How does a network determine if you are using a router or not?

eli2k

Member
Jun 14, 2007
124
3
81
The resnet at my university requires that all routers be placed in bridged-mode (I think that's what they indicated), or that routers have to be 'pass-through' meaning that all of your devices can connect to the router, but each device has to get a different IP from resnet. ie the router is not allowed to have an IP address, devices are not allowed to connect through the router to resnet via NAT translation.

I have one router that seemed to work fine on the network, placing it in gateway mode. My friend has a Linksys WRT54G2, the newer slim black model. Somehow, using that router, resnet is immediately able to determine that his computer his behind a router, and at the same time force a sort of pop-up page to load up saying this message, without me having to do anything. Any clues how they are able to do this, are they doing some sort of active scan to determine this?

Thanks,
- Eli
 

ScottMac

Moderator<br>Networking<br>Elite member
Mar 19, 2001
5,471
2
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Well, speculating (and it happens to be *my* favorite) ... setting the TTL (Time to Live) on their gateway router, such that putting another router in the line causes the "lifetime" of the packet to expire, which causes an ICMP message to be sent to the sender (probably a proxy on teh other side of the router). Since they know they don't have a loop (the other usual reason for TTL to expire) they can assume that there's a router in-place, and send you a love note.

There's lots of other ways too; and, don't bother asking how to get around them, that is a no-no here and will likely put you in disfavor with teh many members that are system / network admins that put these kinds of systems up.

Good Luck
 

tomt4535

Golden Member
Jan 4, 2004
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If you plug a SOHO router in right out of the box without disabling the DHCP server, they could also be detecting that and shutting down the port.
 

spidey07

No Lifer
Aug 4, 2000
65,469
5
76
Also mac address and any control protocols the router sends out. This is a VERY big deal to campus and other large networks. Plugging unauthorized devices can bring down large sections of the network. On simple misconfiguration on your part can wreak havoc - don't plug these things into a network you do not own or operate. They have very good reasons for not allowing them and there are tons of features and systems to detect them.
 

skyking

Lifer
Nov 21, 2001
22,008
4,773
146
What they all said above. If I were you, I'd take that router down to the IT guys and let them set it up as they like. That gets you another port/wireless in your room, IT is happy and you are happy.