1

Really stuck on this one I want to change the gateway on my pc to go through my phone connected to the same wifi network in my home. When hotspot is on and I connect to it with another device it works as expected. When i disable hotspot and change my pc gateway to the phone and enable forwarding the counters on iptables FORWARD table don't change.

I have enabled all of the portforwarding that hotspot uses please see sysctl output below

PC IP is 192.168.0.57

The phone/tablet i am trying to connect through is a Samsung Galaxy TAB A6 currently running lineage custom rom Android 9 wasn't successful with the 8.1 stock image either

Running ping 8.8.8.8 from the pc gives these results:

The tcpdump reply when hotspot is on

06:55:33.500227 IP 19.18.39.197 > dns.google: ICMP echo request, id 21730, seq 1, length 64
06:55:33.528039 IP dns.google > 19.18.39.197: ICMP echo reply, id 21730, seq 1, length 64
06:55:34.578520 IP 19.18.39.197 > dns.google: ICMP echo request, id 21731, seq 1, length 64
06:55:34.603065 IP dns.google > 19.18.39.197: ICMP echo reply, id 21731, seq 1, length 64
06:55:35.717137 IP 19.18.39.197 > dns.google: ICMP echo request, id 21732, seq 1, length 64
06:55:35.763043 IP dns.google > 19.18.39.197: ICMP echo reply, id 21732, seq 1, length 64

The tcpdump reply without the hotspot

06:58:16.566254 IP 19.18.39.197 > 192.168.0.57: ICMP net cache2.service.virginmedia.net unreachable, 
length 68

Which just keeps repeating

ifconfig and ip r:

:/ # ip r
19.0.0.0/8 dev rmnet1 proto kernel scope link src 19.18.39.197
192.168.0.0/24 dev wlan0 proto kernel scope link src 192.168.0.50
:/ # ifconfig
wlan0     Link encap:Ethernet  HWaddr 08:78:08:ac:fc:04  Driver ar6k_wlan
          inet addr:192.168.0.50  Bcast:192.168.0.255  Mask:255.255.255.0
          inet6 addr: fe80::a78:8ff:feac:fc04/64 Scope: Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:5876 errors:0 dropped:0 overruns:0 frame:0
          TX packets:3845 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:3000
          RX bytes:935863 TX bytes:580125

rmnet1    Link encap:Point-to-Point Protocol
          inet addr:19.18.39.197  P-t-P:19.18.39.197  Mask:255.0.0.0
          UP POINTOPOINT RUNNING NOARP MULTICAST  MTU:1500  Metric:1
          RX packets:3351 errors:0 dropped:0 overruns:0 frame:0
          TX packets:3383 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:1564260 TX bytes:1053730

lo        Link encap:Local Loopback
          inet addr:127.0.0.1  Mask:255.0.0.0
          inet6 addr: ::1/128 Scope: Host
          UP LOOPBACK RUNNING  MTU:65536  Metric:1
          RX packets:5435 errors:0 dropped:0 overruns:0 frame:0
          TX packets:5435 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0
          RX bytes:869529 TX bytes:869529

iptables - Nothing changes on the forward chain with hotspot off

:/ # iptables -vnL
Chain INPUT (policy ACCEPT 6542 packets, 1282K bytes)
 pkts bytes target     prot opt in     out     source               destination

Chain FORWARD (policy ACCEPT 1666 packets, 346K bytes)
 pkts bytes target     prot opt in     out     source               destination

Chain OUTPUT (policy ACCEPT 6514 packets, 925K bytes)
 pkts bytes target     prot opt in     out     source               destination

:/ # iptables -t nat -vnL
Chain PREROUTING (policy ACCEPT 3072 packets, 203K bytes)
pkts bytes target     prot opt in     out     source               destination

Chain INPUT (policy ACCEPT 2074 packets, 126K bytes)
pkts bytes target     prot opt in     out     source               destination

Chain OUTPUT (policy ACCEPT 199 packets, 13593 bytes)
pkts bytes target     prot opt in     out     source               destination

Chain POSTROUTING (policy ACCEPT 62 packets, 4337 bytes)
pkts bytes target     prot opt in     out     source               destination
710 58824 MASQUERADE  all  --  *      rmnet1  0.0.0.0/0            0.0.0.0/0

Sysctl (These get disabled when you turn hotspot off and i reenable them)

  :/ # sysctl -a |grep forward
    net.ipv4.conf.all.forwarding = 1
    net.ipv4.conf.all.mc_forwarding = 0
    net.ipv4.conf.default.forwarding = 1
    net.ipv4.conf.default.mc_forwarding = 0
    net.ipv4.conf.ip6tnl0.forwarding = 1
    net.ipv4.conf.ip6tnl0.mc_forwarding = 0
    net.ipv4.conf.lo.forwarding = 1
    net.ipv4.conf.lo.mc_forwarding = 0
    net.ipv4.conf.rmnet0.forwarding = 1
    net.ipv4.conf.rmnet0.mc_forwarding = 0
    net.ipv4.conf.rmnet1.forwarding = 1
    net.ipv4.conf.rmnet1.mc_forwarding = 0
    net.ipv4.conf.rmnet2.forwarding = 1
    net.ipv4.conf.rmnet2.mc_forwarding = 0
    net.ipv4.conf.rmnet3.forwarding = 1
    net.ipv4.conf.rmnet3.mc_forwarding = 0
    net.ipv4.conf.rmnet4.forwarding = 1
    net.ipv4.conf.rmnet4.mc_forwarding = 0
    net.ipv4.conf.rmnet5.forwarding = 1
    net.ipv4.conf.rmnet5.mc_forwarding = 0
    net.ipv4.conf.rmnet6.forwarding = 1
    net.ipv4.conf.rmnet6.mc_forwarding = 0
    net.ipv4.conf.rmnet7.forwarding = 1
    net.ipv4.conf.rmnet7.mc_forwarding = 0
    net.ipv4.conf.sit0.forwarding = 1
    net.ipv4.conf.sit0.mc_forwarding = 0
    net.ipv4.conf.umts_dm0.forwarding = 1
    net.ipv4.conf.umts_dm0.mc_forwarding = 0
    net.ipv4.conf.wlan0.forwarding = 1
    net.ipv4.conf.wlan0.mc_forwarding = 0
    net.ipv4.ip_forward = 1
    net.ipv4.ip_forward_use_pmtu = 0
    sysctl: key 'net.ipv4.route.flush': Permission denied
    net.ipv6.conf.all.forwarding = 1
    net.ipv6.conf.default.forwarding = 1
    net.ipv6.conf.ip6tnl0.forwarding = 1
    net.ipv6.conf.lo.forwarding = 1
    net.ipv6.conf.rmnet0.forwarding = 1
    net.ipv6.conf.rmnet1.forwarding = 1
    net.ipv6.conf.rmnet2.forwarding = 1
    net.ipv6.conf.rmnet3.forwarding = 1
    net.ipv6.conf.rmnet4.forwarding = 1
    net.ipv6.conf.rmnet5.forwarding = 1
    net.ipv6.conf.rmnet6.forwarding = 1
    net.ipv6.conf.rmnet7.forwarding = 1
    net.ipv6.conf.sit0.forwarding = 1
    net.ipv6.conf.umts_dm0.forwarding = 1
    net.ipv6.conf.wlan0.forwarding = 1

Feels like hotspot is changing an option that i don't know about when it turns on that enables forwarding hoping someone has an idea?

EDIT

Thanks to the help from A.B this problem was due to ip rules being created by android when the hotspot is on that i wasn't replicating when the hotspot is off.

As requested here are the tables first with the hotspot off

:/ # ip link
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN mode DEFAULT group default
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
2: umts_dm0: <POINTOPOINT,MULTICAST,NOARP> mtu 1500 qdisc noop state DOWN mode DEFAULT group default qlen 1000
    link/ppp
3: rmnet0: <POINTOPOINT,MULTICAST,NOARP> mtu 1500 qdisc noop state DOWN mode DEFAULT group default qlen 1000
    link/ppp
4: rmnet1: <POINTOPOINT,MULTICAST,NOARP,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UNKNOWN mode DEFAULT group default qlen 1000
    link/ppp
5: rmnet2: <POINTOPOINT,MULTICAST,NOARP> mtu 1500 qdisc noop state DOWN mode DEFAULT group default qlen 1000
    link/ppp
6: rmnet3: <POINTOPOINT,MULTICAST,NOARP> mtu 1500 qdisc noop state DOWN mode DEFAULT group default qlen 1000
    link/ppp
7: rmnet4: <POINTOPOINT,MULTICAST,NOARP> mtu 1500 qdisc noop state DOWN mode DEFAULT group default qlen 1000
    link/ppp
8: rmnet5: <POINTOPOINT,MULTICAST,NOARP> mtu 1500 qdisc noop state DOWN mode DEFAULT group default qlen 1000
    link/ppp
9: rmnet6: <POINTOPOINT,MULTICAST,NOARP> mtu 1500 qdisc noop state DOWN mode DEFAULT group default qlen 1000
    link/ppp
10: rmnet7: <POINTOPOINT,MULTICAST,NOARP> mtu 1500 qdisc noop state DOWN mode DEFAULT group default qlen 1000
    link/ppp
11: sit0@NONE: <NOARP> mtu 1480 qdisc noop state DOWN mode DEFAULT group default
    link/sit 0.0.0.0 brd 0.0.0.0
12: ip6tnl0@NONE: <NOARP> mtu 1452 qdisc noop state DOWN mode DEFAULT group default
    link/tunnel6 :: brd ::
13: wlan0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP mode DORMANT group default qlen 3000
    link/ether 08:78:08:ac:fc:04 brd ff:ff:ff:ff:ff:ff
14: p2p0: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN mode DEFAULT group default qlen 3000
    link/ether 0a:78:08:ac:fc:04 brd ff:ff:ff:ff:ff:ff

1|:/ # ip address
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
       valid_lft forever preferred_lft forever
    inet6 ::1/128 scope host
       valid_lft forever preferred_lft forever
2: umts_dm0: <POINTOPOINT,MULTICAST,NOARP> mtu 1500 qdisc noop state DOWN group default qlen 1000
    link/ppp
3: rmnet0: <POINTOPOINT,MULTICAST,NOARP> mtu 1500 qdisc noop state DOWN group default qlen 1000
    link/ppp
4: rmnet1: <POINTOPOINT,MULTICAST,NOARP,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UNKNOWN group default qlen 1000
    link/ppp
    inet 19.18.242.148/8 scope global rmnet1
       valid_lft forever preferred_lft forever
5: rmnet2: <POINTOPOINT,MULTICAST,NOARP> mtu 1500 qdisc noop state DOWN group default qlen 1000
    link/ppp
6: rmnet3: <POINTOPOINT,MULTICAST,NOARP> mtu 1500 qdisc noop state DOWN group default qlen 1000
    link/ppp
7: rmnet4: <POINTOPOINT,MULTICAST,NOARP> mtu 1500 qdisc noop state DOWN group default qlen 1000
    link/ppp
8: rmnet5: <POINTOPOINT,MULTICAST,NOARP> mtu 1500 qdisc noop state DOWN group default qlen 1000
    link/ppp
9: rmnet6: <POINTOPOINT,MULTICAST,NOARP> mtu 1500 qdisc noop state DOWN group default qlen 1000
    link/ppp
10: rmnet7: <POINTOPOINT,MULTICAST,NOARP> mtu 1500 qdisc noop state DOWN group default qlen 1000
    link/ppp
11: sit0@NONE: <NOARP> mtu 1480 qdisc noop state DOWN group default
    link/sit 0.0.0.0 brd 0.0.0.0
12: ip6tnl0@NONE: <NOARP> mtu 1452 qdisc noop state DOWN group default
    link/tunnel6 :: brd ::
13: wlan0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP group default qlen 3000
    link/ether 08:78:08:ac:fc:04 brd ff:ff:ff:ff:ff:ff
    inet 192.168.0.50/24 brd 192.168.0.255 scope global wlan0
       valid_lft forever preferred_lft forever
    inet6 fe80::a78:8ff:feac:fc04/64 scope link
       valid_lft forever preferred_lft forever
14: p2p0: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN group default qlen 3000
    link/ether 0a:78:08:ac:fc:04 brd ff:ff:ff:ff:ff:ff

130|:/ # ip route
19.0.0.0/8 dev rmnet1 proto kernel scope link src 19.18.242.148
192.168.0.0/24 dev wlan0 proto kernel scope link src 192.168.0.50
:/ # ip rule
0:      from all lookup local
10000:  from all fwmark 0xc0000/0xd0000 lookup legacy_system
10500:  from all iif lo oif wlan0 uidrange 0-0 lookup wlan0
10500:  from all iif lo oif rmnet1 uidrange 0-0 lookup rmnet1
13000:  from all fwmark 0x10063/0x1ffff iif lo lookup local_network
13000:  from all fwmark 0x10065/0x1ffff iif lo lookup wlan0
13000:  from all fwmark 0x50064/0x5ffff iif lo lookup rmnet1
14000:  from all iif lo oif wlan0 lookup wlan0
14000:  from all fwmark 0x40000/0x40000 iif lo oif rmnet1 lookup rmnet1
15000:  from all fwmark 0x0/0x10000 lookup legacy_system
16000:  from all fwmark 0x0/0x10000 lookup legacy_network
17000:  from all fwmark 0x0/0x10000 lookup local_network
19000:  from all fwmark 0x65/0x1ffff iif lo lookup wlan0
22000:  from all fwmark 0x0/0xffff iif lo lookup wlan0
32000:  from all unreachable
:/ #
1|:/ # ip rule show table local
0:      from all lookup local
:/ # ip rule show table rmnet1
10500:  from all iif lo oif rmnet1 uidrange 0-0 lookup rmnet1
13000:  from all fwmark 0x50064/0x5ffff iif lo lookup rmnet1
14000:  from all fwmark 0x40000/0x40000 iif lo oif rmnet1 lookup rmnet1
:/ # ip rule show table wlan0
10500:  from all iif lo oif wlan0 uidrange 0-0 lookup wlan0
13000:  from all fwmark 0x10065/0x1ffff iif lo lookup wlan0
14000:  from all iif lo oif wlan0 lookup wlan0
19000:  from all fwmark 0x65/0x1ffff iif lo lookup wlan0
22000:  from all fwmark 0x0/0xffff iif lo lookup wlan0
:/ # ip rule show table local_network
13000:  from all fwmark 0x10063/0x1ffff iif lo lookup local_network
17000:  from all fwmark 0x0/0x10000 lookup local_network
:/ # ip rule show table legacy_network
16000:  from all fwmark 0x0/0x10000 lookup legacy_network
:/ # ip rule show table legacy_system
10000:  from all fwmark 0xc0000/0xd0000 lookup legacy_system
15000:  from all fwmark 0x0/0x10000 lookup legacy_system
:/ #
:/ # ip route show table local
broadcast 19.0.0.0 dev rmnet1 proto kernel scope link src 19.18.242.148
local 19.18.242.148 dev rmnet1 proto kernel scope host src 19.18.242.148
broadcast 19.255.255.255 dev rmnet1 proto kernel scope link src 19.18.242.148
broadcast 127.0.0.0 dev lo proto kernel scope link src 127.0.0.1
local 127.0.0.0/8 dev lo proto kernel scope host src 127.0.0.1
local 127.0.0.1 dev lo proto kernel scope host src 127.0.0.1
broadcast 127.255.255.255 dev lo proto kernel scope link src 127.0.0.1
broadcast 192.168.0.0 dev wlan0 proto kernel scope link src 192.168.0.50
local 192.168.0.50 dev wlan0 proto kernel scope host src 192.168.0.50
broadcast 192.168.0.255 dev wlan0 proto kernel scope link src 192.168.0.50
:/ # ip route show table rmnet1
default via 19.18.242.1 dev rmnet1 proto static
19.18.242.1 dev rmnet1 proto static scope link
19.18.242.148 dev rmnet1 proto static scope link
:/ # ip route show table wlan0
default via 192.168.0.1 dev wlan0 proto static
192.168.0.0/24 dev wlan0 proto static scope link
:/ # ip route show table local_network
:/ # ip route show table legacy_network
:/ # ip route show table legacy_system
:/ #

And with the hotspot on

:/ # ip link
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN mode DEFAULT group default
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
2: umts_dm0: <POINTOPOINT,MULTICAST,NOARP> mtu 1500 qdisc noop state DOWN mode DEFAULT group default qlen 1000
    link/ppp
3: rmnet0: <POINTOPOINT,MULTICAST,NOARP> mtu 1500 qdisc pfifo_fast state DOWN mode DEFAULT group default qlen 1000
    link/ppp
4: rmnet1: <POINTOPOINT,MULTICAST,NOARP,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UNKNOWN mode DEFAULT group default qlen 1000
    link/ppp
5: rmnet2: <POINTOPOINT,MULTICAST,NOARP> mtu 1500 qdisc noop state DOWN mode DEFAULT group default qlen 1000
    link/ppp
6: rmnet3: <POINTOPOINT,MULTICAST,NOARP> mtu 1500 qdisc noop state DOWN mode DEFAULT group default qlen 1000
    link/ppp
7: rmnet4: <POINTOPOINT,MULTICAST,NOARP> mtu 1500 qdisc noop state DOWN mode DEFAULT group default qlen 1000
    link/ppp
8: rmnet5: <POINTOPOINT,MULTICAST,NOARP> mtu 1500 qdisc noop state DOWN mode DEFAULT group default qlen 1000
    link/ppp
9: rmnet6: <POINTOPOINT,MULTICAST,NOARP> mtu 1500 qdisc noop state DOWN mode DEFAULT group default qlen 1000
    link/ppp
10: rmnet7: <POINTOPOINT,MULTICAST,NOARP> mtu 1500 qdisc noop state DOWN mode DEFAULT group default qlen 1000
    link/ppp
11: sit0@NONE: <NOARP> mtu 1480 qdisc noop state DOWN mode DEFAULT group default
    link/sit 0.0.0.0 brd 0.0.0.0
12: ip6tnl0@NONE: <NOARP> mtu 1452 qdisc noop state DOWN mode DEFAULT group default
    link/tunnel6 :: brd ::
13: wlan0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP mode DEFAULT group default qlen 3000
    link/ether 08:78:08:ac:fc:04 brd ff:ff:ff:ff:ff:ff
:/ #
:/ # ip address
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
       valid_lft forever preferred_lft forever
    inet6 ::1/128 scope host
       valid_lft forever preferred_lft forever
2: umts_dm0: <POINTOPOINT,MULTICAST,NOARP> mtu 1500 qdisc noop state DOWN group default qlen 1000
    link/ppp
3: rmnet0: <POINTOPOINT,MULTICAST,NOARP> mtu 1500 qdisc pfifo_fast state DOWN group default qlen 1000
    link/ppp
4: rmnet1: <POINTOPOINT,MULTICAST,NOARP,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UNKNOWN group default qlen 1000
    link/ppp
    inet 19.18.242.148/8 scope global rmnet1
       valid_lft forever preferred_lft forever
5: rmnet2: <POINTOPOINT,MULTICAST,NOARP> mtu 1500 qdisc noop state DOWN group default qlen 1000
    link/ppp
6: rmnet3: <POINTOPOINT,MULTICAST,NOARP> mtu 1500 qdisc noop state DOWN group default qlen 1000
    link/ppp
7: rmnet4: <POINTOPOINT,MULTICAST,NOARP> mtu 1500 qdisc noop state DOWN group default qlen 1000
    link/ppp
8: rmnet5: <POINTOPOINT,MULTICAST,NOARP> mtu 1500 qdisc noop state DOWN group default qlen 1000
    link/ppp
9: rmnet6: <POINTOPOINT,MULTICAST,NOARP> mtu 1500 qdisc noop state DOWN group default qlen 1000
    link/ppp
10: rmnet7: <POINTOPOINT,MULTICAST,NOARP> mtu 1500 qdisc noop state DOWN group default qlen 1000
    link/ppp
11: sit0@NONE: <NOARP> mtu 1480 qdisc noop state DOWN group default
    link/sit 0.0.0.0 brd 0.0.0.0
12: ip6tnl0@NONE: <NOARP> mtu 1452 qdisc noop state DOWN group default
    link/tunnel6 :: brd ::
13: wlan0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP group default qlen 3000
    link/ether 08:78:08:ac:fc:04 brd ff:ff:ff:ff:ff:ff
    inet 192.168.43.241/24 brd 192.168.43.255 scope global wlan0
       valid_lft forever preferred_lft forever
    inet6 fe80::a78:8ff:feac:fc04/64 scope link
       valid_lft forever preferred_lft forever
:/ #
:/ # ip route
19.0.0.0/8 dev rmnet1 proto kernel scope link src 19.18.242.148
192.168.43.0/24 dev wlan0 proto kernel scope link src 192.168.43.241
:/ # ip rule
0:      from all lookup local
10000:  from all fwmark 0xc0000/0xd0000 lookup legacy_system
10500:  from all iif lo oif rmnet1 uidrange 0-0 lookup rmnet1
10500:  from all iif lo oif wlan0 uidrange 0-0 lookup local_network
13000:  from all fwmark 0x10063/0x1ffff iif lo lookup local_network
13000:  from all fwmark 0x10064/0x1ffff iif lo lookup rmnet1
14000:  from all iif lo oif rmnet1 lookup rmnet1
14000:  from all iif lo oif wlan0 lookup local_network
15000:  from all fwmark 0x0/0x10000 lookup legacy_system
16000:  from all fwmark 0x0/0x10000 lookup legacy_network
17000:  from all fwmark 0x0/0x10000 lookup local_network
18000:  from all iif wlan0 lookup rmnet1
19000:  from all fwmark 0x64/0x1ffff iif lo lookup rmnet1
22000:  from all fwmark 0x0/0xffff iif lo lookup rmnet1
32000:  from all unreachable
:/ # ip rule show table local
0:      from all lookup local
:/ # ip rule show table rmnet1
10500:  from all iif lo oif rmnet1 uidrange 0-0 lookup rmnet1
13000:  from all fwmark 0x10064/0x1ffff iif lo lookup rmnet1
14000:  from all iif lo oif rmnet1 lookup rmnet1
18000:  from all iif wlan0 lookup rmnet1
19000:  from all fwmark 0x64/0x1ffff iif lo lookup rmnet1
22000:  from all fwmark 0x0/0xffff iif lo lookup rmnet1
:/ # ip rule show table wlan0
Error: argument "wlan0" is wrong: table id value is invalid

255|:/ # ip rule show table local_network
10500:  from all iif lo oif wlan0 uidrange 0-0 lookup local_network
13000:  from all fwmark 0x10063/0x1ffff iif lo lookup local_network
14000:  from all iif lo oif wlan0 lookup local_network
17000:  from all fwmark 0x0/0x10000 lookup local_network
:/ # ip rule show table legacy_network
16000:  from all fwmark 0x0/0x10000 lookup legacy_network
:/ # ip rule show table legacy_system
10000:  from all fwmark 0xc0000/0xd0000 lookup legacy_system
15000:  from all fwmark 0x0/0x10000 lookup legacy_system
:/ #
:/ # ip route show table local
broadcast 19.0.0.0 dev rmnet1 proto kernel scope link src 19.18.242.148
local 19.18.242.148 dev rmnet1 proto kernel scope host src 19.18.242.148
broadcast 19.255.255.255 dev rmnet1 proto kernel scope link src 19.18.242.148
broadcast 127.0.0.0 dev lo proto kernel scope link src 127.0.0.1
local 127.0.0.0/8 dev lo proto kernel scope host src 127.0.0.1
local 127.0.0.1 dev lo proto kernel scope host src 127.0.0.1
broadcast 127.255.255.255 dev lo proto kernel scope link src 127.0.0.1
broadcast 192.168.43.0 dev wlan0 proto kernel scope link src 192.168.43.241
local 192.168.43.241 dev wlan0 proto kernel scope host src 192.168.43.241
broadcast 192.168.43.255 dev wlan0 proto kernel scope link src 192.168.43.241
:/ # ip route show table rmnet1
default via 19.18.242.1 dev rmnet1 proto static
19.18.242.1 dev rmnet1 proto static scope link
19.18.242.148 dev rmnet1 proto static scope link
:/ # ip route show table wlan0
Error: argument "wlan0" is wrong: table id value is invalid

255|:/ # ip route show table local_network
192.168.43.0/24 dev wlan0 proto static scope link
:/ # ip route show table legacy_network
:/ # ip route show table legacy_system
:/ #

I think I can make it work by deleting everything and simplifying but I think Android will just recreate the rules with a network change so want to work out the rule that i have overlooked

EDIT

All resolved. The hotspot adds routing between the two interfaces which is missing when the hotspot disconnects. Just need to add these 2 rules back:

:/ # ip rule add from all iif rmnet1 lookup wlan0
:/ # ip rule add from all iif wlan0 lookup rmnet1
9
  • I didn't understand well your setup explanation, but I can see that the default route is missing.
    – A.B
    Apr 7 '20 at 11:33
  • I do find that strange but the output of "ip r" is the same with or without the hotspot it doesn't list a default route either way. I don't know how android is knowing to send things out of the 4G connection but it does Apr 7 '20 at 11:57
  • try ip rule. there might be additional rules
    – A.B
    Apr 7 '20 at 12:54
  • Ah I get a bit lost without the route command. Thank you thats a massive help. I've added 18000: from all iif wlan0 lookup rmnet1 which is a rule that is present when the hotspot is on and now the responses in tcpdump are correct but the pc isnt getting the reply. The PC now thinks its connected to the internet using the phone as the gateway but the connection isnt working. I guess i just need to understand the rules a bit more. Thanks again Apr 7 '20 at 14:34
  • also stop using ifconfig: use ip link; ip address; ip route; ip rule but not ifconfig or route. On linux they are obsolete. Then can't handle ip rule and routing tables. some have the -brief option to make them less verbose
    – A.B
    Apr 7 '20 at 14:47

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