there have been similar questions on StackExchange but none of the answers helped me, so I'll try a question of my own.

I have a VPN connection via OpenVPN. By default, all traffic is redirected through the tunnel using OpenVPN's "two more specific routes" trick, but I disabled that. My routing table is like this:

198.144.156.141 192.168.2.1     255.255.255.255 UGH   0      0        0 eth0
10.30.92.5      0.0.0.0         255.255.255.255 UH    0      0        0 tun1
10.30.92.1      10.30.92.5      255.255.255.255 UGH   0      0        0 tun1
192.168.2.0     0.0.0.0         255.255.255.0   U     0      0        0 eth0
0.0.0.0         10.30.92.5      0.0.0.0         UG    0      0        0 tun1
0.0.0.0         192.168.2.1     0.0.0.0         UG    0      0        0 eth0

And the interface configuration is like this:

# ifconfig
eth0      Link encap:Ethernet  HWaddr XX-XX-
          inet addr:192.168.2.100  Bcast:192.168.2.255  Mask:255.255.255.0
          inet6 addr: fe80::211:9ff:fe8d:acbd/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:394869 errors:0 dropped:0 overruns:0 frame:0
          TX packets:293489 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:388519578 (370.5 MiB)  TX bytes:148817487 (141.9 MiB)
          Interrupt:20 Base address:0x6f00

tun1      Link encap:UNSPEC  HWaddr 00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00  
          inet addr:10.30.92.6  P-t-P:10.30.92.5  Mask:255.255.255.255
          UP POINTOPOINT RUNNING NOARP MULTICAST  MTU:1500  Metric:1
          RX packets:64 errors:0 dropped:0 overruns:0 frame:0
          TX packets:67 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:100 
          RX bytes:9885 (9.6 KiB)  TX bytes:4380 (4.2 KiB)

plus the lo device.

The routing table has two default routes, one via eth0 through my local network router (DSL modem) at 192.168.2.1, and another via tun1 through the VPN's gateway. With this configuration, if I connect to a site, the route chosen is the direct one (because it has less hops?):

# traceroute 8.8.8.8 -n
traceroute to 8.8.8.8 (8.8.8.8), 30 hops max, 60 byte packets
 1  192.168.2.1  0.427 ms  0.491 ms  0.610 ms
 2  213.191.89.13  17.981 ms  20.137 ms  22.141 ms
 3  62.109.108.48  23.681 ms  25.009 ms  26.401 ms
...

This is fine, because my goal is to send only traffic from specific applications through the tunnel (esp. transmission, using its -i / bind-address-ipv4 option). To test whether this can work at all, I check it first with traceroute's -s option:

# traceroute 8.8.8.8 -n -s 10.30.92.6
traceroute to 8.8.8.8 (8.8.8.8), 30 hops max, 60 byte packets
 1  * * *
 2  * * *
 3  * * *
...

This I take to mean that connection using the tunnel's local address as source is not possible.

What is possible (though only as root) is to specify the source interface:

# traceroute 8.8.8.8 -n -i tun1
traceroute to 8.8.8.8 (8.8.8.8), 30 hops max, 60 byte packets
 1  10.30.92.1  129.337 ms  297.758 ms  297.725 ms
 2  * * *
 3  198.144.152.17  297.653 ms  297.652 ms  297.650 ms
...

So apparently the tun1 interface is working and it is possible to send packets through it.

But selecting the source interface is not implemented in my actual target application (transmission), so I would like to get source address selection to work.

What am I doing wrong?