Invalid checksum calculated for ipv4 SYN+ACK when opt.srr is set?

From: Jeroen van Bemmel
Date: Thu Aug 01 2013 - 17:27:20 EST


Although rarely used and considered plain evil by some, the kernel
code allows one to use IP source routing for TCP.

In tcp_v4_send_synack the following two lines of code are used:
__tcp_v4_send_check(skb, ireq->loc_addr, ireq->rmt_addr);
err = ip_build_and_send_pkt(skb, sk, ireq->loc_addr,ireq->rmt_addr,ireq->opt);

In ip_build_and_send_pkt the 'daddr' parameter is used as follows:
iph->daddr = (opt && opt->opt.srr ? opt->opt.faddr : daddr);

It looks like the checksum may be off when (opt && opt->opt.srr ) is true?

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