secure SSTP tunnel when using self-signed certificate is by importing the "server" certificate on SSTP server and on the Windows PC adding CA certificate in trusted root.

Note: If your server certificate is issued by a CA which is already known by Windows, then the Windows client will work without any additional certificates.


Warning: RSA Key length must be at least 472 bits if certificate is used by SSTP. Shorter keys are considered as security threats.


Similar configuration on RouterOS client would be to import the CA certificate and enabling verify-server-certificate option. In this scenario Man-in-the-Middle attacks are not possible.

Between two Mikrotik routers it is also possible to set up an insecure tunnel by not using certificates at all. In this case data going through SSTP tunnel is using anonymous DH and Man-in-the-Middle attacks are easily accomplished. This scenario is not compatible with Windows clients.

It is also possible to make a secure SSTP tunnel by adding additional authorization with a client certificate. Configuration requirements are:

  • certificates on both server and client
  • verification options enabled on server and client

This scenario is also not possible with Windows clients, because there is no way to set up client certificate on Windows.

Certificate error messages

When ssl handshake fails, you will see one of the following certificate errors:

  • certificate is not yet valid - notBefore certificate date is after the current time.
  • certificate has expired - notAfter certificate expiry date is before the current time.
  • invalid certificate purpose - the supplied certificate cannot be used for the specified purpose.
  • self signed certificate in chain - the certificate chain could be built up using the untrusted certificates but the root could not be found locally.
  • unable to get issuer certificate locally - CA certificate is not imported locally.
  • server's IP address does not match certificate - server address verification is enabled, but address provided in certificate does not match server's address.

Hostname verification

Server certificate verification is enabled on SSTP client, additionally if IP addresses or DNS name found in certificate's subjectAltName or common-name then issuer CN will be compared to the real servers address. v5.7 adds new parameter verify-server-address-from-certificate to disable/enable hostname verification.

SSTP Client

Sub-menu: /interface sstp-client

Properties

Property Description
add-default-route (yes | no; Default: no) Whether to add SSTP remote address as a default route.
authentication (mschap2 | mschap1 | chap | pap; Default: mschap2, mschap1, chap, pap) Allowed authentication methods.
certificate (string | none; Default: none)
comment (string; Default: ) Descriptive name of an item
connect-to (IP:Port; Default: 0.0.0.0:443) Remote address and port of SSTP server.
default-route-distance (byte [0..255]; Default: 1) sets distance value applied to auto created default route, if add-default-route is also selected
dial-on-demand (yes | no; Default: no) connects to AC only when outbound traffic is generated. If selected, then route with gateway address from 10.112.112.0/24 network will be added while connection is not established.
disabled (yes | no; Default: yes) Whether interface is disabled or not. By default it is disabled.
http-proxy (IP:Port; Default: 0.0.0.0:443) Address and port of HTTP proxy server.
keepalive-timeout (integer | disabled; Default: 60) Sets keepalive timeout in seconds.
max-mru (integer; Default: 1500) Maximum Receive Unit. Max packet size that SSTP interface will be able to receive without packet fragmentation.
max-mtu (integer; Default: 1500) Maximum Transmission Unit. Max packet size that SSTP interface will be able to send without packet fragmentation.
mrru (disabled | integer; Default: disabled) Maximum packet size that can be received on the link. If a packet is bigger than tunnel MTU, it will be split into multiple packets, allowing full size IP or Ethernet packets to be sent over the tunnel. Read more >>
name (string; Default: ) Descriptive name of the interface.
password (string; Default: "") Password used for authentication.
pfs (yes | no; Default: no) Enables "Perfect Forward Secrecy" which will make sure that private encryption key is generated for each session. Must be enabled on both server and client to work.
profile (name; Default: default-encryption) Used PPP profile.
user (string; Default: ) User name used for authentication.
tls-version (any | only-1.2; Default: any) Specifies which TLS versions to allow
verify-server-certificate (yes | no; Default: no) If set to yes, then client checks whether certificate belongs to the same certificate chain as server's certificate. To make it work CA certificate must be imported.
verify-server-address-from-certificate (yes | no; Default: yes) If set to yes, server's IP address will be compared to one set in certificate. Read More >>

Quick example

This example demonstrates how to set up SSTP client with username "sstp-test", password "123" and server 10.1.101.1 

/interface sstp-client add user=sstp-test password=123 connect-to=10.1.101.1 disabled=no

/interface sstp-client print
Flags: X - disabled, R - running
 0  R name="sstp-out1" max-mtu=1500 max-mru=1500 mrru=disabled connect-to=10.1.101.1:443 
      http-proxy=0.0.0.0:443 certificate=none verify-server-certificate=no 
      verify-server-address-from-certificate=yes user="sstp-test" password="123" 
      profile=default keepalive-timeout=60 add-default-route=no dial-on-demand=no
      authentication=pap,chap,mschap1,mschap2

SSTP Server

Sub-menu: /interface sstp-server

This sub-menu shows interfaces for each connected SSTP client.

An interface is created for each tunnel established to the given server. There are two types of interfaces in SSTP server's configuration

  • Static interfaces are added administratively if there is a need to reference the particular interface name (in firewall rules or elsewhere) created for the particular user.
  • Dynamic interfaces are added to this list automatically whenever a user is connected and its username does not match any existing static entry (or in case the entry is active already, as there can not be two separate tunnel interfaces referenced by the same name).

Dynamic interfaces appear when a user connects and disappear once the user disconnects, so it is impossible to reference the tunnel created for that use in router configuration (for example, in firewall), so if you need a persistent rules for that user, create a static entry for him/her. Otherwise it is safe to use dynamic configuration.

Note: in both cases PPP users must be configured properly - static entries do not replace PPP configuration.



Server configuration

Sub-menu: /interface sstp-server server

Properties:

Property Description
authentication (pap | chap | mschap1 | mschap2; Default: pap,chap,mschap1,mschap2) Authentication methods that server will accept.
certificate (name | none; Default: none) Name of the certificate that SSTP server will use.
default-profile (name; Default: default)
enabled (yes | no; Default: no) Defines whether SSTP server is enabled or not.
force-aes (yes | no; Default: no) Force AES encryption (AES256 is supported). If enabled windows clients (supports only RC4) will be unable to connect.
keepalive-timeout (integer | disabled; Default: 60) If server during keepalive period does not receive any packet, it will send keepalive packets every second five times. If the server does not receives response from the client, then disconnect after 5 seconds. Logs will show 5x "LCP missed echo reply" messages and then disconnect.
max-mru (integer; Default: 1500) Maximum Receive Unit. Max packet size that SSTP interface will be able to receive without packet fragmentation.
max-mtu (integer; Default: 1500) Maximum Transmission Unit. Max packet size that SSTP interface will be able to send without packet fragmentation.
mrru (disabled | integer; Default: disabled) Maximum packet size that can be received on the link. If a packet is bigger than tunnel MTU, it will be split into multiple packets, allowing full size IP or Ethernet packets to be sent over the tunnel. Read more >>
pfs (yes | no; Default: no) Enables "Perfect Forward Secrecy" which will make sure that private encryption key is generated for each session. Must be enabled on both server and client to work.
port (integer; Default: 443) Port for SSTP service to listen on.
tls-version (any | only-1.2; Default: any) Specifies which TLS versions to allow
verify-client-certificate (yes | no; Default: no) If set to yes, then server checks whether client's certificate belongs to the same certificate chain. 


/interface sstp-server server set certificate=server
/interface sstp-server server set enabled=yes

/interface sstp-server server print 
                    enabled: no
                       port: 443
                    max-mtu: 1500
                    max-mru: 1500
                       mrru: disabled
          keepalive-timeout: 60
            default-profile: default
             authentication: pap,chap,mschap1,mschap2
                certificate: none
  verify-client-certificate: no
                  force-aes: no

Warning: It is very important that the date on the router is within the range of the certificate's date of expiration. To overcome any certificate verification problems, enable NTP date synchronization on both server and client.


Monitoring

Monitor command can be used to monitor status of the tunnel on both client and server. 

/interface sstp-server monitor 0
     status: "connected"
     uptime: 17m47s
  idle-time: 17m47s
       user: "sstp-test"
  caller-id: "10.1.101.18:43886"
        mtu: 1500

Read-only properties

Property Description
status () Current SSTP status. Value other than "connected" indicates that there are some problems estabising tunnel.
uptime (time) Elapsed time since tunnel was established.
idle-time (time) Elapsed time since last activity on the tunnel.
user (string) Username used to establish the tunnel.
mtu (integer) Negotiated and used MTU
caller-id (IP:ID)

Application Examples

Connecting Remote Client

The following example shows how to connect a computer to a remote office network over secure SSTP encrypted tunnel giving that computer an IP address from the same network as the remote office has (without the need for bridging over EoIP tunnels)


Consider following setup:

Office router is connected to internet through ether1. Workstations are connected to ether2. Laptop is connected to the internet and can reach Office router's public IP (in our example it is 192.168.80.1).


Before you begin to configure SSTP you need to create a server certificate and import it into the router (instructions here).

Now it is time to create a user: 

/ppp secret add name=Laptop service=sstp password=123 local-address=10.1.101.1 \
    remote-address=10.1.101.100

/ppp secret print detail
Flags: X - disabled
  0   name="Laptop" service=sstp caller-id="" password="123" profile=default
      local-address=10.1.101.1 remote-address=10.1.101.100 routes==""

Notice that SSTP local address is the same as the router's address on the local interface and the remote address is from the same range as the local network (10.1.101.0/24).

Next step is to enable SSTP server and SSTP client on the laptop: 

/interface sstp-server server set certificate=server
/interface sstp-server server set enabled=yes
/interface sstp-server server set authentication=mschap2

/interface sstp-server server print
                     enabled: yes
                        port: 443
                     max-mtu: 1500
                     max-mru: 1500
                        mrru: disabled
           keepalive-timeout: 60
             default-profile: default
                 certificate: server
  verify-client-certificate: no
              authentication: mschap2

Notice that authentication is set to mschap. These are the only authentication options that are valid to establish a secure tunnel.


SSTP client from the laptop should connect to routers public IP which in our example is 192.168.80.1.

Please, consult the respective manual on how to set up a SSTP client with the software you are using. If you set up SSTP client on Windows and self-signed certificates are used, then CA certificate should be added to trusted root.

Note: Currently, SSTP is only fully supported on recent Windows OS releases such as Vista SP1, Windows 7, Windows 8, Windows 2008 etc. With other OS's such as Linux, results cannot be guaranteed.



To verify if SSTP client is connected 

/interface sstp-server print 
Flags: X - disabled, D - dynamic, R - running 
 #     NAME      USER         MTU        CLIENT-ADDRESS    UPTIME   ENCODING   
 0  DR <sstp-... Laptop     1500       10.1.101.18:43886 1h47s  

/interface sstp-server monitor 0
     status: "connected"
     uptime: 1h45s
  idle-time: 1h45s
       user: "Laptop"
  caller-id: "192.168.99.1:43886"
        mtu: 1500

At this point (when SSTP client is successfully connected) if you try to ping any workstation from the laptop, ping will time out, because Laptop is unable to get ARPs from workstations. Solution is to set up proxy-arp on local interface 

/interface ethernet set ether2 arp=proxy-arp

/interface ethernet print
Flags: X - disabled, R - running
  #    NAME                 MTU   MAC-ADDRESS         ARP
  0  R ether1              1500  00:30:4F:0B:7B:C1 enabled
  1  R ether2              1500  00:30:4F:06:62:12 proxy-arp

After proxy-arp is enabled client can successfully reach all workstations in the local network behind the router.

Site-to-Site SSTP

The following is an example of connecting two Intranets using SSTP tunnel over the Internet.

Consider following setup:

Office and Home routers are connected to internet through ether1, workstations and laptops are connected to ether2. In this example both local networks are routed through SSTP client, thus they are not in the same broadcast domain. To overcome this problem as with any other ppp tunnel, SSTP also supports BCP which allows it to bridge SSTP tunnel with a local interface.

First step is to create a user: 

/ppp secret add name=Home service=sstp password=123 local-address=172.16.1.1 \
  remote-address=172.16.1.2 routes="10.1.202.0/24 172.16.1.2 1"

/ppp secret print detail
Flags: X - disabled
  0   name="Home" service=sstp caller-id="" password="123" profile=default
      local-address=172.16.1.1 remote-address=172.16.1.2 routes=="10.1.202.0/24 172.16.1.2 1"

Notice that we set up SSTP to add a route whenever the client connects. If this option is not set, then you will need a static routing configuration on the server to route traffic between sites through the SSTP tunnel.

Now we need to upload and import CA and server/client certificates. Assuming that the files are already uploaded use following commands: 

/certificate import file-name=ca.crt
passphrase: 
/certificate import file-name=server.crt
passphrase: ****
/certificate import file-name=server.key
passphrase: ****

Edit names to something more meaningful: 

/certificate set 0 name=CA
/certificate set 1 name=server

/certificate print 
Flags: K - decrypted-private-key, Q - private-key, R - rsa, D - dsa 
 0  D name="CA" subject=C=LV,ST=RI,L=Riga,O=MT,CN=MT CA,emailAddress=xx@mt.lv 
      issuer=C=LV,ST=RI,L=Riga,O=MT,CN=MT CA,emailAddress=xx@mt.lv 
      serial-number="DF626FA846090BCC" email=xx@mt.lv invalid-before=jun/25/2008 07:23:50 
      invalid-after=jun/23/2018 07:23:50 ca=yes 

 1 KR name="server" subject=C=LV,ST=RI,L=Riga,O=MT,CN=server,emailAddress=xx@mt.lv 
      issuer=C=LV,ST=RI,L=Riga,O=MT,CN=MT CA,emailAddress=xx@mt.lv serial-number="01" 
      email=xx@mt.lv invalid-before=jun/25/2008 07:24:33 invalid-after=jun/23/2018 07:24:33 
      ca=yes

Do the same on client side, but instead of server's certificate import client's certificate.


Next step is to enable SSTP server on the office router: 

/interface sstp-server server set certificate=server
/interface sstp-server server set enabled=yes
/interface sstp-server server set verify-client-certificate=yes

/interface sstp-server server print
                     enabled: yes
                        port: 443
                     max-mtu: 1500
                     max-mru: 1500
                        mrru: disabled
           keepalive-timeout: 60
             default-profile: default
                 certificate: server
  verify-client-certificate: yes
              authentication: pap,chap,mschap1,mschap2

Now configure SSTP client on the Home router: 

/interface sstp-client add user=Home password=123 connect-to=192.168.80.1 disabled=no \
  certificate=client verify-server-certificate=yes

/interface sstp-client print
Flags: X - disabled, R - running
 0  R name="sstp-out1" max-mtu=1500 max-mru=1500 mrru=disabled connect-to=192.168.80.1:443 
       user="Home" password="123" proxy=0.0.0.0:443 profile=default certificate=client
       keepalive-timeout=60 add-default-route=no dial-on-demand=no 
       authentication=pap,chap,mschap1,mschap2 verify-server-certificate=yes

Now we need to add static route on Home router to reach local network behind Office router: 

/ip route add dst-address=10.1.101.0/24 gateway=sstp-out1

After tunnel is established you should be able to ping remote network.

Troubleshooting

After Windows 7 upgrade SSTP is unable to connect (windows error 631) ?
MS Patch KB2585542 changes cypher to RC4 which was not supported on RouterOS. Starting from RouterOS v5.13 RC4 is the preferred cipher and AES will be used only if peer does not advertise RC4.
I get following error when trying to connect Windows 7 client. Error 0x80070320 The oplock that was associated with this handle is now associated with a different handle.
Disable verify-client-certificate option on the server.
I get following error "Encryption negotiation rejected”.
Disable use-encryption option in ppp profile.

Read More


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