Say that you'd like to create an El-Cheapo server accessible from the Internet, or just want to access your home PC from work AND that you have some reason to have two "routers" (i.e. NAT-firewall gizmos, commonly but erroneously named "routers").
Me for example, I don't have any physical line as the wireless broad-band (3G/4G) in the neighborhood is very good, though not as reliable as a physical line would be. I'm also very much into the idea of personal freedom and I'm into coastal sailing and to like the idea of being able to bring my stuff with me.
(I also don't want to pay the network-provider extra for housing servers, which I'm lucky not to have to from a legal standpoint because my provider is very liberal. But some others Fascist bastards in this country do have the stomach to charge for inbound traffic and they have even set up limitations for it already. Officially the reason is that they want to get payed for their services, especially for voice traffic. WTF - who cares about voice nowadays!? There's no doubt in my mind the real reason is to limit file-sharing using torrents, like this is going to be such a big hindrance. Bah, twisted trolls. May they rot in Hell... Anyway, back to the story...)
The plans for my accounts are very different. One is very fast and quite cheap downstream and has also unlimited quota - to my kids enormous joy and pleasure). The other costs virtually nothing, but is very throttled downstream, but not upstream. This is probably a mistake by the operator, but who am I to correct their mistakes. They're not correcting mine :-P. Furthermore, I'd like my "server" to be accessible by name(s), not by number and the IP-numbers change from time-to-time each time there's been a glitch an a "router" restarts.
Add to this the fact that radio-based networks of various flavors are by nature not as stable as their wired cousins, what do you do? Hmm... Interesting. Challenge accepted :-)
What you could do is use the old spare router and account you have left over and attach it to an extra NIC, and then use that extra router for inbound traffic (i.e. traffic initiated from the outside in) and the other one for outbound traffic (i.e. the other way around).
But here's the problem. Since these "routers" are not real routers, they change the packets IP headers address fields and piggy-back onto another field (proc ID) to translate back and forth. This is called NAT and is the trick behind why IPV4 has survived for so many years longer was anticipated. In practice, each household has become a network of it's own where all inside nodes share the same public IP-number.
This works extremely well, so to the point that what was once believed to be a IP-number shortage and the end of IPV4, and the big motivation behind the gigantic infrastructure undertaking of IPV6 a decade ago, now mostly seems to have become just a fart up-wind.
It was originally thought that the Net-traffic would be distributed fairly equal, which in a sense is somewhat true. But it seems no-one in the late 90' or early 20' though about that there would be that much more consumers (clients) than data-providers (servers), and that initiation of traffic would be mostly unidirectional. Big data-providers hosting your e.mail and so forth has certainly helped in that direction, even if their motivation IMO is quite questionable. NAT solved this problem over-night, and whats even better, the infrastructure cost was completely shifted away from back-bones and sub-nets, down to the end users (IPv6 would had meant a global paradigm shift, affecting each router and each computer in the world. Some people still believe IPv6 is coming, personally I consider it stillborn). Not realizing this in time and before shouting "wolf!", scaring up the whole world, Now that's what I'd call a "once in a life-time blunder"... (only superseded bu the Y2K ditto). The only outcome of such mistakes is a total loss of trustworthiness and even if there's a real need behind, any solution will be greatly delayed.
However there's a few real technical down-sides about keeping IPv4 via NAT:s in favor for IPv6, and this writing is about one of them:
For multi-homed hosts and concerning in-bound traffic: What comes in on one NIC, must go back via the same NIC (!).
This won't happen auto-magically unless you do some wizardry. The reason is that the default route in your server will always choose one NIC over the other, and if it's not the same as where it came from, the second "router" will do a NAT, and the originating outside client-socket will not be able to pair the returning package with the one it sent. Normally, it would be perfectly alright for packets to take different routes one way than the other. But if there's a NAT in the way, this just won't work and all returning packets will be lost in void.
Here's how you solve it:
Create (a) specific route(s) and rules.
This needs to be done eithe for the inferior router, or can be replicated for all your routers. The "inferior" router in this context is the one which don'y not have your servers default route set up first in it's routing table. The order of the list may differ, it's up to the OS to decide, but for computers the primary NIC is often choosen by time it got ready. Note however that this may vary a lot. Smart-phones for example tend to prefer WLAN over WAN in the (sometimes incorrect assumption) that the user would prefer WLAN or that your WLAN is always faster (which in my case for example is almost never true).
Before we continue, lets assume the following for your multi-homed host. Each NIC belongs to a separate test-networ. Networks in 192.168/16 are used for this for this purpost, so:
NIC1: 192.168.0.X / IF1: eth0 / GW1: 192.168.0.1
NIC2: 192.168.1.Y / IF2: wlan0 / GW2: 192.168.1.1
Lets furthermore assume that each "router" runs a DHCP-serice and that X and Y are any number in the allowed subnet's DHCP range ( 192.168.0. and 192.168.1. respectively). I've found it best to either allocate a DHCP entry in each "router" or to let it be fixed close to either range-end of the sub-net. For one because normally your server would be headless and besides it saves you the trouble of physically change work-plae just to look up the IP when something isn't working as it should. Except for choosing different network numbers for each subnet and make sure they don't get in contact with each other (your DHCP:s will fail if they do), the actual numbers are not that important as you will soon see. The above is just some friendly advice prto help debugging should you be needing it doing so. Personally I prefer reserve a number in the DHCP, but not some really old routers won't allow you to do that and your only choice will be the second option of narrowing the subnet down and to allocate a fixed address outside of the DHCP range, but still within the subnet range.
There's yet besides routing still the problem about the dynamic DNS thingy and the wish to reach the server's NIC:s by name, not number. Most "routers" nowadays have a dynamic-DNS updater built in. But what if your favorite dyn-DNS provider isn't supported by your router? Then you will have to run the updating from a host in the inside. We're going to do that in a cron script at the host that's supposed to be always up: teh Seeer-Veeer. Note that many of the once free-as-in-free-beer dyn-DNS service providers (like one of the true originals "DynDNS" for example to many peoples dismay) has started to charge for their service, way too much considering the actual "service" if you ask me. Remember that our "server" belongs to the fine category of El-Cheapos...
Set up a free dyn-DNS account at any of the free providers - for example: http://freedns.afraid.org/ and create two host-entries (under the same account if you wish). This service-provider has the elegant solution considering updating DNS-entries. The only thing needed is to access a randomly generated and lightly encrypted URL, known only by you, once in a while or when changes of your IP:s occur. But you want to do that with the right NIC.
So we start by creating a specific route via the right IF connected to the subnet where the intended router is at. Here's a couple of scripts doing just that for you. Note that only the first one contain the logic, the two following are just some helper scripts that can be reused whence we get as far as to routing. Put the following somewhere root can use them (/root/bin perhaps?).
Export your SHA1:s that you've got from freedns to make life easier. For the sake of it, I'm using my own case as example. SHA1:s obfuscated naturally:
export PI="aSecretString1="
export KATO="aSecretString2="
Now you can test it it works:
dyndns_update.sh eth0 $KATO
dyndns_update.sh wlan1 $PI
The scripts should be fairly scilent, unless something goes wrong or when a IP address is changed. You can try repeate the above but swap interfaces. Try pinging the names using an externally connected computer (use a shell in your smart-phone for example), ICMP will probably not be replied until the next part with routing, but the IP-addresses should be updated.
Add the corresponding information in a crontab running each 5 minutes. Remember that cron lines must be unbroken and that crontab has only a very limited set of variables. I.e. you can't use shell-expansion and hence you need to put the complete SH1-strings in there:
3,8,13,18,23,28,33,38,43,48,53,58 * * * * dyndns_update.sh eth0 aSecretString1=
4,9,14,19,24,29,34,39,44,49,54,59 * * * * dyndns_update.sh wlan1 aSecretString2=
Note that freedns has more then your hearts desire when it comes to registered suitable domain names. There's is no actual need to consider any other, even if you already have one. It's hard to compete with free. Besides freedns is also very good: Any needed updates usually propagate world-wide within seconds.
Next update will be about the actual routing...
Me for example, I don't have any physical line as the wireless broad-band (3G/4G) in the neighborhood is very good, though not as reliable as a physical line would be. I'm also very much into the idea of personal freedom and I'm into coastal sailing and to like the idea of being able to bring my stuff with me.
(I also don't want to pay the network-provider extra for housing servers, which I'm lucky not to have to from a legal standpoint because my provider is very liberal. But some others Fascist bastards in this country do have the stomach to charge for inbound traffic and they have even set up limitations for it already. Officially the reason is that they want to get payed for their services, especially for voice traffic. WTF - who cares about voice nowadays!? There's no doubt in my mind the real reason is to limit file-sharing using torrents, like this is going to be such a big hindrance. Bah, twisted trolls. May they rot in Hell... Anyway, back to the story...)
The plans for my accounts are very different. One is very fast and quite cheap downstream and has also unlimited quota - to my kids enormous joy and pleasure). The other costs virtually nothing, but is very throttled downstream, but not upstream. This is probably a mistake by the operator, but who am I to correct their mistakes. They're not correcting mine :-P. Furthermore, I'd like my "server" to be accessible by name(s), not by number and the IP-numbers change from time-to-time each time there's been a glitch an a "router" restarts.
Add to this the fact that radio-based networks of various flavors are by nature not as stable as their wired cousins, what do you do? Hmm... Interesting. Challenge accepted :-)
What you could do is use the old spare router and account you have left over and attach it to an extra NIC, and then use that extra router for inbound traffic (i.e. traffic initiated from the outside in) and the other one for outbound traffic (i.e. the other way around).
But here's the problem. Since these "routers" are not real routers, they change the packets IP headers address fields and piggy-back onto another field (proc ID) to translate back and forth. This is called NAT and is the trick behind why IPV4 has survived for so many years longer was anticipated. In practice, each household has become a network of it's own where all inside nodes share the same public IP-number.
This works extremely well, so to the point that what was once believed to be a IP-number shortage and the end of IPV4, and the big motivation behind the gigantic infrastructure undertaking of IPV6 a decade ago, now mostly seems to have become just a fart up-wind.
It was originally thought that the Net-traffic would be distributed fairly equal, which in a sense is somewhat true. But it seems no-one in the late 90' or early 20' though about that there would be that much more consumers (clients) than data-providers (servers), and that initiation of traffic would be mostly unidirectional. Big data-providers hosting your e.mail and so forth has certainly helped in that direction, even if their motivation IMO is quite questionable. NAT solved this problem over-night, and whats even better, the infrastructure cost was completely shifted away from back-bones and sub-nets, down to the end users (IPv6 would had meant a global paradigm shift, affecting each router and each computer in the world. Some people still believe IPv6 is coming, personally I consider it stillborn). Not realizing this in time and before shouting "wolf!", scaring up the whole world, Now that's what I'd call a "once in a life-time blunder"... (only superseded bu the Y2K ditto). The only outcome of such mistakes is a total loss of trustworthiness and even if there's a real need behind, any solution will be greatly delayed.
However there's a few real technical down-sides about keeping IPv4 via NAT:s in favor for IPv6, and this writing is about one of them:
For multi-homed hosts and concerning in-bound traffic: What comes in on one NIC, must go back via the same NIC (!).
This won't happen auto-magically unless you do some wizardry. The reason is that the default route in your server will always choose one NIC over the other, and if it's not the same as where it came from, the second "router" will do a NAT, and the originating outside client-socket will not be able to pair the returning package with the one it sent. Normally, it would be perfectly alright for packets to take different routes one way than the other. But if there's a NAT in the way, this just won't work and all returning packets will be lost in void.
Here's how you solve it:
Create (a) specific route(s) and rules.
This needs to be done eithe for the inferior router, or can be replicated for all your routers. The "inferior" router in this context is the one which don'y not have your servers default route set up first in it's routing table. The order of the list may differ, it's up to the OS to decide, but for computers the primary NIC is often choosen by time it got ready. Note however that this may vary a lot. Smart-phones for example tend to prefer WLAN over WAN in the (sometimes incorrect assumption) that the user would prefer WLAN or that your WLAN is always faster (which in my case for example is almost never true).
Before we continue, lets assume the following for your multi-homed host. Each NIC belongs to a separate test-networ. Networks in 192.168/16 are used for this for this purpost, so:
NIC1: 192.168.0.X / IF1: eth0 / GW1: 192.168.0.1
NIC2: 192.168.1.Y / IF2: wlan0 / GW2: 192.168.1.1
Lets furthermore assume that each "router" runs a DHCP-serice and that X and Y are any number in the allowed subnet's DHCP range ( 192.168.0. and 192.168.1. respectively). I've found it best to either allocate a DHCP entry in each "router" or to let it be fixed close to either range-end of the sub-net. For one because normally your server would be headless and besides it saves you the trouble of physically change work-plae just to look up the IP when something isn't working as it should. Except for choosing different network numbers for each subnet and make sure they don't get in contact with each other (your DHCP:s will fail if they do), the actual numbers are not that important as you will soon see. The above is just some friendly advice prto help debugging should you be needing it doing so. Personally I prefer reserve a number in the DHCP, but not some really old routers won't allow you to do that and your only choice will be the second option of narrowing the subnet down and to allocate a fixed address outside of the DHCP range, but still within the subnet range.
There's yet besides routing still the problem about the dynamic DNS thingy and the wish to reach the server's NIC:s by name, not number. Most "routers" nowadays have a dynamic-DNS updater built in. But what if your favorite dyn-DNS provider isn't supported by your router? Then you will have to run the updating from a host in the inside. We're going to do that in a cron script at the host that's supposed to be always up: teh Seeer-Veeer. Note that many of the once free-as-in-free-beer dyn-DNS service providers (like one of the true originals "DynDNS" for example to many peoples dismay) has started to charge for their service, way too much considering the actual "service" if you ask me. Remember that our "server" belongs to the fine category of El-Cheapos...
Set up a free dyn-DNS account at any of the free providers - for example: http://freedns.afraid.org/ and create two host-entries (under the same account if you wish). This service-provider has the elegant solution considering updating DNS-entries. The only thing needed is to access a randomly generated and lightly encrypted URL, known only by you, once in a while or when changes of your IP:s occur. But you want to do that with the right NIC.
So we start by creating a specific route via the right IF connected to the subnet where the intended router is at. Here's a couple of scripts doing just that for you. Note that only the first one contain the logic, the two following are just some helper scripts that can be reused whence we get as far as to routing. Put the following somewhere root can use them (/root/bin perhaps?).
Export your SHA1:s that you've got from freedns to make life easier. For the sake of it, I'm using my own case as example. SHA1:s obfuscated naturally:
export PI="aSecretString1="
export KATO="aSecretString2="
Now you can test it it works:
dyndns_update.sh eth0 $KATO
dyndns_update.sh wlan1 $PI
The scripts should be fairly scilent, unless something goes wrong or when a IP address is changed. You can try repeate the above but swap interfaces. Try pinging the names using an externally connected computer (use a shell in your smart-phone for example), ICMP will probably not be replied until the next part with routing, but the IP-addresses should be updated.
Add the corresponding information in a crontab running each 5 minutes. Remember that cron lines must be unbroken and that crontab has only a very limited set of variables. I.e. you can't use shell-expansion and hence you need to put the complete SH1-strings in there:
3,8,13,18,23,28,33,38,43,48,53,58 * * * * dyndns_update.sh eth0 aSecretString1=
4,9,14,19,24,29,34,39,44,49,54,59 * * * * dyndns_update.sh wlan1 aSecretString2=
Note that freedns has more then your hearts desire when it comes to registered suitable domain names. There's is no actual need to consider any other, even if you already have one. It's hard to compete with free. Besides freedns is also very good: Any needed updates usually propagate world-wide within seconds.
Next update will be about the actual routing...
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