At first blush, it looks like Debian is stretching the boundaries for sending an ICMP redirect; quoting RFC 792 (Internet Protocol).
In this case, G1 is ( above), X is and G2 is , and the source is (i.e. ). Maybe this has been historically interpreted differently in the case of interface aliases (or secondary addresses) on the same interface, but strictly speaking I'm not sure we should see Debian send that redirect.
Depending on your requirements, you might be able to solve this by making the subnet for something like (host addresses from - ) instead of using interface aliases for individual blocks (, , , ); if you did this, you'll need to change the netmask of all hosts attached and you wouldn't be able to use 10.1.4.x unless you expanded to a .
We're venturing a bit outside the scope of the original question, but I'll help work through the design/security issues mentioned in your comment.
If you want to isolate users in your office from each other, let's step back for a second and look at some security issues with what you have now:
You currently have four subnets in one ethernet broadcast domain. All users in one broadcast domain doesn't meet the security requirements you articulated in the comments (all machines will see broadcasts from other machines and could spontaneously send traffic to each other at Layer2, regardless of their default gateway being , , or ). There is nothing your Debian firewall can do to change this (or maybe I should say there is nothing your Debian firewall should do to change this :-).
- You need to assign users into Vlans, based on security policy stated in the comments. A properly-configured Vlan will go a long way to fixing the issues mentioned above. If your ethernet switch doesn't support Vlans, you should get one that does.
- With respect to multiple security domains accessing , you have a couple of options:
- Option 1: Given the requirement for all users to access services on , you could put all users in one IP subnet and implement security policies with Private Vlans (RFC 5517), assuming your ethernet switch supports this. This option will not require rules to limit intra-office traffic from crossing security boundaries (that is accomplished with private Vlans).
- Option 2: You could put users into different subnets (corresponding to Vlans) and implement rules to deploy your security policies
- After you have secured your network at the Vlan level, set up source-based routing policies to send different users out your multiple uplinks.
FYI, if you have a router that supports VRFs, some of this gets even easier; IIRC, you have a Cisco IOS machine onsite. Depending on the model and software image you already have, that Cisco could do a fantastic job isolating your users from each other and implement source-based routing policies.
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