Syn-Ack dropped for port forwarding service at 80

[smanikandan]

be Hi ,

I have installed the latest firmware V1.0.0.52 in the router.

I have setup two port forwarding to Linux web server (apache) namely 80 and 443.

Port forwarding for 443 works well and I am able to connect from external IP address.

But, for port 80:80, I see the SYN coming all the way to Linux web server box with proper NATing, apache replies with a SYN-ACK and that never reaches the external IP address. See below for tcpdump,

01:30:50.170126 IP (tos 0x0, ttl 51, id 0, offset 0, flags [DF], proto TCP (6), length 64)
<external address>.35330 > <internal address>.http: Flags [S], cksum 0x13a8 (correct), seq 2224545020, win 65535, options [mss 1360,nop,wscale 5,nop,nop,TS val 2264106004 ecr 0,sackOK,eol], length 0
01:30:50.170254 IP (tos 0x0, ttl 64, id 0, offset 0, flags [DF], proto TCP (6), length 60)
<internal address>.http > <external address>.35330: Flags [S.], cksum 0x0268 (correct), seq 2619162788, ack 2224545021, win 28960, options [mss 1460,sackOK,TS val 130337831 ecr 2264106004,nop,wscale 7], length 0

01:30:50.276935 IP (tos 0x0, ttl 51, id 0, offset 0, flags [DF], proto TCP (6), length 64)

Just to eliminate any other network issue, I tried port forwarding 8080 (external):80 (internal) and it works well.

So, clearly, the router is dropping the SYN-ACK packets. Is there any workarounds or solution for this issue?