Linux Ntl FAQ

NTL & Linux FAQ

NTL own a lot wires in Hampshire, typically the ones running past your home. Lots of Hampshire Linux users have NTL as their ISP, and while the call centre won’t be able to help you with Linux, the NTL service is fully compatible with Linux and other Unixes.

Binary-only diallers?

When some diallup NTL users who have BT phones had to change dial-up numbers (because of BT), NTL brought out a Binary-only dialler program for Windows and Mac OS. In fact all this does is substitute one diallup number with another, and all users of any OS need to do is use the new number. (Which is?)

Arp Spam

People with NTL cable BroadBand often complain about receiving very large amounts of ARP traffic. In fact this is perfectly normal. The packets, although large in number, are small in size, and they are a essential on a large Ethernet network, which is what NTL city-wide DocSis networks resemble. They are also local to the IP network – they don’t get out onto the Internet and don’t contribute to daily data-transfer limits. If you weren’t getting ARP traffic there would be something wrong with your connection

USB or Ethernet?

NTL cable modems typically come with USB and Ethernet ports. Use Ethernet. USB is vastly heavier on your CPU (due to the nature of the USB itself) and a much inferior networking system. Ethernet is a lot more reliable.

(Nothing to do with NTL, but try to use a decent Ethernet NIC in your PC. Brand new £30 3Com ones are really good, and vastly outclass £10 Realtek and most second hand NICs.)

Using Ethernet

The cable modems (in fact they are ethernet bridges) are wired CDI (straight through), so of you are connecting your PC (i.e. your Firewall or personal PC) to it directly you muct use a cross-over ethernet cable. If you are connecting several devices directly (i.e. you are a premium customer of some kind, like you have an XBox account or you are a business customer, and NTL give you more than one global IP address) to the Internet (they aren’t on a seperate LAN behind your firewall) you should use a straight-through ethernet cable to connect your cable modem to an ethernet switch, to which all the devices with public addresses should be attached.

=== Registration === Using the sign-up CD isn’t compulsory. Last time I looked it just directed people to https://autoreg.autoregister.net/. If you change cable modems you must reregister, as a new cable modem or set top box has a different MacAddress and without registering it, the only site on the Internet your uBR will actually let you use is the registration site!

Stealth Proxying and Wccp

NTL do StrealthProxying (using Wccp) on the core routers of some metropolitan networks, diverting Tcp Port 80 traffic (i.e web sites) to various local caches. If this happens on the network you are on, and it doesn’t suit you, your only real option is to specify another proxy, either nearby (for best performance) or elsewhere on the Internet for improved privacy. If this cost-saving measure (saves off-network data tranfers, hence saves of peering bills to other ISPs) bothers you, why not suggest to NTL they offer a premium service to power users who want that kind of sevice for the same bandwidth?

=== Static Addresses === IP addresses are semi-static via DHCP. If you leave your directly connected equipment (i.e. your CM and you PC) switched on and hooked up, it should keep its IP addres up until NTL have to do a network re-seg, changing the layout and distrubtion of IP address block. This happens everywhere in the end, and possibly as often as every 2 months if the number of customers is growing particularly rapidly, or a previous re-seg didn’t allow enough address space for future growth. If you have a business account then you keep your 5 IPs even when the PCs are switched off, although re-seging still has to happen occasionally.

=== DHCP or Static IPs === You have to use DHCP, even if your address stays the same. The uBR (which is a Default gateway for cable modems on DocSis networks, as well as being a bit like a wireless base-station (but with wires)) will check with the DHCP server to see if you have a valid DHCP Lease. If you manually set your IP address on the PC connected to your cable modem it will work for as long as the lease remains valid, then it will stop working as your lease won’t get renewed at the end of the day/week/time period. DHCP is compulsory. That does not stop your IP address from remaining essentially fixed for often very long periods of time.

Port Blocking

Do NTL block any TCP or UDP ports? Yes they do, TCP and UDP Port 135. This is because too many unfirewalled and unpatched Windows users get cracked using exploits on these ports, and this fills NTL’s network and email systems with unwanted and hostile traffic. If you need to use this port for your l33t Perl-based client-server prog you wrote, and your service level is below that of LeasedLine, you will have to find a different port, as TCP & UDP 135 is closed.

Firewalls and Home LANs

Home LANs are allowed. Look in the Terms And Conditions. It actually says no more than 3 PCs behind the firewall, but IMO that should be interpreted as not drawing attention to yourself, and if necessary using something like DummyNet to rate-limit your users under certain conditions at you firewall. If you hammer the UBR for 3000 other people to the extent that NTL notice the UBR is slower than all its peers and has a large proportion of traffic coming from one or two cable modems, with lots of diverse simultaneous (Kazaa) TCP sessions and UDP frags all at once, then they will catch you. FreeBSD‘s DummyNet allows all kinds of clever per-port and per-IP restrictions in both directions, and it really is something to consider if your flatmates are like mine.