[Qemu-devel] Slirp/uIP incompatibility

[Filip Navara <filip.navara@gmail.com>]

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Hello,
I'm not sure this worth looking into or fixing, but there is an
incompatibility between the uIP TCP/IP stack and Slirp as used with -net
user option.

Guest runs uIP in an emulated ARM machine with the AT91SAM7-EK board:

$ arm-softmmu/qemu-system-arm.exe -M at91sam -kernel
tests/basic-emac-uip-helloworld-at91sam7x-ek-at91sam7x256-sram.elf -net
nic,model=at91sam,macaddr=00:45:5
6:78:9a:bc -net user -redir tcp:1000::1000

The guest successfully receives the IP addresses using DHCP and starts to
listen on port 1000. Now I try to connect to it from the host machine. When
the SYN packet is received in the guest code, the SYN-ACK packet is
prepared. Due to an implementation error it is never sent though and ARP
request is sent instead, if the ARP tables are not filled already (which is
always the case for the first connection). After a while Slirp figures out
that no reply was received and sends the SYN again. The guest code thinks
that it already sent SYN-ACK and replies with ACK, probably due to
mismatched sequence numbers at that point. This repeats practically forever
and none of the parties can recover from it (by the means of RST or other
way).

If anyone can offer some advice how to resolve it or how the TCP/IP stacks
should behave if the first SYN-ACK during the connection establishment is
lost I'd be glad. Attached is a packet dump from QEMU VLAN.

Best regards,
Filip Navara

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