A question that seems to come up often, is how to provide a static IPs to UEs on Open5GS EPC.
By default all UEs are allocated an internal IP that the server the P-GW is running on NATs out, but many users want to avoid NAT, for obvious reasons.
Open5GS has the ability to set a Static IP for each APN a subscriber has, but let’s get one thing out of the way first;
LTE is not Ethernet. No broadcast, no multicast. Each IP Address is best thought of as a single /32 network.
This means you can’t have the UEs in your LTE network in the same 192.168.1.x subnet as your home network along with your laptop and printer, it’s not how it works.
So with that out of the way, let’s talk about how to do static IP address allocation in Open5GS EPC.
Assigning a Subscriber a Static IP Address
From the HSS edit the Subscriber and in the UE IPv4 or UE IPv6 address, set the static address you want to use.
You can set any UE IP Address here and it’ll get allocated to that UE.
But – there’s an issue.
The problem is not so much on the Open5GS P-GW implementation, but just how TCP/IP routing works in general.
Let’s say I assign the UE IPv4 address 220.127.116.11 to my UE. From the UE it sends a packet with the IPv4 Source address of 18.104.22.168 to anywhere on the internet, the eNB puts the packet in GTP and eventually the it gets to the P-GW which sends it out onto the internet from the source address 22.214.171.124.
The problem is that the response will never get back to me, as 126.96.36.199 is not allocated to me and will never make it back to my P-GW, so never relayed back to the UE.
For TCP traffic this means I can send the SYN with the source address of 188.8.131.52, but the SYN/ACK will be routed back to the real 184.108.40.206, and not to me, so the TCP socket will never get opened.
So while we can set a static IPs to be allocated to UEs in Open5GS, unless the traffic can be routed back to these IPs it’s not much use.
So let’s say we have assigned IP 220.127.116.11 to the UE, we’d need to put a static route on our routers to route traffic to the IP of the PGW. In my case the PGW is 10.0.1.121, so I’ll need to add a static route to get traffic destined 18.104.22.168/32 to 10.0.1.121.
In a more common case we’d assign internal IP subnets for the UE pool, and then add routes for the entire subnet to the IP of the PGW.