tcpdump is a network traffic monitoring tool. At its core, it uses libpcap which in turn uses a technology called Extended Berkley Packet Filter (eBPF).

BPF was first proposed around 1995 when filtering mechanisms (and firewalls) were still a novel concept and were based on interpreters. BPF (now referred to as Classic BPF - cBPF) was the initial version of a Motorola inspired virtual ISA (i.e.: had no hardware implementation – think CHIP-8). eBPF is basically still BPF but more compatible with 64-bit architectures so that Just In Time (JIT) translators have an easier time running the code.

At first, the whole idea was to compile packet filtering programs and attach them to sockets in kernelspace. These programs would filter out packets that userspace processes would not be interested in. Consequently, this would reduce the quantity of data copied over the kernelspace/userspace boundary, only to ultimately be discarded.

Today, eBPF is used heavily for system profiling by companies such as Netflix and Facebook. Linux has had a kernel VM capable of running and statically analyzing eBPF code since around 2006. tcpdump is one of the few examples that still use it for its original purpose.

Use tcpdump to output outgoing NTP queries and incoming http(s) responses. Use the -d flag to see an objdump of the filter program's code.

Complete the tcpdump command in order to satisfy the following formatting requirements:

• print the packet number
• print the elapsed time (in nanoseconds) since the first packet was captured
• print the content of each packet (without l2 header) in hex and ASCII
• do not resolve IP addresses to names

How to test:

$ ntpdate -q ro.pool.ntp.org
$ curl ocw.cs.pub.ro

iptables is a configuration tool for the kernel packet filter.

The system as a whole provides many functionalities that are grouped by tables: filter, nat, mangle, raw, security. If you want to alter a packet header, you place a rule in the mangle table. If you want to mask the private IP address of an internal host with the external IP address of the default gateway, you place a rule in the nat table. Depending on the table you choose, you will gain or lose access to some chains. If not specified, the default is the filter table.

Chains are basically lists of rules. The five built-in chains are PREROUTING, FORWARD, POSTROUTING, INPUT, OUTPUT. Each of these corresponds to certain locations in the network stack where packets trigger Netfilter hooks (here is the PREROUTING kernel hook as an example – not that hard to add one, right?) For a selected chain, the order in which the rules are evaluated is determined primarily by the priority of their tables and secondarily by the user's discretionary arrangement (i.e.: order in which rules are inserted).

A rule consists of two entities: a sequence of match criteria and a jump target.

The jump target represents an action to be taken. You are most likely familiar with the built-in actions such as ACCEPT or DROP. These actions decide the ultimate fate of the packet and are final (i.e.: rule iteration stops when these are invoked). However, there are also extended actions (see man iptables-extensions(8)) that are not terminal verdicts and can be used for various tasks such as auditing, forced checksum recalculation or removal of Explicit Congestion Notification (ECN) bits.

The match criteria of every rule are checked to determine if the jump target is applied. The way this is designed is very elegant: every type of feature (e.g.: Layer 3 IP address vs Layer 4 port) that you can check has a match callback function defined in the kernel. If you want, you can write your own such function in a Linux Kernel Module (LKM) and thus extend the functionality of iptables (Writing Netfilter Modules with code example). However, you will need to implement a userspace shared library counterpart. When you start an iptables process, it searches in /usr/lib/xtables/ and automatically loads certain shared libraries (note: this path can be overwritten or extended using the XTABLES_LIBDIR environment variable). Each library there must do three things:

• define iptables flags for the new criteria that you want to include.
• define help messages for when iptables --help is called (its help message is an amalgamation of each library's help snippet).
• provide an initialization function for the structure containing the rule parameters; this structure will end up in the kernel's rule chain.

So when you want to test the efficiency of the iptables rule evaluation process, keep in mind that each rule may imply the invocation of multiple callbacks such as this.

Write an iptables rule according to the following specifications:

• chain: OUTPUT
• match rule: TCP packets originating from ephemeral ports bound to a socket created by root
• target: enable kernel logging of matched packets with the “EP: ” prefix

How to test:

$ sudo curl www.google.com
$ sudo dmesg

$ man 8 iptables-extensions

Write an iptables rule according to the following specifications:

• chain: OUTPUT
• match rule: BPF program that filters UDP traffic to port 53 (try bash command substitution)
• target: set TTL to 1 (initially)

Continue appending the same rule with incremented TTL value until the DNS request goes through.

How to test:

$ dig +short fep.grid.pub.ro @8.8.8.8

$ man 8 iptables-extensions nfbpf_compile

$ sudo apt install libpcap-dev
$ wget https://raw.githubusercontent.com/netgroup-polito/iptables/master/utils/nfbpf_compile.c
$ gcc -o nfbpf_compile nfbpf_compile.c -lpcap

Give an example when iptables is unable to catch a packet.

The Address Resolution Protocol (ARP) resolves layer 2 addresses (MAC) from layer 3 addresses (e.g.: IP). Normally, all hosts are compelled to reply to ARP requests, but this can be fiddled with using tools such as arptables. You can show the currently known neighbors using iproute2.

$ ip -c neigh show

# alias for iproute2 color output
alias ip='ip -c'

The Internet Control Message Protocol (ICMP) is an ancillary protocol meant mainly to report errors between hosts. Sometimes it can also be used to perform measurements (ping) or to inform network participants of better routes (Redirect Messages). There are many ICMP functionalities, most of which are now deprecated. Note that some network equipment may not be capable of understanding new and officially recognized protocols, while other may not even recognize experimental ICMP codepoints (i.e.: type=253,254) and simply drop the packet. Because ICMP can be used to stage attacks in a network, some operating systems (e.g.: Windows ≥7) went so far as to disable Echo Replies by default.

Use arp-scan to scan your local network while monitoring ARP traffic with wireshark to get a sense of what's going on. After that, use the following script to identify hosts discoverable via ARP but not ICMP.

nmap is a network exploration tool and a port scanner. Today, we will look only at a specific functionality that it shares with the traceroute utility.

Route discovery is simple in principle: IPv4 packets have a Time to Live (TTL) field that is decremented by 1 with each hop, thus ensuring a limited packet lifespan (imagine routing loops without TTL). Even if the TTL is 0, the layer 3 network equipment must process the received packet (the destination host can accept a packet with TTL=0). Routers may check the TTL field only if they are to forward the packet. If the TTL is already 0, the packet is dropped and a ICMP Time-To-Live Exceeded message is issued to the source IP. By sending packets with incrementally larger TTL values, it is possible to obtain the IP of each router on the path (at least in theory).

With 8.8.8.8 as a target, use wireshark to view the traffic generated by both nmap and traceroute. What differences can you find in their default mode of operation?

$ sudo nmap                            \
    -sn     `# disable port scan`      \
    -Pn     `# disable host discovery` \
    -tr     `# perform traceroute`     \
    8.8.8.8
$ traceroute 8.8.8.8

If we do allow for a port scan by removing -sn (default is a TCP-based scan; use -sU for a UDP scan), this will take place before the actual traceroute. What changes does this bring?

When doing the TCP scan with nmap, you may have noticed a weird field in the TCP header: Options. Generate some TCP traffic with curl and look at the SYN packet in wireshark. What options do you see there?

Here is a quick break down of the more common TCP options and how they are used to overcome protocol limitations and improve throughput. Take a quick look if you want, then move on. We'll dive deeper into protocol options in the next task.

Earlier in Ex. 1, we mentioned that eBPF is used for more than traffic filtering. Some of you may have heard of the eXpress Data Path (XDP) or the more recent eXpress Resubmission Path (XRP). Both of these are eBPF-powered shunts of kernel data paths that are used to optimize the system for very specific types of workloads. We'll return to these in a future lecture (and maybe a lab as well) since they can be considered advanced topics. For now, we'll focus on the third purpose eBPF can serve: execution tracing.

pwru is a tool created by Cilium to help trace network packets in the kernel's network stack and debug network connectivity issues. It does this by attaching simple eBPF programs to certain function entry points. These programs can report back to a userspace process different kinds of information, including the function that was reached, the arguments that were passed, and a CPU clock timestamp. The method used for instrumenting kernel code is based on kprobes. Ask your assistant for more information.

Install pwru on your system. Check that the minimum requirements stated on the Github page are met. Note that this tool is already provided by some public package repos (e.g.: pacman: extra/).

Now, trace all outgoing DNS queries to the Google DNS (i.e.: 8.8.8.8) and perform one using dig. Add relative timestamps to the individual trace entries, to get an idea of the computational cost of each operation.

Finally, insert an iptables rule on the OUTPUT chain that drops DNS queries to 8.8.8.8 and redo the experiment. Check where the packet's path is cut short (the reason should be obvious :p).

Analyze the call path in the kernel network stack for the first scenario (when the packet actually made it out). Explain each step of the packet's journey.

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