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Debugging why dpinger does not work over t-mobile 5g but ping from the command line does work. It appears that t-mobile drops icmp echo requests with less that 4 bytes of data.

Four bytes of data works ...

[25.07.1-RELEASE][root@gatekeeper.no-such-agency.net]/etc: ping -s 4 4.2.2.2
PING 4.2.2.2 (4.2.2.2): 4 data bytes
12 bytes from 4.2.2.2: icmp_seq=0 ttl=51
12 bytes from 4.2.2.2: icmp_seq=1 ttl=51
12 bytes from 4.2.2.2: icmp_seq=2 ttl=51
^C
--- 4.2.2.2 ping statistics ---
3 packets transmitted, 3 packets received, 0.0% packet loss

Three (or less) bytes does not work

[25.07.1-RELEASE][root@gatekeeper.no-such-agency.net]/etc: ping -s 3 4.2.2.2
PING 4.2.2.2 (4.2.2.2): 3 data bytes
^C
--- 4.2.2.2 ping statistics ---
3 packets transmitted, 0 packets received, 100.0% packet loss


Patching /etc/inc/gwlb.inc to change dpinger to -d 4 got it working.

Two possible fixes
1) change the default to >= 4
2) make the data length a config item

Options 2 is more work but better.