Süsteemihaldus 2018/19 kevad

0. Overview of Lab 4 :

Welcome to 4th lab. Here the short action list we will do in this lab:

1. Verifying previous lab tasks
2. Installing updates
3. Performing backup
4. Installing Domain Name Server
5. Configuring own local resolver server (allowing recursion and caching)
6. Configuring own domain zone, understanding world and local resolvers

1.Make sure you did finish all the tasks of the previous week.

PS! Before continuing please make sure you have completed all previous labs.

Additionally - Securing your ssh server

One thing that makes SSH server more secure is disabling password authentication. We disable it since we use our private key to log in and we disable password login, because people often use bad (easy to quess) passwords. We want to elminate that risk and since we use key, we disable it.

2. Check for the updates using debian package manager

We have worked with package manager apt during Lab1 (installing the system). Moreover, in the Lab3 we did upgrade packages, therefore the exact commands we expect you to know.

After upgrading the system it wise to clean the left-overs: the downloaded deb packages. Moreover it is wise to remove the packages that are not used anymore. The are two commands responsible for performing these actions:

# Remove the downloaded packages
apt autoclean 

# Uninstall the not used packages
apt autoremove

3. Perform backup

We are about to introduce some portion of changes to our system during this lab therefore it is wise to backup the current state first. Pay attention that one of the changes we do is the Firewall configurations, mistake in iptables syntax or wrong order of the rules in iptables chains might result in locking the network access (deny-all rule). Once network access is lost - you can't control your VM anymore over SSH. There is no recovery console (no VNC access allowed this year) - VM will be considered lost once inaccessible from the network. The ETAIS infrastructure does allows to perform the snapshots of the hard-drives. By doing the snapshot you literally saving the state of the whole hard drive up to date. Once the snapshot is done you can proceed with dangerous actions knowing that you can always recover in case you misconfigure something.

5. Disable IPv6

The ETAIS cloud does not suggest any IPv6 network providers, therefore is is pointless to keep the IPv6 stack up in our VMs. In this section we are going to disable the IPv6 stack by alternating kernel runtime parameters.

System wide configuration variables are managed using sysctl utility. Example:

• To print the whole list of system settings use:
  • # sysctl -A
• To see specific variable value:
  • # sysctl variableName
• To set a specific variable to a new value use:
  • # sysctl <variable>=<value>

This will however change only running system setting and will reset to default values after system restart. In order to alter the default values put the corresponding variables in the end of /etc/sysctl.conf file, example content of /etc/sysctl.conf (only few last lines shown):

...

 # My default value of aa.bb.cc_var1 variable
 # NB! This is an example, do not use this!
 aa.bb.cc_var1=1

...

During this Lab we do not configure any service to rely on IPv6 hence we do not need the system wide IPv6 stack. Let's disable it using well studied sysctl utility.

Now if you check your network interfaces using ip addr l command, there should be no inet6 prefixed lines.

6. Domain Name System (DNS) using BIND software

DNS (Domain Name Service) is the main name service used in the Internet. Name services are used to translate names meaningful to humans into the numerical/binary addresses/numbers (and vice versa). Practically every system relies on a working name service. The name service has not always to be set up in-house, most often it is provided by the Internet Service Provider (ISP).

Extra materials

• Wikipedia: Domain Name System
• DNS for Rocket Scientists
• BIND 9 Administrator Reference Manual
• Just a Good BIND9 Manual

Domain Hierarchy for System Administration Labs

For our practicals we will use our own domain hierarchy starting from the top level domains. DNS management is based on strict delegation, so we are unable to introduce our "fake" top level domains into the "real" DNS system used in the Internet. That is why our top level domains will only work on the lab systems.

The domain hierarchy is divided into zones based on authority:

• The authority over root domain and top level domains will belong to the teaching staff. 2019 we will be using top level domain .est.
• Every participant/system will have authority over a second-level domain vm_name.est, under which one or more names may be defined. "vm_name" refers to the name to the machine you've set inside minu.etais.ee. It is mandatory to have a name for every host/sub system in the lab group under each of these second-level domains.
• The general form for these names is <service>.vm_name.est Here "<service>" refers to any services we will be setting up in the future.

For example:

• If student John Doe has his UT study number book B12345 and his VM was named B12345 then full hostname is B12345.est. If you've named your VM something else, use that name.

Changing hostname

By default all the debian instances are set by ETAIS during VM setup as a host name. During this lab we are going to setup a personal DNS server (serving personal domain <vm_name>.est). Our VM will join the domain by name <vm_name>.est. Therefore next we prepare VM by alternating its host name from short notation to full qualified domain notation, Example for B12345 in B12345.est domain:

* short notation: B12345
* fqdn notation: B12345.est

 ...  and its output is exactly the host name in short notation:
myhost

The host name is held in file /etc/hostname, by altering the single line in this file you will alter the name of your host. In addition there is a /etc/hosts file, which provides fixed host name <-> ip address mappings, and has the following content (only lines we are interested in are shown):

 ...

• 127.0.0.1 localhost
• 127.0.1.1 myhost.novalocal myhost

 ... 

The shown mappings allows local resolving of host names to IP address (without any requests to external services like DNS). This especially critical when testing your services locally, if you consider the simple command, like:

 ... this will actually resolve localhost to  127.0.0.1 using /etc/hosts file.

There is another line in /etc/hosts associating your current hostname to 127.0.1.1, in case we change our domain we need to modify this line. Example:

• my short host name is myhost, and my new domain is .est, so the full domain name myhost.est
• the line in /etc/hosts should rewritten as follows:
  • 127.0.1.1 myhost.est myhost

7. Configuring the BIND Name Server

First install the Bind 9 DNS server related packages.

As the BIND name server runs with the rights of the bind user and group (not root), you should always make sure that the main configuration file and zone files have the correct permissions and group ownership. Every file to be read by the BIND DNS server must be readable by bind group. If you get "permission denied" errors, use the following commands to set the owner or group ownership, example:

• # chgrp bind /etc/bind/targetConfFile.conf
• # chown bind folder/file

and following commands to set the correct permissions: (Learn More)

• # chmod 640 /etc/bind/targetConfFile.conf
  • Will set rights rw- r-- ---
• # chmod g+rX /etc/bind/targetConfFile.conf
  • Will just set read right for the group
• Use chown and chgrp to change owner/group. Use ls -la to check permissions/owner/group.

The main configuration file for BIND is spread across multiple files:

• /etc/bind/named.conf Main configuration file
• /etc/bind/named.conf.local Local zone declarations
• /etc/bind/named.conf.options Bind service options

The named.conf file is the main one, which includes all the others and which is loaded by the bind9 server. The named.conf.local describes the local views or zone definitions (the zones that this name server knows about). The named.conf.local.options overall behavior of the name server. The contents of the zones (resource records) will be located in the /etc/bind/zones directory, which we create later.
We are going to add another one called named.conf.logging which will contain logging configurations and will make bind write it's logs into files we have just created in previous task.

From now on, the main configuration files are referred to as:

• named.conf - main file
• named.conf.local - local definitions
• named.conf.options - options file
• named.conf.logging - logging options

Add the logging configuration to the logging options file:

logging {
        channel update_debug {
                file "/var/log/bind9/update_debug.log" versions 3 size 100k;
                severity debug;
                print-severity  yes;
                print-time      yes;
        };
        channel security_info {
                file "/var/log/bind9/security_info.log" versions 1 size 100k;
                severity info;
                print-severity  yes;
                print-time      yes;
        };
        channel bind_log {
                file "/var/log/bind9/bind.log" versions 3 size 1m;
                severity info;
                print-category  yes;
                print-severity  yes;
                print-time      yes;
        };

        category default { bind_log; };
        category lame-servers { null; };
        category update { update_debug; };
        category update-security { update_debug; };
        category security { security_info; };
};

Now we add the simple caching DNS configuration to our bind9 server. This in order to get rid of the forwarding the DNS resolv queries to 10.10.10.1. Having the caching DNS server running locally on the same host will to the trick. Instead of forwarding the queries to 10.10.10.1 we will use our own resolver on 127.0.0.1 which has recursive resolving (and also caching) enabled.
Edit the options file named.conf.options:

acl goodclients{
    127.0.0.0/8;
};

dnssec-validation auto;
auth-nxdomain no;    # conform to RFC1035
listen-on { 127.0.0.1; };
listen-on-v6 { none; };
recursion yes;
allow-query { goodclients; };

Regarding the previous configuration we first of all declare our white list of clients allowed to send the resolve queries to our bind9 server by having 127.0.0.0/8 localhost subnet declaration in acl section.
In the options section we leave dnssec-validation and auth-nxdomain untouched allowing default DNS security options and RFC conformance. Next we disable bind9 from listening to the IPv6, as we to not use system wide anyway. In addition we limit the scope of IPv4 by putting bind9 listen on localhost only (the default port is 53 UDP). Next we enable recursion and apply the white-list with 2 last options. The recursion is needed in order to let bind9 act as standard caching recursive resolver (like those assigned by DHCP or put manually in /etc/resolv.conf).

• Example query "where is courses.cs.ut.ee" is then resolved as follows:
  • "where is .ee"
  • "where is ut.ee"
  • "where is cs.ut.ee"
  • "where is courses.cs.ut.ee"
• The often made queries are cached and answered explicitly

Now you can check your configuration using named-checkconf utility

No output means configuration is OK.

Now query the service status and if needed, check the log files (those we have created in the beginning)

If everything works fine and no error is reported, you may check how your local resolver is performing:

It should answer with list of IP addresse for courses.cs.ut.ee like this:

; <<>> DiG 9.10.3-P4-Debian <<>> @localhost courses.cs.ut.ee
; (1 server found)
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 40914
;; flags: qr rd ra; QUERY: 1, ANSWER: 3, AUTHORITY: 2, ADDITIONAL: 4

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
;; QUESTION SECTION:
;courses.cs.ut.ee.              IN      A

;; ANSWER SECTION:
courses.cs.ut.ee.       4289    IN      CNAME   web.cs.ut.ee.
web.cs.ut.ee.           4289    IN      A       193.40.36.57
web.cs.ut.ee.           4289    IN      A       193.40.36.55

;; AUTHORITY SECTION:
cs.ut.ee.               10769   IN      NS      ns.ut.ee.
cs.ut.ee.               10769   IN      NS      ns2.ut.ee.

;; ADDITIONAL SECTION:
ns.ut.ee.               10769   IN      A       193.40.5.99
ns2.ut.ee.              10769   IN      A       193.40.5.76
ns2.ut.ee.              10769   IN      AAAA    2001:bb8:2002:500::76

;; Query time: 0 msec
;; SERVER: 127.0.0.1#53(127.0.0.1)
;; WHEN: Mon Mar 04 09:04:58 UTC 2019
;; MSG SIZE  rcvd: 190

If you get answer like this for courses.cs.ut.ee then local resolver is performing well. You may want to enable it for auto-starting at the boot time:

Modifying DHCP client configuration

In our current setup VM gets it's IP automatically assigned (internal address) by DHCP server which resides in cloud router. In addition to IP address the DHCP server provision VM with several other options. You can list those options by looking into /etc/dhcp/dhclient.conf file and locating the request option, by default it looks something like:

request subnet-mask, broadcast-address, time-offset, routers,
        domain-name, domain-name-servers, domain-search, host-name,
        dhcp6.name-servers, dhcp6.domain-search, dhcp6.fqdn, dhcp6.sntp-servers,
        netbios-name-servers, netbios-scope, interface-mtu,
        rfc3442-classless-static-routes, ntp-servers;

For example the options domain-name, domain-name-servers, domain-search will be used to auto-configure the /etc/resolv.conf . The last one contains the information regarding the resolver address the OS should contact to resolve host names to IP addresses. By default it looks as follows:

domain openstacklocal
search openstacklocal
nameserver 8.8.8.8
nameserver 8.8.4.4

In previous steps we have configure a local resolver which resides inside our VM (listening on 127.0.0.1:53). It is configure with recursive queries enable and therefore can resolve any world addresses (try dig @127.0.0.1 google.com). Next we would like to rely on our local resolver in place of one provisioned by DHCP server. Therefore we need to disable some options of DHCP client first. Namely in /etc/dhcp/dhclient.conf file and request variable, the following options we need to exclude, in order to disable the automatic provisioning of DNS resolver:

domain-name, domain-name-servers, domain-search, host-name,

Additionally we may disable the IPv6 related options (as we are not using IPv6)

Now the /etc/resolv.conf will be not anymore rewritten by DHCP client (next time we reboot) and stays static. So we can safely edit it to rely on your own local resolver for doing DNS queries by replacing the namesever variable value with 127.0.0.1

Now your local resolver is used system wide, so you can test it by just pinging some hosts by name: