The candidate should be able to configure a network device to implement various network authentication schemes. This objective includes configuring a multi-homed network device, configuring a virtual private network and resolving networking and communication problems.
Utilities to manipulate routing tables
Utilities to configure and manipulate ethernet network interfaces
Utilities to analyse the status of the network devices
Utilities to monitor and analyse the TCP/IP traffic
A VPN (Virtual Private Network) allows you to connect two or more remote networks securely over an insecure connection, for example over the public Internet. To do this an encrypted secure tunnel is created: all data will be encrypted before being sent over the insecure network. The resulting network connection acts and feels like a physical connection, but actually may traverse many physical networks and systems. Hence its name: "virtual".
VPNs are frequently used to save costs. In olden days physical connections had to be leased from telecom providers or you had to use POTS or ISDN lines. This was a costly business. Nowadays the Internet is omnipresent, and almost always available at a low fixed monthly price. However, the Internet can be sniffed and intruders might inspect and/or intercept your traffic. A VPN shields you from most of the problems you might have otherwise.
A use case might be to integrate LANs in several offices or branches. A user that works in the Los Angeles office hence can access the network services of the department in New York vice versa. In most cases, offices already have an Internet connection, so no additional investments need to be made.
There are many ways to implement a VPN, although most solutions either use IPSEC or SSL/TLS as their basis for encryption. Some companies use proprietary software implementations. Many routers have IPSEC based VPN support built in. A usable VPN can be built using a simple SSH tunnel or by using a more sophisticated dedicated solution. Some VPN implementations include:
Many Cisco Routers (or other proprietary implementations)
This book will outline the implementations of OpenVPN and IPSEC. OpenVPN is covered separately in the chapter on System Security.
IPSEC provides encryption and authentication services at the IP (Internet Protocol) level of the network protocol stack. It replaces/enhances the regular level 3 IP layer so all packets are encrypted, including for example UDP packets. The IPSEC layer has been standardized by the IETF in RFCs 2401–2412. Implementing IPSEC is an option for IPv4 but is mandatory in IPv6 stacks.
In a regular IPv4 network you might set up dedicated IPSEC gateway machines to provide encrypted IP network connections when needed. IPSEC can run on routers, firewall machines, various application servers and on end-user desktop or laptop machines - any system that has an IP stack.
Using IPSEC is simple, as the protocol is built-in into the IP stack. But there are additional tasks in comparison with a regular IPv4 connection, for example encryption keys need to be exchanged between the end-points before an encrypted tunnel can be set up. It was decided to handle this over a higher-level protocol, the Internet Key Exchange protocol (IKE). After IKE has done its work, the IP level services ESP and AH know which keys to use to do their work.
The full names of the three protocols that are used in an IPSEC implementation are:
Encrypts and/or authenticates data;
Provides a packet authentication service;
Negotiates connection parameters, including keys, for the protocols mentioned above. The IKE protocol ensures authentication of the peers and exchange of their symmetric keys. The IKE protocol is usually implemented by a user space daemon that uses port 500/udp.
IPSEC standards define all three protocols, but in some contexts people use the term IPSEC to refer to just AH and ESP.
OpenSwan, formerly known as FreeS/WAN, is a complete IPSEC implementation for Linux 2.0 - 2.6 kernels. StrongSwan (also derived from FreeS/WAN) is another implementation that also supports the 3.x kernel. Both OpenSwan and FreeSwan implement all three protocols mentioned earlier. The Openswan implementation has several main parts:
KLIPS (KerneL IPSec) which implements generic IPSEC packet handling, AH and ESP on the kernel level, for all kernels before version 2.5.47. KLIPS has been superseded by native IPSEC kernel support (NETKEY).
NETKEY is the Kernel IPSec implementation included with the 2.6 kernel.
Pluto (an IKE daemon) implements IKE, negotiating connections with other systems.
various scripts provide an administrator interface to the machinery.
The config file contains three parts:
Each connection section starts with
ident is an arbitrary name
which is used to identify the connection.
This section starts with
For each parameter in it,
any section which does not have a parameter of the
same name gets a copy of the one from the
%default section. There may
be multiple %default sections, but only one
default may be supplied for any specific parameter
name and all
sections must precede all
%default sections of
A sample configuration file is shown below:
# basic configuration config setup interfaces="ipsec0=eth0" klipsdebug=none plutodebug=none plutoload=%search plutostart=%search # Close down old connection when new one using same ID shows up. uniqueids=yes # defaults that apply to all connection descriptions conn %default # How persistent to be in (re)keying negotiations (0 means very). keyingtries=0 # How to authenticate gateways authby=rsasig # VPN connection for head office and branch office conn head-branch # identity we use in authentication exchanges email@example.com leftrsasigkey=0x175cffc641f... left=184.108.40.206 leftnexthop=220.127.116.11 leftsubnet=192.168.11.0/24 # right s.g., subnet behind it, and next hop to reach left firstname.lastname@example.org rightrsasigkey=0xfc641fd6d9a24... right=18.104.22.168 rightnexthop=22.214.171.124 rightsubnet=192.168.0.0/24 # start automatically when ipsec is loaded auto=start
In a typical setup you have two interconnected gateways
that both run IPSEC and route packets.
One of these gateways can be seen as 'the one on the left',
the other as 'the one on the right'. Hence specifications
are written in terms of
right participants. There is no special
meaning attached to either name, they are just labels - you
might have defined the 'left' host to be the 'right' host and
Normally, you would use the exact same configuration file on both sides. Interpretation of that file is done by checking the local configuration. For example if it was stated in the configuration file that the IP address for 'left' is 126.96.36.199, the software assumes that it runs on the left node if it finds that IP address configured on one of its network devices. The same file is interpreted differently on the other node, as that hosts configuration differs.
leftnexthop (and the
right... counterparts) determine
the layout of the connection:
leftrsasigkey are used in
authenticating the left participant. The
leftrsasigkey is the public key of
the left participant (in the example above the RSA keys
are shortened for easy display). The private key is stored
/etc/ipsec.secrets file and
should be kept secure.
The keys can be generated on both client and server with the command:
# ipsec rsasigkey --verbose 2048 > rsa.key
The IKE-daemon of IPSEC is called pluto. It will authenticate and negotiate the secure tunnels. Once the connections is set up, the kernel implementation of IPSEC routes traffic through the tunnel if need be.
the pluto daemon to search the
configuration file for
statements. All connections with
auto=add will be loaded in the
pluto database. Connections with
auto=start will also be started.
Network troubleshooting is a very broad subject with thousands of tools available. There are some very good books available on this topic, so we will limit our discussion to an overview of some of the tools which can be used to solve or detect network problems.
Typing ifconfig without additional parameters displays the configuration for all network interfaces on the system. You might use this command to verify the configuration of an interface if the user experiences connectivity problems, particularly when their system has just been (re)configured.
When ifconfig is entered with an interface name and
no other arguments, it displays the current values assigned
to that particular interface. For example, checking interface
eth0 system gives this report:
$ ifconfig eth0 eth0 Link encap:Ethernet HWaddr 00:10:60:58:05:36 inet addr:192.168.2.3 Bcast:192.168.2.255 Mask:255.255.255.0 UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 RX packets:1398185 errors:0 dropped:0 overruns:0 frame:0 TX packets:1411351 errors:0 dropped:0 overruns:0 carrier:0 collisions:829 txqueuelen:100 RX bytes:903174205 (861.3 Mb) TX bytes:201042106 (191.7 Mb) Interrupt:11 Base address:0xa000
The ifconfig command displays a few lines of output. The third line of the display shows the characteristics of the interface. Check for these characteristics:
The interface is enabled for use. If the interface is “down”, bring the interface “up” with the ifconfig command (e.g. ifconfig eth0 up).
This interface is operational. If the interface is not “running”, the driver for this interface may not be properly installed.
The second line of ifconfig output shows the IP address, the subnet mask and the broadcast address. Check these three fields to make sure the network interface is properly configured.
Two common interface configuration problems are misconfigured subnet masks and incorrect IP addresses. A bad subnet mask may be the case when the host can reach some hosts on its local subnet but is unable to reach other hosts, even if they are on the same subnet. ifconfig quickly reveals if a bad subnet mask is set.
An incorrectly set IP address can be a subtle problem. If the network part of the address is incorrect, every ping will fail with the “no answer” error; because the IP address is unfamiliar to the other hosts on the network, return packets will be directed to their default gateway (often leading to the internet) or even dropped. In this case, using ifconfig may reveal the incorrect address. However, if the host part of the address is wrong, the problem can be more difficult to detect. A small system, such as a PC that only connects out to other systems and never accepts incoming connections, can run for a long time with the wrong address without its user noticing the problem. Additionally, the system that suffers the ill effects may not be the one that is misconfigured. It is possible for someone to accidentally use your IP address on his own system and for his mistake to cause intermittent communication problems to your system. This type of configuration error cannot be discovered by ifconfig, because the error is on a remote host. IP conflicts like this can be discovered using the arp command, which will show two alternating MAC addresses for the same IP address.
The ifconfig command can be used to set up multihomed network device. There are two ways a host can be multihomed.
Two Or More Interfaces To The Same Network: Devices such as servers or high-powered workstations may be equipped with two physical interfaces to the same network for performance and/or reliability reasons. They will have two IP addresses on the same network with the same network ID.
Interfaces To Two Or More Different Networks: Devices may have multiple interfaces to different networks. The IP addresses will typically have different network IDs in them.
$ ping [-c
host$ ping6 [-c
The hostname or IP address of the remote host being tested. Note that you cannot ping from an IPv4 host to an IPv6 host or vice versa. Both ends need to use the same IP version.
The number of packets to be sent in the test. Use the count field and set the value low. Otherwise, the ping command will continue to send test packets until you interrupt it, usually by pressing CTRL+C (^C).
$ ping -c 4 www.sue.nl PING home.NL.net (188.8.131.52): 56 data bytes 64 bytes from 184.108.40.206: icmp_seq=0 ttl=245 time=32.1 ms 64 bytes from 220.127.116.11: icmp_seq=1 ttl=245 time=32.1 ms 64 bytes from 18.104.22.168: icmp_seq=2 ttl=245 time=37.6 ms 64 bytes from 22.214.171.124: icmp_seq=3 ttl=245 time=34.1 ms --- home.NL.net ping statistics --- 4 packets transmitted, 4 packets received, 0% packet loss round-trip min/avg/max = 32.1/33.9/37.6 ms
This test shows a good wide-area network link to
with no packet loss and fast response. A small
packet loss, and a round-trip time an order of magnitude
higher, would not be abnormal for a connection made across a
wide-area network. The statistics displayed by the ping
command can indicate a low-level network problem. The key
How long it takes a packet to make the round trip, displayed in milliseconds after the string time=;
The percentage of packets lost, displayed in a summary line at the end of the ping output.
If the packet loss is high, the response time is very high or packets are arriving out of order, there could be a network hardware or link problem. If you see these conditions when communicating over great distances on a wide area network, there is nothing to worry about. TCP/IP was designed to deal with unreliable networks, and some wide-area networks suffer from a moderate level of packet loss. But if these problems are seen on a local-area network, they indicate trouble.
On a local-network cable segment, the round-trip time should be close to zero; there should be little or no packet loss and the packets should arrive in order. If these conditions are not met, there is a problem with the network hardware or with the links connecting them. On an Ethernet, the problem could be improper cable termination, a bad cable segment or a bad piece of “active” hardware, such as a hub, switch or transceiver.
The results of a simple ping test, even if the ping is successful, can help direct you to further testing toward the most likely causes of the problem. But other diagnostic tools are needed to examine the problem more closely and find the underlying cause.
$ route -n Kernel IP routing table Destination Gateway Genmask Flags Metric Ref Use Iface 192.168.11.0 0.0.0.0 255.255.255.0 U 0 0 0 eth0 126.96.36.199 0.0.0.0 255.255.252.0 U 0 0 0 eth1 0.0.0.0 188.8.131.52 0.0.0.0 UG 0 0 0 eth1
This host has two interfaces, one on subnet 192.168.11.0/24
the other on subnet 184.108.40.206/22. There is also a default
route out on
eth1 to 220.127.116.11
(denoted by the
G under “Flags” and a
“Destination” and “Genmask” of 0.0.0.0).
To be able to troubleshoot this information you need to know what the routing should be, perhaps by saving the routing information when the system is known to work.
The two most common mistakes are:
No network entry for an interface. When a network interface is configured a routing entry should be automatically added. This informs the kernel about the network that can be reached through the interface.
No default route (or two default routes). There should be exactly one default route. Note that two default gateways can go undetected for a long time because the routing could “accidentally” use the proper gateway.
In general, if there is a routing problem, it is better to first locate the part of the network where the problem originates, e.g. with ping or traceroute and then use route as part of the diagnostics.
traceroute and traceroute6 are tools used to discover the gateways along a path. Path discovery is an essential step in diagnosing routing problems. Note that traceroute6 is equivalent to traceroute -6
The traceroute command is based on a clever use of the Time-To-Live (TTL) field in the IP packet's header. The TTL field is used to limit the lifetime of a packet. When a router fails or is misconfigured, a routing loop or circular path may result. The TTL field prevents packets from remaining on a network indefinitely should such a routing loop occur. A packet's TTL field is decremented each time the packet crosses a router on its way through a network. When its value reaches 0, the packet is discarded rather than forwarded. When discarded, an ICMP TIME_EXCEEDED message is sent back to the packet's source to inform the source that the packet was discarded. By manipulating the TTL field of the original packet, the program traceroute uses information from these ICMP messages to discover paths through a network.
traceroute sends a series of UDP packets with the destination address of the device you want a path to. By default, traceroute sends sets of three packets to discover each hop. traceroute sets the TTL field in the first three packets to a value of 1 so that they are discarded by the first router on the path. When the ICMP TIME_EXCEEDED messages are returned by that router, traceroute records the source IP address of these ICMP messages. This is the IP address of the first hop on the route to the destination.
Next, three packets are sent with their TTL field set to 2. These will be discarded by the second router on the path. The ICMP messages returned by this router reveal the IP address of the second router on the path. The program proceeds in this manner until a set of packets finally has a TTL value large enough so that the packets reach their destination. Most implementations of traceroute default to trying only 30 hops before halting.
An example traceroute on linux looks like this:
$ traceroute vhost2.cistron.net traceroute to vhost2.cistron.net (18.104.22.168), 30 hops max, 38 byte packets 1 gateway.kabel.netvisit.nl (22.214.171.124) 56.013 ms 19.120 ms 12.642 ms 2 126.96.36.199 (188.8.131.52) 138.138 ms 28.482 ms 28.788 ms 3 asd-dc2-ias-ar10.nl.kpn.net (184.108.40.206) 102.338 ms 240.596 ms 253.462 ms 4 asd-dc2-ipc-dr03.nl.kpn.net (220.127.116.11) 95.325 ms 76.738 ms 97.651 ms 5 asd-dc2-ipc-cr01.nl.kpn.net (18.104.22.168) 61.378 ms 60.673 ms 75.867 ms 6 asd-sara-ipc-dr01.nl.kpn.net (22.214.171.124) 111.493 ms 96.631 ms 77.398 ms 7 asd-sara-ias-pr10.nl.kpn.net (126.96.36.199) 78.721 ms 95.029 ms 82.613 ms 8 ams-icr-02.carrier1.net (188.8.131.52) 90.179 ms 80.634 ms 112.130 ms 9 184.108.40.206 (220.127.116.11) 49.521 ms 80.354 ms 63.503 ms 10 18.104.22.168 (22.214.171.124) 94.528 ms 60.698 ms 103.550 ms 11 vhost2.cistron.net (126.96.36.199) 102.057 ms 62.515 ms 66.637 ms
Again, knowing what the route through your network should be helps to localize the problem. Note that not all network problems can be detected with a traceroute, because of some complicating factors. First, the router at some hop may not return ICMP TIME_EXCEEDED messages. Second, some older routers may incorrectly forward packets even though the TTL is 0. A third possibility is that ICMP messages may be given low priority and may not be returned in a timely manner. Finally, beyond some point of the path, ICMP packets may be filtered by a firewall.
The traceroute command is a great tool to narrow down the possible causes of a network problem.
arp is used to manipulate the kernel's ARP cache. The primary options are clearing an address mapping entry and manually setting one up. For debugging purposes, the arp program also allows a complete dump of the ARP cache.
If you know what the MAC address of a specific host should be, the dump may be used to determine a duplicate IP-address, but running arpwatch on a central system might prove more effective.
IP address conflicts are often the result of configuration errors including:
assignment of the same static IP address by a network administrator
assignment of a static IP address within a DHCP range (dynamic range) resulting in the same address being automatically assigned by the local DHCP server
an error in the DHCP server
a system coming back online after an extended period in stand-by or hibernate mode with an IP address that has been reassigned and is in use on the network.
Detection of duplicate IP addresses can be very hard even with arpwatch. IP address conflicts occur when two devices on a network are assigned the same IP address, resulting in one or both being disabled and losing connectivity until the conflict is resolved.
If, for example, a rogue host uses the IP address of the host running the arpwatch program or never communicates with it, a duplicate IP address will go unnoticed. Still, arpwatch is a very useful tool in detecting networking problems.
arpwatch keeps track of ethernet/IP address pairings. Changes are reported via syslog and e-mail.
arpwatch will report a “changed ethernet address” when a known IP address shows up with a new ethernet address. When the old ethernet address suddenly shows up again, a “flip flop” is reported.
tcpdump is a program that enables network administrators to inspect every packet going through the network in real-time. This tool is typically used to monitor active connections or troubleshoot occasional network connectivity problems. In order to see traffic, however, the host running tcpdump must be somewhere along the path between two (or more) hosts exchanging traffic. This may prove to be difficult in a fully switched network. An easy solution is to run tcpdump on the host that needs to send or receive traffic. Another option is to configure a port on one of the switches where a copy of traffic from certain source and destination ports is sent; this is called a SPAN port.
tcpdump can generate a lot of output, so
it is useful to narrow the scope of packets captured by
specifying the interface you want to listen on using
-i. In addition, you can specify the source,
destination, protocol type and/or port number of the traffic
you want to see joined by boolean AND and OR statements if
necessary. An example command could be:
tcpdump -i eth0 src 10.10.0.1 and dst 10.10.0.254
and tcp port 80).
Other useful options are
-n to turn of name
-w to write captured packets
to a file for later inspection (e.g. in Wireshark).
An example of this command and output is:
$ nmap -A localhost Starting Nmap 7.60 ( https://nmap.org ) at 2020-05-08 12:56 CEST Nmap scan report for localhost (127.0.0.1) Host is up (0.00015s latency). Other addresses for localhost (not scanned): ::1 Not shown: 994 closed ports PORT STATE SERVICE VERSION 22/tcp open ssh OpenSSH 7.6p1 Ubuntu 4ubuntu0.3 (Ubuntu Linux; protocol 2.0) | ssh-hostkey: | 2048 fc:4c:2d:44:51:30:9e:8a:56:0b:a9:29:fa:e3:d7:21 (RSA) | 256 83:bb:46:7c:fc:61:9a:61:26:25:87:89:1c:43:a6:6c (ECDSA) |_ 256 bd:16:c8:7c:3e:29:00:6d:cc:79:d8:80:4a:92:0b:d7 (EdDSA) 25/tcp open smtp Postfix smtpd |_smtp-commands: oeboe.udonix.nl, PIPELINING, SIZE 10240000, ETRN, AUTH DIGEST-MD5 NTLM CRAM-MD5 LOGIN PLAIN, AUTH=DIGEST-MD5 NTLM CRAM-MD5 LOGIN PLAIN, ENHANCEDSTATUSCODES, 8BITMIME, DSN, SMTPUTF8, | smtp-ntlm-info: |_ Target_Name: OEBOE.UDONIX.NL 111/tcp open rpcbind 2-4 (RPC #100000) | rpcinfo: | program version port/proto service | 100000 2,3,4 111/tcp rpcbind | 100000 2,3,4 111/udp rpcbind | 100003 3 2049/udp nfs | 100003 3,4 2049/tcp nfs | 100005 1,2,3 42208/udp mountd | 100005 1,2,3 48465/tcp mountd | 100021 1,3,4 37657/tcp nlockmgr | 100021 1,3,4 38288/udp nlockmgr | 100227 3 2049/tcp nfs_acl |_ 100227 3 2049/udp nfs_acl 143/tcp open imap Dovecot imapd (Ubuntu) |_imap-capabilities: SASL-IR more Pre-login post-login LITERAL+ IDLE listed capabilities ID have LOGIN-REFERRALS OK IMAP4rev1 AUTH=PLAINA0001 STARTTLS ENABLE | ssl-cert: Subject: commonName=oeboe.udonix.nl | Not valid before: 2018-06-23T13:16:13 |_Not valid after: 2028-06-20T13:16:13 |_ssl-date: TLS randomness does not represent time 993/tcp open ssl/imap Dovecot imapd (Ubuntu) |_imap-capabilities: SASL-IR more IMAP4rev1 LITERAL+ IDLE post-login listed ID have LOGIN-REFERRALS Pre-login OK AUTH=PLAINA0001 capabilities ENABLE | ssl-cert: Subject: commonName=oeboe.udonix.nl | Not valid before: 2018-06-23T13:16:13 |_Not valid after: 2028-06-20T13:16:13 |_ssl-date: TLS randomness does not represent time 2049/tcp open nfs_acl 3 (RPC #100227) Service Info: Host: no; OS: Linux; CPE: cpe:/o:linux:linux_kernel Service detection performed. Please report any incorrect results at https://nmap.org/submit/ . Nmap done: 1 IP address (1 host up) scanned in 14.54 seconds
Some of the nmap command options require root
privileges, consult the
for more information
Wireshark is an open source network protocol analyzer. It allows you to examine data from a live network or from a capture file on disk. You can interactively browse the capture data, delving down into just that level of packet detail you need.
Wireshark has several powerful features, including a rich display filter language and the ability to view the reconstructed stream of a TCP session. It also supports hundreds of protocols and media types. A tcpdump-like console version named tethereal is also included. One word of caution is that Ethereal has suffered from dozens of remotely exploitable security holes, so stay up-to-date and be wary of running it with root privileges on untrusted or hostile networks (such as security conferences).
The lsof command lists all open files on a system. Since Linux treats everything as a file, it also shows open network sockets. It can be used to find open ports on a system as well as determining the origin of a network connection.
Options of the lsof used for network troubleshooting.
List IP sockets. The
-i option also takes
arguments to restrict the sockets listed, like
Do not resolve hostnames; can make lsof run faster.
Do not resolve port names; can make lsof run faster.
Resolve port name to protocol; default behaviour.
Useful options of the ss for network troubleshooting include:
List all sockets. This includes sockets in listening state.
Do not resolve port names.
Only display listening sockets.
Show processes using the sockets
Restrict output to TCP sockets
Restrict output to UDP sockets
Frequently used options are:
List all sockets.
List only IP connections
Only show listening sockets
Show IP numbers instead of resolving them to hostnames.
Show the PID number and name of the process that is holding the socket.
Show the kernel routing table. Is the same as the route command.
nc is used for establishing TCP and UDP connections between arbitrary ports on either end. After opening a port, it can listen for input, which can be passed through to another command for further processing. Note that you need to have administrative privileges on the system you're running this command on for opening a listening port below 1024. The command allows the user to set up, analyze and read connections between systems. It is a very useful tool in the troubleshooting toolbox. nc can be used to open up any port.
Because nc is capable of connecting and listening to any port it can be used to transfer files between systems that lack Samba, FTP or SSH etc.
if nmap is not available the nc command can be used to check for open ports: Run nc command with -z flag. You need to specify host name / ip along with the port range to limit and speedup operation. eg.
# nc -z localhost 1-1023
The mtr is extremely helpful for troubleshooting network problems, because it combines the functionality of ping and traceroute. Rather than provide a simple outline of the route that traffic takes across the internet like traceroute, mtr collects additional information regarding the state, connection, and responsiveness of the intermediate hosts.
mtr can be used without any options. Just type: mtr> host to get a visual display of the path between you and the host and per-hop statistics. Like ping, mtr will continue to send ICMP packets indefinitely by default. Useful options are:
Do not reverse resolve IP addresses to hostnames
Send count number of probe packets, then stop