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I have a couple of 3G modems installed into my RaspberryPI via USB hub with external power adapter. The goal is to create several ppp connection via these modems and share the internet via socks5 proxy (1 carrier -> 1 dedicated port).

root@raspberrypi:/etc# ifconfig
eth0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500
 inet 192.168.88.81 netmask 255.255.255.0 broadcast 192.168.88.255
 inet6 fe80::be35:5f3a:e942:e39a prefixlen 64 scopeid 0x20<link>
 ether b8:27:eb:92:b1:0b txqueuelen 1000 (Ethernet)
 RX packets 8747 bytes 702623 (686.1 KiB)
 RX errors 0 dropped 2448 overruns 0 frame 0
 TX packets 1452 bytes 183993 (179.6 KiB)
 TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0

lo: flags=73<UP,LOOPBACK,RUNNING> mtu 65536
 inet 127.0.0.1 netmask 255.0.0.0
 inet6 ::1 prefixlen 128 scopeid 0x10<host>
 loop txqueuelen 1000 (Local Loopback)
 RX packets 4 bytes 156 (156.0 B)
 RX errors 0 dropped 0 overruns 0 frame 0
 TX packets 4 bytes 156 (156.0 B)
 TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0

ppp-kyivstar0: flags=4305<UP,POINTOPOINT,RUNNING,NOARP,MULTICAST> mtu 1500
 inet 10.203.1.38 netmask 255.255.255.255 destination 10.64.64.64
 ppp txqueuelen 3 (Point-to-Point Protocol)
 RX packets 16 bytes 382 (382.0 B)
 RX errors 0 dropped 0 overruns 0 frame 0
 TX packets 16 bytes 514 (514.0 B)
 TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0

ppp-vodafone0: flags=4305<UP,POINTOPOINT,RUNNING,NOARP,MULTICAST> mtu 1440
 inet 100.87.250.240 netmask 255.255.255.255 destination 10.64.64.65
 ppp txqueuelen 3 (Point-to-Point Protocol)
 RX packets 7 bytes 58 (58.0 B)
 RX errors 0 dropped 0 overruns 0 frame 0
 TX packets 7 bytes 82 (82.0 B)
 TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0

The config file of dante-server looks like this:

root@raspberrypi:/etc# grep -v "^#" /etc/danted-vodafone0.conf | grep -v "^$"
debug: 0
logoutput: stderr stdout
internal: 192.168.88.81 port = 50002
internal: 127.0.0.1 port = 50002
external: ppp-vodafone0
socksmethod: none
clientmethod: none
user.privileged: proxy
user.unprivileged: nobody
user.libwrap: nobody
client pass 
 from: 192.168.88.0/24 port 1-65535 to: 0.0.0.0/0

client pass 
 from: 127.0.0.0/8 port 1-65535 to: 0.0.0.0/0

client block 
 from: 0.0.0.0/0 to: 0.0.0.0/0
 log: connect error

socks block 
 from: 0.0.0.0/0 to: lo
 log: connect error

socks pass 
 from: 192.168.88.0/24 to: 0.0.0.0/0
 protocol: tcp udp

socks pass 
 from: 127.0.0.0/8 to: 0.0.0.0/0
 protocol: tcp udp

socks block 
 from: 0.0.0.0/0 to: 0.0.0.0/0
 log: connect error

When I try to test that with curl - nothing happens:

# curl -v --socks5 192.168.88.81:50002 http://ifconfig.co
* Rebuilt URL to: http://ifconfig.co/
* Trying 192.168.88.81...
* TCP_NODELAY set
* SOCKS5 communication to ifconfig.co:80
* SOCKS5 connect to IPv4 104.27.140.78 (locally resolved)
* Can't complete SOCKS5 connection to 0.0.0.0:0. (6)
* Closing connection 0
curl: (7) Can't complete SOCKS5 connection to 0.0.0.0:0. (6)

From other side curl is able to make a connection through that interface when I soecify that as a parameter:

# curl -v --interface ppp-vodafone0 http://ifconfig.co
* Rebuilt URL to: http://ifconfig.co/
* Trying 104.27.140.78...
* TCP_NODELAY set
* Local Interface ppp-vodafone0 is ip 100.120.201.176 using address family 2
* Local port: 0
* Connected to ifconfig.co (104.27.140.78) port 80 (#0)
> GET / HTTP/1.1
> Host: ifconfig.co
> User-Agent: curl/7.52.1
> Accept: */*
>
< HTTP/1.1 200 OK
< Date: Sat, 08 Sep 2018 00:20:50 GMT
< Content-Type: text/plain; charset=utf-8
< Content-Length: 14
< Connection: keep-alive
< Set-Cookie: __cfduid=df1da9ab56a621c1a8e3a1e75faac555c1536366050; expires=Sun, 08-Sep-19 00:20:50 GMT; path=/; domain=.ifconfig.co; HttpOnly
< Via: 1.1 vegur
< Server: cloudflare
< CF-RAY: 456d4066058483ee-KBP
<
46.133.227.38
* Curl_http_done: called premature == 0
* Connection #0 to host ifconfig.co left intact 

What can be wrong here?

I found the problem. First of all, during ppp session I especially set up the ppp peer without default route provisioning. Because of this the outgoing packet from pppX interface had no idea how to be routed. The situation was resolved with so called policy based routing.

First of all you need to pre-define all your additional routing tables in /etc/iproute2/rt_tables. My files looks like this:

# Ansible managed
#
# reserved values
#
255 local
254 main
253 default
0 unspec
#
# local
#
#1 inr.ruhep
103 kyivstar0
101 lifecell0
102 vodafone0

These 101..103 are the routing tables for PPP interfaces. After that you need to set up the policy. It looks like this:

ip rule add from 100.83.31.204 table vodafone0
ip route add 0.0.0.0/0 via 10.64.64.64 dev ppp-vodafone0 table vodafone0

Where:

• 
  100.83.31.204 - Local PPP IP address


• 
  10.64.64.64 - Remote PPP IP address


• 
  ppp-vodafone0 - the name of PPP interface


• 
  vodafone0 - the route table name predefined in /etc/iproute2/rt_tables
  

After tuning up policy based routing everything works as expected.