Comments for Is DHCP safe?

Comment from Ziggie on 2008-07-03

2008-07-04T02:20:37Z

I don't have a way of posting a screen shot, but if you go into your TCP/IP settings (In Vista, you have to select Version 4), you have the general tab which allows you to set your static IP address. There is also a tab named "Alternate Configuration". If Windows cannot get a DHCP address, this is what it will use. You can choose 'Automatic private IP address' or specify your own.

I ran into this once at a convention that was not using DHCP. It took forever to track down when they kept getting AN ip address, but not the right one.

--ziggs

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Comment from Leo on 2008-07-03

2008-07-03T20:41:35Z

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I don't think so. Windows assigns a 169.x.x.x when DHCP
fails.

Leo

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Comment from Robert on 2008-07-02

2008-07-02T21:39:28Z

Leo:
Since he got a 192.168.x.x address back which is a private address, it probably means he had a DHCP failure and so Windows defaulted to giving the machine the 192.168 address. If you go into the Internet Protocol (TCP/IP) Properties page and look at Alternate Configuration, you will probably see it set to Automatic Private IP Address.

So what probably happened is the machine requested a DHCP assigned IP address, got no response and assigned the private IP address.

There is probably no rogue device on the network.

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Comment from Bud on 2008-07-01

2008-07-01T21:31:21Z

PC World had a very good article in their July 2006 magazine giving instructions for tracing addresses. They reference Microsoft's documentation page on their site at: "find.pcworld.com/52612".

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Comment from Octav on 2008-07-01

2008-07-01T17:20:10Z

Let's be on the same page here: "a website reporting their IP address back to them" is unlikely to report a 10.0.0.0/8 address since, per [RFC 1918], this range, together with 172.16.0.0/12 and 192.168.0.0/16, are reserved for LAN use only, which means no router will forward this source IP over the Internet (though the address will be reported if the website in question is in the same LAN as the user, of course).

Similarly, and for completeness, per [RFC 3330], 127.0.0.0/8 is "assigned for use as the Internet host loopback address. A datagram sent by a higher level protocol to an address anywhere within this block should loop back inside the host. This is ordinarily implemented using only 127.0.0.1/32 for loopback, but no addresses within this block should ever appear on any network anywhere [RFC1700, page 5]."

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Comment from Leo on 2008-06-29

2008-06-29T23:50:47Z

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My assumption (adminitedly an assumption) is that he's
connecting directly to the internet connection provided, OR
he's reporting the IP address assigned to his router. The
KEY clue is that he *sometimes* get what he expects (10.)
and othertimes not (192.). Regardless of what interface he's
looking at or how, the issue seems to be a rogue DHCP
server.

Regardless, the questions asked are still valid - rogue DCHP
servers can cause ... issues. :-)

Leo

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Comment from Greg Bulmash on 2008-06-29

2008-06-29T09:37:17Z

The reader may not have anything unconventional happening at all. Let's say they're connecting via a home router. If they check their IP via their network status on their machine, it will be 192.168.xxx.xxx, which is normal for home routers. It's actually one of the firewall methods you recommend most... NAT.

If they check their IP address online, via a website reporting their IP address back to them, it's going to be 10.xxx.xxx.xxx, because only their PC and the router see the 192.168.xxx.xxx address. The rest of the world sees the 10.xxx.xxx.xxx coming from the router.

It would be good to know how they're connecting to their ISP and whether it's hardwired or wireless. You could probably get some good clues right there.

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