Category Archives: Terminal & Co

jailed openvpn server

As there is no real progress in my piano sessions due to the lack of time and blah :) I just have another nice topic which costs me a couple of hours I want you to spend more efficient. But there will definitely be piano contest soon I promise, so please stay patient.

Migrating your openvpn server to run within a jail on freeBSD

You might have recognized that I am moving servcies from an old and long used linux host to clients they’re more suitable today.
So I did with my running openvpn server which should stay openvpn and will be replaced by cisco ipsec/vpn later on; so look out for another post then ­čśë

First I wanted to put the openvpn server into a jail which I use a lot to keep the system clean and opeational for each of the servcies:

[root@zolga:~] jls
   JID  IP Address      Hostname                      Path
     1        xterm                         /usr/jails/xterm
     2       tor                           /usr/jails/tor
     3      syslog                        /usr/jails/syslog
     4       sql                           /usr/jails/sql
     5      nethack                       /usr/jails/nethack
     6                 /usr/jails/mail
     7                  /usr/jails/http
     8       erl                           /usr/jails/erl
     9             /usr/jails/ejabber
    10                  /usr/jails/dns
    11       dhcp                          /usr/jails/dhcp
    29      openvpn                       /usr/jails/openvpn

On the setup of the new server there was not a single issue to solve: just install the server, take your old configuration files and keys with you and everything will be fine. So. I. thought.
But as one of the features a jail cannnot do what it wants with the host it is sitting on, and so the first run of the server failed by dynamically create the tunnel interface which is needed to set up the site-2-site connection.

And this is prohibited by default, so this step has to done manually once, but there is a good howto in freebsd forums.

The configuration file of the openVPN configutation itself is not topic of this post, because there are several ways to set up a VPN connection. All you have to take care of in this step is, to use the same interface (“dev tun0” or “dev tap0”) in your /usr/local/etc/openvpn/server.conf as you created before.

How to get familiar with IPv6

As there is no real progress in my piano sessions due to the lack of time and blah :) I just have another nice topic which costs me a couple of hours I want you to spend more efficient.

This is meant to be a double post, as it is handling the IPv6 tunneling topic in two variations: as a setup on a linux host and as one on a Cisco router.

– the linux part:

I was using a DSL modem in bridged mode with ppp dailup on a linux host behind becuase the modem was not able to handle IPv6 not even in an unstable mode.

As I now got a Cisco 877VA as my new CPE I am now able to use IPv6 natively if my provider would also support IPv6 on the dialup accounts. I was to nosy to see what IPv6 can do for me and my annoying NAT issueus on the Cisco router that I decided to go for an 6to4-tunnel for the first sight.
There the services of SixXS ( came in place because they are giving you your first tunnel for free.
To asure that you are using IPv6 they are monitoring your first connection and your’re earning ISK for keepong up the tunnel to get more services as a /64 net and another tunnel and more.

So first of all (before the Cisco setup) I built the tunnel with the recommended linux client aiccu which is well documented and easy to set up


username your-SIXXS-user
password your-SIXXS-pass
protocol tic

ipv6_interface sixxs
tunnel_id your-SIXXS-tunnel-id

verbose true
daemonize true
automatic true
requiretls false
pidfile /run/
defaultroute true

If you’re happy to only have an tunnel running to repack your IPv4 traffic to the IPv6 network on your default gateway your’re fine by now. If not, you’re a bit like me: I was also interesed in running my own v6 network to shut down IPv4 step by step as most of my devices at home are able to run on v6.

So there your earned ISK’s are good to check out a /64 net and set up radvd as a route advertiser to spread IPv6 addresses to all devices which are responding properly on a v6 broadcast. This radvd is also easy to set up and can autoconfigure your devices in a single second.


interface eth1
	AdvSendAdvert on;
        MinRtrAdvInterval 3;
        MaxRtrAdvInterval 10;
	prefix 2001:your-SIXXS-IPv6-prefix::/64
                AdvOnLink on;
                AdvAutonomous on;
                AdvRouterAddr on;

– the cisco part:

there is a pretty good documentation in the wiki of SixXS ( itself, so I just give you a quick look on the tunnel interface and the _really needed_ access-lists in the ZBF (zone based firewall) on the router which are covering the security issues you’ll have as there is no NAT in a default IPv6 network.

interface Tunnel6
 description +++ 4to6 Tunnel to SIXXS +++
 no ip address
 ip tcp adjust-mss 1420
 ipv6 address 2001:your-SIXXS-Linknet::2/64
 ipv6 enable
 ipv6 inspect cbac-ipv6 out
 ipv6 traffic-filter ipv6-internet-in in
 tunnel source Dialer0
 tunnel mode ipv6ip
 tunnel destination your-SIXXS-ipv4-tunneldest
ipv6 route ::/0 Tunnel6

to let the garbage outside you should apply at least this

ipv6 access-list ipv6-internet-in
 remark allow ping by SixXS PoP to determine tunnel status
 permit icmp host 2001:your-SIXXS-Linknet::1 host 2001:your-SIXXS-Linknet::2 echo-request
 remark Prevent spoofing
 deny ipv6 2001:your-SIXXS-Prefix::/64 any log
 remark prevent ingress of all addresses except global unicast and multicast
 deny ipv6 ::/3 any log
 deny ipv6 8000::/2 any log
 deny ipv6 C000::/3 any log
 deny ipv6 E000::/4 any log
 deny ipv6 F000::/5 any log
 deny ipv6 F800::/6 any log
 deny ipv6 FC00::/7 any log
 deny ipv6 FE00::/8 any log
 permit icmp any any time-exceeded
 permit icmp any any packet-too-big
 permit icmp any any echo-request
 permit icmp any any echo-reply
 deny ipv6 any any log

So if all is aisd and done, you can check on if your turtle is dancing; if so, you did everything right, congratulations! Have fun using next genration IP network.

die MTU, dein Freund und Kopfzerbrecher

Liebe Leute lasst euch sagen: ppp encapsultaion kann schon gl├╝cklich machen im ├Âsterreichischen ADSL-Land, dann wenn man vergisst, dass die Einkapselung die MTU/MSS-Sizes automatisch repariert, wenn man via pptp-client ├╝ber ein (gutes) altes Thomson Modem einw├Ąhlt.

Ersetzt man nun, wie in meinem Fall, dieses “Linux-Host macht Einwahl ├╝ber Modem”-Konstrukt durch einen Cisco als Zugang ins weite Internetz, dann muss man den Tanz der Segments, Frames & Packets tanzen:

MTU 1500 ist im LAN noch ok, aber ├╝ber WAN meist ung├╝nstig, daher pr├╝ft man am besten mittels swipe-ping, welche maximale Gr├Â├če denn ├╝berhaupt unterst├╝tzt wird:

Protocol [ip]:
Target IP address:
Repeat count [5]: 1
Datagram size [100]:
Timeout in seconds [2]:
Extended commands [n]: y
Source address or interface:
Type of service [0]:
Set DF bit in IP header? [no]:
Validate reply data? [no]:
Data pattern [0xABCD]:
Loose, Strict, Record, Timestamp, Verbose[none]:
Sweep range of sizes [n]: y
Sweep min size [36]: 1400
Sweep max size [18024]: 1520
Sweep interval [1]:
Type escape sequence to abort.
Sending 121, [1400..1520]-byte ICMP Echos to, timeout is 2 seconds:
Success rate is 38 percent (47/121), round-trip min/avg/max = 124/132/136 ms

Hier sieht man also sch├Ân, da├č bei 1415 byte die ersten Packets gedropped werden, was also die max. MTU darstellt…

daher hab ich auf die Schnelle in meinem Dialer-Interface am Cisco 887 VA folgende Zeilen konfiguriert, um den Bits wieder uneingeschr├Ąnkten, aber fragmentierten Lauf zu lassen:

interface Dialer0
mtu 1415
ip tcp adjust-mss 1349

Der Wert der max. Segmentation Size (MSS sollte 40 byte unter der MTU liegen, da auf Layer 4 noch weitere Bytes f├╝r Falgging und Tagging verwendet werden – wenn man IPSec/GRE einsetzten will, dann sollte man – wie ich auch – nochmal 4 Bytes, also insgesamt 44 Bytes geringer ┬ásegmentieren.┬áDann funktionieren wieder alle Webseiten- und Datenaufrufe, welche die Payload des ETH-Frames mit den standardm├Ąssigen 1500 Bytes bef├╝llen wollen wieder!

Ein Beispiel aus der Wikipedia, welches die Fragmentierung, Segemtierung und Tagging anschaulich darstellt:

# ping -s 1472
          1472 bytes Nutzlast des ICMP-Protokolles (Transportschicht)
        +    8 bytes ICMP-Header (Transportschicht)
        +   20 bytes IPv4-Header (der Vermittlungsschicht)
        = 1500 bytes (Nutzlast von Ethernet)
        +   14 bytes (Header der Sicherungsschicht)
        +    4 bytes (Frame Check Sequence)
        = 1518 bytes (kompletter Ethernetrahmen)