openvpn
Karl Denninger
karl at denninger.net
Thu Apr 25 20:43:30 UTC 2019
On 4/24/2019 00:42, Doug Hardie wrote:
>> On 23 April 2019, at 19:16, Karl Denninger <karl at denninger.net> wrote:
>>
>>
>> On 4/22/2019 19:53, Doug Hardie wrote:
>>> I am trying to setup an openvpn server on my home network. Home machines are all running FBSD 12.0 Release. openvpn was installed as a package. The results are quite confusing. Ping from an external device works correctly to all the home machines. I can use tcpdump to see the request packets arriving at the openvpn server, being sent to the recipient machine, the response packets being sent from the recipient machine to the openvpn server, and then sent to the external device. The external device shows that the response was received with a reasonable response time given that it is a cell phone.
>>>
>>> However, when I try to access a web page on any of the servers, I see the same set of packets via tcpdump. In addition if I run ktrace on the openvpn server, I see the encrypted packets from the client being received. The decrypted packets sent to the home server. The unencrypted response from the home server, and the encrypted response sent to the phone. However, the phone says that the server dropped the connection, or it shows a blank page.
>>>
>>> My first thought was that there was an encryption issue, but if that were the case, ping would not work. Checking the ping packets shows that they are encrypted between the phone and the openvpn server. Likewise a routing issue in the home network does not seem to be the problem for the same reason. All the info I have found on the web about vpn indicates that a ping test should be sufficient. But, in this case it is not.
>>>
>>> Any ideas on how to track down the problem, or fix it? Thanks,
>>>
>>> -- Doug
>> IMHO -- post your configuration file to the list....
>>
>> I use OpenVPN with ipfw's internal NAT and it works fine, but the config
>> file is a bit wonky and if you get it wrong you'll either have no DNS on
>> the client or packets won't get routed. Either way the connection comes
>> up but it doesn't work.
>>
> mail# more server.conf
> #################################################
> # Sample OpenVPN 2.0 config file for #
> # multi-client server. #
> # #
> # This file is for the server side #
> # of a many-clients <-> one-server #
> # OpenVPN configuration. #
> # #
> # Comments are preceded with '#' or ';' #
> #################################################
>
> # Which local IP address should OpenVPN
> # listen on? (optional)
> ;local a.b.c.d
>
> # Which TCP/UDP port should OpenVPN listen on?
> # If you want to run multiple OpenVPN instances
> # on the same machine, use a different port
> # number for each one. You will need to
> # open up this port on your firewall.
> port 1194
>
> # TCP or UDP server?
> ;proto tcp
> proto udp
>
> # "dev tun" will create a routed IP tunnel,
> # "dev tap" will create an ethernet tunnel.
> # Use "dev tap0" if you are ethernet bridging
> # and have precreated a tap0 virtual interface
> # and bridged it with your ethernet interface.
> # If you want to control access policies
> # over the VPN, you must create firewall
> # rules for the the TUN/TAP interface.
> # On non-Windows systems, you can give
> # an explicit unit number, such as tun0.
> # On Windows, use "dev-node" for this.
> # On most systems, the VPN will not function
> # unless you partially or fully disable
> # the firewall for the TUN/TAP interface.
> ;dev tap
> dev tun
>
> # SSL/TLS root certificate (ca), certificate
> # (cert), and private key (key). Each client
> # and the server must have their own cert and
> # key file. The server and all clients will
> # use the same ca file.
> #
> # See the "easy-rsa" directory for a series
> # of scripts for generating RSA certificates
> # and private keys. Remember to use
> # a unique Common Name for the server
> # and each of the client certificates.
> #
> # Any X509 key management system can be used.
> # OpenVPN can also use a PKCS #12 formatted key file
> # (see "pkcs12" directive in man page).
> ca ca.pem
> cert vpn_server.pem
> key vpn_server.key # This file should be kept secret
>
> # Diffie hellman parameters.
> # Generate your own with:
> # openssl dhparam -out dh2048.pem 2048
> dh dh2048.pem
>
> # Network topology
> # Should be subnet (addressing via IP)
> # unless Windows clients v2.0.9 and lower have to
> # be supported (then net30, i.e. a /30 per client)
> # Defaults to net30 (not recommended)
> ;topology subnet
>
> # Configure server mode and supply a VPN subnet
> # for OpenVPN to draw client addresses from.
> # The server will take 10.8.0.1 for itself,
> # the rest will be made available to clients.
> # Each client will be able to reach the server
> # on 10.8.0.1. Comment this line out if you are
> # ethernet bridging. See the man page for more info.
> server 10.8.0.0 255.255.255.0
I assume your internal network is on 10.0.0.0/8 (and NOT on
10.8.0.0/24); right?
>
> # Maintain a record of client <-> virtual IP address
> # associations in this file. If OpenVPN goes down or
> # is restarted, reconnecting clients can be assigned
> # the same virtual IP address from the pool that was
> # previously assigned.
> ifconfig-pool-persist ipp.txt
>
> # Configure server mode for ethernet bridging.
> # You must first use your OS's bridging capability
> # to bridge the TAP interface with the ethernet
> # NIC interface. Then you must manually set the
> # IP/netmask on the bridge interface, here we
> # assume 10.8.0.4/255.255.255.0. Finally we
> # must set aside an IP range in this subnet
> # (start=10.8.0.50 end=10.8.0.100) to allocate
> # to connecting clients. Leave this line commented
> # out unless you are ethernet bridging.
> ;server-bridge 10.8.0.4 255.255.255.0 10.8.0.50 10.8.0.100
>
> # Configure server mode for ethernet bridging
> # using a DHCP-proxy, where clients talk
> # to the OpenVPN server-side DHCP server
> # to receive their IP address allocation
> # and DNS server addresses. You must first use
> # your OS's bridging capability to bridge the TAP
> # interface with the ethernet NIC interface.
> # Note: this mode only works on clients (such as
> # Windows), where the client-side TAP adapter is
> # bound to a DHCP client.
> ;server-bridge
>
> # Push routes to the client to allow it
> # to reach other private subnets behind
> # the server. Remember that these
> # private subnets will also need
> # to know to route the OpenVPN client
> # address pool (10.8.0.0/255.255.255.0)
> # back to the OpenVPN server.
> ;push "route 192.168.10.0 255.255.255.0"
> ;push "route 192.168.20.0 255.255.255.0"
> push "route 10.0.1.0 255.255.255.0"
Why? This route is not necessary assuming you push default, which you
do below.
> # To assign specific IP addresses to specific
> # clients or if a connecting client has a private
> # subnet behind it that should also have VPN access,
> # use the subdirectory "ccd" for client-specific
> # configuration files (see man page for more info).
>
> # EXAMPLE: Suppose the client
> # having the certificate common name "Thelonious"
> # also has a small subnet behind his connecting
> # machine, such as 192.168.40.128/255.255.255.248.
> # First, uncomment out these lines:
> ;client-config-dir ccd
> ;route 192.168.40.128 255.255.255.248
> # Then create a file ccd/Thelonious with this line:
> # iroute 192.168.40.128 255.255.255.248
> # This will allow Thelonious' private subnet to
> # access the VPN. This example will only work
> # if you are routing, not bridging, i.e. you are
> # using "dev tun" and "server" directives.
>
> # EXAMPLE: Suppose you want to give
> # Thelonious a fixed VPN IP address of 10.9.0.1.
> # First uncomment out these lines:
> ;client-config-dir ccd
> ;route 10.9.0.0 255.255.255.252
> # Then add this line to ccd/Thelonious:
> # ifconfig-push 10.9.0.1 10.9.0.2
>
> # Suppose that you want to enable different
> # firewall access policies for different groups
> # of clients. There are two methods:
> # (1) Run multiple OpenVPN daemons, one for each
> # group, and firewall the TUN/TAP interface
> # for each group/daemon appropriately.
> # (2) (Advanced) Create a script to dynamically
> # modify the firewall in response to access
> # from different clients. See man
> # page for more info on learn-address script.
> ;learn-address ./script
>
> # If enabled, this directive will configure
> # all clients to redirect their default
> # network gateway through the VPN, causing
> # all IP traffic such as web browsing and
> # and DNS lookups to go through the VPN
> # (The OpenVPN server machine may need to NAT
> # or bridge the TUN/TAP interface to the internet
> # in order for this to work properly).
> push "redirect-gateway def1 bypass-dhcp"
>
> # Certain Windows-specific network settings
> # can be pushed to clients, such as DNS
> # or WINS server addresses. CAVEAT:
> # http://openvpn.net/faq.html#dhcpcaveats
> # The addresses below refer to the public
> # DNS servers provided by opendns.com.
> ;push "dhcp-option DNS 208.67.222.222"
> push "dhcp-option DNS 10.0.1.230"
I know this should be obvious but your DNS is on 10.0.1.230, right?
> # Uncomment this directive to allow different
> # clients to be able to "see" each other.
> # By default, clients will only see the server.
> # To force clients to only see the server, you
> # will also need to appropriately firewall the
> # server's TUN/TAP interface.
> ;client-to-client
>
> # Uncomment this directive if multiple clients
> # might connect with the same certificate/key
> # files or common names. This is recommended
> # only for testing purposes. For production use,
> # each client should have its own certificate/key
> # pair.
> #
> # IF YOU HAVE NOT GENERATED INDIVIDUAL
> # CERTIFICATE/KEY PAIRS FOR EACH CLIENT,
> # EACH HAVING ITS OWN UNIQUE "COMMON NAME",
> # UNCOMMENT THIS LINE OUT.
> ;duplicate-cn
>
> # The keepalive directive causes ping-like
> # messages to be sent back and forth over
> # the link so that each side knows when
> # the other side has gone down.
> # Ping every 10 seconds, assume that remote
> # peer is down if no ping received during
> # a 120 second time period.
> keepalive 10 120
>
> # For extra security beyond that provided
> # by SSL/TLS, create an "HMAC firewall"
> # to help block DoS attacks and UDP port flooding.
> #
> # Generate with:
> # openvpn --genkey --secret ta.key
> #
> # The server and each client must have
> # a copy of this key.
> # The second parameter should be '0'
> # on the server and '1' on the clients.
> ;tls-auth ta.key 0 # This file is secret
>
> # Select a cryptographic cipher.
> # This config item must be copied to
> # the client config file as well.
> # Note that 2.4 client/server will automatically
> # negotiate AES-256-GCM in TLS mode.
> # See also the ncp-cipher option in the manpage
> cipher AES-256-CBC
>
> # Enable compression on the VPN link and push the
> # option to the client (2.4+ only, for earlier
> # versions see below)
> compress lz4-v2
> push "compress lz4-v2"
>
> # For compression compatible with older clients use comp-lzo
> # If you enable it here, you must also
> # enable it in the client config file.
> ;comp-lzo
Are you sure your clients are compatible with the newer compression?
> # The maximum number of concurrently connected
> # clients we want to allow.
> max-clients 5
>
> # It's a good idea to reduce the OpenVPN
> # daemon's privileges after initialization.
> #
> # You can uncomment this out on
> # non-Windows systems.
> user nobody
> group nobody
>
> # The persist options will try to avoid
> # accessing certain resources on restart
> # that may no longer be accessible because
> # of the privilege downgrade.
> persist-key
> persist-tun
>
> # Output a short status file showing
> # current connections, truncated
> # and rewritten every minute.
> status openvpn-status.log
>
> # By default, log messages will go to the syslog (or
> # on Windows, if running as a service, they will go to
> # the "\Program Files\OpenVPN\log" directory).
> # Use log or log-append to override this default.
> # "log" will truncate the log file on OpenVPN startup,
> # while "log-append" will append to it. Use one
> # or the other (but not both).
> ;log openvpn.log
> ;log-append openvpn.log
>
> # Set the appropriate level of log
> # file verbosity.
> #
> # 0 is silent, except for fatal errors
> # 4 is reasonable for general usage
> # 5 and 6 can help to debug connection problems
> # 9 is extremely verbose
> ;verb 3
> verb 4
>
> # Silence repeating messages. At most 20
> # sequential messages of the same message
> # category will be output to the log.
> ;mute 20
>
> # Notify the client that when the server restarts so it
> # can automatically reconnect.
> explicit-exit-notify 1
> mail#
How are you NATting your internal network on the way out? When you get
packets into the machine that is running openvpn they then have to be
NAT'd to get out onto the Internet as a whole. In addition you have
10.8.0.0/24 and (it appears) 10.0.1.0/24 at least. Is all of your
"other" stuff on 10.0.1.0/24, and are they all consistent in netmask
(that is, /24)?
In general the config looks ok but I'm suspicious on that compression
setting, especially considering that you say that when you connect to
your Postfix server (which I assume is on that same box) and it's seeing
invalid characters.
I have the older compression enabled (which is in your file but
commented out) and do NOT have the newer specified, and it's working fine.
However, before playing with that check the following.
You state the receiving box is single-interface. This implies it is NOT
your network's edge gateway (e.g. to the Internet.) If the receiving
box is single-interface and is NOT the box that talks outbound to the
rest of the world then when the connection comes up you now have packets
on 10.8.0.0/24 being emitted on your LAN. To get them beyond the OpenVPN
machine (the one where openvpn is running) the rest of your network's
topology has to know how to get *back* to 10.8.0.0/24.
Example from here (I've also got IPv6 configured but we'll leave that
out of the discussion since it's not material here) -- this is a box
that provides a number of services to local hosts on the LAN on
192.168.10.100/24, and also has a VLAN interface running on 192.168.4/24.:
Destination Gateway Flags Netif Expire
default 192.168.10.200 UGS em0
127.0.0.1 link#3 UH lo0
192.168.4.0/24 link#5 U em0.3
192.168.4.100 link#5 UHS lo0
192.168.10.0/24 link#1 U em0
192.168.10.100 link#1 UHS lo0
The edge device is on 192.168.10.200. That device has a routing table
that looks like this:
Internet:
Destination Gateway Flags Netif Expire
default 68.1.56.1 UGS igb0
68.1.56.0/21 link#1 U igb0
68.1.57.197 link#1 UHS lo0
127.0.0.1 link#3 UH lo0
192.168.2.0/24 link#2 U igb1
192.168.2.200 link#2 UHS lo0
192.168.4.0/24 link#5 U igb1.3
192.168.4.200 link#5 UHS lo0
192.168.6.0/24 192.168.6.2 UGS tun0
192.168.6.1 link#6 UHS lo0
192.168.6.2 link#6 UH tun0
192.168.10.0/24 link#2 U igb1
192.168.10.200 link#2 UHS lo0
192.168.6.0/24 is my OpenVPN subnet (tun0) and default points outward to
the Internet on this box. If I talk to 192.168.10.100 from an OpenVPN
client all is well because when packets are sent back to the client
machine they get tossed at default (192.168.10.200) and that device's
routing table knows how to get back t0 192.168.6.0/24 and thus can send
them onward to the client.
But in your case if the OpenVPN server is NOT the edge device -- so if
you talk to something on your LAN from the OpenVPN client the server you
send packets to has no idea how to get back to 10.8.0.0/24.
Without explicit routes back to the OpenVPN server when your client
attempts to connect to some resource on 10.0.1.0/24 the packets coming
back from that resource will be sent to wherever default points (likely
your edge router that connects to the Internet) since there is no
SPECIFIC route for 10.8.0.0/24 back to the OpenVPN gateway from that
particular on-network resource. The edge router will either toss that
packet out ITS default (to the Internet) which will get black-holed by
your ISP that (we all hope) blackballs all 10./8 (and the other private
IP ranges) or, if it thinks it should do translation on the packet first
since it's from a private address it will toss it right in the trash
since there's no matching entry and it's not a SYN (e.g. not a
connection setup, but rather a reply.)
If OpenVPN runs on your edge gateway then this problem doesn't come up
since default on all the other hosts on your LAN point to that same
place so when the packets come for IP addresses on that block the
gateway knows where they go. But in the circumstance where this is not
true you have to make sure all your hosts that you wish to reach from
OpenVPN clients have a route for the OpenVPN subnet back to the OpenVPN
server machine.
--
Karl Denninger
karl at denninger.net <mailto:karl at denninger.net>
/The Market Ticker/
/[S/MIME encrypted email preferred]/
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