UPDATE #1
After having applied this patch, the system continues to suffer of the problem and therefore I had to investigate the root cause. First of all, I started to play with the iptables and I discovered that the source and destination ports for offering the DHCP service are inverted, in that way the ports are for using the DHCP (client).
As you can see the second rule in the INPUT chain is the same which is set in connman-INPUT but the one that works for offering the service is the first. Both are necessary, obviously and thus I updated my patch as well.
- Project: udhcpd-tether-config-fixing (v0.0.2, updated after a test)
2 638 ACCEPT udp -- * * 0.0.0.0/0 0.0.0.0/0 udp spt:68 dpt:67
0 0 ACCEPT udp -- * * 0.0.0.0/0 0.0.0.0/0 udp spt:67 dpt:68
I am quite surprised to found out something like this because I cannot believe that nobody noticed before. Possibly, I am still overlooking something.
iptables investigation
iptables -nvL; iptables -nvL -t nat;
Chain INPUT (policy ACCEPT 46 packets, 3931 bytes)
pkts bytes target prot opt in out source destination
4 1312 ACCEPT udp -- * * 0.0.0.0/0 0.0.0.0/0 udp spt:68 dpt:67
0 0 ACCEPT udp -- * * 0.0.0.0/0 0.0.0.0/0 udp spt:67 dpt:68
Chain FORWARD (policy ACCEPT 1292 packets, 830K bytes)
pkts bytes target prot opt in out source destination
Chain OUTPUT (policy ACCEPT 40 packets, 8071 bytes)
pkts bytes target prot opt in out source destination
1250 157K connman-OUTPUT all -- * * 0.0.0.0/0 0.0.0.0/0
Chain connman-INPUT (0 references)
pkts bytes target prot opt in out source destination
4 240 ACCEPT tcp -- tether * 0.0.0.0/0 0.0.0.0/0 tcp flags:0x17/0x02 multiport dports 22,2222
0 0 ACCEPT all -- rndis0 * 0.0.0.0/0 0.0.0.0/0
0 0 ACCEPT tcp -- lo * 0.0.0.0/0 0.0.0.0/0 tcp flags:0x17/0x02 multiport dports 22,2222
0 0 DROP tcp -- !rndis0 * 0.0.0.0/0 0.0.0.0/0 tcp flags:0x17/0x02 multiport dports 22,2222
0 0 ACCEPT udp -- * * 0.0.0.0/0 0.0.0.0/0 udp spt:67 dpt:68
0 0 ACCEPT icmp -- * * 0.0.0.0/0 0.0.0.0/0 icmp !type 8 code 0
0 0 ACCEPT 33 -- * * 0.0.0.0/0 0.0.0.0/0 multiport dports 1024:65535
0 0 ACCEPT sctp -- * * 0.0.0.0/0 0.0.0.0/0 multiport dports 1024:65535
0 0 ACCEPT tcp -- * * 0.0.0.0/0 0.0.0.0/0 multiport dports 1024:65535
0 0 ACCEPT udplite-- * * 0.0.0.0/0 0.0.0.0/0 multiport dports 1024:65535
0 0 ACCEPT udp -- * * 0.0.0.0/0 0.0.0.0/0 multiport dports 1024:65535
0 0 ACCEPT 47 -- * * 0.0.0.0/0 0.0.0.0/0
0 0 ACCEPT esp -- * * 0.0.0.0/0 0.0.0.0/0
0 0 ACCEPT ah -- * * 0.0.0.0/0 0.0.0.0/0
0 0 ACCEPT all -- lo * 0.0.0.0/0 0.0.0.0/0
374 31159 ACCEPT all -- * * 0.0.0.0/0 0.0.0.0/0 ctstate RELATED,ESTABLISHED
Chain connman-OUTPUT (1 references)
pkts bytes target prot opt in out source destination
0 0 ACCEPT all -- * rndis0 0.0.0.0/0 0.0.0.0/0
81 11775 ACCEPT icmp -- * * 0.0.0.0/0 0.0.0.0/0 icmp !type 0 code 0
Chain PREROUTING (policy ACCEPT 651 packets, 55779 bytes)
pkts bytes target prot opt in out source destination
Chain INPUT (policy ACCEPT 123 packets, 11059 bytes)
pkts bytes target prot opt in out source destination
Chain OUTPUT (policy ACCEPT 9 packets, 626 bytes)
pkts bytes target prot opt in out source destination
Chain POSTROUTING (policy ACCEPT 7758 packets, 493K bytes)
pkts bytes target prot opt in out source destination
101 6824 connman-POSTROUTING all -- * * 0.0.0.0/0 0.0.0.0/0
Chain connman-POSTROUTING (1 references)
pkts bytes target prot opt in out source destination
94 6340 MASQUERADE all -- * rmnet_data1 172.28.172.0/24 0.0.0.0/0