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This is a corollary to the question Why don't ISPs filter on source address to prevent spoofing?.

Are there valid reasons to spoof an address?

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I would take a look at the question what-security-risks-does-ip-spoofing-bring . I feel these questions are very similar, if not duplicate? –  Chris Andrè Dale Dec 9 '10 at 15:37
    
I think this question is different than your question. At first glance I only see risks; I'd like to know how a Corporate IT environment can benefit from IP address spoofing. Link: security.stackexchange.com/questions/1009/… –  makerofthings7 Dec 9 '10 at 15:40
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The two could easily meet in the middle, but the origins are different enough that the answers could be interesting. –  Scott Pack Dec 9 '10 at 15:51

3 Answers 3

up vote 6 down vote accepted

I found an article here which describes some legit examples for spoofing IP:

  • In mobile IP environments, where a roaming host must use a "home" IP address in a foreign network (ref. C. Perkins, "IP Mobility Support for IPv4)
  • virtual private networks that set the host IP to an address local to the organization's network
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I updated the answer and added the old as a comment :) –  Chris Andrè Dale Dec 9 '10 at 15:49
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I don't see enough real-world justification for spoofing yet, and still don't think the need outweighs the risks involved. I hope more people post more details on how an application uses spoofing, and why those relying apps can't be upgraded to use something higher in the OSI stack. –  makerofthings7 Dec 9 '10 at 23:56
    
which VPN do that? –  curiousguy Jun 25 '12 at 19:38
    
If I'm not mistaken, listening for and responding to Anycast addresses might be considered spoofing as it appears that an otherwise unique address is present on two sides of a router –  freddyb Oct 5 '12 at 20:06

Mobile IP networks are not really a justification for spoofing. RFC 2344 Reverse Tunneling provides an answer to allow Mobile IP to work with ingress filtering / antispoofing protection.

I'm not sure of current recommendations but old (2000) RFCs like RFC 3013 ISP recommendations recommend ingress and egress filtering to stop spoofing.

I don't think there is a real legitimate reason for spoofing on the public internet. Occasionally, private intranets might have a reason, just like they have a reason to do arp-proxying (a router masquerades as a host and forwards the packets) sometimes.

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"Mobile IP networks are not really a justification for spoofing. RFC 2344 Reverse Tunneling provides an answer to allow Mobile IP to work with ingress filtering / antispoofing protection." An answer for antispoofing hardly make spoofing not justified. –  curiousguy Jun 25 '12 at 19:40
    
Huh? Mobile IP was put forth as a justification for spoofing, but the justification doesn't exist because there are ways to do mobile IP without spoofing. –  rox0r Jun 27 '12 at 19:42
    
"Mobile IP was put forth as a justification for spoofing" yes, and it is a valid justification. "there are ways to do mobile IP without spoofing" so? –  curiousguy Jun 27 '12 at 19:43
    
What are you confused about? –  rox0r Jun 27 '12 at 19:48
    
What constitutes a "justification" for something? –  curiousguy Jun 27 '12 at 20:19

One possible usage scenario is a corporate internet filtering environment which is not configured inline (that is between the internet and users) but monitors traffic off a network SPAN/TAP.

In this scenario, when a user visits a site the web filtering environment has listed on a block list, the web filtering application may spoof the source IP of the web server and send a TCP reset packet back to the client, web server, or both, to kill the connection.

Websense web filtering products can operate in this way, for example.

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oh. That's pretty nasty. I can see people just constantly hitting refresh and being frustrated as to why the internet doesn't work. –  rox0r Jun 27 '12 at 19:43
    
Typically you would only block certain sites, otherwise you would just block web access for those users completely on a firewall/router. Most web filtering products will serve a block page as well, so the user gets some feedback. Also, if it only sends the RST back to the user, you should be able to drop those packets using a host-based firewall which can bypass the block. –  lew Jun 28 '12 at 6:20

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