a question about Slirp usage

Hi,

I came across this tool slirp, which seems very powerful and interesting, and fits my needs perfectly. However, with all the information (very little) I can find online, I could not get it working.

e.g. I have tried the instructions in the doc HOWTO-TEST in the package slirp on Ubuntu - 20.04 and Debian bullseye

#pppd nodefaultroute noauth nobsdcomp nodetach debug :10.1.2.1 pty "ssh -t <email address hidden> slirp ppp"
using channel 1
Using interface ppp0
Connect: ppp0 <--> /dev/pts/2
sent [LCP ConfReq id=0x1 <asyncmap 0x0> <magic 0xdc7000b9> <pcomp> <accomp>]
rcvd [LCP ConfReq id=0x1 <mru 1500> <magic 0xed08dc07> <pcomp> <accomp>]
sent [LCP ConfAck id=0x1 <mru 1500> <magic 0xed08dc07> <pcomp> <accomp>]
rcvd [LCP ConfAck id=0x1 <asyncmap 0x0> <magic 0xdc7000b9> <pcomp> <accomp>]
sent [LCP EchoReq id=0x0 magic=0xdc7000b9]
sent [CCP ConfReq id=0x1 <deflate 15> <deflate(old#) 15>]
sent [IPCP ConfReq id=0x1 <compress VJ 0f 01> <addr 0.0.0.0>]
rcvd [LCP EchoRep id=0x0 magic=0xed08dc07]
rcvd [IPCP ConfReq id=0x1 <addr 10.0.0.4> <compress VJ 0f 01>]
sent [IPCP ConfNak id=0x1 <addr 10.1.2.1>]
rcvd [CCP ConfReq id=0x1]
sent [CCP ConfAck id=0x1]
rcvd [CCP ConfRej id=0x1 <deflate 15> <deflate(old#) 15>]
sent [CCP ConfReq id=0x2]
rcvd [IPCP ConfNak id=0x1 <addr 10.0.2.15>]
sent [IPCP ConfReq id=0x2 <compress VJ 0f 01> <addr 10.0.2.15>]
rcvd [IPCP ConfReq id=0x2 <addr 10.1.2.1> <compress VJ 0f 01>]
sent [IPCP ConfAck id=0x2 <addr 10.1.2.1> <compress VJ 0f 01>]
rcvd [CCP ConfAck id=0x2]
rcvd [IPCP ConfAck id=0x2 <compress VJ 0f 01> <addr 10.0.2.15>]
local IP address 10.0.2.15
remote IP address 10.1.2.1
Script /etc/ppp/ip-up started (pid 3219)
Script /etc/ppp/ip-up finished (pid 3219), status = 0x0

The interface is up, but the network is not working:
# route -n
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
0.0.0.0 10.211.55.1 0.0.0.0 UG 100 0 0 eth0
10.1.2.1 0.0.0.0 255.255.255.255 UH 0 0 0 ppp0
10.211.55.0 0.0.0.0 255.255.255.0 U 0 0 0 eth0
10.211.55.1 0.0.0.0 255.255.255.255 UH 100 0 0 eth0

ICMP is not working

# ping 10.1.2.1
PING 10.1.2.1 (10.1.2.1) 56(84) bytes of data.
^C
--- 10.1.2.1 ping statistics ---
8 packets transmitted, 0 received, 100% packet loss, time 7168ms

TCP is not working either

# route add -host www.google.com dev ppp0
# route -n
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
0.0.0.0 10.211.55.1 0.0.0.0 UG 100 0 0 eth0
10.1.2.1 0.0.0.0 255.255.255.255 UH 0 0 0 ppp0
10.211.55.0 0.0.0.0 255.255.255.0 U 0 0 0 eth0
10.211.55.1 0.0.0.0 255.255.255.255 UH 100 0 0 eth0
142.251.43.4 0.0.0.0 255.255.255.255 UH 0 0 0 ppp0

# telnet 142.251.43.4 80
Trying 142.251.43.4...
^C

But if I add 10.0.2.0/24 to route to ppp0, ICMP is working
# route add -net 10.0.2.0/24 dev ppp0
# route -n
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
0.0.0.0 10.211.55.1 0.0.0.0 UG 100 0 0 eth0
10.0.2.0 0.0.0.0 255.255.255.0 U 0 0 0 ppp0
10.1.2.1 0.0.0.0 255.255.255.255 UH 0 0 0 ppp0
10.211.55.0 0.0.0.0 255.255.255.0 U 0 0 0 eth0
10.211.55.1 0.0.0.0 255.255.255.255 UH 100 0 0 eth0
142.251.43.4 0.0.0.0 255.255.255.255 UH 0 0 0 ppp0

# ping 10.0.2.2
PING 10.0.2.2 (10.0.2.2) 56(84) bytes of data.
64 bytes from 10.0.2.2: icmp_seq=1 ttl=255 time=148 ms
64 bytes from 10.0.2.2: icmp_seq=2 ttl=255 time=151 ms
64 bytes from 10.0.2.2: icmp_seq=3 ttl=255 time=150 ms
64 bytes from 10.0.2.2: icmp_seq=4 ttl=255 time=155 ms
64 bytes from 10.0.2.2: icmp_seq=5 ttl=255 time=153 ms
^C
--- 10.0.2.2 ping statistics ---
5 packets transmitted, 5 received, 0% packet loss, time 4013ms
rtt min/avg/max/mdev = 148.017/151.304/154.708/2.284 ms

TCP forwarding is still not working, nor is 10.0.2.2 itself.

I spent a lot of time on finding the answer, it seems all the info is from the last century, and it is not working anymore.

Any suggestion is appreciated.

Best regards,