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3. IP


3.1 What are source routed packets?

Source route is an option in the IP header that allows the sender to override some or all of the routing decisions. Normally, routers between the source and destination decide how to route the packet.

There are a couple of network management uses of this packet, such as testing to see if two computers can talk to each other. A network manager at point A may send a packet to B through point C. This tells A if B & C can talk to each other.

The same technique can be used to evade firewalls, subvert trust relationships, and communicate with machines using "private" address (10.x.x.x, 192.168.x.x, 172.[16-31].x.x).

Let's say you are a hacker/cracker on the Internet and you want to talk to some machines behind a firewall who use 10.x.x.xcker/cracker on the Internet and you want to talk to some machines behind a firewall who use 10.x.x.x as their IP addresses. Since the routers on the Internet do not know where this subnet is located, they will drop your packets. However, you put a loose source route option in the IP packet and tell all the Internet routers to first forward to the firewall. Since the firewall straddles both the Internet and the private network, it will know how to forward the packet appropriately. Thus, you can carry on a conversation with the victim by bouncing all packets through the firewall.

This can be used with IP spoofing. You pretend to be a router (like the firewall mentioned above) and pretend that somebody else is bouncing packets through you. Thus, pick some random machine on the Internet (ALICE) as the spoofee, then send packets from ALICE to your victim BOB. BOB will think the packets are coming from ALICE, but in reality they are coming from you. This masks the real source of the attack.

This is even better if you know that BOB trusts ALICE. IP addresses are often used as part of authentication. Let's say the firewall has a rule allowing all traffic from ALICE into the network. By forging all IP packets to be from ALICE (but being source routed through your own machine), then you get free access to the victim network.

More and more core Internet routers are disabling source routed packets. They slow down routing anyway, but they are a huge security risk. There is also no real need for them. Managers should do the same and disable source routing everywhere: on firewalls, on routers, and even on end-nodes so that they won't even accept incoming source routed packets.

See Microsoft Knowledge Base article Q217336 for setting the "DisableIPSourceRouting" on WinNT SP5 systems

3.2 I'm seeing the IP address in my reject log

This is happening a lot these days as more and more people use DSL or cable-modem connections. The reason is that unlike point-to-point connections (like T-1, frame relay, etc.), these new high-speed technologies drop you onto an ATM VLAN, which is a single broadcast domains. In fact, many cable-modem users are seeing multiple megabytes of traffic per day simply from such broadcasts.

You must remember that such packets MUST be local. Routers (generally) refuse to forward packets with the IP address of This address is known as a "local broadcast" for this reason: it never travels past the local segment (or these days, the local "virtual" segment).

What are these packets for?

Check the list of ports at the top of this document. If it is not listed there, then the only way to figure this out is to capture them with a sniffer and view their contents.

For example, a common service that runs with a random port number is CORBA IIOP packets. Many services run at port 535, but it is frequently reconfigured to broadcast on other ports. If you look at the hex dump in the sniffer, you will see the letters "IIOP" somewhere in the contents.

In any case, this is rarely something to be concerned about. In fact, it advertises something about the person sending the traffic that can be used to hack them. Hackers rarely attack their own neighborhoods (because it is easy to detect), so it probably is accidental, not malicious.

It should be noted that with today's ATM networks, the source of the broadcast may not even be in the same state as you are; they may be hundreds of miles away. The word "local" means in terms of the network topology, not distance.

3.3 How do I track down the owner of an IP address?

Remember that IP addresses can be spoofed, so that the "owner" of an IP address may be innocent. Increasingly, attacks are coming from compromised machines. The owner of the IP may actually be grateful! Both of these statements come to the same conclusion: be polite and professional.

Many companies have established the e-mail address "" (replace "example" with the proper company). This e-mail role is for both e-mail abuse (such as spam) as well as for network abuse. When you find the owner of the IP address, you should probably compose a message including the evidence of the attack.

Registrar Databases

In the past, all the IP address owners were kept by the Internic. A database built from that information is at There are now 3 official registrars for North America, Asia, and Europe. Unfortunately, you will have to query each individual database. However, if you start with the North America registrar, it will tell you if the address belongs to one of the other three. Warning: The returned information is fragile; so don't send flames to these people because you have only about 30% chance of reaching the right people.

ARIN (American Registry for Internet Numbers)

RIPE (Reseaux IP Europeens)

Asia and Pacific
APNIC (Asia Pacific Network Information Centre)


Japanese English


Running traceroute will often find at least the ISP who is hosting the IP address. A reverse DNS lookup on the actual IP address is easy to spoof, but the route to the machine will reveal who is hosting the possible intruder.

Common IP addresses

Many attacks are now coming from cable-modem subscribers in the 24.x.x.x range. These are probably from machines who have been compromised by a Remote Access Trojan (RAT). (While hackers/crackers frequently use dial-up lines because they don't care if their account gets canceled, few users want to have their cable-modem accounts canceled).

Another important range is the "private address" ranges of 10.x.x.x, 192.168.x.x, and 172.16.x.x-172.31.x.x. See 3.4 below.

Addresses like 127.x.x.x indicate "localhost" and should never be seen on the Internet.

The address range 192.0.2.x has been designated for "examples", like "".

3.4 I'm seeing packets from "private" addresses (10.x.x.x et al.) on the Internet side of my firewall

The "private address" ranges are 10.x.x.x, 192.168.x.x, and 172.16.x.x-172.31.x.x.

I've been seeing these in three cases:

Core routers on the Internet are increasingly being assigned IP address in this range. There is really no need for a router to be reached from the Internet. The "forwarding" function really is independent from "sending/receiving" packets. When a router drops a packet and sends back a "ICMP TTL Exceeded" message, it will use the private address. Note that some routers are multi-homed with both private and non-private addresses. Other routers have only private addresses.
cable-modem, DSL
Most cable-modem and DSL connections are on virtual LANs over ATM. You will often see broadcast packets from neighboring machines with these private addresses.
Very rarely, you may see an address from a hackers who are spoofing addresses in this range.

3.5 What kind of scans should I expect to see from quasi-legitimate sources?

You will often see scans from somewhat legitimate sources. What I mean by this is that people will scan you without the intention of actually hacking you. For example, search engines will index your site, but it isn't an attack.
Sends echos to people in order to redirect them to a nearer server for their advertising.
Scans websites looking for illegal activities, such as copyrighted items.

3.6 I'm seeing source IP address of

If the port is also 0, then this is probably an attempt to fingerprint your system.

3.7 What are directed broadcasts and what do they mean?

  • Often indicate people scanning your subnet
  • Hackers looking for smurf amplifiers

3.8 I'm seeing strange addresses like 169.254.x.x?

From a draft document on auto-configuration of IP addresses when DHCP fails:
    Once a DHCP Client has determined it must auto-configure an IP
    address, it chooses an address.  The algorithm for choosing an
    address is implementation dependant.  The address range to use MUST
    be "169.254/16", which is registered with the IANA as the LINKLOCAL

This only happens when the normal DHCP process fails.

This new technique was introduced with Microsoft Win98 and Apple MacOS 8.5.

Also see:



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